Paleontology, Taphonomy, and

Stratigraphy of the

Browns Park Formation

(Oligocene and Miocene)

Near Maybell, Moffat County, Colorado

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1358
Paleontology, Taphonomy, and

Stratigraphy of the

Browns Park Formation

(Oligocene and Miocene)

Near Maybell, Moffat County, Colorado

By JAMES G. HONEY and GLEN A. IZETT

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 135

Descriptions of fossil mammals from the Browns 

Park Formation in northwestern Colorado, 

with a discussion of the stratigraphy of the 

formation and the burial mechanics of the fossils

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Honey, James G.

Paleontology, taphonomy, and stratigraphy of the Browns Park Formation (Oligocene 

and Miocene) near Maybell, Moffat County, Colorado.

(U.S. Geological Survey professional paper ; 1358) 

Bibliography: p.

Supt. of Docs, no.: I 19.16:1358

1. Paleontology Oligocene. 2. Paleontology Miocene. 3. Paleontology 

Colorado Moffat County. I. Izett, Glen Arthur, 1928- . II. Title. III. Series.

QE738.H66 1988 560'.9788'12 86-600107

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225

Any use of trade names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey.
CONTENTS

Page

Abstract ........................................... 1

Introduction ....................................... 1

Background .................................... 1

Purpose ........................................ 2

Location ....................................... 2

Previous investigations ........................... 2

Abbreviations ................................... 2

Fieldwork and acknowledgments ................... 3

Stratigraphy of the Browns Park Formation ............ 4

Distribution .................................... 4

Physical characteristics and outcrop pattern ......... 4

Geologic setting and contact relations .............. 4

Lithology and sedimentary structures .............. 5

Basal conglomerate unit ....................... 5

White, crossbedded sandstone unit .............. 5

White, parallel-bedded sandstone unit ........... 9

Tan sandstone unit ........................... 9

Volcanic ash ................................. 10

Thickness of the Browns Park Formation ........... 10

Source area and mode of deposition ................ 11

Vertebrate taphonomy of the Browns Park Formation .... 11

Introduction .................................... 11

On the role of water as a dispersive agent ........... 12

Bone orientations ............................ 12

Relative abundance of bones ................... 13

Comparisons with locality 7498 ................ 14

Processes affecting destruction and dispersal of bones at

locality D856 .................................. 15

Conclusions ..................................... 18

Vertebrate fauna of the Browns Park Formation ......... 18

Locations of fossil sites ........................... 18

Mammalian paleontology ......................... 20

Order Carnivora Bowdich ...................... 20

Family Canidae Gray ...................... 20

Page

Vertebrate fauna Continued

Mammalian paleontology Continued 

Order Carnivora Bowdich Continued 

Family Canidae Gray Continued

Genus Tomarctus Cope ................. 20

Tomarctus cf. T. paula Henshaw ..... 20

Order Perissodactyla Owen .................... 24

Family Equidae Gray ..................... 24

Genus Megahippus McGrew ............ 24

Megahippus sp. .................... 24

Genus Parahippus Leidy ............... 26

Parahippus cf. P. leonensis Sellards ... 26

Order Artiodactyla Owen ...................... 29

Family Camelidae Gray .................... 29

Subfamily Stenomylinae Matthew ........ 29

Genus Blickomylus Frick and Taylor . . 29 

Blickomylus galushai Frick and

Taylor ...................... 29

Subfamily Camelinae Gray .............. 31

Genus cf. Protolabis Cope ........... 31

Genus Michenia Frick and Taylor .... 32

Michenia sp. ................... 32

Subfamily Camelinae or Aepycamelinae ... 35

Procamelus sp. or Aepycamelus sp. ... 35

Subfamily Camelinae sp. ............... 39

Family Antilocapridae Gray ................ 40

Subfamily Merycodontinae Matthew ..... 40

Genus Meryceros Frick ............. 40

Meryceros warreni (Leidy) ........ 40

Meryceros sp. .................. 41

Merycodontinae sp. .................... 44

Mammalian age and correlation ....................... 45

Isotopic age ........................................ 48

References cited .................................... 48

ILLUSTRATIONS

FIGURE 1. Index map showing distribution of the Browns Park Formation in northwest Colorado

2. Photograph of Browns Park Formation at locality D854 ..........................

3. Rose diagrams of dip direction measurements in Browns Park sandstone ...........

4. Stereographic and rose diagrams showing bone orientations at locality D856 quarry . .

5. Stereographic and rose diagrams showing bone orientations at UALP locality 7498 . . .

6. Photograph of weathering characteristics of bone from locality D856 quarry .........

7. Map showing fossil mammal localities southwest of Maybell, Colo.

3

6

7

12

14

17

19

in
IV CONTENTS

	Page

FIGURE 8-18. Photographs:

8. Tomarctus cf. T. paula ...................................................................... 22

9. Megahippus sp. ............................................................................ 25

10. Parahippus cf. P. leonensis .................................................................. 27

11. Blickomylus galushai ....................................................................... 30

12. cf. Protolabis sp. ........................................................................... 33

13. Michenia sp. .............................................................................. 34

14. Procamelus sp. or Aepycamelus sp. ........................................................... 37

15. Camelinae sp. .............................................................................. 39

16. Meryceros warreni and Merycodontinae sp. .................................................... 40

17. Meryceros sp. ............................................................................. 41

18. Meryceros sp. and Merycodontinae sp. ........................................................ 43

TABLES

Page

TABLE 1. Bones from locality D856 and their distribution into Voorhies Groups ...................................... 13

2. Observed and expected frequencies of bones at locality D856 .............................................. 13

3. Bones from UALP locality 7498 and their distribution into Voorhies Groups ................................. 15

4. Observed and expected frequencies of bones for Michenia, Protolabis, and Equidae from UALP locality 7498 ..... 16

5. Data used to calculate observed frequencies given in table 4 ............................................... 17

6. Miocene fossil mammals from the Browns Park Formation ................................................ 20

7. Skull and dental measurements of Tomarctus cf. T. paula ................................................. 21

8. Dental and skeletal measurements of Megahippus sp. .................................................... 25

9. Skull and dental measurements of Parahippus cf. P. leonensis ............................................. 26

10. Dental and skeletal measurements of Blickomylus galushai ................................................ 29

11. Skeletal measurements of cf. Protolabis sp. ...................................................... 32

12. Dental and skeletal measurements of Michenia sp. ....................................................... 33

13. Dental and skeletal measurements of Procamelus sp. or Aepycamelus sp. .................................... 36

14. Skeletal measurements of Camelinae sp. ............................................................. 39

15. Comparative measurements of Meryceros warreni and Meryceros crucensis .................................. 40

16. Dental and skeletal measurements of Meryceros sp. and comparison with M. hookwayi ........................ 42

17. Fission-track age data for zircon microphenocrysts ....................................................... 48
PALEONTOLOGY, TAPHONOMY, AND STRATIGRAPHY OF THE

BROWNS PARK FORMATION (OLIGOCENE AND MIOCENE)

NEAR MAYBELL, MOFFAT COUNTY, COLORADO

By JAMES G. HONEY and GLEN A. IZETT

ABSTRACT

Fossils from four sites in the sparsely fossiliferous Browns Park 

Formation provide a new record of mammalian life in the middle 

Miocene in northwestern Colorado. The Browns Park fossils allow 

f aunal dating of the upper part of the formation in the Maybell, Colo. 

area. Measurements of the orientation, relative abundance, and preservation of the fossil mammal bones at one locality indicate that the 

bones were not subjected to water currents before burial. The fracturing, destruction, and random orientation of the bones were probably caused mainly by intense carnivore activity; weathering may also 

have contributed to the bone destruction.

The Browns Park Formation crops out extensively in northwestern 

Colorado, and locally is more than 555 m thick near the town of 

Maybell. In the area we studied, which is southwest of Maybell, the 

formation is composed primarily of sandstone that contains a layer 

of conglomerate and conglomeratic sandstone about 9 m thick at its 

base. Pebbles and cobbles in the conglomerate were derived from adjacent topographic highs or from more distant sources. The overlying sandstone comprises three informal units. A white, crossbedded 

sandstone is the most widely exposed; the dip of its large-scale, highangle sets indicates that it was deposited by winds blowing from the 

southwest. A white, parallel-bedded sandstone lacks eolian crossstrata. A tan, parallel-bedded silty and clayey sandstone contains some 

mudstone and is probably of fluvial origin. The distribution of the 

basal conglomerate and the white, crossbedded sandstone suggests 

that the Browns Park Formation, at least in the Maybell area, was 

deposited in late Oligocene and Miocene time by the ancestral Yampa 

River flowing west from the northern Park Range in Colorado. Volcanic 

ash beds as thick as 3 m occur locally in the formation and range in 

age from 23.3±3.7 m.y. near the base of the formation to 11.3± 

0.8 m.y. near the highest preserved parts of the formation.

Despite its thickness and extent, the Browns Park Formation contains few fossils. In Colorado, only small collections from two localities 

had been reported before our study. In 1972, we discovered fossil mammals at four localities, comprising two local faunas, west of Maybell. 

The older local fauna, which we name the Cross Mountain local fauna 

(locality D857), is early to late Hemingfordian in age and contains the 

horse, Parahippus cf. P. leonensis, and the camel, Blickomylus 

galushai. The younger local fauna, which we name the Cedar Springs 

Draw local fauna (Iocs. D854, D855, D856), is late Barstovian to early 

Clarendonian in age. It contains the following fossil mammals:

the dog, Tomarctus cf. T. paula

the horse, Megahippus sp.

the camel, cf. Protolabis

the camel, Michenia sp.

the camel, Procamelus sp. or Aepycamelus sp.

a camel, genus and species indeterminate

an antilocaprid, Meryceros warreni

an antilocaprid, Meryceros sp.

an antilocaprid, genus and species indeterminate 

The Cedar Springs Draw local fauna most closely resembles a largely 

undescribed late Barstovian to possibly early Clarendonian fauna from 

the Pojoaque Member of Galusha and Blick (1971) in the Tesuque Formation near Espanola, N. Mex. A volcanic ash in the Browns Park 

Formation that projects to the stratigraphic level of locality D855 

has a fission-track age of 11.3±0.8 m.y. This age is latest Barstovian 

or early Clarendonian.

No generic similarity exists between the assemblages found in the 

Browns Park Formation at the Cross Mountain and Cedar Springs 

Draw local faunas of the Maybell area and Peterson's Douglas Mountain fauna of the Browns Park Formation farther northwest in Colorado; accordingly, comparisons among the faunas cannot be made. 

The Douglas Mountain fauna is difficult to date precisely owing to 

the small number of genera, but the fauna is considered to be of late 

Barstovian age.

INTRODUCTION

BACKGROUND

The Browns Park Formation was deposited in northwest Colorado, northeast Utah, and south-central Wyoming during latest Oligocene and Miocene time. The 

formation is as much as 600 m thick. Isotopic ages 

(Izett, 1975, p. 186) determined on minerals from 

volcanic ash beds interlayered in the formation range 

from about 26 m.y. near the base of the formation to 

about 9 m.y. in the highest preserved parts of the formation. The Oligocene and Miocene time boundary, as 

used herein, is 23.7 m.y. based on a recent estimate 

made by Berggren and others (1985, p. 1409).

Fossils are rare in the Browns Park Formation, 

despite its extent and thickness. Previous discoveries 

of fossil vertebrates in the formation in Colorado have 

been reported from only two localities one of unrecorded stratigraphic position and another from the

1
BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

middle part of the formation (Peterson, 1924,1928). At 

both localites the fossil collections are small. In 1972, 

we discovered fossil vertebrates in the upper part of the 

formation at several localities west of Maybell, Colo. 

These fossils provide a significant record of mammalian 

life in northwest Colorado in medial Miocene time and 

allow faunal dating of the upper part of the formation 

in the Maybell area.

PURPOSE

The primary purpose of this paper is to describe the 

fossil mammals we collected from the Browns Park Formation between Maybell and Elk Springs, Colo. A second purpose is to compare the Browns Park fossil 

assemblages with other North American fossil mammal 

assemblages, thereby temporally placing the Browns 

Park fossils within the biochronological framework of 

North American land mammal ages. A third purpose 

is to propose a mechanism for burial (taphonomy) of the 

fossils. Brief descriptions of the rocks of the Browns 

Park Formation in the study area are included.

LOCATION

Most exposures of the Browns Park Formation 

studied, and all the fossil localities discussed in this 

report are between 40 and 63 km west of Craig, Colo. 

The area studied is in the northern half of the Citadel 

Plateau 15-minute quadrangle, and in the northeast part 

of the Elk Springs 15-minute quadrangle (fig. 1). The 

study area for the purpose of this report includes exposures of Browns Park Formation from Juniper Mountain on the east to locality D857 on the west, and from 

the town of Maybell on the north (located just outside 

the Citadel Plateau quadrangle), to the southernmost 

limit of Browns Park exposures in the Citadel Plateau 

quadrangle.

Access to the northern part of the study area is by 

U.S. Highway 40, which connects Craig, Maybell, and 

Elk Springs, Colo. The Deception Creek road, which 

connects Maybell with Meeker, Colo., extends along the 

eastern margin of the study area. The Cedar Springs 

Draw road, which runs south from U.S. Highway 40 

and connects with the Deception Creek road, crosses 

the western part of the study area. Numerous unimproved dirt roads provide easy access to other points 

within the study area. Many exposures of Browns Park 

Formation were studied along U.S. Highway 40 between Craig and Elk Springs, Colo., a distance of about 

69 km.

PREVIOUS INVESTIGATIONS

The Browns Park Formation was named by Powell 

(1876) for outcrops of upper Cenozoic rocks exposed 

along the Green River in Browns Park in western Colorado and eastern Utah. Numerous geologists since 

Powell have studied the Browns Park Formation, and 

a thorough discussion of the older studies can be found 

in Buffler (1967, p. 9).

Several studies have been made of the Browns Park 

Formation in northwestern Colorado. Winkler (1970) 

described a white, sandstone unit near Maybell and concluded that it is of eolian origin. Izett and others (1970) 

determined an isotopic age of 24.8±0.8 m.y. for the 

lower part of the Browns Park Formation along the 

Little Snake River. Later, Izett (1975) reported a 

9.1± 1.0 m.y. isotopic age for the upper part of the formation west of the Little Snake River in sec. 21, T. 9 N., 

R. 101 W., Moffat Co., Colo., and briefly discussed 

folding and faulting in the Browns Park Formation in 

northwest Colorado. Grutt and Whalen (1955), Bergin 

and Chisholm (1956), and Lewis (1977), discussed 

uranium mineralization in the Browns Park Formation 

in the Maybell-Lay area. Luft and Thoen (1981) 

measured a number of partial stratigraphic sections in 

the Browns Park Formation in Moffat County, Colo. 

Mapping by Sears (1924b), Hancock (1925), Bergin 

(1959), Dyni (1968), McKay (1974), McKay and Bergin 

(1974), Brownfield and Anderson (1979), Brownfield and 

Prost (1979), Rowley and others (1979), Reheis (1981), 

and Izett and others (1985) delineated the distribution 

of the Browns Park Formation in northwest Colorado.

Peterson (1924) described Browns Park fossils found 

near Sunbeam, Colo., and other fossils found a short 

distance farther west along Sand Wash. Later (1928) 

he described fossils collected from north of Douglas 

Mountain near Greystone, Colo. McGrew (1951, 

p. 55-56), Skinner (1968), and Frick and Taylor (1968) 

reexamined some of Peterson's Browns Park fossils; 

McGrew also reported on a small fauna from rocks near 

Saratoga, Wyo. that he assigned to the Browns Park 

Formation. An undescribed collection of Browns Park 

fossil mammals collected near Peterson's Douglas 

Mountain locality is in the Frick collection of the 

American Museum of Natural History.

ABBREVIATIONS

The following abbreviations are used in this report: 

Institutions

AMNH Department of Vertebrate Paleontology, 

American Museum of Natural History
INTRODUCTION

WYOMING 

COLOiADO

R 103W R 102W FM01W R 100W R 99W R 98W R 97W R 96W R 95W R 94W R 93W R.92W R91W R 90W

FIGURE 1. Distribution of the Browns Park Formation (patterned area) in northwest Colorado and location of the Citadel Plateau, Elk 

Springs, Lone Mountain, and Maybell 15-minute quadrangles, and the Lay and Lay SE TVfe-minute quadrangles. Letters designate fossil 

localities: a, D854; b, D857; c, D855 and D856 of this report; d, Peterson's Weller Ranch locality near Douglas Mountain; e, Peterson's 

Sunbeam locality near Maybell. DCR, Deception Creek road. Distribution of Browns Park Formation modified from Tweto (1979).

BEG University of Texas, Bureau of Economic

Geology 

LACM-CIT formerly the California Institute of

Technology; the collection is now at the Los

Angeles County Museum 

D854, D855, D856, D857-U.S. Geological Survey

vertebrate locality numbers, Denver register 

ESP Field designation on some specimens in the

AMNH 

F:AM Frick American Mammals, Department of

Vertebrate Paleontology, American Museum of

Natural History

FGS Florida Geological Survey 

UNSM Nebraska State Museum 

UALP University of Arizona, Laboratory of

Paleontology 

UCMP University of California, Museum of

Paleontology, Berkeley 

USNM U.S. National Museum 

Anatomy 

C canine

d deciduous, as in dl, deciduous incisor 

I incisor 

L-left 

M molar 

P premolar 

R right 

Measurements and statistics

AP anteroposterior measurement

EF expected frequency

n sample size

OF observed frequency

O.R. observed range

TR transverse measurement

X mean

m.y. million years

FIELDWORK AND ACKNOWLEDGMENTS

This paper resulted from a study undertaken as 

part of a U.S. Geological Survey mapping program
BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

conducted by G. A. Izett, J. G. Honey, and M. E. 

Brownfield in the Citadel Plateau 15-minute quadrangle, 

Moffat County, Colo. Fossil and stratigraphic data were 

collected in the summers of 1972 and 1973 and during 

one short trip to the area in the summer of 1975.

We appreciate advice and criticism concerning the 

mammalian paleontology given by E. H. Lindsay and 

G. G. Simpson (deceased) of the'University of Arizona. 

L. L. Jacobs, L. H. Taylor, L. J. Flynn, and J. Baskin, 

all formerly of the University of Arizona, aided with 

quarrying at the University of Arizona's locality 7498 

(discussed in this report). We owe special thanks to 

R. H. Tedford, B. E. Taylor, and M. F. Skinner of the 

American Museum of Natural History for allowing us 

to examine specimens in their care, for the loan of 

fossils, and for information concerning the fossils. We 

thank C. B. Schultz and L. G. Tanner of the University 

of Nebraska for showing us fossils in their care. M. E. 

Brownfield of the U.S. Geological Survey helped us collect fossils on several occasions.

STRATIGRAPHY OF THE 

BROWNS PARK FORMATION

DISTRIBUTION

The Browns Park Formation is extensively exposed 

in northwest Colorado and eastern Utah, and other 

sandstone outcrops in southern Wyoming have been 

correlated with the formation. The formation crops out 

in a continuous west-trending belt from an area just 

west of Craig, Colo., to its type area in Browns Park, 

which straddles the Utah-Colorado state line, a distance 

of about 136 km (Hansen, 1965, p. 115). In this belt the 

Browns Park Formation was deposited in a paleovalley 

cut on the Uinta Mountain arch, which was formed more 

than 26 m.y. ago (Izett, 1975, p. 189). Rocks referred 

to the Browns Park Formation are mapped on the north 

and south flanks of the Uinta Mountains, Utah, on the 

west flank of the Sierra Madre and Park Range in Colorado and Wyoming, on the White River Plateau in Colorado, and in the Poison Basin area, Wyoming (Izett, 

1975, p. 184-185).

PHYSICAL CHARACTERISTICS AND 

OUTCROP PATTERN

The sandstone of the Browns Park Formation erodes 

easily and is poorly exposed in the Maybell, Colo. area. 

The formation forms a blanketlike deposit of low relief 

in the northwest quarter of the Citadel Plateau 

quadrangle. The formation generally lies near the

surface beneath a soil, but in many places it is covered 

by Pleistocene and Holocene eolian deposits, as south 

of Maybell, or by alluvial deposits, as along the Yampa 

River. At lower elevations, the formation is covered by 

grass and sage; at higher elevations, the formation is 

covered by shrubs and pinyon pine. The best exposures 

are in roadcuts, along arroyos, and occasionally, on 

steep slopes. The Browns Park forms some cliffs along 

the Yampa River between Juniper and Cross Mountains, and a small area of badlands on the west flank 

of Juniper Mountain. We examined all outcrops within 

the Citadel Plateau quadrangle, and numerous others 

outside the study area.

GEOLOGIC SETTING AND 

CONTACT RELATIONS

The study area includes the southern half of the westtrending depositional basin of the Browns Park Formation. The deepest part of the depositional basin is probably north of the study area, near Maybell, Colo., where 

more than 555 m of Browns Park have been drilled (S. J. 

Luft, written commun., 1980); the greatest thicknesses 

in the Citadel Plateau quadrangle, within the study 

area, are 510 m at the Texaco No. 1 State-K well in the 

NW1/4SE1/4 sec. 16, T. 6 N., R. 96 W., and 533 m at 

a core hole near this well. In the northeast corner of the 

Citadel Plateau quadrangle, the Browns Park paleovalley is split by the Juniper Mountain uplift, which exposes Precambrian and Paleozoic rocks surrounded by 

sandstone outcrops of the Browns Park Formation. To 

the west, the Browns Park Formation extends uninterrupted north and south of the Cross Mountain anticline.

The Browns Park Formation unconformably overlies 

all older formations and rests on rocks ranging in age 

from Precambrian to middle Eocene. Along the southern edge of the depositional basin, the Browns Park 

overlaps Mesozoic rocks, and farther west lies against 

the upturned edges of Cretaceous and Eocene strata. 

At the Texaco No. 1 State-K well, in the northwest part 

of the Citadel Plateau quadrangle, the Browns Park 

rests on Cretaceous Mancos Shale.

The Browns Park Formation within the study area 

is mostly flat lying. Along the southern edge of the 

Browns Park depositional basin, however, bedding dips 

from about 5 ° to 30 ° to the north, and along the northern edge of the depositional basin the rocks locally dip 

30° to the south (McKay and Bergin, 1974). PostBrowns Park faulting has considerably steepened the 

southerly dips along the northern edge of the basin 

(Sears, 1924a, p. 288).

A few small normal faults with throws generally less 

than 50 m locally cut the Browns Park in the Citadel
STRATIGRAPHY

Plateau quadrangle. Larger unseen faults may cut the 

formation in the study area, but because much of the 

formation is covered by surficial deposits their locations 

and throws are unknown.

LITHOLOGY AND SEDIMENTARY STRUCTURES

BASAL CONGLOMERATE UNIT

At surface exposures and in boreholes, the base of the 

Browns Park Formation commonly consists of a conglomerate or conglomeratic sandstone unit with clasts 

derived both from local bedrock and from distant 

source-rock areas. Just east of the Citadel Plateau 

quadrangle, near Juniper Hot Springs, Colo., the basal 

conglomerate consists of well-rounded pebbles and cobbles of Precambrian rocks composed of schist, gneiss, 

coarse- to fine-grained granite, white and reddish 

quartzite, and white and reddish vein quartz (Hancock, 

1925, p. 24). This may be the same conglomerate containing varicolored pebbles that was mentioned by 

Sears (1924a, p. 285) as being north of the Yampa River 

upstream from Juniper Canyon. South of Juniper Hot 

Springs, Reheis (1981) also reported igneous and metamorphic cobbles in the basal sandstone of the Browns 

Park. Presumably, all these pebbles and cobbles were 

derived from outcrops of Precambrian rocks in the Park 

Range east of Steamboat Springs, Colo. (Izett, 1975, p. 

203). Along the south flank of Juniper Mountain in the 

northeast quarter of the Citadel Plateau quadrangle, the 

basal conglomerate unit contains quartzite boulders as 

much as 0.9 m in diameter of the Middle Proterozoic 

Uinta Mountain Group, perhaps locally derived from 

outcrops at Juniper Mountain. At surface exposures 

throughout much of the western part of the Citadel 

Plateau quadrangle (for example, in sec. 18, T. 5 N., R. 

96 W.), the basal unit of the Browns Park consists chiefly of conglomerate and conglomeratic sandstone containing clasts of quartzite from the Middle Proterozoic 

Uinta Mountain Group, Paleozoic carbonate rocks, and 

chert, the last possibly of Paleozoic age. No igneous or 

high-grade metamorphic clasts were seen. However, in 

the Pan-American Petroleum No. 1 George Norvell well, 

in the NEl/4 sec. 3, T. 5 N., R. 96 W., igneous rock 

fragments were recorded in the basal conglomeratic 

unit. North and northwest of the Citadel Plateau 

quadrangle, in the parts of the Maybell and Lone Mountain quadrangles east of the Little Snake River, the 

basal conglomerate unit also contains pebbles of mafic 

and felsic igneous and metamorphic rocks, quartzite, 

chert, and quartz (Sears, 1924b, p. 295; McKay, 1974; 

McKay and Bergin, 1974). West of the Little Snake 

River, in Colorado, the conglomerate consists only of

red quartzite from the Uinta Mountain Group and some 

gray Paleozoic carbonate rock and chert. In the Elk 

Springs quadrangle west of the Citadel Plateau 

quadrangle, Dyni (1968) noted only carbonate and chert 

cobble conglomerate and red conglomeratic sandstone 

in the basal part of the Browns Park; these clasts were 

probably locally derived from outcrops on Cross and 

Juniper Mountains and the eastern Uinta Mountains. 

In the Park Range to the east, Crews (1963, p. 66) and 

Buffler (1967, p. 24) reported granite and high-grade 

metamorphic rocks in the basal conglomerate of the 

Browns Park Formation.

WHITE, CROSSBEDDED SANDSTONE UNIT

White, crossbedded sandstone in the Browns Park 

Formation is widely distributed in northwest Colorado, 

and is locally well exposed in the Maybell area. In most 

areas, poor exposures make it impossible to determine 

the precise distribution of these rocks, but topographic 

expression and weathering characteristics suggest that 

the unit occurs at the surface over the entire area west 

of the Deception Creek road and north of T. 5 N. (except for a small area of Pleistocene and Holocene dunes 

in the northeast corner of this area). Rocks of this unit 

apparently blanket large areas to the north and west 

of the Citadel Plateau quadrangle in the Maybell, Lone 

Mountain, and Elk Springs quadrangles, and they extend eastward nearly to Craig, Colo.

This unit consists of gray to white quartzose sandstone that locally contains some interbedded volcanic 

ash beds and tuffaceous sandstone. The quartzose sandstone is fine to medium grained, well sorted, and the 

grains are subrounded to rounded and generally frosted. 

The sandstone contains more than 90 percent quartz 

grains, with black opaque mineral grains and dark lithic 

grains composing the remainder. Calcite locally cements 

this sandstone, which then forms resistant ledges. Beds 

of the white, crossbedded sandstone unit usually show 

high-angle cross-stratification, as in roadcuts along U.S. 

Highway 40 (fig. 2) and in cliffs along the Yampa River 

north of Maybell. Low-angle bounding surfaces commonly separate individual sets of high-angle crossstrata. Horizontal lamination is also present, as at fossil 

quarry D856. The white, crossbedded sandstones are 

generally friable, and in a few places have been a sediment source for active or recently stabilized dunes.

We examined crossbedding in the white, crossbedded 

sandstone unit to determine the transport direction of 

the sand grains. Crossbedding is best seen in roadcuts, 

and consists mainly of wedge-planar and tabular-planar 

(McKee, 1979, fig. 137) sets, which are medium to large 

scale, and straight to concave upward. These large-scale 

cross-strata were observed in roadcuts from Craig west
BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

FIGURE 2. View of roadcut exposure at locality D854 along U. S. Highway 40, at the west edge of the Citadel Plateau 15-minute quadrangle, 

Moffat County, Colo., showing large-scale, cross-stratified sandstone in the upper part of the Browns Park Formation. Person at center 

of photograph shows scale.

to Elk Springs, Colo. Dips recorded at these roadcuts 

generally range from 2° to 34°, with most high-angle 

dips clustering between 24° and 29°. Dips greater than 

30 ° in crossbedded sandstone of the Browns Park also 

were noted by Buffler (1967, p. 43) in the Elkhead region 

east of Craig, Colo.

The large-scale, high-angle, tabular- and wedge-planar 

sets in the Browns Park Formation are foresets formed 

on the slip faces of dunes. The large size and high angle 

of the individual sets, the low-angle bounding surfaces 

between some of the sets (McKee, 1979, p. 89), and the 

lack of coarse-grained channel-lag deposits suggest that 

the sandstone was deposited by eolian processes. A 

sample of medium-grained, well-sorted sandstone, with 

grains subrounded and frosted, was taken from foresets 

at roadcut locality D854. Most of the sandstone at D854 

is not cemented by calcite, and the frosting of the grains 

probably was caused by abrasion during transport 

rather than by dissolution of grains by intrastratal 

solutions.

Winkler (1970, p. 48) also suggested an eolian origin 

for the crossbedded sandstone unit of the Browns Park 

in the Maybell area. Earlier, Bradley (1936, p. 182) 

thought that the Browns Park Formation west of Craig, 

Colo. and north of the Citadel Plateau quadrangle was 

in part eolian in origin. In addition, Chisholm (1963, 

p. 353) suggested that parts of the Browns Park Formation in northwestern Colorado and south-central 

Wyoming are eolian, and Buffler (1967, p. 60) suggested 

an eolian origin for his white sandstone facies of the 

Browns Park Formation in the Elkhead Mountains east 

of Craig, Colo.

The transport direction of the white, crossbedded 

sandstone was determined by 191 dip direction measurements taken on cross-strata along the 69-km stretch 

of highway between Craig and Elk Springs, Colo. Dip 

measurements taken from both high-and low-angle sets 

indicate an average transport direction of about 

N. 45 °E. (fig. 3A). Laminae dipping less than 15° might 

have been deposited on the backs and sides of dunes,
STRATIGRAPHY

so a second average was calculated from the 135 dip 

measurements that were greater than 15°. The transport direction calculated was N. 47 °E., nearly the same 

as before (fig. 3B). Chisholm and others (1961, p. 84) obtained a south-to-north sediment transport direction for 

the Browns Park Formation, derived from more than 

600 measurements made over a wide area. In the 

Elkhead region, Buffler (1967, fig. 10) obtained a 

bimodal distribution of dip maxima based on 250 bedding measurements in his white sandstone facies of the 

Browns Park Formation. His dip directions are also 

spread over a range of nearly 180 °, but the mean of the 

measurements demonstrates a southwest to northeast 

transport direction, as in the area west of Craig, Colo. 

Figure 3 is interpreted to indicate a unimodal wind 

regime. Both diagrams show the dominant northeast 

direction of sand transport; 87 percent of all the dips 

are within a 140° arc in fig. 3A, and 93 percent of the 

high-angle dips (fig. 3B) are within the same arc. These 

dip direction distributions are similar to those reported 

for some barchan and dome dunes (Glennie, 1970, figs. 

82 and 83; Ahlbrandt and Fryberger, 1980, p. 8; McKee, 

1966, table IV). Parabolic dunes have a wide distribution (180°-270°) of dip directions which may be strongly 

bimodal or trimodal (Ahlbrandt and Andrews, 1978,

fig. 5; McKee and Bigarella, 1979, fig. 78; McKee, 1979, 

p. 195). The dip directions shown in figures 3A and 3B 

show no strong bimodality, but this may be partly 

because most of the dip readings were taken in an eastwest section along Highway 40, where roadcuts gave 

the best exposures of crossbedding. Sample bias may 

thus have increased the number of measurements taken 

of high-angle strata dipping parallel to the highway and 

decreased the number of measurements taken on strata 

dipping normal to the highway. The spread of Browns 

Park dip directions is wider than that reported for 

transverse dunes (Ahlbrandt and Fryberger, 1980, fig. 6). 

The distribution of dip directions also does not correspond to expected distributions for linear (seif) dunes, 

which should have two clusters of dips opposite one 

another (McKee and Tibbitts, 1964; McKee, 1979), or 

dunes formed from scattered wind directions (star and 

reversing), in which a significant number of high-angle 

dips should be opposite the main northeast transport 

direction (McKee, 1966; McKee and Bigarella, 1979). 

Stratigraphic sections were measured at fossil sites 

D855 and D856 in the white, crossbedded sandstone 

unit. Locality D855 is stratigraphically 15 m below 

locality D856. The following stratigraphic section shows 

the lithology at locality D855:

340

320

GO0

300

FIGURE 3. Rose diagrams of dip direction measurements of the white, crossbedded sandstone unit of the Browns Park Formation in 

roadcuts along U.S. Highway 40 west of Craig, Colo. Concentric circles and associated values represent percent of total number of 

observations. Numbers on periphery of each circle show dip direction. A, plot of 191 dip directions. B, plot of 135 dip directions (dips 

equal to or greater than 15°).
8 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

Section of the Browns Park Formation (top to bottom) exposed on 

northwest slope of a ridge in the NE1/4 sec. 36, T. 6 N., R. 97 W., 

Moffat County, Colorado

Thickness 

Meters Feet

Sandstone, quartzose, in part tuffaceous, very fine 

grained to fine-grained, well-sorted, grains frosted; 

less than 1 percent dark lithic fragments. Alternating resistant and nonresistant layers about 6.4 

cm thick; resistant layers calcareous. Vertebrate 

fossils found beneath a thin calcareous mudstone 

at top of unit (D855) ....................... 1.07 3.5

Tuff and tuffaceous sandstone, white, thinly 

laminated, highly calcareous ............ ... 0.76 2.5

Volcanic ash, blue-gray, vitric, very finely laminated 0.15 0.5

Sandstone and tuffaceous sandstone, very fine grained 

to fine-grained, highly calcareous; individual 

laminae 0.10-0.51 cm thick; more tuffaceous parts 

weather as resistant white laminae .......... 1.22 4.0

Tuffaceous sandstone and tuff, gray at bottom of unit 

and white at top; no calcite cement; individual 

laminae 0.10-0.51 cm thick . ... ........ 0.15 0.5

Quartzose sandstone, tan, very fine grained, moderately well sorted; individual laminae 0.10-0.30 cm 

thick .......... ....................... 0.15 0.5

Bottom of exposure ....................... 0.15 0.5

Total thickness of section .......... 3.50 11.5

At locality D856 the 1-m-thick unit contains four noncrossbedded layers. The only stratification observed 

was in the fossiliferous layer, where thin, horizontal 

laminae 3.8 cm thick were seen. Each of the four layers 

is composed of fine-grained quartzose sandstone; the individual grains are subrounded to rounded and well 

sorted. Larger grains tend to be more rounded than 

smaller grains. The quartz grains are frosted and are 

cemented by calcite. Many short, white, noncalcareous 

stringers, interpreted as root casts, are present in 

several of the layers. No sedimentary structures other 

than the horizontal lamination and white stringers were 

observed. The following measured section shows the 

lithology at locality D856:

Measured section of the Browns Park Formation (top to bottom) exposed near top of a west-facing slope of a hill (1981 m) in the NW1/4 

sec. 31, T. 6 N., R. 96 W., Moffat County, Colorado

Thickness 

Meters Feet

Sandstone, brown; to top of hill; not measured

Sandstone, grayish-white, slightly green, quartzose, 

fine-grained, well-sorted; grains subrounded to 

rounded, frosted, and calcite cemented; root casts 

present near bottom; becomes broken into balls and 

grades into brown sandstone above .......... 0.46 1.5

Sandstone, greenish-white, quartzose, fine-grained; 

dark grains rounded, well sorted, and frosted; less 

than 1 percent lithic fragments, highly calcareous 

and indurated; root casts common ..... ..... 0.18 0.58

Sandstone, greenish-gray, quartzose, fine-grained, 

well-sorted, less than 1 percent lithic fragments; 

grains subrounded and frosted, calcite cemented; 

thinly laminated; root casts common; fossil horizon 0.13 0.42

Sandstone, greenish-gray, quartzose, fine-grained, 

well-sorted, less than 10 percent dark lithic 

fragments; grains subrounded to rounded, calcite 

cemented; unit massive; root casts common near 

top ................................. 0.25

Base of excavation

Total thickness of section .............. 1.02

0.83

3.33

The sandstone at locality D856 is probably of eolian 

origin. Some evidence is inconclusive: the frosting of the 

grains, for example, could result from either eolian 

transport or chemical solution. But most of the evidence 

suggests eolian origin: horizontal sand laminae 

associated with eolian deposition have been described 

from modern interdune, sheet sand, and wadi-fill 

deposits (Ahlbrandt and Fryberger, 1981, and references therein), and from ancient eolian deposits (Adams 

and Patton, 1979; McKee, 1979, and references therein). 

No silt or clay interbeds were seen at D856, and visual 

examination suggests that the sand is nearly as well 

sorted as in nearby Browns Park dune deposits. Glennie (1970, p. 39 and 106) mentioned and illustrated the 

occurrence of horizontally laminated, well-sorted sands 

in both desert wadis and sheet sands. Ahlbrandt and 

Fryberger (1981, fig. 7) interpreted some horizontally 

laminated sandstones of the Browns Park Formation 

near Maybell, Colo., as interdune deposits. Horizontal 

laminae could have been present in the other layers at 

locality D856, but may have been destroyed due to 

bioturbation by plant roots.

Horizontal stratification also occurs in lacustrine 

deposition, but no lacustrine fossils such as freshwater 

mollusks or fish were found in the rocks at locality 

D856. Also, no load structures, mudcracks, or any other 

features that might have formed in a lacustrine environment were found.

Horizontal stratification may also occur locally due 

to flood deposition, and may constitute the bulk of a 

flood deposit (McKee and others, 1967, p. 829). In the 

Bijou Creek, Colo., flood deposit studied by McKee and 

others (1967), the sediment was fine- to coarse-grained 

quartzose sand. In contrast to the sandstone at D856, 

the sorting usually was only fair; rarely was the sorting 

good. Williams (1971, p. 35) however, noted that upper 

flow regime plane beds were the best sorted in the flood 

deposit he studied. The D856 locality lacks the armored 

mud balls noted at Bijou Creek. The fine-grained sandstone at D856 would not suggest high water velocities; 

however, the grain size in flood deposits partially 

depends on the size of the material available for 

transport (McKee and others, 1967; Williams, 1971). If 

the layers at locality D856 are flood deposits, it might 

be supposed that the fossils represent the larger material transported during the flood. However, the fossils 

show no evidence of transport in their orientation or
STRATIGRAPHY 9

abundance. The field evidence then, suggests that the 

sandstone at locality D856 is of eolian rather than of 

fluvial or lacustrine origin.

WHITE, PARALLEL-BEDDED SANDSTONE UNIT

White, parallel-bedded sandstone crops out west of 

the Deception Creek road in sees. 1, 2, and 11, T. 5 N., 

R. 96 W., south of Juniper Mountain, and along the 

north bank of the Yampa River just north of Juniper 

Mountain. Rocks at these exposures lack the high-angle 

cross-stratification of the white, crossbedded unit. The 

parallel-bedded unit is generally highly calcareous, resistant, and laterally continuous. In the study area, 

white, parallel-bedded sandstone is typically exposed 

near the base of the Browns Park Formation, and is 

older than the eolian crossbedded sandstone. South of 

Juniper Mountain, for example, the Browns Park is only 

152 m thick in the Moore and Gilmore No. 1 Seeley well 

drilled in the NE1/4SE1/4SE1/4, sec. 2, T. 5 N., 

R. 95 W., and rocks of the white, parallel-bedded unit 

are exposed near this location. Along the Deception 

Creek road, white, parallel-bedded sandstone directly 

overlies Mancos Shale of Cretaceous age. In contrast, 

rocks of the white, crossbedded unit occur at the surface at the Texaco No. 1 State-K well in the 

NW1/4SE1/4 sec. 16, T. 6 N., R. 96 W., and are 

underlain by about 510 m of Browns Park Formation.

TAN SANDSTONE UNIT

Rocks of the tan sandstone unit of the Browns Park 

crop out on Cedar Knob in sees. 6 and 7, T. 5 N., R. 96 

W. These rocks contain more mudstone than do rocks 

of the white, crossbedded sandstone unit; bedding is 

generally parallel, and no large-scale, high-angle crossstratification is present. The measured section that 

follows gives the general characteristics of rocks of this 

unit.

Section of the Browns Park Formation (top to bottom) on south side 

of Cedar Knob in the NE1/4 sec. 7, T. 5 N., R. 96 W., Moffat County, Colorado

Thickness 

Meters Feet

Sandstone, tan, fine-grained, friable; forms slope; fossil 

turtle found near top of unit ................ 2.8 9.3

Sandstone, grayish-white, calcareous, fine- to mediumgrained, poorly sorted with dark lithic fragments, 

thick to massive bedded; forms top of resistant 

ledge .................................... 0.5 1.5

Mudstone, grayish-white, tuffaceous ........... 0.4 1.2

Volcanic ash, white, vitric, thinly laminated near base, 

becoming calcareous near top; forms bottom of 

resistant ledge ............................ 0.6 1.8

Mudstone, pinkish-tan, calcareous and indurated; contains root casts ........................... 0.8 2.5

Sandstone, pinkish-tan, friable; discontinuous 

calcareous mudstone ledge near base of unit; 0.4 m 

resistant but discontinuous calcareous sandstone 

ledge 1.5 m below top of unit ............... 3.1 10.0

Sandstone, greenish-gray, fine- to coarse-grained, poorly sorted, calcareous; contains coarse, subangular 

lithic fragments; a few root casts present; forms 

resistant ledge ............................ 0.9 3.0

Sandstone, pinkish-tan, medium-grained, friable; top 

0.61 m calcareous and thinly laminated ....... 5.2 17.0

Calcareous mudstone, white at base, becoming darker 

gray with medium-grained sandstone in upper 15.3 

cm; forms resistant ledge ................... 0.3 1.0

Volcanic ash, blue-gray, and tuffaceous sandstone; 

thinly laminated near base; calcareous near top of 

unit ..................................... 1.1 3.7

Sandstone with calcareous interbeds, fine-grained; 

forms slope except for more resistant interbedded 

calcareous units ........................... 2.4 7.9

Calcareous mudstone, grayish-tan .............. 3.1 10.0

Sandstone, tan, medium-grained, moderately well 

sorted, subangular, unconsolidated; forms slope at 

base of hill ............................... 1.8 5.9

Total thickness of section .............. 23.0 74.8

Surficial deposits hide the exact stratigraphic relationships between the tan sandstone unit and the white, 

crossbedded sandstone unit. On Cedar Knob, only rocks 

of the tan sandstone unit are exposed, and this unit was 

not recognized elsewhere in the study area. The section 

measured on Cedar Knob is topographically from about 

46 m to more than 61 m above locality D856, and 

bedding-plane attitudes were nearly horizontal. Cedar 

Knob may have been faulted up relative to the rest of 

the Browns Park Formation, as suggested by the gross 

lithologic similarity of the rocks at Cedar Knob to the 

rocks exposed nearly 48 km farther west at the northern 

base of Douglas Mountain. There, Peterson's (1928) 

fossil locality lies stratigraphically lower in the Browns 

Park Formation than fossil locality D856. Near Douglas 

Mountain, Peterson (1928, p. 93) measured 224 m from 

the base of the Browns Park Formation to the fossil 

horizon; the total thickness of the Browns Park Formation at locality D856 is unknown, but it is 533 m thick 

north of D856, in SE1/4 sec. 16, T. 6 N., R. 96 W. If, 

however, the rocks at Cedar Knob have not been faulted 

up relative to the rest of the formation, then the tan 

sandstone unit would be the highest preserved part of 

the Browns Park Formation. This relationship corresponds to the stratigraphic relations noted by Buffler (1967, p. 46) in the Elkhead region east of Craig, 

Colo., where he described a Miocene brown sandstone 

f acies overlying an eolian crossbedded white sandstone 

facies. Buffler assigned an Arikareean to Hemingfordian age to the white sandstone facies, which would 

make it older than the white, crossbedded unit in the
10 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

Maybell area. Buffler's age assignment however, was 

based only on lithologic similarity of the Miocene 

rocks of the Steamboat Springs, Colo., area with the 

Arikareean to Hemingfordian Split Rock Formation 

(Love, 1961) in central Wyoming.

VOLCANIC ASH

The Browns Park contains lenticular beds of vitric, 

friable volcanic ash from the base to the highest preserved parts of the formation. The thickness of these 

beds ranges from several centimeters to several meters. 

Many of the ash beds are nearly pure in that they contain a high proportion of glass shards, and are not contaminated by sediments. Some of the ash beds are 

indurated and blocky, probably caused by incipient 

alteration of the glass shards.

In the eastern part of the Citadel Plateau quadrangle 

(SW1/4 sec. 10 and SE1/4 sec. 14, T. 5 N., R. 95 W.) one 

or more white, biotite-bearing ash beds, about 15 cm 

thick, occur about 9 m above the base of the Browns 

Park Formation. A gray ash bed 15 cm thick occurs 

3.0-3.7 m below this white ash. In sec. 10, T. 5 N., R. 

95 W., three white ash beds occur in a 1.4-m-thick interval directly overlying a gray ash. Farther west, in 

the Wl/2 sec. 9 and NE1/4 sec. 10, T. 5 N., R. 96 W., 

a biotite-bearing white ash occurs about 9 m above the 

base of the Browns Park, but gray ash beds are apparently missing. In sec. 10, this white ash was dated 

at 23.3±3.7 m.y. (Izett, 1975, p. 187, table 1), and may 

correlate with a white, biotitic ash about 30 m above 

the base of the Browns Park at the Little Snake River, 

dated at 24.8±0.08 m.y. (Izett and others, 1970, 

p. C150-C152).

Ash beds that are stratigraphically higher in the 

Browns Park Formation include two gray volcanic ash 

beds on a hill in the NW1/4SE1/4 sec. 25, T. 6 N., R. 

97 W. These ash beds are in the upper part of the white, 

crossbedded sandstone unit, and the higher of the two 

is the same elevation as the D855 quarry. The upper 

ash is 23 m stratigraphically above the lower ash, and 

has been dated at 11.3±0.8 m.y. (see Isotopic Age). Two 

bluish-gray ash beds, presumably of medial Miocene 

age, occur on the south side of Cedar Knob. These two 

ashes lie in the tan sandstone unit of the Browns Park 

Formation, and are about 10m stratigraphically apart. 

Izett (1975) reported other volcanic ash beds within the 

Browns Park Formation elsewhere in northwestern 

Colorado.

THICKNESS OF THE BROWNS PARK FORMATION

The Browns Park Formation thickens from zero along 

the southern edge of the Browns Park depositional

trough south of Cedar Knob to more than 555 m near 

Maybell (S. J. Luft, written comm., 1980). Similar maximum thicknesses were recorded at drill holes located 

west of the Deception Creek road and north of T. 5 N. 

South of Juniper Mountain, however, the formation is 

locally considerably thinner 153 m thick at the Moore 

and Gilmore No. 1 Seeley well in the NE1/4SE1/4SE1/4 

sec. 2, T. 5 N., R. 95 W. In the Elk Springs quadrangle, 

Dyni (1968) recorded a maximum thickness of about 

457 m east of Cross Mountain. In the Maybell quadrangle, McKay and Bergin (1974) recorded a maximum 

thickness of 488 m. In the Lone Mountain quadrangle, 

McKay (1974) recorded a similar thickness. To the east 

of the study area, the maximum thickness of the 

Browns Park Formation is 259 m in the Lay quadrangle 

(Brownfield and Prost, 1979) and 274 m in the Lay SE 

quadrangle (Brownfield and Anderson, 1979). Buffler 

(1967, p. 38) recorded a maximum thickness of 671 m 

at Sand Mountain in the Elkhead region east of Craig, 

Colo. In the type area of the formation in Browns Park, 

the total thickness of the formation is unknown, owing 

to lack of subsurface information, but Hansen (1965, 

p. 127) obtained a partial thickness of 250 m for the formation by measuring surface exposures in the type area, 

and S. J. Luft (written commun., 1980) reported at least 

364 m in the Vermillion Mesa 7 1/2-minute quadrangle, 

Moffat County, Colo.

In the Citadel Plateau quadrangle, a maximum thickness of about 9 m was measured for the basal conglomerate at surface exposures. In the subsurface, the 

basal conglomeratic sandstone ranges from less than 

6.1 m in the Utah Southern Kingston No. 2 well, in the 

center of the NW1/4NW1/4 sec. 11, T. 5 N., R. 96 W., 

to at least 27.4 m in the Moore and Gilmore No. 1 Seeley 

well.

In the Elk Springs quadrangle, Dyni (1968) recorded 

an average thickness of 12.2-27.4 m for the basal 

conglomerate unit and a maximum thickness of 41.1 m. 

In the Maybell quadrangle, McKay and Bergin (1974) 

reported a thickness for the basal conglomeratic sandstone of only 0.3-0.9 m, yet farther west, in the Lone 

Mountain quadrangle, McKay (1974) reported a 

thickness of as much as 91 m for the basal conglomerate. In the Elkhead region to the east, Buffler (1967, 

p. 20) reported a similar range of thicknesses 0 to 91 m.

The total thicknesses for the white, crossbedded sandstone unit, the white, parallel-bedded unit, and the tan 

sandstone unit were not determined in the study area. 

About 88 m of sandstone separates the base of the 

eolian sandstone at D854 from the top of the hill at 

quarry D856, which represents a minimum thickness 

for the crossbedded sandstone unit the maximum 

thickness is probably much greater. At the Texaco No. 

1 State-L well, just north of the study area, frosted
VERTEBRATE TAPHONOMY 11

quartz grains were present in the Browns Park sandstone down to a depth of at least 168 m. Winkler (1970, 

p. 50) reported a thickness of about 122 m for the white, 

crossbedded unit in the Maybell area, but the method 

of calculating this thickness was not given. Buffler 

(1967, p. 41) measured more than 183 m of sandstone 

containing large-scale cross-stratification in his white 

sandstone facies in the Elkhead region.

On Cedar Knob in the Citadel Plateau quadrangle, 

about 23 m of tan sandstone unit is exposed, but the 

total thickness of that unit is probably greater. The 

thickness of the white, parallel-bedded unit was not 

determined.

SOURCE AREA AND MODE OF DEPOSITION

As mentioned, the basal conglomerate in the study 

area is derived from both local and distant sources. Field 

evidence for derivation of the cobbles in the conglomerate unit from nearby topographic highs is found 

on the south flank of Juniper Mountain in the 

NE1/4NE1/4 sec. 34, T. 6 N., R. 95 W., where the base 

of the Browns Park consists of a fine-grained sandstone 

containing scattered quartzite boulders of the Middle 

Proterozoic Uinta Mountain Group. Some of these 

boulders are 0.9 m in diameter. About 3.2 km southeast 

of this locality, at the Moore and Gilmore No. 1 Seeley 

well, at least 27 m of slightly conglomeratic sandstone 

(but no distinct conglomerate beds) was penetrated at 

the base of the Browns Park. This conglomeratic sandstone may have been derived from nearby Precambrian 

and Paleozoic rocks at Juniper Mountain.

Outcrops of Precambrian and Paleozoic rocks at Cross 

Mountain, Colo., probably contributed more to the basal 

conglomerate in the Citadel Plateau quadrangle than 

did similar outcrops at Juniper Mountain. Quartzite 

pebbles and cobbles of the Uinta Mountain Group are 

larger and more numerous in the basal conglomerate in 

the western part of the Citadel Plateau quadrangle, indicating a western source for some of this material. As 

mentioned, the total thickness of the basal conglomerate increases to the west in the Elk Springs quadrangle, suggesting that the basal conglomerate 

extended east and south from near Cross Mountain, and 

formed an alluvial apron of gradually decreasing 

thickness. The eastern extent of this alluvial apron is 

unknown, but it probably terminated near Deception 

Creek, west of the barrier formed by Jumper Mountain. 

Igneous rock pebbles in the basal conglomerate in Pan 

American Petroleum No. 1 Norvell well, and the mafic 

and felsic igneous and high-grade metamorphic pebbles 

of the basal conglomerate in the Maybell and Lone 

Mountain quadrangles east of the Little Snake River, 

were derived from source areas other than the Uinta

Mountains. These pebbles were possibly derived from 

the northern Park Range, as suggested by Izett (1975, 

p. 203) for the pebbles at Juniper Hot Springs. This 

derivation suggests that a west-flowing river the 

ancestral Yampa originated in the Park Range and 

flowed at least as far west as the Little Snake River during late Oligocene and early Miocene time. The course 

of the ancestral Yampa west of the Lone Mountain 

quadrangle is uncertain, as only quartzite, chert, and 

limestone cobbles in the basal Browns Park conglomeratic unit have been reported west of the Little 

Snake River (Sears, 1924b; McKay, 1974).

It is possible that the extensive eolian dunes in the 

Maybell area were formed from sand deposited along 

the course of the ancestral Yampa River. Alien (1965, 

p. 162-163) noted that migrating dunes sometimes form 

belts along the course of a river and are formed from 

sand winnowed from channel deposits at times of low 

water. A sand belt is present today in northeast Colorado along the south side of the South Platte River, 

and its distribution can be seen on the Geologic Map 

of Colorado (Tweto, 1979). The white, eolian crossbedded sandstone of the Browns Park Formation is 

widespread between Craig and Cross Mountain, Colo., 

and may reflect the course of the ancestral Yampa. If 

so, then in Miocene time the Yampa was a west-flowing 

stream probably following a course similar to its modern 

course between Craig and Cross Mountain. Bergin and 

Chisholm (1965, p. 195) had previously suggested a 

possible Sierra Madre-Park Range origin for their nontuffaceous sandstone facies of the Browns Park in the 

Maybell area. Such a source would also indicate 

westward-flowing streams or a river from the Park 

Range in Miocene time. Southwest of Cross Mountain, 

eolian sandstone is also present at Elk Springs, Colo., 

and (S. J. Luft, written commun., 1980) eolian sandstone 

also occurs at places northwest of Cross Mountain to 

the Utah state line. What the relation of these occurrences of eolian sandstone are to the inferred location 

of the ancestral Yampa is unknown.

VERTEBRATE TAPHONOMY OF THE 

BROWNS PARK FORMATION

INTRODUCTION

The orientation of the fossil bones at locality D856, 

the relative abundance of the different bone elements, 

and the character of bone preservation indicates how 

the bones were buried and what may have altered them 

after burial. As stated by Voorhies (1969, p. 2), taphonomy "* * * is concerned with the factors intervening 

between a living fauna and the fossilization of a
12 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

fraction of it. It deals mainly with the post-mortem history of animal remains (especially their decay, transportation, and burial) but also with cause of death."

The bones at locality D856 are mainly disarticulated, 

fragmentary, and dispersed. Carnivore utilization of a 

limited food resource, with associated trampling and 

kicking, best accounts for the observed bone fragmentation and dispersal. Weathering at the time of deposition probably accounts for some of the surface texture 

observed on the bones. The bones do not appear to have 

been reworked or transported by water, based on bone 

abundance and orientation data, including comparisons 

with a site in Arizona where the bones apparently were 

deposited by running water. Other possible influences, 

including sediment compaction, tectonism, and calcite 

deposition, do not account for the condition and distribution of the bones at locality D856.

ON THE ROLE OF WATER AS A DISPERSIVE AGENT

BONE ORIENTATIONS

Orientation data help determine whether bones were 

aligned in a current of water. Stream-table experiments

by Voorhies (1969, p. 66) showed that water-transported 

bones tend to be deposited parallel or transverse to current direction, depending on the type of bone and the 

depth of water; long bones tend to align transverse to 

the current in shallow water and parallel to the current 

in deeper water. Bones that parallel the current direction have their large ends downstream. In his analysis 

of the Verdigre quarry flood deposit in Nebraska, 

Voorhies (1969, fig. 9) found two mutually perpendicular 

directions for the bones which he interpreted as parallel 

and transverse to the current direction. In the Verdigre 

deposit, many bones had high angles of plunge, which 

Voorhies attributed to two factors: rapid burial in a 

high-energy flood situation, with little chance to return 

to the more stable horizontal position; imbrication in 

an upstream direction. At locality D856, the orientation 

of fossil vertebrate bones was determined in the field 

and later plotted on a stereonet (fig. 4A). Most bones 

lie in a horizontal plane, and are represented on the 

stereo diagram by two points 180 degrees removed from 

each other on the periphery. A rose diagram constructed 

from the same data (fig. 4B), showing only the azimuth 

of each bone, indicates no strong preferred orientations 

that might be interpreted as parallel and transverse to 

a current. Most of the bones are near horizontal, and

300

60°

120°

140°

FIGURE 4. Orientation of fossil mammalian bones from the Browns Park Formation at locality D856 in the Citadel Plateau 15-minute 

quadrangle, Moffat County, Colo. A, Stereographic projection; poles of bones plotted on the lower surface of a stereonet; filled circle, 

one pole; open circle, two poles at same position; cross, three poles at same position. Observations at periphery of diagram indicate 

horizontal bones. B, Rose diagram showing azimuths of bones; observed asymmetry results from bones with plunge greater than 

0 degrees; values for these bones plot only in one quadrant. Concentric circles and associated values represent percent of total number 

of observations.
VERTEBRATE TAPHONOMY 

TABLE 1. Bones from locality D856 and their distribution into Voorhies Groups

13

Group I

6 vertebrae 

4 ribs

Group I/II

27 phalanges 

4 calcanea 

6 astragali 

7 podials 

1 scapula

Group II

6 distal humeri 

7 proximal radii-ulnae 

7 distal radii -ulnae 

2 distal tibiae 

5 distal metapodials 

7 proximal metapodials 

1 acetabulum

Group II/III

5 rami and partial rami

Group III

1 skull 

1 fragment skull 

1 mandible

none plunge at high angles. At the time of excavation, 

large ends were distinguished from small ends on only 

eight bones. Seven of these bones plunge northeast, 

with the large ends oriented northeast on five and 

southwest on two; the eighth bone plunges northwest, 

with the small end pointed northwest. These few bones 

indicate a slightly greater tendency for the large ends 

to be pointed north. The northwest and northeast 

plunge of the bones suggests that the upstream direction would be to the north if the bones were imbricated. 

However, only three of the eight bones have the large 

ends pointed south, which would be in the downstream 

direction. This suggests that the bones at locality D856 

were not acted upon by a current of water.

RELATIVE ABUNDANCE OF BONES

The relative abundance of the different skeletal 

elements at locality D856 (table 1) provides further information on whether the bones were influenced by 

water currents. The relative abundances are determined 

by listing each skeletal element and the number of times 

it occurs. The bones listed in table 1 are divided into 

five Voorhies Groups (Behrensmeyer 1975, p. 484) according to their susceptibility to water transport. 

Voorhies (1969, p. 16) noted that ribs, vertebrae, sacra, 

and sterna (Group 1) tend to be easily removed in a current; others, principally long bones, (Group II) are 

removed gradually. Rami, skulls, and mandibles (Group 

II/III, III) form a lag deposit. All three groups are present at locality D856. Group I is underrepresented owing to collecting bias. The observed and expected 

frequencies of selected skeletal elements are given in 

table 2. The observed frequencies of most of the long 

bones are only slightly greater than would be expected 

if these bones were not transported. On the other hand, 

the observed frequency of metapodials is slightly less 

than expected. The observed frequencies of proximal 

phalanges, calcanea, and astragali are very near the expected values if they had not been transported, whereas 

the observed frequencies of skulls and mandibles are

slightly less than expected. The sample is small, which 

has probably caused some slight differences in observed 

frequencies. For example, the observed frequencies of 

calcanea and astragali should be the same, but they are 

slightly different. Similarly, there are 22 complete and 

partial proximal phalanges preserved, whereas only 5 

medial phalanges and no distal phalanges were recognized. The larger size of the proximal phalanges probably has resulted in their better preservation and 

consequent recognition.

Behrensmeyer (1975, fig. 5) showed that a bone 

assemblage composed of Groups I, II, and III is an undisturbed concentration. The relative numbers of bones 

in each group, though, is important for interpretation. 

At locality D856, the members of Group II are the most 

abundant, but not disproportionately so. The observed

TABLE 2. Observed and expected frequencies of bones from locality

D856

[The number of rami is given the frequency of 1.0; the number of identifiable parts of other 

bones is then divided by the number of rami. For example, in a complete skeleton there 

are 1 skull, 2 distal humeri, and 8 proximal phalanges per 2 rami, resulting in expected 

frequencies of 0.5, 1.0, and 4.0. There were excavated 2 skulls, 6 distal humeri, and 22 

proximal phalanges per 5 rami, resulting in observed frequencies of 0.4, 1.2. and 4.4]

rl . Observed 

Elements frequencies

Rami 

Skulls 

Mandibles 

Distal humeri 

Proximal radii-ulnae 

Distal radii -ulnae 

Distal tibiae ---------

Proximal metapodials 

Distal metapodials - 

Proximal phalanges 

Astragali- ------------

Cal canea 

1.0 

0.4 

0.2 

1.2

1.4 

1.4 

0.4 

1.4

1.4 

4.4 

1.2 

0.8

Expected 

frequencies

1.0 

0.5 

0.5 

1.0

1.0 

1.0 

1.0 

2.0

2.0 

4.0 

1.0 

1.0
14 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

320

300'

240'

220'

FIGURE 5. Bone orientations at UALP locality 7498, Yavapai Co., Ariz. A, Stereographic projection; poles of bones plotted on lower 

surface of a stereonet. Each filled circle indicates one pole. Observations on periphery of diagram indicate horizontal bones. B, Rose 

diagram; concentric circles and associated values represent percent of total number of observations.

frequency of mandibles is low because the mandibles 

easily disassociate into individual rami. The lack of 

preferred orientation to the bones and the high number 

of bones in each Voorhies Group (remembering that collecting bias has lowered the number of bones in Group 

I) indicates that they were not transported by water.

COMPARISONS WITH LOCALITY 7498

The bone orientation and relative abundance data at 

the Browns Park locality D856 may be usefully compared with similar data collected by J. G. Honey at the 

UALP (University of Arizona Laboratory of Paleontology) locality 7498, about 15 km south of Prescott, 

Ariz. Comparison of the two data sets suggests that 

water currents influenced bone deposition at the UALP 

locality in contrast to the Browns Park locality. The late 

Clarendonian UALP bone assemblage is in an area of 

combined fluvial and eolian sediments of the Milk Creek 

Formation and is similar to the bone assemblage at 

locality D856 in that it contains a preponderance of 

camelid fossils. For details of the stratigraphy and 

depositional interpretations, see Hook, 1956; Honey, 

1977; and Honey and Taylor, 1978. The bone orientations and relative abundances, however, differ significantly between the two deposits.

In the UALP sample, the orientations of the long axes 

of 42 bones were plotted on a stereonet (fig 5A). About

half of the bone orientations fall on the periphery of the 

circle, indicating that the bones were deposited horizontally. The rest of the bones have shallow dips, none exceeding 21 °. The points tend to cluster in the northeast 

and northwest sections of the stereonet. The data were 

replotted as a rose diagram (fig. 5B), which can be compared with the rose diagram for the Browns Park bones 

(fig. 4B). At the UALP locality the bones form four 

distinct clusters. The mean of each cluster is within 7 ° 

of being 90 ° removed from its two nearest neighbors. 

The polarity of 14 of the bones was recorded; the heavy 

ends of 13 of the bones are distributed almost equally 

among the northwest, northeast, and southeast 

clusters.

The tendency for preferred orientations is strong 

evidence that the UALP bones have been aligned by 

water currents. Most significant is that the means of 

the four distinct clusters are within 7° of being 90° 

removed from each other; that is, the long axes of these 

bones tend to fall into two mutually perpendicular directions. These directions are interpreted as parallel and 

transverse to a water current. Generally, on bones 

parallel to the current direction, the large ends tend to 

point downstream (Voorhies, 1969); however, the orientations of the UALP bones indicate only that the current probably did not flow southwest.

All skeletal elements from UALP locality 7498 are 

listed in table 3, which can be compared with table 1
VERTEBRATE TAPHONOMY 

TABLE 3. Bones from UALP locality 7498, Yavapai County, Arizona, and their distribution into Voorhies Groups

15

Group I Group I/II

1 patella 4 podials 

1 rib

Group II

3 distal metapodia 

2 proximal metapodia 

2 metapodial shafts 

3 partial innominates 

3 distal tibiae 

2 radii -ulnae

Group II/III

23 partial rami

Group III

1 skull 

1 complete mandible

for the Browns Park (locality D856) bones. A glance 

reveals the preponderance of ramal fragments (Group 

II/III) in the sample. Partial palates of at least six individuals were also recovered but are not included in 

the table because their transport susceptibilities were 

not examined by Voorhies (1969) or Behrensmeyer 

(1975). The preponderance of ramal fragments is strikingly illustrated by calculating the observed frequencies of cranial parts relative to the postcranial parts. 

In a complete skeleton, the number of rami will equal 

the number of tibiae, or humeri, or metatarsals, or any 

other paired limb element. The expected frequencies of 

these elements is therefore equal to one. In camels, there 

are two indistinguishable proximal (or medial or distal) 

phalanges per metapodial. Because it is nearly impossible on isolated specimens to tell front from hind 

phalanges, for practical purposes a complete skeleton 

will have eight indistinguishable proximal phalanges. 

Thus, the expected frequency of rami to proximal (or 

medial or distal) phalanges is 1:4. The frequency of 

palates or skulls to phalanges should be 1:8.

Table 4 shows the radical departure of the observed 

frequencies from the expected frequencies for three taxa 

found at UALP locality 7498. Table 5 gives some of the 

data used to calculate these ratios. On Protolabis, for 

example, the rami occur 16 times more frequently than 

most of the long bones. Palates of Protolabis are 1.5 

times more frequent than astragali, whereas theoretically the astragali should be twice as abundant as the 

palates. On the horse, a single metapodial is present; 

theoretically, metapodials should be twice as abundant 

as rami, but here they are equal. Most of the UALP 

locality 7498 skeletal elements belong to Group III, the 

lag group, or to the intermediate Group II/III. Only a 

single rib and patella have been recovered from Group I.

The relative abundance of the bones in the different 

Voorhies Groups is further evidence that the UALP 

bones were subject to water current action. Group I is 

probably slightly underrepresented, as the field notes 

show that a fragmentary rib shaft was found and 

discarded. Otherwise, collecting bias has been minimal, 

and the lower number of bones in Groups I, I/II, and

II reflects a real scarcity in the deposit. These bones, 

which are lighter than those of Groups II/III and III, 

would tend to be winnowed out in a water current, and 

the members of Groups II/III and III would remain 

behind as a lag deposit, as shown by the experiments 

of Voorhies (1969). Although carnivores might selectively drag off and eat the remains of other animals, the 

evidence for preferred orientations of the cranial and 

ramal remains argues against removal of the lighter 

postcranial elements by agents other than water. The 

nonrandom orientations of the bones and the preponderance of lag elements at UALP locality 7498 indicates 

the presence of a water current strong enough to both 

orient the bones and winnow out some of the lighter 

elements, as at the Verdigre deposit (Voorhies, 1969). 

This situation differs somewhat from that reported by 

Hunt (1978) at the Harper Quarry (Miocene) in 

Nebraska, in which a water current was strong enough 

to orient bones preferentially in two perpendicular directions, but not strong enough to winnow out the lighter 

bones. Thus, the orientations and observed ratios of the 

bones at UALP locality 7498 are distinctly different 

from those at locality D856, and indicate the difference 

between a lag deposit and a sedentary accumulation.

PROCESSES AFFECTING DESTRUCTION AND 

DISPERSAL OF BONES AT LOCALITY D856

The preservation of the bones at locality D856 gives 

further evidence on post-mortem events. Damage to 

bones shortly after an animal's death may usually be 

attributed to processes of weathering (climatic weathering, chemical effects of soils and roots, and bacterial activity) or to mechanisms of physical damage (carnivore 

activity, trampling, fluvial transport, and diagenesis) 

(Behrensmeyer, 1978, p. 153; Hill, 1980, p. 143). Sediment compaction and tectonism can also damage bones. 

Of these, carnivore activity and weathering best account for the condition of the bones at locality D856.

The bones from locality D856 vary in their preservation. The bones are white, gray, or brown. Tooth enamel
16 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

TABLE 4. Observed and expected frequencies of bones for Michenia, Protolabis, and Equidae from UALP locality

7498, Yavapai County, Arizona

[OF, observed frequency, EF, expected frequency; dist, distal; (prox), proximal]

Michenia OF EF Protolabis OF EF Equidae OF EF

ramus 

dist tibia

radiusul nametatarsal 

palatephalanges

(prox) 

1

0

0

0

1

0

.00

.66

.33

.33

.66

.00

1

1

1

1

0

4

.00

.00

.00

.00

.5

.00

ramus 

dist tibia--

dist radiusul na- -

metapodial--

astragalus--

palate 

phalanges

(prox)- 

1

0

0

0

0

0

0

.00

.06

.06

.19

.125

.375

.00

1

1

1

2

1

0

4

.00

.00

.00

.00

.00

.5

.00

ramus - 1.00

metapodial--- 1.00

phalanges

(prox) 0.00

1.00

2.00

4.00

is bluish brown. Greenish-gray bones are best preserved; 

white and brown bones are more split and fragmented. 

The white color of some bones is due to a thin layer of 

calcium carbonate deposited on the outside surface; 

underneath this layer the bone is usually a shade of 

gray.

The greenish-gray bone is little damaged on its surface, although a few specimens (D856-32, 41) have faint, 

parallel cracks, and one (D856-56) is heavily damaged 

on the medial condyle and navicular facet. Some bones 

are bicolored (gray and brown, gray and white, brown 

and white); in nearly all cases the brown and white bone 

is more deeply split and fragmented than the gray bone. 

Several of the white bones from D856 (D856-77, 78, 28) 

are deeply pitted, and most show thin, parallel crackssome rather prominently (D856-25, 35). When recovered, the brown bone was commonly partially matted 

by living roots. In several bones, brown organic matter was concentrated along splits or in cavities. Roots 

growing parallel to the bedding planes sometimes led 

to and surrounded bone. Bone highly disintegrated by 

roots left a brown stain in the enclosing sediments.

Many of the bones at locality D856 have surface textures similar to those produced by weathering. For example, some bones are nearly unweathered, and have 

only minute cracks (D856-30, 32, 65, 82). Other bones 

have thin, parallel cracks and some have a fibrous texture: they resemble bones that have dried and 

weathered on the surface of the ground (Behrensmeyer, 

1975, pi. 1, fig. A). Other quarry bones (fig. 6, E and 

F) are deeply weathered with cancellous bone exposed.

There is, however, a strong overprint on many D856 

bones caused by the destructive action of living roots. 

Root acids can create a pattern of etch marks on the 

surface of bone, or roots can grow into and split bones 

(Behrensmeyer, 1978, p. 154; Haynes, 1980, p. 349).

Some of the bones at locality D856 were etched and split 

by Holocene roots, which may have created some of the 

apparent Miocene weathering textures such as the 

shrinkage cracks. The bones with the spongiosa exposed 

(fig. 6, E and F) however, show no traces of Holocene 

roots, and their texture is thus more likely to have been 

caused in the Miocene.

The bones vary in completeness. None of the long 

bones or metapodials are completely represented; most 

are proximal or distal ends. The ribs are represented 

only by their heads; fragmentary shafts probably were 

destroyed during collection. Complete bones include 

some phalanges, podials, and vertebrae. The horn core 

and jaws are partially broken, the scapula is extremely 

broken, and the pelvis is represented only by the 

acetabulum.

In general, the breaks and morphological details of 

the bones are sharp and clear. Several bones have spiral 

fractures (D856-62, 55, 64; figs. 6£, 6C, 6Z>), and a few 

bones (D856-25, 19; fig. 6A) have splintered fractures 

(compare Behrensmeyer, 1975, pi. 2, figs. A, B, C). Some 

bones (D856-28, 77, 78; figs. 6E, 6F) have a roughened 

and pitted surface similar to bones illustrated by 

Behrensmeyer (1975, pi. 1, figs. A, B, and C). Other 

bones are crushed or have broken, depressed areas. For 

example, a distal radius-ulna (D856-25; fig. 6H) has the 

posterior part of the shaft crushed inward, yet the distal 

end is undisturbed. A phalanx (D856-68; fig. 6G) has 

a crushed shaft although the proximal and distal ends 

are undisturbed. Nearly all bones are broken or cracked, 

or have rough, pitted surfaces. No bones larger than 

camel phalanges are preserved intact.

Some of the damage on the bones at locality D856 is 

attributable to breakage rather than weathering, and 

includes the previously mentioned crushing of several 

of the bones, the destruction of the more fragile shafts
VERTEBRATE TAPHONOMY 17

TABLE 5. Data used to calculate observed frequencies given in table 4

Michenia

rami: 3 separate rami; 1-1/2 individuals maximum, 

distal tibiae: 2 separates; 2 individuals minimum, 

radius-ulna: 1 separate; 1 individual, 

metatarsal: 1 separate; 1 individual, 

palates: at least five "1/2 palates"; 

2-1/2 individuals minimum.

Protolabis

rami: 16 separates; 9 individuals minimum, 

distal tibia: 1 separate; 1 individual, 

radius-ulna: 1 separate; 1 individual, 

metapodials: 3 separates at most; 2(?) individuals, 

astragali: 2 separates; 2 individuals, 

palates: six "1/2 palates"; 3 individuals.

Equidae

rami: 1 separate; 1 individual, 

metapodial: 1 separate; 1 individual.

^Refers to one-half of a complete palate, 

broken in the sagittal plane.

of the long bones, and spiral and splintered fracturing. 

According to Behrensmeyer (1975, p. 480), spiral and 

splintered fractures occur only on fresh bone, and their 

presence on several locality D856 bones may indicate 

that fracturing occurred before burial Fluvial transport 

is one possible mechanism of physical damage. The 

edges of the bones at locality D856 are not rounded or 

polished, which indicates no prolonged current transport. Current transport of bones can result in extensive 

breakage without polishing, provided the transport has 

not been of long duration or over a great distance (compare Behrensmeyer, 1975, pi. 1, fig. B, bones a and b). 

However, the lack of fluvial deposits at locality D856, 

and the lack of indications of water transport in either 

the orientations or abundance of the bones, suggests no 

fluvial transport.

Sediment compaction or tectonic processes probably 

do not account for the fragmentation of the bones. The 

presence of dune foresets greater than 30 ° in some of 

the Browns Park roadcuts suggests little or no postMiocene compaction of the sediments. Although sediment compaction or tectonism could crush the relatively 

fragile shafts of the long bones, these processes would 

probably not destroy all traces of these shafts while

preserving the ends, nor would they preserve, undistorted, several delicate cervical vertebrae while 

crushing the larger and stronger phalanx (D856-68) and 

radius-ulna (D856-55). There is no direct evidence for 

tectonic activity in and around locality D856. Dips 

taken on quarry strata range from 3° SE to 4° NW; 

slight irregularities on the dip surface and the original 

surface of deposition can account for these slight variations from horizontal. In the area around Maybell, the 

rocks are flat lying.

According to Behrensmeyer (1975, p. 482), CaCO3 

deposition can destroy a bone through the outward 

growth of a calcite nodule. At locality D856, however, 

there is no evidence for CaCO3 nodule formation and 

the subsequent "explosion" of bone; instead, the calcite 

is a deposit on the surface of the bone. In some instances

FIGURE 6. Weathering characteristics of bones from locality D856, 

Citadel Plateau 15-minute quadrangle, Moffat County, Colo. A, 

posterior view of distal right humerus showing splintered fractures; 

cf. Protolabis, specimen D856-19. B, anterior view of metapodial 

shaft showing spiral fracture along top; Procamelus sp. or 

Aepycamelus sp., D856-62. C, posterior view of distal metapodial 

showing spiral fracture along top; Procamelus sp. or Aepycamelus 

sp., D856-62. D, anterior view of right distal radius-ulna, showing 

spiral fracture along top; Procamelus sp. or Aepycamelus sp., 

D856-55. E, anterior view of right distal radius-ulna showing pitted 

surface; Camelinae sp., D856-77. F, anterior view of left distal radiusulna showing pitted surface; Camelinae sp., D856-78. G, posterior 

view of proximal phalanx showing crushed central part of shaft; Procamelus sp. or Aepycamelus sp., D856-68. H, posterior view of left 

distal radius-ulna showing crushed shaft; cf. Protolabis, D856-25. 

Scale is in centimeters.
18 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

(D856-37, 53) gray bone is evident underneath this surface veneer, whereas in other instances (D856-68, 77) 

the bone is soft and powdery. The fossil layer itself 

becomes more firmly cemented with calcite near its contact with the overlying highly calcareous layer, and it 

was from the bottom of this overlying layer that the 

Tomarctus skull and jaws were found. This specimen 

consists of gray-green bone that is remarkably well 

preserved, suggesting that CaCO3 deposition has not 

destroyed the bone.

Animal trampling and associated kicking could explain some of the features of bones from quarry D856. 

Trampling would tend to fragment the more fragile 

shafts of long bones, leaving the ends and the denser 

bones intact. Incidental kicking is a random dispersive 

process (Hill, 1979, p. 270) which, like carnivore activity, could scatter and randomly orient the bones at locality D856. Trampling, however, often orients bones at 

high angles (Andrews and others, 1981, p. 45; Behrensmeyer and Boaz, 1980, p. 87); because most bones at 

locality D856 are horizontal and none dip at high angles, 

trampling probably was not the major cause of the 

observed bone destruction.

Carnivore activity is probably the main cause of bone 

destruction and dispersal at locality D856. Although 

there are tooth marks on only one bone (a camel ulna 

(D856-23) has a probable gnawed area immediately 

below the semilunar notch), Haynes (1980, p. 343) noted 

that carnivore gnawing does not always leave identifiable tooth marks on bones. He also noted that removal 

of the outermost lamellar tissue of bone can eliminate 

the shallower tooth marks (1980, p. 349). Carnivore activity is indirectly suggested by the absence of proximal 

humeri, tibiae, proximal and distal femora, and the presence of distal humeri, radii-ulnae, tibiae, and proximal 

and distal metapodials. The proximal ends of the humerus and tibia, and both ends of the femur, have a high 

fat content, and are therefore readily eaten by carnivores 

(Voorhies, 1969, p. 20). Haynes (1980, p. 345-347) reported extensive carnivore gnawing of these same areas 

on Pleistocene and Holocene bones, but notes little 

damage to distal ends of long bones except for the femur. 

According to Voorhies (1969) scavenging by carnivores 

possibly accounted for the low number of femora, and 

proximal humeri and tibiae in the Verdigre sample; Hunt 

(1978, p. 35) also suggested that carnivore activity accounted for the presence of broken limb bones, isolated 

teeth, vertebrae and ribs at Harper Quarry. The abundance of metapodials, podials, and phalanges at locality D856 could be explained by their having been covered 

by less meat, thus making them less attractive dietary 

items to carnivores. The greater density of the podials 

and the proximal and distal ends of the metapodials and 

phalanges would also help explain their preservation.

CONCLUSIONS

Fluvial transport, sediment compaction, tectonism, 

and calcium carbonate deposition do not account for the 

observed fragmentation and destruction of the bones 

at locality D856. The physical damage to these bones 

was most likely caused by carnivore activity, with a 

possible minor component due to trampling. The surface textures of some of the bones may be due to 

Miocene weathering phenomena, but the strong overprint caused by etching and splitting due to growth of 

modern plant roots makes tenuous any interpretation 

of ancient weathering textures.

The pattern of highly fragmented bones, missing their 

shafts and nutrient-rich ends, and which are disarticulated and highly dispersed, indicates intense utilization of the original carcasses by carnivores. This 

suggests a relatively low number of carcasses available 

to predators when the locality D856 bone deposit accumulated. Where there is an abundance of carcasses, 

as can occur in catastrophic mortality, there is less 

utilization of each carcass by carnivores, resulting in 

less bone destruction and increased chance for burial 

of intact bones (Western, 1980, p. 48; Coe, 1980, p. 59; 

Behrensmeyer and Boaz, 1980, p. 85). It is unlikely that 

the bone assemblage at locality D856 is the result of 

the catastrophic death of many mammals, but is more 

likely the result of slow, attritional mortality. With 

relatively few carcasses available for the number of 

predators, the bones were more thoroughly consumed, 

resulting in a fossil deposit of fragmentary, dispersed 

bones.

VERTEBRATE FAUNA OF THE 

BROWNS PARK FORMATION

LOCATIONS OF FOSSIL SITES

The fossils described are from four localities (fig. 7; 

table 6): D854, south side of roadcut in the NE1/4NE1/4 

sec. 23, T. 6 N., R. 97 W., Citadel Plateau 15-minute 

quadrangle; D855, on a hill in the NE1/4NE1/4 sec. 36, 

T. 6 N., R. 97 W., Citadel Plateau quadrangle; D856, 

on the same hill in NW1/4NW1/4 sec. 31, T. 6 N., R. 96 

W., Citadel Plateau quadrangle; D857, in a roadcut in 

SW1/4NW1/4 sec. 34, T. 6 N., R. 97 W., Elk Springs 

15-minute quadrangle. Locality D855 is 15.2 m 

stratigraphically below locality D856. Extensive plant 

cover prevents direct stratigraphic correlation between 

the rocks at localities D854 and D855. D854 lies 

topographically 32.6 m below D855 at an elevation of 

1,930 m. Similarly, fossil locality D857 cannot be correlated stratigraphically with the other fossil localities.
VERTEBRATE FAUNA 19

R 97 W R 96 W 

2 3 MILES

A KILOMETERS

FIGURE 7. Fossil mammal localities and dated volcanic ash southwest of Maybell, Colo. A, locality D857; B, D854; C, dated 

ash; D, D855; E, D856. Topographic base from Citadel Plateau and Elk Springs 15-minute quadrangles, Moffat County, 

Colo. Elevations are in feet; to convert to meters, multiply by 0.3048. Double dashed lines indicate unimproved dirt road.

Its elevation is about 1,963 m, which is the same as at 

D855 (1,962 m).

The fossils found at these four sites are assigned to 

two local faunas distinguished by taxonomic and temporal differences. The term local fauna, as used in this

paper, follows that given by Tedford (1970, p. 682-683). 

He stated that "the local fauna may be represented by 

samples from a single site or a series of closely 

associated sites having a limited geographic and 

stratigraphic distribution." Although the four sites in
20 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

TABLE 6. Miocene fossil mammals from the Browns Park Formation

[Locality numbers not arranged in stratigraphic order]

Locality 

No.

Lithology Fossils

Cedar Springs Draw local fauna

D854 Sandstone, white, quartzitic, mediumgrained, subrounded, friable.

D855 Sandstone, white, very fine to finegrained, silty; in part calcareous.

D856 Sandstone, light greenish-gray, finegrained, subrounded; in part calcareous,

Meryceros sp.

Procamelus sp. or 

Aepycamelus sp.

Tomarctus cf. _T. paula 

Hegahip])us sp. 

cf. Protolabis 

Michema sp. 

Procamefus sp. or 

Aepycame"lus sp. 

Camelinae sp. 

Meryceros warreni 

Merycodorfti naif sp.

Cross Mountain local fauna

D857 Sandstone, greenish-gray, quartzitic, finegrained, subangular to subrounded, 

well-sorted.

Parahippus cf. _P_.

Teonensis 

Blickomylus galushai

the Maybell area are close geographically and 

topographically, the fauna at one site is older than the 

other three. The older fossil assemblage is here named 

the Cross Mountain local fauna, based on its nearness 

to the old settlement of Cross Mountain in the Elk 

Springs 15-minute quadrangle; the younger fossil 

assemblage is here named the Cedar Springs Draw local 

fauna, based on its proximity to a dry wash of that 

name on the eastern edge of the Elk Springs 15-minute 

quadrangle and the western edge of the Citadel Plateau 

15-minute quadrangle.

MAMMALIAN PALEONTOLOGY

Class MAMMALIA Linnaeus 

Order CARNIVORA Bowdich

Family CANIDAE Gray 

Subfamily CANINAE Gray

Genus TOMARCTUS Cope 

Tomarctus cf. T. paula Henshaw

Table 7, Figure 8

Tomarctus paulus Henshaw (1942, p. 105-108, pi. 2, 

figs. 3, 4, 4a).

Holotype. CIT 1229, palate with P-M2 , and left 

ramus with I2 -M3 , from CIT locality 172, Esmeralda 

Formation, 9 miles north of Tonopah, Nye County, Nev.

Referred material. D856-1: broken skull with 

fragmentary LM1 and complete LM2 , a right dentary 

with canine root, P: alveolus, P2 roots, fragmentary P3 , 

M1-3 (Mj.g damaged); D856-2, right premaxillary and 

maxillary with I3 , C, P1 ; D856-3, condyle of left dentary; D856-4, temporal fossa of left dentary; D856-88, 

isolated, broken teeth.

Distribution and age. Holotype from Tonopah 

fauna, Nevada; Barstovian land mammal age.

Description. The skull of Tomarctus cf. T. paula is 

smaller than that of Canis latrans, with a relatively 

shorter preorbital portion, and the braincase is less expanded. The zygomatic arches are widely flaring which, 

coupled with the small braincase, results in temporal 

fossae relatively larger than in C. latrans.

The width across the piriform aperture is the same 

as in C. latrans. A small tuberosity on the dorsal border 

of the premaxillary is more prominent on specimen 

D856-1 than on C. latrans. The incisor alveoli are transversely narrow, and the alveolus for the I3 is much 

larger than that for either the I 1 or I2 .
ORDER CARNIVORA BOWDICH 21

The muzzle is short and deep, with the maxillary nearly as deep but only about half as long antero-posteriorly 

as in some specimens of C. latrans. The muzzle is less 

broad than that of C. latrans. The lateral edge of the 

palatine between P4 and M1 is strongly depressed.

A large maxillary foramen is located anteriorly on the 

orbital border of the maxillary. Alveolar foramina are 

present on the dorsal surface of the maxillary tuberosity. The sphenopalatine and posterior palatine foramina 

are located 14 mm posteromesial to the maxillary 

foramen, on the dorsal part of the palatine bone. The 

rest of the medial wall of the orbit has been destroyed.

The jugal-maxillary contact is V-shaped, with a prominent ventral maxillary process on the jugal. The scar 

for the origin of the masseteric muscle is pronounced, 

with the upper margin marked by a distinct ridge. The 

frontal process of the jugal is mesially inclined.

On the partial right frontal, the external frontal crest 

trends posteromesially from the zygomatic process to 

meet its counterpart at the midline and form a strong 

sagittal crest. The parietals are less inflated than in C. 

latrans. The lambdoidal crest has a more strongly 

developed posterior flare than in C. latrans, and forms 

a broad, inverted U. Owing to this marked flaring, the 

posterior surface of the supraoccipital is highly concave 

on T. cf. T. paula. The external occipital crest is poorly 

developed, whereas on C. latrans it is a marked swelling above the foramen magnum. Several dorso-ventrally 

trending ridges and depressions, for the insertion of the 

splenius muscle, are present on the supraoccipital.

The foramen magnum and basioccipital are destroyed. 

Laterally, the lambdoidal crest extends anteroventrally as a strong shelf to join the posterior root of the 

zygomatic arch. A prominent tubercle is located on the 

crest and posterodorsal to the auditory meatus. The partially broken bulla is mesiolaterally elongated and less 

expanded than in C. latrans. In C latrans, the bulla is 

inflated so that its lower border lies below the lowermost portion of the retroglenoid process, whereas on 

specimen D856-1 the bottom of the bulla is level with 

the bottom of the process. Owing to this expansion, in 

C. latrans the bullar fusion of the squamosal occurs further down on the retroglenoid process than on specimen 

D856-1. The lower lip of the external auditory meatus, 

which on specimen D856-1 lies above the opening of the 

retroglenoid foramen, is level with or below the opening for this foramen in C. latrans. Owing to the less expanded bulla in specimen D856-1, the stylomastoid 

process is much more prominent and extends below the 

level of the bulla.

Parts of the basisphenoid and orbitosphenoid are 

preserved. The posterior opening of the alar canal and 

the foramen rotundum are smaller, but situated much 

as in C. latrans. On specimen D856-1, a tiny foramen

TABLE 7. Skull and dental measurements o/Tomarctus cf. T. paula 

(specimen D856-1) from locality D856 in the Citadel Plateau 

15-minute quadrangle, Moffat County, Colorado 

[Measurements in millimeters]

Distance from anterior edge of orbit 

to anterior edge of premaxillary ....

Distance from anterior edge of orbit to 

posterior edge of interparietal process,

Distance from anterior edge of premaxillary to 

posterior edge of palate behind third molar.,

Width between frontal processes of

zygomatic bone...............................

Width between infraorbital foramina. 

Maximum width across parietals......

Width of piriform aperture..........

Distance from infraorbital foramen 

to anterior border of maxillary..

Height of premaxillary bone at 

i nfraorbital foramen.............

Outside width at narrowest point of muzzle.

 5

Palatal width between anterior roots of P . 

Palatal width between M3 ............ .......

I 3 APxTR...................................

C (upper) APxTR............................

M 2 APxTR...................................

C (lower) APxTR. 

Mj APxTR........

Mj heel TR......

M2 APxTR........

M 0 APxTR........

55.5

107.0

67.5

72.0

30.7

50.0

21.2

23.5

32.0

25.9

19.0

23.6

4.0x4.0 

6.8x4.0 

5.9x9.1

6.9x5.7 

15.7x6.8

6.5

8.9x5.6 

4.0x3.5

C-M3 .......................................

P rM 3 . .....................................

..... 63.9

..... 54.7

..... 15.0

..... 9.2

..... 105.0

is present in the position of the hypophyseal fossa; the 

dorsum sellae is broad and flat.

An isolated I1 (specimen D856-88) possesses a broad 

central cusp bounded laterally by a smaller accessory 

cusp. The small mesial accessory cusp may have been 

eliminated by wear. The lingual surface of the tooth is 

concave and bounded by a broad cingulum. The I3 

(specimen D856-2) is larger and more caniniform than 

the I 1 and has a minute posterior swelling. There is a 

mesial cingulum at the base of the tooth. The canine 

is robust and laterally compressed.

The premolars of specimen D856-1 are highly fragmented. A partial P4 consists of a paracone and metastyle separated by a notch. A strong cingulum runs 

along the base of the lingual wall. A fragmentary M1 

shows no differences from that of C. latrans. The M2
22 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

FIGURE 8. Tomarctus cf. T. paula, from locality D856, Citadel Plateau 15-minute quadrangle, Moffat County, Colo. Upper left, ventral view 

of palate; specimen D856-1. Lower left, buccal view of right dentary showing premolar (PJ and molars (M: g ) of specimen D856-1. Upper right, lateral view of rostrum showing right incisor (I3 ), canine (C), and premolar (P*) (broken) of specimen D856-2. Lower right, 

lingual view of right dentary showing Pg and MI g of specimen D856-1.

metacone is smaller in relation to the paracone, and the 

M2 buccal cingulum is weaker and the posterior edge 

of the tooth is less concave than that of C. latrans.

The lower canine root on specimen D856-1 indicates 

a large, vertically oriented tooth. The C-P1 diastema is 

slightly shorter than that of C. latrans. The premolars 

are too fragmented to be described. The M1 is broken 

anterobuccally. The protoconid, paraconid, and metaconid are all strongly developed; the metaconid more 

so than that of C latrans. Small lingual and buccal accessory cusps are present on the posterior walls of the 

metaconid and protoconid, respectively. On specimen 

D856-1, the lingual accessory cusp is more closely appressed to the metaconid than on C. latrans. The 

posterior wall of the Ml talonid is broken, but it appears that the posterior cingulum is less well developed 

than in C. latrans. The M2 has some slight anterobuccal breakage. The protoconid and metaconid are well 

developed on specimen D856-1, and in addition there 

is a small anterior paraconid a cusp absent on the M2 

of C latrans. The M3 is round and simple with two 

shallow anterior and posterior basins.

The dentary of specimen D856-1 is short and slender. 

The ventral border is convex beneath the molars, and 

concave beneath the masseteric fossa. Owing to the 

shortness of the jaw, the ventral border anterior to the 

molars is less rounded than in C. latrans. A large mental foramen is located beneath the anterior root of the 

P2 . The masseteric fossa is deeply excavated. The articular condyle is slightly less elongate laterally than 

in C. latrans. The posterior border of the symphysis lies 

between the Pl and P2 .

Comparisons and discussion. Specimen D856-1 

resembles Tomarctus rather than Canis in having a 

short facial region, small braincase, strong lambdoidal 

crest (McGrew, 1935, p. 305) and a distinct paraconid 

on the M2 (Merriam, 1913, p. 360). The presence of M1 

accessory cusps distinguishes specimen D856-1 from 

Cynodesmus (Olsen, 1956, p. 2).

Specimen D856-1 is about 25 percent smaller than 

Tomarctus rurestris (Condon, 1896), figured by Downs 

(1956, figs. 10 and 11). The dentary is more robust and 

more strongly convex anterior to the masseteric fossa 

in T. rurestris.
ORDER CARNIVORA BOWDICH 23

The type of Tomarctus brevirostris (Cope, 1873) is a 

fragmentary jaw with the M1 present; the M1 is about 

40 percent larger than that of specimen D856-1. The 

jaws from the Lower Snake Creek fauna (early Barstovian) referred by Matthew (1924, p. 91) to T. brevirostris, average about 20 percent larger than specimen 

D856-1. The teeth are wider and more massive and the 

dentary is more robust in T. brevirostris. The skull is 

larger, the palatal width at the M: is about 29 percent 

greater, and the auditory bulla is more inflated in T. 

brevirostris.

The type lower dentition of Tomarctus optatus Matthew, 1924 (Matthew, 1924, fig. 18) is only slightly 

larger than that of specimen D856-1. The molar widths 

are the same, and the referred j aws in the AMNH do 

not differ from specimen D856-1. However, the upper 

dentition of undescribed T. optatus in the AMNH collection is larger than that of specimen D856-1. In particular, the M2 of T. optatus is less reduced than that 

of specimen D856-1, and consequently, T. optatus more 

closely resembles other small species of Tomarctus 

which have nonreduced dentitions.

Tomarctus confertus (Matthew, 1918) has a smaller, 

shorter skull, with narrower palate and less expanded 

braincase, than specimen D856-1. The mandible of T. 

confertus (AMNH 17203; Matthew (1918), fig. 1) is 

shorter and narrower dorsoventrally, the premolars are 

shorter, the canine smaller, and the M1 narrower than 

that of specimen D856-1.

Tomarctus euthos (McGrew, 1935), has a bigger skull 

with wider rostrum and palate than that of specimen 

D856-1, and the rostrum is not sharply constricted 

anterior to the P4 as it is in specimen D856-1. The 

teeth of T. euthos are more robust than those of 

specimen D856-1.

Tomarctus propter Cook and Macdonald, 1962, 

based only on an Mlt is distinguished from specimen 

D856-1 by being much larger and having a reduced 

metaconid.

The tooth row of Tomarctus canavus (Simpson, 1932), 

is 8-13 percent longer than specimen D856-1. Although 

the morphology of the lower molars is similar in 

specimen D856-1 and T. canavus, the molars of T. 

canavus are slightly larger than those of specimen 

D856-1. The upper teeth of T. canavus (including the 

types of T. thomasi White, 1941 and Nothocyon insularis White, 1942) are distinctly larger than those of 

specimen D856-1. In particular, the M2 of specimen 

D856-1 is much reduced compared to the M2 of T. 

canavus (Olsen, 1956, tables 1 and 2). Although the M1 

of specimen D856-1 is broken, the transverse width is 

less than that of T. canavus.

Tomarctus casei (Wilson, 1939), is smaller than T. 

canavus, and has a shorter dentition than that of

specimen D856-1 (measured from the back of the canine 

to the back of the M3 ). Although the transverse width 

of the M1 of T. casei is less than that of specimen 

D856-1, the M2 of T. casei is wider about the size of 

the M2 of T. canavus. On T. casei the M2 ectocingulum 

is stronger than on specimen D856-1, and there is a 

marked concavity on the posterior edge of the tooth not 

present on specimen D856-1.

Tomarctus thomsoni (Matthew, 1907) is distinguished 

from specimen D856-1 by having a less reduced M2 , 

with a more expanded posterointernal corner. In specimen D856-1 the premolar row is longer, the M: is 

slightly longer, the M2 is longer and wider with a narrower talonid, and the M3 is slightly more reduced 

than that of T. thomsoni.

Tomarctus kelloggi (Merriam, 1911) is distinguished 

from specimen D856-1 in having a more robust jaw 

and an enlarged M2 with a well developed paraconid. 

On a referred M2 (UCMP 10651), the paraconid is nearly as large as the metaconid, whereas on specimen 

D856-1 the paraconid is relatively small. The trigonid 

basin on T. kelloggi is wider than that of specimen 

D856-1.

Specimen D856-1 is quite similar to Tomarctus paula 

Henshaw, 19421 . T. paula resembles specimen D856-1 

in the reduction of the M2 , which is greater than in the 

other small species of Tomarctus. The morphology of 

the M2 is also similar; although T. paula appears to 

have a stronger lingual cingulum, that of specimen 

D856-1 is broken and thus reduced in size. As indicated 

by the length of the lower dentition, T. paula is slightly shorter faced than specimen D856-1, and the P^Mg 

length of T. paula is about 7 percent shorter than that 

of specimen D856-1. The P^Mg length is probably a 

truer measure of the relative size of these two animals 

than is the length given for the condyle to the 1^ in 

both fossils this latter measurement is approximate and 

Henshaw's (1942, pi. 2, fig. 4) drawing shows that the 

position of the condyle is hypothetical. As in specimen 

D856-1, the posterior part of the palate on T. paula is 

relatively wide. The lower teeth of T. paula are slightly 

smaller than those of specimen D856-1.

Henshaw's (1942, pi. 2, fig. 4) illustration shows that 

the M! metaconid on T. paula is the size of that on 

specimen D856-1. The M2 of T. paula is not elongated 

in relation to the M: as it is in T. kelloggi. Henshaw 

(1942, p. 106) noted the presence of a "distinguishing 

paraconid" on the M2 , but his illustration (1942, pi. 2,

Tomarctus paula was originally named Tomarctus paulus. According to G. G. Simpson 

(written commun., 19771, Tomarctus is feminine in gender in Latin and paulus is masculine. 

According to Article 30 of the 1971 International Code of Zoological Nomenclature, adjectival specific names must agree with the generic name in gender, and (Article 34) must be 

changed, if necessary, to conform to the gender of the generic name. Thus, the change from 

paulus to paula is a "justified emendation" (Article 33).
24 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

fig. 4a) shows only a strong crest anterior to the protoconid and metaconid. Wear sometimes causes the 

paraconid to lose its cuspate form and become incorporated into an anterior ridge. Other small differences 

with specimen D856-1 include a slightly greater depth 

anteriorly on the dentary, a greater inclination from the 

horizontal of the ascending part of the ramus, and a 

slightly narrower masseteric fossa.

Canid indet. BEG 31081-1255 (Wilson, 1960, p. 994, 

Fig. 5e) is similar to both specimen D856-1 and T. paula. 

The partial palate of Wilson's specimen has a reduced 

M2 similar in size to that of D856-1.

Specimen D856-1 and the type lower jaw of Tomarctus temerarius (Leidy, 1858) are almost identical; there 

are no significant size differences in either the M1 or 

the dentary. Although a labial accessory cusp does not 

appear in Leidy's (1869, pi. 1, fig. 12) illustration of the 

Mp it may have been eliminated by wear.

Similarly, the type of Tomarctus marylandica2 

Berry, 1938 cannot be distinguished from the M^ of 

specimen D856-1. Berry (1938, p. 159) noted that in T. 

marylandica, "the protoconid, the most outstanding 

feature of the tooth, is about twice as high as the 

metaconid." The M: metaconid on specimen D856-1 

stands only slightly over half the height of the protoconid, and is thus similar to that of T. marylandica. 

On the M2 of T. marylandica a tiny basin is present 

posterior to the ridge between the entoconid and 

hypoconid. This depression was probably caused by 

overlap by the succeeding M3, and is therefore due to 

crowding of the last two molars. On specimen D856-1 

the M3 is closely appressed against the M2 . The upper 

teeth of T. marylandica are unknown.

Although the M1 of specimen D856-1 cannot be 

distinguished from that of T. temerarius, or the M1 

heel and the M2 of specimen D856-1 cannot be 

distinguished meaningfully from T. marylandica, 

specimen D856-1 skull and jaws are not referred to 

either of these species: specimen D856-1 in fact closely resembles the more completely known T. paula. T. 

paula and specimen D856-1 are both short-faced forms 

with reduced M2 , and the measurements on the upper 

molars of the two are nearly identical. Nothing is 

published of the skull proportions of T. temerarius or 

T. marylandica, so it is premature to assign the 

brachycephalic specimen D856-1 skull to either of these 

species. For this reason, specimen D856-1 is assigned 

to Tomarctus cf. T. paula.

Tedford and Hunter (1984) suggested that T. marylandica is a primitive member of the 

genus Cynarctus.

Order PERISSODACTYLA Owen

Suborder HIPPOMORPHA Wood

Family EQUIDAE Gray

Genus MEGAHIPPUS McGrew 

Megahippus sp.

Table 8, Figure 9

Referred material. D856-5, right premaxillary; 

D856-6, 10 incisors; D856-7, dPx(?); D856-8, right and 

left dP: ; D856-9, right dentary with dP^, Mx ; 

D856-10, LdPx(?).

Description. The right premaxilla preserves the 

symphysis and the three incisor alveoli. The anterodorsal border of the premaxilla is elevated posterior to the 

symphysis. The alveoli for the I 1 and I 2 are subequal 

in size and larger than the alveolus for the I3.

None of the incisors were found socketed, but their 

number suggests that some came from the lower jaw. 

The incisors, all probably deciduous, are low crowned 

and strongly cupped, and have long, strong roots. Five 

slightly worn incisors possess narrow, discontinuous 

lingual cingula. The labial surface is smooth or may 

have a cingulum running continuously along three sides. 

The broken dPx(?) is low crowned with a cingulum 

anterior to the slightly worn protocone, and between 

the hypocone and hypostyle.

The dP: is large and single rooted, consisting of a 

prominent central cusp, swollen basally, from which 

sharp anterior and posterior crests descend. A strong 

basal cingulum is present buccally. The cingulum is 

more shelflike anteriorly and posteriorly than centrally.

The dP2 is elongate and relatively narrow anteriorly. 

A labial basal cingulum runs from the cusp-like 

parastylid to the hypostylid, and is more strongly 

developed on the hypolophid than on the protolophid. 

A short, shallow groove about 2 mm long separates the 

metaconid and metastylid. The dP3 and dP4 each have 

a strong, shelflike buccal cingulum running from the 

paralophid to the hypostylid. The hypostylid has a 

short, posteriorly directed shelf that does not reach the 

base of the tooth. The metaconid and metastylid are 

separated by a groove 2 mm long. The dP2 paralophid 

and protolophid are more widely separated than those 

of the dP3 and dP4. The anterior and posterior walls of 

the dP2 protoconid join to form a sharp buccal ridge; 

on the dP3 and dP4 this junction is wide and rounded. 

None of the deciduous premolars have a lingual 

cingulum. The M1 is unerupted and incomplete. The 

metaconid and metastylid are only faintly separated 

lingually. A remnant of a cingular structure is present 

lingually at the bottom of the postflexid.

The mandible is deep dorsoventrally and flat ventrally. It is eroded anteriorly, but there is evidence of strong
ORDER PERISSODACTYLA OWEN 25

TABLE 8. Dental measurements of Megahippus sp. from locality 

D856, Citadel Plateau 15-minute quadrangle, Moffat County, 

Colorado

[Measurements in millimeters]

Incisor length.................. n=6; O.R.=11.8-13.2; X=12.3

?dPx AP length at protocone 

and hypocone.................. 26

dPj Ap.......................... n=2; O.R.=11.0-11.2; X=11.3

dPj TR.......................... n=2; O.R.=7.8-8.3; X=8.1

dP 2 APxTR....................... 28.7 x 18.7

dP3 APxTR....................... 26.6 x 19.5

dP4 APxTR....................... 27.2 x 19.5

?dPx APxTR...................... 29.5 x 21.3

Mj Height of unworn metaconid... 19.4

Narrowest depth of ramus

between dC and dPj............ 29.2

Narrowest depth of ramus

below dp. protocone........... 41.0

upturning at the symphysis. The post-canine diastema 

is short. The mental foramen is 14.5 mm below the 

dPr

Comparisons and discussion. McGrew (1938, p. 315) 

defined Megahippus on the basis of M. matthewi found 

at the Burge quarry in the Burge Member of the Valentine Formation, Neb. Although about one-fourth smaller 

than M. matthewi, and thus similar in size to Hypohippus affinis, specimen D856-9 differs from//, affinis in 

having a strongly upturned symphysis, short postcanine diastema, and more hypsodont Mr A strongly 

upturned symphysis, short post-canine diastema, and 

more hyposodont dentition were among the characteristics considered diagnostic of Megahippus by 

McGrew. M. F. Skinner of the AMNH (oral commun., 

1975), notes that the lower cheek teeth of later Megahippus possess lingual cingula, whereas those of Hypohippus do not. Examination of anchitheriines in the Frick 

collection corroborates Skinner's statement. In New 

Mexico, Megahippus from the Pojoaque Member of 

Galusha and Blick (1971) of the Tesuque Formation 

possess the lower lingual cingula on the permanent dentition. In contrast, specimens from the stratigraphically 

lower Skull Ridge Member of Galusha and Blick (1971) 

of the Tesuque Formation lack the cingula. The small 

cingular structure below the Mx postflexid of specimen 

D856-9 is suggestive of, but not so well developed as, 

the cingular structures seen on the Clarendonian 

Megahippus cf. M. matthewi from the Esmeralda Formation at Cedar Mountain, Nev. (Mawby, 1968, fig. 1).

The lower lingual cingula of Megahippus progressively increased in size during the Barstovian. This increase 

in size probably correlates with the progressive development of the lingual cingula on the upper premolars of 

Megahippus. Tedford and Alf (1962, p. 119) pointed out 

that the upper premolar cingula of the late Barstovian 

form M. mckennai are less well developed than those 

of the latest Barstovian and Clarendonian form M. 

matthewi.

McGrew (1938, p. 315-317) considered the absence of 

P! diagnostic of Megahippus. However, specimens in 

the Frick collection show that some early Barstovian 

(Pawnee Creek Formation of Galbreath (1953) in

FIGURE 9. Megahippus sp., from locality D856, Citadel Plateau 

15-minute quadrangle, Moffat County, Colo. A, buccal view of right 

dentary with deciduous premolars (dP1 _4) of specimen D856-9. B, 

labial (1,2) and occlusal (3-5) views of incisors; D856-6. C, occlusal 

view of right deciduous premolars (dP1 _4) of specimen D856-9.
26 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

northeast Colorado) forms have a well-developed Pr 

Some late Barstovian to possibly early Clarendonian 

(Pojoaque Member of the Tesuque Formation) forms 

have a reduced Plt and some latest Barstovian (Burge 

quarry)3 forms lack the Pr Similarly, the dPj is well 

developed in the early Barstovian forms and is small 

or absent in latest Barstovian and Clarendonian forms.

Cupped incisors are characteristic of adult Megahippus, but are also occasionally found in adult Hypohippus. Deciduous incisors of both genera are cupped.

The features listed above and comparison of fossil 

horse D856-9 with specimens in the Frick collection 

suggest that specimen D856-9 should be referred to 

Megahippus. Specimen D856-9 matches a deciduous 

dentition in the AMNH (field number ESP 399-1842) 

of Megahippus from the Pojoaque Member of the Tesuque Formation. The eroded symphysis of specimen 

D856-9 was probably strongly upturned, which suggests Megahippus affinities: the more complete jaw 

from New Mexico (ESP 399-1842) has a strongly upturned symphysis. The New Mexico specimen of 

Megahippus has a short diastema, strong dP1? and 

lacks lingual cingula on the dP2_4, as does specimen 

D856-9. The height of the M1 metaconid of specimen 

D856-9 is greater than that of H. affinis. Webb (1969, 

p. 77) notes that the metaconid on the unworn M1 of 

H. affinis is 15 mm high; on specimen D856-9 it is 

19.4 mm high. Thus, the teeth of specimen D856-9 are 

the same length but higher crowned than those of H. 

affinis.

The deciduous dentition found in the Browns Park 

cannot at present be allocated to any previously named 

species of Megahippus.

Genus PARAHIPPUS Leidy 

Parahippus cf. P. leonensis Sellards

Table 9, Figure 10

Parahippus leonensis Sellards (1916, p. 83-87, pi. 11, 

figs. 3, 6-9, 12).

Holotype. FGS 5084, right upper cheek tooth, probably M1 .

Referred material. D857-1: crushed skull with 

RP-3 , LI3 , R and L canine, RP2-3 , R and LM1 -3.

Distribution and age. Thomas Farm, Griscom Plantation, Tallahassee, Alum Bluff, and Chattahoochee 

faunas, all in Florida; Hemingfordian land mammal age.

Description. The skull is highly eroded with many 

of the cranial features destroyed. It is long and narrow.

The Burge fauna was considered Clarendonian by Wood and others (1941) and Webb 

(1969). Recently, the Burge fauna has been placed in the latest Barstovian (Tedford, R. H., 

and others, unpub. data, cited in Skinner and Johnson, 1984).

TABLE 9. Skull and dental measurements of Parahippus cf. P. 

leonensis (specimen D857-1) from locality D857 in the Elk 

Springs 15-minute quadrangle, Moffat County, Colorado 

[Measurements in millimeters]

Distance from anterior edge of

orbit to nasal tubercle........ 155.0

Height of infraorbital foramen

above alveolar border........... 28.5

Width of palate between canines... 26.4

Width of palate between M 3 ........ 35.5

I 3 -C diastema..................... 14.0

C-P1 diastema..................... 37.5

Length P 2 "4 ....................... 53.5

Length M 1 '3 ....................... 47.3

Length P 2 -M3 ...................... 99.4

1 2 APxTR.......................... 8.1x6.6

1 3 APxTR.......................... 7.8x5.9

C APxTR.......................... 4.2x3.8

P2 APXTR.......................... 18.6x18.0

P 3 APxTR.......................... 17.1x18.7

M 1 APxTR.......................... 14.1x21.1

M2 APxTR.......................... 16.8x20.8

M 3 APxTR.......................... 17.3x20.4

Anteriorly, the premaxillaries are preserved. The incisive foramen, present between and dorsal to the alveoli 

for the first two incisors, is anterior to a small tubercle 

situated in the anterior opening of the narial cavity. 

Ventrally are remnants of the palatine processes of the 

premaxillaries. The nasal processes of the premaxillaries 

are broken approximately at their sutures with the 

nasals. The infraorbital foramen lies above the anterior 

moiety of the P4. Mesial to the molars, palatine 

grooves are preserved on the palatine processes of the 

maxillaries. The anterior edge of the U-shaped posterior 

nares is opposite the anterior edge of the M2 . 

Posteriorly, and forming two small tubercles on either 

side of the narial opening, are remnants of the pterygoid 

hamuli.

The skull is broken posterior to the basisphenoid. The 

temporal condyle and part of the glenoid cavity of the 

temporal are preserved. Owing to crushing, the malar 

part of the zygomatic arch is slightly flared out in the 

region of the facial crest.
ORDER PERISSODACTYLA OWEN 27

FIGURE W. Parahippus cf. P. leonensis, specimen D857-1, from locality D857, Elk Springs 15-minute quadrangle, Moffat County, Colo. Top, lateral view of right side of skull showing incisors (I 2~3 ), canine (C), premolars (P2~ 3 ), and 

molars (M1 '3 ); O, orbit; P, preorbital fossa. Middle, palatal view of skull. Bottom, closeup view of right P2"3 , M1 "3 .
28 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

Although partially crushed, the bones of the orbital 

region reveal significant details. Anteriorly, the facial 

portion of the lacrymal is depressed, forming the preorbital fossa. The orbital and facial portions of the 

lacrymal are separated by the preorbital bar, which 

has a distinct tubercle on its anterodorsal margin. On 

the orbital surface is a small foramen mesial, and 

slightly ventral, to a larger lacrymal foramen. The nasolacrymal duct leads forward from the lacrymal foramen 

and passes beneath the wall of the preorbital fossa. 

Examination of some skulls of Merychippus in the 

American Museum of Natural History, collected from 

the middle part (Skinner and others, 1977) of the Sheep 

Creek Formation (Matthew and Cook, 1909), reveals 

that the small foramen below the lacrymal foramen apparently passes anteriorly and mesially into the maxillary sinus. This lower foramen was not seen on modern 

horse skulls. At the same position in the modern horse 

is the depression for the origin of the obliquus oculi 

inferior muscle. On the Sheep Creek horses, the lower 

foramen is probably too deep to serve as a muscle 

origin. The foramen is not a second opening into the 

naso-lacrymal canal as it is in the tapir, in which it 

merges with the upper naso-lacrymal canal and empties 

into the nares. On the Sheep Creek horses, this smaller 

foramen passes medially as well as anteriorly and would 

not merge with the lower lacrymal duct to exit in the 

region of the external nares. Possibly, one of the 

branches of the maxillary part of the trigeminal nerve 

enters the maxillary sinus through this foramen. 

According to Getty (1975, p. 655), "small caudal maxillary alveolar branches [of the infraorbital branch of the 

maxillary branch of the trigeminal nerve] are given off 

in the pterygopalatine fossa, pass through small 

foramina in the maxillary tuber and supply the caudal 

molar teeth and the maxillary sinus. The middle maxillary alveolar branches are given off in the infraorbital canal and constitute the chief nerve supply to 

the cheek and the maxillary sinus." Because this 

foramen on the Sheep Creek horses courses into the 

maxillary sinus, it possibly carried the posterior or middle alveolar nerve.

A small sphenopalatine foramen is present 34 mm 

posterior to the preorbital bar and midway up the mesial 

wall of the orbit. The large vacuity for the optic foramen 

is 22 mm posterior to the sphenopalatine foramen. Ventrally, the optic foramen is separated by a sharp ridge 

of bone from the smaller orbital foramen. The foramen 

rotundum lies ventral and slightly posterior to the latter. A small supraorbital foramen is present on the dorsal margin of the orbit. On the dorsal surface of the 

parietal, a crest curves mesially and posteriorly from 

a small supraorbital tuberosity located above the

supraorbital foramen. The frontal-parietal suture is apparent down the midline.

The incisors are worn nearly to their roots, and their 

occlusal surfaces are flat with centrally located, narrow 

lakes. The I3 is more transversely compressed and 

smaller than the I2. A short diastema separates the I3 

from the small, peglike canine. A longer diastema separates the canine from the P1 , which apparently has two 

closely appressed roots.

The cheek teeth are worn nearly to the roots, and the 

diagnostic occlusal pattern is nearly obliterated. The 

P2 is subtriangular and anteroposteriorly longer than 

wide. The prominent mesostyle is much larger than the 

diminutive metastyle. Owing to wear, the pre- and postfossettes are flattened ovals near the lingual side of the 

crown; consequently, the ectoloph is broad. The borders 

of the lophs are smooth, owing to destruction of the 

plications. At this stage of wear the protocone and 

hypocone are indistinctly separated by a small 

medivallum. The hypocone is larger than the protocone. 

Labially, two widely separated roots are visible. The 

P3 is transversely elongated with the buccal side wider 

than the lingual. The parastyle and metastyle are subequal in size, and the parastyle slightly overlaps the 

metastyle of the P2. The fossettes are nearly obliterated. The protocone is larger and more mesially projecting than the hypocone, and the posterolingual border 

of the tooth is reduced in relation to the anterolingual 

border. In contrast, the anterolingual borders of the 

molars are reduced in relation to the posterolingual 

borders. The P4 is missing.

The M1 -2 are transversely elongate and posteriorly 

concave. The transverse elongation is due to the 

anteroposterior length decreasing toward the root and 

the transverse width remaining constant. Being less 

worn, the M3 is less transverse than the M1 or M2. The 

molar parastyles are prominent and overlap the 

preceding metastyles. Ribs are only faintly expressed 

between the styles. The mesostyles are larger than the 

parastyles, which are larger than the metastyles. Ectocingula run along the base of the teeth and are confluent with the styles. Wear has simplified the molar 

occlusal patterns, resulting in the loss of the plicaballinae. The M3 retains a small pli-hypostyle on the 

post-fossette and has a hypoconal groove which extends 

nearly to the post-fossette. The teeth are worn so that 

the buccal edges are higher than the lingual edges. The 

molars have long, widely flaring roots and buccal 

coatings of cement. The M3 is less worn than the other 

teeth and has better preserved buccal cement.

Comparisons and discussion. Comparisons with 

other horses are difficult owing to the old age of the individual and the crushing and erosion of the skull. In
ORDER ARTIODACTYLA OWEN 29

size and morphology, however, the worn dentition of 

specimen D857-1 compares most favorably with those 

of advanced species oi Parahippus and primitive species 

of Merychippus. Merychippus and advanced species of 

Parahippus have cementum on the teeth. Downs (1956, 

table 9) suggested that the thickness of this coating is 

useful in distinguishing between Merychippus and 

Parahippus; according to his comparison, the amount 

of cement on the teeth of advanced Parahippus is less 

than that on the teeth of two of the most primitive 

species of Merychippus, M. primus and M. gunteri. 

Simpson (1932, p. 23), however, noted that some of the 

P. leonensis from the Thomas Farm quarries in Florida 

"have as much cement as the less coated Midway [fossil 

quarries in Florida] M. gunteri specimens" and White 

(1942, p. 22) noted that P. leonensis occasionally has 

more cement than on M. primus.

Specimen D857-1 has less cement on the buccal side 

of the teeth than some Merychippus from the Sheep 

Creek Formation (Matthew and Cook, 1909) of late 

Hemingfordian age. The amount of cement on specimen 

D857-1 is similar to that on P. avus (Marsh, 1874) (= P. 

brevidens (Marsh, 1874) according to Downs (1956, 

p. 240)) and also to that on P. leonensis SeUards, 1916. 

The type of P. avus is similar in size to specimen 

D857-1; on the type of P. brevidens the M3 is equal in 

size and the M2 is larger than that of specimen 

D857-1. The amount of cement on specimen D857-1 is 

equal to that on the type of P. avus. A partial skull of 

P. avus (AMNH 14182) illustrated by Osborn (1918, pi. 

6) has no features preserved useful for comparison with 

specimen D857-1. P. leonensis is slightly smaller than 

P. avus. Downs (1956, table 6) listed statistics on the 

lengths and widths of the cheek teeth of P. avus and 

P. leonensis; for length and width, specimen D857-1 

falls nearer the means for P. leonensis. Downs (1956, 

p. 243) noted that P. avus and P. leonensis are similar 

in the amount of cement present on the teeth.

P. crenidens (Scott, 1893), and P. coloradensis Gidley, 

1907, also have cement on the teeth. According to 

Downs (1956, table 9), P. crenidens has less cement than 

P. avus and, therefore, the cement is also less than on 

specimen D857-1. Downs (1956, table 9) characterized 

P. coloradensis as having only a light coat of cement. 

In the original diagnosis of P. coloradensis, no mention 

of cement was made by Gidley; Gazin (1932, p. 77) referred some teeth from Skull Spring, Oreg., to this 

species, noting that they lack cement.

The size of the teeth and the amount of cement on 

them closely matches that of P. leonensis, although the 

difference with P. avus is not great. For this reason, 

specimen D857-1 is assigned to Parahippus cf. P. 

leonensis.

Order ARTIODACTYLA Owen

Suborder TYLOPODA Uliger

Family CAMELIDAE Gray

Subfamily STENOMYLINAE Matthew

Genus BLICKOMYLUS Frick and Taylor

Blickomylus galushai Frick and Taylor

Table 10, Figure 11

Blickomylus galushai Frick and Taylor (1968, p. 23-36, 

figs 8-11).

Holotype. F:AM 50840, a nearly complete skull with 

F-M3 , from Blick Quarry in the Chamisa Mesa 

Member of the Zia Sand of Galusha (1966), in Sandoval 

County, N. Mex.

Referred material. D857-2,- LP4 , LM1 '2, LP4 ; 

D857-3, distal fragment right radius-ulna; D857-4, 

proximal fragment right metacarpus; D857-5, fragmentary distal metapodial.

Distribution and age. Chamisa Mesa Member of 

Galusha (1966) and Canada Pilares Member of Gawne 

(1981b) of the Zia Sand of Galusha (1966); Arikaree Formation, Fremont County, Wyo.; Browns Park Formation, Moffat County, Colo.; Hemingfordian land 

mammal age.

Description. The P4 is small and heavily worn, and 

the fossette between the buccal and lingual cusps has 

disappeared. The M1 is short and transversely narrow 

and tapers strongly antero-posteriorly toward the roots. 

The pre- and post-fossettes are present as shallow 

depressions lacking internal enamel rings. Ribs and 

styles are absent on the flat buccal wall, and the 

posterior lobe of the tooth is longer than the anterior 

lobe. The M2 is transversely narrow and hypsodont 

with only the the anterior lobe completely preserved. 

A fossette is present on this lobe. Except for a faint 

parastyle, the buccal wall is smooth. On the M1 and

TABLE 10. Dental and skeletal measurements of Blickomylus 

galushai from locality D857, Elk Springs 15-minute quadrangle, 

Moffat County, Colorado

[Measurements in millimeters]

APxTR, I.I x 5.0

M 1 APxTR..................... 15.0 x 9.6

M2 APxTR of first lobe....... 13.5 x 10.1

P 4 TR........................ 3.5

Radius-ulna shaft TR width

above distal end........... 16.3

Proximal metacarpus APxTR.... 15.8 x 19.8
30 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

FIGURE 11. Blickomylus galushai, specimen D857-2, from locality D857, Elk Springs 15-minute quadrangle, Moffat County, Colo. Left 

photo: A, occlusal view of left molars (M1 '2); B, occlusal view of left premolar (P4); C, occlusal view of left P4. Right photo: D, lingual 

view of left M1 "2; E, buccal view of left P4; F, buccal view of left P4. Both scales in centimeters.

M2 the wall of the metaconule is flatter and does not 

project as far lingually as does the wall of the protocone. 

The P4 is compressed laterally and tapers markedly 

anteriorly. It lacks a paraconid inflection.

The shaft of the distal part of the radius-ulna is relatively slender. Only half of the distal end is preserved. 

The lunar and cuneiform condyles are subequal in size, 

and the lunar condyle has a proximolaterally trending 

trough on its medial side. Anteriorly, the lateral and 

medial carpal extensor grooves are present, the medial 

groove being deeper and narrower than the lateral. A 

prominent tuberosity lies between them. Anterodorsal 

to the cuneiform condyle is a distal interosseous 

foramen. A posterior distal interosseous foramen is also 

present with a vertical groove extending approximately 20 mm proximally. The lateral surface of the shaft 

immediately above the cuneiform condyle is highly 

rugose.

The proximal metacarpal preserves the trapezoid, 

magnum, and unciform facets. Rugose depressions for 

metacarpals 2 and 5 are present posteriorly beneath the 

trapezoid and unciform facets respectively. Metacarpals 

3 and 4 are completely fused proximally.

Comparisons and discussion. The narrow, hypsodont teeth with smooth buccal walls clearly identify this

camel as a member of the subfamily Stenomylinae. 

Direct comparisons of many specimens, including the 

types of Blickomylus andRakomylus, establish that the 

Browns Park stenomyline is a Blickomylus. Both 

specimen D857 and Blickomylus differ from most 

Stenomylus in having more solidly fused metacarpals. 

The metacarpals are completely uncoossified on S. 

hitchcocki Loomis, 1910, and on an unnamed Stenomylus from Wellton, Ariz. In S. gracilis Peterson, 1906, 

the proximal halves of the third and fourth metacarpals 

are fused, but the line of fusion is usually very prominent, as in the type. On specimen D857-4 and 

Blickomylus, this line of fusion is very faint. A few specimens of Stenomylus though, such as F:AM 36815 

(Frick and Taylor, 1968, fig. 6), have strongly coossified 

metacarpals which resemble those of Blickomylus. The 

P4 of specimen D857-2 and Blickomylus is more 

reduced than that of Stenomylus. The length of the P4 

of specimen D857-2 is 8.1 mm; that of Blickomylus 

ranges from 7.8 to 9.0 mm. The length of the P4 of S. 

gracilis ranges from 10.0 to 10.3 mm. Although the 

length of the P4 of the Wellton Stenomylus is the same 

as that of specimen D857-2, the P4 of the Wellton 

camel is 2 mm wider. The molars of the Wellton 

Stenomylus are shorter than those of specimen D857-2
ORDER ARTIODACTYLA OWEN 31

and Blickomylus. Similarly, though the molars of S. 

gracilis have the same length as those of specimen 

D857-2 and Blickomylus, the P4 is longer.

The crown patterns and tooth sizes of specimen 

D857-2 match those on some Blickomylus from the 

Chamisa Mesa Member of the Zia Sand and differ 

significantly from those of Rakomylus. Both specimen 

D857-2 and Blickomylus differ from Rakomylus in having a relatively larger M1 . The M1 of Rakomylus is 

9.3 mm long (Frick and Taylor, 1968, table 6), whereas 

that of Blickomylus ranges from an estimated 10 mm 

to 17.5 mm (Frick and Taylor, 1968, table 4); specimen 

D857-2 measures 15.0 mm. The P4 of specimen 

D857-2 also lies within the size range of Blickomylus 

given by Frick and Taylor, and is larger than the P4 of 

Rakomylus. Also, the concave wear facets on the 

anterior lobes of the two teeth match those on 

Blickomylus.

The metacarpal of Rakomylus is about 30 percent 

shorter than that of Blickomylus, but the metacarpal 

of specimen D857-2 is too incomplete to offer any information about size. However, the dental evidence is 

conclusive that this camel is Blickomylus. The only 

known species of Blickomylus is B. galushai. The 

Browns Park camel cannot be distinguished morphologically or metrically from B. galushai and is therefore 

assigned to that species.

Subfamily CAMELINAE Gray 

Tribe PROTOLABIDINI Cope

Genus cf. PROTOLABIS Cope

Table 11, Figure 12

Referred material. D856-17, distal fragment left 

humerus; D856-18, distal fragment left humerus; 

D856-19, distal fragment right humerus; D856-20, 

proximal fragment left radius-ulna; D856-21, proximal 

left radius-ulna; D856-22, proximal fragment left 

radius-ulna; D856-23, proximal fragment right radiusulna; D856-24, distal fragment right radius-ulna; 

D856-25, distal fragment left radius-ulna; D856-26, 

distal fragment left radius-ulna; D856-27, distal fragment left tibia; D856-28, left astragalus; D856-29, left 

astragalus; D856-30, left astragalus; D856-31, left 

astragalus; D856-32, right astragalus; D856-33, right 

calcaneum; D856-34, right proximal fragment 

calcaneum; D856-35, left calcaneum; D856-36, left 

calcaneum; D856-37, left distal fragment calcaneum; 

D856-38, right proximal fragment metatarsal; 

D856-39, right proximal fragment metatarsal; 

D856-40, right proximal fragment metatarsal; 

D856-41, distal fragment metapodial; D856-42, distal 

fragment metapodial; D856-43, distal fragment

metapodial; D856-44, proximal phalanx; D856-45, 

proximal phalanx; D856-46, medial phalanx; D856-47, 

medial phalanx; D856-48, proximal phalanx; D856-49, 

proximal phalanx; D856-50, proximal phalanx; 

D856-51, proximal phalanx.

Comparisons and discussion. The postcranial 

elements are practically indistinguishable from those 

of Protolabis coartatus. The humeri, radii-ulnae, and 

tibiae are completely within the range of morphological 

and size variation of that species. The calcanea, 

astragali, and metapodials of the locality D856 camel 

are somewhat larger than those of most specimens of 

P. coartatus, but the difference is insignificant for example, the proximal widths of the metatarsals range 

from no greater to at most 8 percent wider than the 

largest known metatarsal of P. coartatus. The 

phalanges from locality D856 are similar in length, but 

slightly more slender, than those of P. coartatus. The 

anterior limb elements from locality D856 are, like those 

of the P. coartatus specimens from the Milk Creek Formation (Clarendonian) of Arizona, slightly smaller than 

those of the type of P. coartatus.

None of the various limb elements were associated. 

However, they can be assumed to belong to the same 

species because of their close match in size and morphology to similar limb elements of P. coartatus. 

Because of this close correspondence, the limbs from 

locality D856 probably belong to a species of Protolabis, 

although not necessarily P. coartatus. At present, there 

are few characteristics other than size and proportions, 

that can be used to separate the postcrania of various 

genera of Miocene camels. For the sake of completeness, 

the following comparisons are offered.

The locality D856 camel limb elements are smaller 

than those of Aepycamelus, Hesperocamelus, 

Megatylopus, Titanotylopus, Pliauchenia magnifontis, 

Hemiauchenia, Alforjas, and Procamelus grandis. The 

limbs of Procamelus occidentalis cannot be adequately 

compared with those from locality D856. Oxydactylus, 

Priscocamelus, Miolabis, Nothotylopus, and probably 

Aguascalientia have unfused metapodials. Tanymykter 

has unfused metacarpals and only weakly fused metatarsals, in contrast to the strongly fused metapodials 

of D856. The limb bones of Stenomylus are smaller and 

the metapodials generally more slender and less 

coossified than those from locality D856. Frick and 

Taylor (1968) illustrated the metapodials of Blickomylus 

and Rakomylus, and they belong to smaller animals 

than the locality D856 camel (the Blickomylus distal 

radius from the Browns Park Formation also supports 

smaller size for these camels). The limbs of Australocamelus, Nothokemas, and Floridatragulus have not 

been adequately described. Michenia has smaller, 

slenderer limbs than the locality D856 camel. Thus,
32 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

TABLE 11. Skeletal measurements of cf. Protolabis sp. from locality D856, Citadel Plateau 15-minute

quadrangle, Moffat County, Colorado

[Measurements in millimeters; leaders ( ) indicate no data

Specimen O.R.

Humerus trochlear TR width..................... 3 42.3-43.6 42.9

Humerus trochlear height....................... 3 29.5-32.0 31.13

Radius-ulna TR width distal articular surface.. 2 41.9-43.5 42.7

Radius-ulna AP width distal articular surface.. 3 37.9-39.3 38.4

Tibia TR width distal end...................... 1 41.5 

Tibia AP length distal end..................... 1 25.8 

Astragalus lateral length...................... 5 38.9-44.1 41.1

Astragalus medial length....................... 5 34.3-40.0 36.3

Astragalus minimum length...................... 5 29.7-33.8 30.9

Astragalus greatest TR width................... 5 25.8-28.8 27.4

Astragalus distal trochlear TR width........... 5 23.9-27.1 25.4

Calcaneum maximum length....................... 3 91.2-91.6 91.4

Calcaneum maximum AP length.................... 4 35.9-38.1 37.1

Calcaneum tuberosity TR width.................. 3 21.5-23.6 22.6

Calcaneum tuberosity AP length................. 3 22.0-23.0 22.6

Proximal metatarsal TR width................... 3 29.5-34.0 32.2

Distal metapodial (forefoot?) trochlear

surface APxTR................................ 1 23.1x23.0

Distal metapodial (hindfoot?) trochlear

surface AP................................... 2 19.2-20.2 19.7

Distal metapodial (hindfoot?) trochlear

surface TR................................... 2 18.4-19.8 19.1

Proximal phalanx; TR distal width.............. 6 14.1-15.4 14.7

Proximal phalanx; AP distal length............. 5 11.0-12.6 11.8

Medial phalanx length.......................... 2 37.8-39.0 38.4

Medial phalanx proximal TR width............... 2 13.0-13.3 13.15

Medial phalanx proximal AP length.............. 2 12.8-13.2 13.0

Medial phalanx distal TR width................. 2 12.7-12.9 12.8

Medial phalanx distal AP length................ 2 8.3-8.4 8.35

these limb elements probably belong to Protolabis. 

A thorough study of that genus, however, is necessary before specific assignment of these limbs can be 

made.

Genus MICHENIA Frick and Taylor 

Michenia sp.

Table 12, Figure 13

Referred material. D856-11, RI 2_ 3 roots, RC; 

D856-12, fragmentary symphysis with RP^ D856-13, 

right dentary with Pj_4, M^, M3 (broken); D856-14,

distal fragment right humerus; D856-15, proximal fragment left radius; D856-16, proximal right metacarpal.

Description. The I3 is the only incisor with the 

crown preserved. In early wear it is highly spatulate and 

transversely flattened with a crown 20.2 mm high. The 

incisor roots are small. The canine is separated from the 

I3 by a 4.0 mm diastema. The canine is small, slightly 

recurved, and transversely flattened with the buccal 

side convex and the lingual side concave dorsoventrally. Anterodorsally, a slight pinching of the enamel forms 

a small flange.

The C-Pj diastema is not preserved. The Pj is suppressed; that is, although fully formed, the tooth has
ORDER ARTIODACTYLA OWEN 33

FIGURE 12. Cf. Protolabis, from locality D856, Citadel Plateau 

15-minute quadrangle, Moffat County, Colo. A, right calcaneum, 

lateral view; specimen D856-33. B, left calcaneum, anterior view; 

D856-36. C, left astragalus, anterior view; D856-29. D, left 

astragalus, posterior view; D856-30. E, right astragalus, lateral 

view; D856-32. F, medial phalanx, anterior view; D856-46. G, 

medial phalanx, anterior view; D856-47. H, proximal phalanx, 

anterior view; D856-49.

not erupted in the mature animal. The P: has only one 

root; it cannot be determined whether this root formed 

from a single root or from two fused roots. Excavation 

of the bone shows that the P: is vertical.

The P2 lies 23.5 mm behind the Pr The P2 is short 

and transversely narrow, with the simple crown consisting only of a low protoconid. A shallow median 

sulcus on the single root indicates that it formed from 

the fusion of two separate roots. The P3 is transversely narrow and double-rooted. The protoconid, swollen at 

the base, is the most distinctive cusp. A ridge runs 

anteriorly from the protoconid and terminates in a 

minute and slightly lingually inflected paraconid. 

Posteriorly, another ridge terminates in a small but distinct swelling formed from the entoconid and hypoconid. 

The P4 is transversely narrow and nearly as long as the 

combined lengths of the P2 and P3. The protoconid is 

the highest cusp. A lingual sulcus separates the protoconid from the lingually inflected paraconid. The entoconid and hypoconid have, by wear, united to form a 

posterior swelling as broad as that of the protoconid. 

A broad, shallow buccal sulcus lies between the protoconid and hypoconid. Wear has eliminated the fossettid between the hypoconid and entoconid.

The M1 is worn nearly to the roots. It is relatively 

long, and the posterior lobe is wider than the anterior

lobe. On the M2 , wear has eliminated the anterior 

fossettid, and the posterior fossettid is relatively small. 

The lingual wall is flat, with only a small parastylid present. The M3 is broken behind the anterior moiety. The 

worn crown height measures 17.2 mm on the protoconid. The parastylid is prominent lingually, but the 

mesostylid is only a low ridge. Lingual ribs are absent. 

None of the molars have intercolumnar stylids or 

protostylids.

Erosion has eliminated most of the details of the dentary. The right dentary is narrow transversely 6.3 mm 

wide between the P: and P2. The minimal depth of the 

dentary between the Pl and P2 is not preserved; however, it is less than the depth at the suppressed Plt 

where it is 22.5 mm.

On the basis of size, three fragmentary limb elements 

are referred to this camel. The distal fragment of the 

humerus has the trochlea preserved. Anteriorly, above 

the medial condyle, is a transverse trough for the origin 

of the common digital extensor. The proximal fragment 

of the radius has only the humeral articular surface 

preserved. The lateral tuberosity is smaller than on cf. 

Protolabis. The proximal metacarpal preserves the 

small, triangular trapezoidal facet posteriorly. Anterior 

to the trapezoidal facet is the large, concave magnum 

facet, quadrilateral in shape. Below its anterior edge is

TABLE 12. Dental and skeletal measurements o/Michenia sp. from 

locality D856, Citadel Plateau 15-minute quadrangle, Moffat County, Colorado

[Measurements in millimeters]

Lo

Mo

APxTR..................

APxTR..................

APxTR..................

APxTR..................

APxTR..................

APxTR of first lobe....

......... 6.4

C OyO C

......... 8.4x3.4

......... 12.7x6.2

......... 20.0x11.0

......... 23.9x12.9

......... 13.8x13.6

Depth jaw between and P 2 ........ <22.5

Depth jaw below P^................

Humerus trochlear TR width........

Humerus maximum trochlear height..

Radius; width proximal articular 

surface.......................

22.5

33.7

25.3

33.2
34 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

FIGURE 13. Michenia sp., from locality D856, Citadel Plateau 15-minute quadrangle, Moffat County, Colo. A, lateral view of right symphysis with incisor roots and canine (C) of specimen D856-11. B, lateral view of right dentary with premolars (P1 _4) and molars (Mx 3 ) 

(M3 broken) of specimen D856-13.

a small tuberosity. Only the anterior portion of the unciform facet is preserved. Metacarpals 3 and 4 are completely fused, and the line of fusion is not visible.

Comparisons and discussion. Specimen D856-13 

differs from Oxydactylus, Priscocamelus, Tanymykter 

and Nothokemas in having higher crowned molars, 

reduced premolars, and suppressed Pr Reduced 

premolars and suppressed Pl separate specimen 

D856-13 from Australocamelus, Aepycamelus, and 

Hesperocamelus; in addition, the latter two are much 

larger. Procamelus has relatively nonreduced premolars, 

fully erupted P1? and more hypsodont molars than 

specimen D856-13. Miolabis is distinguished from 

specimen D856-13 by the absence of Pp relatively

shorter and less laterally compressed premolars, and 

lower crowned molars with stronger metastylids. 

Miotylopus is distinguished from specimen D856-13 by 

rather brachyodont, short molars, and nonreduced 

premolars. Nothotylopus differs from specimen 

D856-13 in having more brachyodont molars, larger 

P3 with well-defined cusps, absent P2 , and erupted Pr 

Low-crowned molars, nonreduced premolars, and P2_3 

diastema distinguish Floridatragulus from specimen 

D856-13. Aguascalientia is much smaller than 

D856-13, with non-reduced P2_3 , and with brachyodont, laterally swollen molars with intercolumnar 

tubercles on Ml 2 . The stenomylines possess extremely hypsodont and flat-walled molars. Hemiauchenia
ORDER ARTIODACTYLA OWEN 35

lacks the P2 and has so-called llama buttresses on the 

lower molars; Paleolama has low-crowned molars with 

strong mesostylids.

Pliauchenia magnifontis Gregory, 1942 is larger than 

specimen D856-13 and has a large and erupted P^ the 

P2_4 are longer and broader than on specimen D856-13. 

Pliauchenia singularis Matthew, 1918 differs from specimen D856-13 in having broader P3^, shorter molars, and 

M2 3 with prominent parastylids. Specimen D856-13 

cannot adequately be compared with Pliauchenia 

minima Wortman, 1898, as it has never been illustrated, 

and measurements were not given in the original description. Gregory (1942, p. 379) noted that P. minima 

"looks about as much like an antelope as a camelid 

* * *and cannot be definitely assigned to any family at 

present." Specimen D856-13 differs from Pliauchenia 

humphreysiana Cope, 1875, Protolabis heterodontus 

(Cope, 1874), and Protolabis barstowensis Lewis, 1968, 

in having a small canine and suppressed Pr These 

same criteria separate D856-13 from most individuals 

of Protolabis coartatus (Stirton, 1929), although a few 

individuals of P. coartatus have a suppressed Pr

Specimen D856-13 resembles Michenia in possessing 

a small, semiincisiform lower canine and differs from 

Protolabis which possesses a larger, caniniform canine. 

The sizes of the lower canines in Michenia and Protolabis do not overlap (Honey and Taylor, 1978, p. 418). 

Specimen D856-13 resembles derived forms of Michenia in having a suppressed Pr Although not restricted 

to Michenia, suppression of the Pl is much more common in that genus than in Protolabis, in which it is only known to occur in a few individuals of the derived 

P. coartatus.

Reduction in size of the premolars occurs in Michenia 

and Protolabis, and is greatest in Protolabis coartatus 

and Michenia yavapaiensis Honey and Taylor, 1978. 

The size of the P2 and P3 of specimen D856-13 is close 

to the mean size given for Michenia yavapaiensis, and 

is further removed from the mean size for Protolabis 

coartatus (Honey and Taylor, 1978, tables 4 and 10). The 

P4 however, is relatively long and is closer to the mean 

length for Protolabis coartatus than for Michenia 

yavapaiensis, although it falls within the observed 

range of the latter species. Fracturing has lengthened 

the Ml of specimen D856-13 slightly, but nevertheless 

it is closer to the mean length of P. coartatus than to 

M. yavapaiensis. The M2 length (but not width) is 

closer to the mean length of P. coartatus, but it also falls 

within the observed range of M. yavapaiensis.

The ramal depth of Michenia is less than that of Protolabis. The ramal depth at Pl of specimen D856-13 is 

22.5 mm, which is intermediate between average ramal 

depths for currently recognized species of Michenia and 

Protolabis.

Although some of the above characteristics are 

equivocal, the small canine and suppressed Pl indicate 

that the material from locality D856 belongs to 

Michenia. The limb elements are referred to this genus 

because they cannot be separated from those of 

Michenia yavapaiensis. The reduced premolars and suppressed Px exclude this specimen from M. agatensis 

Frick and Taylor, 1971; the D856 camel is distinct from 

M. exilis (Matthew and Macdonald, 1960) in having a 

suppressed Plt P2_ 3 reduced relative to P4 , more 

elongate molars, and fused metapodials. The depth of 

the jaw is greater than in Michenia yavapaiensis, and 

the lengths of the P4-M2 lie at the extremes of the 

observed ranges of these teeth in Michenia yavapaiensis. For these reasons, this small camel from locality 

D856 is not included within Michenia yavapaiensis. It 

may be a representative of a new species, but a larger 

sample is needed before this can be verified.

Subfamily CAMELINAE or AEPYCAMELINAE 

Frocamelus sp. or Aepycamelus sp.

Table 13, Figure 14

Referred material. D855-1, right ramus with dI 2_3, 

!,(?), dC, dP3 _ 4 , M: ; D855-2, right astragalus; 

D856-52, right distal fragment humerus; D856-53, left 

distal fragment humerus; D856-54, right proximal fragment radius-ulna; D856-55, right distal fragment 

radius-ulna; D856-56, left astragalus; D856-57, right 

lunar, D856-58, right scaphoid; D856-59, right 

cuneiform; D856-60, distal fragment metapodial; 

D856-61, distal fragment metapodial; D856-62, distal 

fragment metapodial; D856-63, distal fragment 

metapodial; D856-64 through D856-72, eight proximal 

phalanges; D856-73, medial phalanx; D856-74, medial 

phalanx; D856-88, medial phalanx.

Description. The spatulate I1 is in the process of 

eruption. It lies beneath the broken, slightly 

anteroposteriorly flattened root of dI 2. The laterally 

flattened root of dI3 follows a diastema of 7.5 mm. The 

broken root of dC is directed anteriorly.

The mandible is broken 26 mm behind the deciduous 

canine and 23 mm in front of the dP3 , and the jaw 

length between these two teeth is not measurable. A 

fragment from the left side of the jaw contains the roots 

of the dP2, and (18 mm anteriorly) the alveolus for the 

P1 (camels lack a deciduous predecessor to the P1? according to Frick and Taylor, 1968, p. 14). The dP3 is 

long, narrow, and double-rooted, consisting of three 

cusps consecutively wider posteriorly; the third cusp is 

much wider than the preceding two. Buccally, two sulci 

separate the three cusps, and the posterior sulcus is 

wider than the anterior sulcus. On the lingual side of
36 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

TABLE 13. Dental and skeletal measurements o/Procamelus sp. or Aepycamelus sp. from localities D855 and D856, Citadel 

Plateau 15-minute quadrangle, Moffat County, Colorado, and comparative measurements on Aepycamelus from the Pojoaque Member of Galusha and Blick (1971) (late Barstovian and possibly early Clarendonian) of the Tesuque Formation

[Measurements in millimeters; leaders ( ) indicate no data. F:AM Frick, American Museum numbers]

Specimen

dP 3 APxTR.................

dP4 APxTR.................

Humerus trochlear TR

width...................

Humerus trochlear height..

Radius midshaft TR........

Radius distal TR width....

Proximal phalanx:

length..................

proximal TR width.......

distal TR width.........

distal AP width. ........

Medial phalanx:

proximal AP width.......

distal TR width.........

distal AP width.........

Astragalus:

greatest TR width.......

distal trochlear

TR width..............

Metapodial midshaft TR....

Distal metapodial

trochlear surface TR....

Distal metapodial

trochlear surface AP...

Spread of distal

metapodi al ..............

n

1

1

1

1

1

1

5

71'

6

6

3

3

3

3

3

2

2

2

2

2

1

4

4

1

O.R.

16.5x8.5

30.4x15.6

55.9

46.2

52.1

61.6

81.4-95.5

26.1-31.2

24.2-27.8

22.9-25.5

20.6-23.0

46.8-54.2

20.9-24.6

16.6-18.7

20.2-20.9

13.6-16.7

61.0-62.2

53.9-56.6

45.5-46.5

38.1-39.9

37.2-37.4

31.7

27.2-28.7

31.8-33.6

58.1

X

___

___

___

90.1

27.9

26.7

24.2

21.7

51.6

22.8

17.5

20.5

15.5

61.6

55.2

46.0

38.9

37.3

___

28.1

32.9

---

F:AM F:AM

38606 24101

___ ___

 

 ._ -__

 

46.7

63.8 60.6

93.4

29.1

25.8

23.7

22.9

55.6

27.6

23.6

23.1

17.0

 

 

 

 

 

39.6

29.5

29.8

F:AM F:AM

38585 68577

_ _ ___

 

56.1

40.5

___ _ 

 

 

 

 

 

 

 

 

58.6

52.6

45.1

38.2

37.1

 

 

the tooth is a narrow, anterior sulcus. Owing to wear, 

all traces of fossettids have been eliminated.

The dP4 is three-lobed with the posterior lobe widest. 

A small tubercle is present in the anterior buccal sulcus 

between the first two lobes. Formed as a small fold on 

the posterior wall of the first lobe, the tubercle disappears before reaching the base of the crown. Traces of 

ribs are preserved on the lingual walls of the second and 

third lobes, and a small stylid is present between the

first and second lobes. Wear has eliminated all 

fossettids.

The second lobe of the Mj is broken. Although the 

tooth is in wear, the prefossettid is still present. An intercolumnar tubercle is present between the protoconid and 

hypoconid near the base of the tooth. Lingually, the parastylid and mesostylid are prominent. The metaconid is 

25.4 mm high and has a faint rib. The M2 is fragmentary, 

revealing nothing diagnostic. The unerupted M3 is lost.
ORDER ARTIODACTYLA OWEN 37

FIGURE l4. Procamelus sp. or Aepycamelus sp., from localities D855 and 856, Citadel Plateau 15-minute quadrangle, Moffat County, Colo. 

Top, lateral view of right dentary; specimen D855-1; M, molar; dP, deciduous premolars. Lower left, occlusal view of right dP3 4, MI 

(broken); D855-1. Lower right, A, anterior view of distal metapodial; D856-61. B, anterior view of right distal radius-ulna; D856-55. 

C, posterior view of proximal phalanx; D856-67. D, anterior view of proximal phalanx; D856-65. E, anterior view of proximal phalanx; 

D856-70. F, anterior view of medial phalanx; D856-74. G, anterior view of medial phalanx; D856-73. H, anterior view of right astragalus; 

D855-2. Scale is in centimeters.
38 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

The dentary is long and deep, with a convex ventral 

edge. A mental foramen is present 18.7 mm below the 

dorsal edge of the dentary and 17 mm posterior to the 

canine. The dentary is thin near the angle. Above the 

angle the camelid hook is broken, and the condyle is 

heavily eroded and possibly laterally flattened. The coronoid process projects approximately 49 mm above its 

junction with the condyle. It has a slight medial inflection and is 25.5 mm wide anteroposteriorly.

Large size is the most distinctive feature of the 

postcranial elements of this camel. On the medial condyle of the humerus is a large tuberosity, more prominent than that found on the cf. Protolabis from this 

fauna. The common digital extensor depression (above 

the medial condyle) is shallower and shorter than that 

of cf. Protolabis.

The humeral articular surface of the radius is broad 

and has a small pit lateral to the median sagittal ridge. 

The lateral tuberosity is small. The shaft of the radius 

is broad about 50 mm wide above the distal extremity. The ridge between the grooves for the carpal digital 

extensor is prominent.

The astragali are much larger than those of cf. Protolabis. Medially, the ridge between the navicular and 

medial malleolar facets is broad, contrasting with the 

narrow ridge on cf. Protolabis. Similarly, the tibial ligament surface is broader than that of cf. Protolabis.

A scaphoid, lunar, and cuneiform are referred to Procamelus sp. or Aepycamelus sp. on the basis of large 

size. The distal metapodials are distinguished from 

those of cf. Protolabis only by larger size. The medial 

phalanges are much larger and more robust than those 

of cf. Protolabis.

Comparisons and discussion. The large camel from 

the Browns Park Formation was compared with camels 

in the Frick collection at the American Museum of 

Natural History. In size, the limb elements correspond 

well with those of undescribed specimens of 

Aepycamelus from the Pojoaque Member (late Barstovian and possibly early Clarendonian) of the Tesuque 

Formation, N. Mex., and with large specimens of Procamelus from the Burge Member (latest Barstovian) of 

the Valentine Formation of Lugn (1938), Nebraska. The 

dentition also closely resembles those of Procamelus 

and Aepycamelus. The crown height of the partially 

worn M! of specimen D855-1 is matched by specimens 

of the large Procamelus from the Burge Member. The 

M: of specimen D855-1 is higher crowned than in 

most Aepycamelus in the Frick collection; a few, however, from the Pojoaque Member of the Tesuque Formation, have teeth as high-crowned as specimen 

D855-1. The rami of these specimens of Aepycamelus 

are shallower than those of specimen D855-1. The ramal

depth of specimen D855-1 matches that of adult 

specimens of Procamelus from the Burge Member. The 

size and morphology of the dP3_4 on specimen D855-1 

was matched by specimens of Procamelus from the 

Clarendon fauna (Clarendonian); on these forms, 

however, the Mx is higher crowned.

The postcranial material of the large Browns Park 

camel is about 12 percent smaller than that of 

Aepycamelus procerus (Matthew and Cook, 1909), 

which is a smaller species than A girafftnus (Matthew, 

1909). Specimen D856 cannot be compared with either 

A. prisons (Matthew, 1924), or A leptocolon (Matthew, 

1924), as they lack adequate description. Camel D856 

limb materials are slightly smaller than those of A(?) 

stocki (Henshaw, 1942), and Procamelus grandis 

Gregory, 1942.

Specimen D855-1 compares favorably with the lower 

jaws UCMP 32312 and UCMP 33420 from the Clarendonian Big Spring Canyon fauna (Gregory, 1942, figs. 

32 and 33b). The individual represented by UCMP 

32312 is younger than D855-1, and the jaw is thus 

slightly narrower dorsoventrally. The crown length of 

the dP4 is 25 percent longer than on specimen D855-1; 

the latter, however, is worn nearly to the roots. Data 

taken on mandibles of Protolabis coartatus show that 

the dP4 undergoes a 35-40 percent change in length 

from the crown to the base. Also, on UCMP 32312 there 

is a small labial intercolumnar tubercle between the 

second and third lobes, as on specimen D855-1. The 

length of the dP4 of UCMP 33420 is similar to that of 

specimen D855-1. The Ml is not associated with these 

jaws. Gregory (1942) was unable to assign these 

specimens to either Procamelus or Pliauchenia. 

However, Loring and Wood (1969, p. 1209) assigned the 

relatively smaller UCMP 33446 from Big Spring Canyon to Pliauchenia because it possessed cement. They 

did not assign the larger deciduous jaws to any genus.

The camel from localities D855 and D856 cannot be 

positively excluded fromHesperocamelus. Comparison 

with the measurements given for H. alexandrae (Davidson, 1923), showed that the distal humeri and radii-ulnae 

are 10-20 percent larger on H. alexandrae. The proximal phalanges, though, are the same size on both 

camels. H. stylodon Macdonald, 1949 is smaller than 

H. alexandrae. The distal metapodials of D856 appear 

to be the same size as those of H. stylodon; other 

measurements cannot be made from Macdonald's illustrations. In most measurements, the large camel 

from D855 and D856 is slightly larger than Pliauchenia 

magnifontis Gregory, 1942. Megatylopus and 

Titanotylopus are much too large for the Browns Park 

camel. Alforjas taylori Harrison, 1979, is larger than 

the Browns Park camel, and evidently lacks a buccal
ORDER ARTIODACTYLA OWEN 39

tubercle on the dP4. Dentary D855-1 is excluded from 

Hemiauchenia because it lacks a so-called llama buttress on the Mjj although a parastylid is present on 

specimen D855-1, the protostylid is absent. Dentary 

D855-1 is higher crowned than Paleolama. Other 

Miocene camels are smaller than the camel from 

localities D855 and D856.

The camel from localities D855 and D856 cannot, with 

certainty, be assigned to either Aepycamelus or Procamelus. Diagnostic differences in the lower deciduous 

teeth have never been established in the literature for 

Aepycamelus or Procamelus. Measurements on selected 

postcranial elements of Aepycamelus from the Pojoaque 

Member of the Tesuque Formation are given in table 

13, along with measurements for the Browns Park 

camel.

Subfamily CAMELINAE sp.

Table 14, Figure 15

Referred material. D856-75, left proximal fragment 

radius-ulna; D856-76, right distal fragment radius-ulna;

FIGURE 15. Camelinae sp., from locality D856, Citadel Plateau 

15-minute quadrangle, Moffat County, Colo. A, anterior view of right 

radius-ulna; specimen D856-77. B, anterior view of left distal radiusulna; D856-78. C, anterior view of proximal phalanx; D856-80. D, 

anterior view of proximal phalanx; D856-81. E, anterior view of 

proximal phalanx; D856-82.

TABLE 14. Skeletal measurements of Camelinae sp. from locality 

D856, Citadel Plateau 15-minute quadrangle, Moffat County, 

Colorado

[Measurements in millimeters; leaders ( ) indicate no data]

Specimen O.R.

Radius-ulna TR width distal 

end...................... 48.5-52.9 51.4

Proximal phalanges (manus?):

length.......... ..........

TR proximal width. ........

AP proximal length........

TR distal width. ..........

AP distal length..........

Proximal phalanx (pes?):

length. ...................

TR proximal width.........

AP proximal length. .......

TR distal width. ..........

AP distal length..........

4

3

4

6

6

1

1

1

1

1

77.0-83.2

21.4-23.0

21.2-22.9

18.5-21.5

14.8-19.2

66.7

19.8

17.6

16.7

13.6

79.6

22.1

21.9

19.7

16.5

---

D856-77, right distal fragment radius-ulna; D856-78, 

left distal fragment radius-ulna; D856-79 through 

D856-83, proximal phalanges; D856-84, distal end of 

proximal phalanx; D856-85, distal end of proximal 

phalanx.

Description and comparisons. A possible fourth 

camel is represented in this fauna. The distal humeri 

are indistinguishable from those of Procamelus or 

Aepycamelus sp. except by size. They are slightly 

smaller and are about the size of the distal humerus of 

Pliauchenia magnifontis.

Some proximal phalanges are indeterminate. The indeterminate phalanges are shorter and more slender 

than those belonging to Procamelus or Aepycamelus sp. 

These phalanges are larger than those referred to 

Michenia sp. and appear to be too large to belong to 

any species of Michenia. The phalanges are excluded 

from cf. Protolabis on the basis of morphology. The 

phalanges of Protolabis coartatus and the Protolabis 

from New Mexico, referred by Cope to Procamelus occidentalis (Cope, 1877, pi. 79, figs. 7 and 7a), are shorter 

and have wider, rounder shafts than the D856 

phalanges. The distal trochlea on these Protolabis 

specimens are also more massive and prominent than 

on D856. These indeterminate phalanges do not necessarily belong to the same camel as the indeterminate 

humeri, but are classified with them for the sake of 

convenience.
40 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

Suborder RUMINANTIA Scopoli

Infraorder PECORA Linnaeus

Superfamily BOVOIDEA Gray

Family ANTILOCAPRIDAE Gray

SubfamUy MERYCODONTINAE Matthew

Genus MERYCEROS Frick

Meryceros warreni (Leidy)

Table 15, Figure 16^4

Cervus warreni Leidy, 1858, p. 23.

Cervus warreni Leidy. Leidy, 1869, pp. 172 and 379, pi.

xxvii, fig. 12.

Meryceros warreni (Leidy). Frick, 1937, p. 361, figs. 2

(in part), 2a (in part), 25B (in part), 28 (in part), 28B,

28C, 35A (in part), 39.

Holotype. U.S.N.M. 149, right horn core.

Referred material. D856-86, right horn core.

Distribution. Niobrara River fauna (including 

Crookston Bridge Quarry) Nebraska; Trail Creek 

Quarry, Wyoming; Kennesaw fauna, Colorado, probable 

occurrence; Devil's Gulch Horse Quarry, Nebraska; 

Horse and Mastodon Quarry, Colorado: all late Barstovian land mammal age; possibly also from Lapara Creek 

fauna, Texas: Clarendonian.

Description. The shaft of the horn core is short and 

transversely flattened and there is no apparent twisting. 

The two partial tines are parallel to the sagittal plane. 

Part of the orbit is preserved and has a small foramen 

on the medial wall. This foramen exits on the frontal 

bone and anteromesial to the base of the horn core.

Comparisons and discussion. The short-shafted, 

transversely flattened horn core is assigned to 

Meryceros Frick, 1937. Meryceros is characterized by 

a single-forked, transversely flattened horn core.

FIGURE 16. Meryceros warreni and Merycodontinae sp., from locality D856, Citadel Plateau 15-minute quadrangle, Moffat County, 

Colo. A, Meryceros warreni, lateral view of right horn core; specimen 

D856-86. B, Merycodontinae sp., buccal view of right dentary with 

molars (M1-3) of specimen D856-87.

TABLE 15. Comparative measurements of Meryceros warreni from locality D856, Citadel Plateau 15-minute 

quadrangle, Moffat County, Colorado, and Meryceros crucensis from New Mexico

[Measurements in millimeters; leaders ( ) indicate no data. F:AM Frick, American Museum numbers; ESP field designation, American

Museum of Natural History]

M. warreni M. crucensis

Specimen

Height of 

Shaft TR 

Shaft AP

fork 

above 

above

above 

orbit, 

orbit,

D856-86

orbit.. 37 

........ 11

........ 14

.0 

.2 

.5

Type

40 

12

.3 

.6 

.6

F:AM 

31319

34 

14

F:AM 

51145

38.7 

12.8 

14.2

ESP 

400

32. 

10. 

12.

3

0 

0

F:AM 

51180

53.0 

13.7 

14.7

 Burr present.
ORDER ARTIODACTYLA OWEN 41

Merycodus, which also has a short-shafted and singleforked horn core, is distinguished from Meryceros by 

a rounded shaft in cross-section.

The D856 horn core is referred to Meryceros warreni, 

as it closely resembles the type of that species (figured 

in Leidy, 1869, pi. 27, fig. 12). Although the shaft of the 

type is slightly less compressed transversely above the 

orbit and is taller than specimen D856-86, referred 

specimens in the American Museum overlap specimen 

D856-86 in shaft thickness just above the orbit. F:AM 

31319 (Frick, 1937, fig. 39), in fact, is close to specimen 

D856-86. M. crucensis Frick, 1937, is a highly variable 

form with horn cores 20 percent shorter to 48 percent 

longer than that of M. warreni. Specimen D856-86 horn 

core cannot be separated from some allocated to M. 

crucensis and illustrated by Frick (1937, fig. 39), The 

range in size of the horn cores allocated to M. crucensis suggests that this species may include individuals 

of more than one species; the smaller individuals do not 

appear to be specifically distinct from M. warreni. M. 

joraki Frick, 1937, is also morphologically close to 

specimen D856-86, but has a wider shaft. M. crucianus 

Frick, 1937, has a smaller horn core than specimen 

D856-86. The horn cores of M. major Frick, 1937, M 

nenzelensis Frick, 1937, and M. hookwayi (Furlong, 

1935), are longer shafted than specimen D856-86.

Meryceros sp.

Table 16, Figures 17, ISA

Referred material. D854-1, left Mx; D854-2, right 

Mx; D854-3, left dentary with LI2(?), LP2_4, LM1 _3; 

D854-4, right scapula; D854-5, right proximal fragment radius; D854-6, left proximal fragment tibia; 

D854-7, left distal fragment tibia; D854-8, left proximal fragment metatarsal and shaft; D854-9, left 

naviculocuboid; D854-10, left astragalus; D854-11, left 

calcaneum; D854-12 through D854-14, three proximal 

phalanges; D854-15, medial phalanx; all associated.

Description. The upper molars are typically antilocaprid, being small and hypsodont. The mesostyle 

is prominent and narrow. The smaller parastyle and 

metastyle are subequal in size. The buccal wall of the 

paracone has a pronounced rib, but the buccal wall of 

the metacone is ribless.

The LI2(?) is small and near vertical. The concave 

lingual side faces posteromesially. The premolars successively increase in size posteriorly. The P2 has the 

posterior part of the tooth much wider than the anterior 

part, whereas the P3 and P4 have the buccal and 

lingual walls more nearly parallel. The P3_4 have a 

mesially-inflected paraconid, separated by a U-shaped 

valley from a larger, mesially directed metaconid. This

valley does not extend to the base of the enamel. A narrower valley separates the metaconid from the smaller, 

posterolingually directed entoconid. A small fossettid 

lies posteriorly on the entoconid and separates it from 

the entostylid on the back margin of the tooth. A small 

hypoconid forms the posterobuccal corner of the tooth, 

and is separated from the much larger protoconid by 

a broad and shallow basin. The protoconid lies opposite 

the valley between the paraconid and metaconid.

The lower molars closely resemble those of other antilocaprids. The lingual walls completely lack 

mesostylids. The metastylid is faintly suggested on the 

M2, and the parastylid is rudimentary on the M2 and 

M3. A weak protostylid is present labially on the M2 

and M3, and extends nearly to the base of the enamel. 

The lingual walls of the metaconid and entoconid are 

convex. The third lobe of the M3 lacks a posterior 

stylid. The dentary is narrow dorsoventrally, especially anterior to the P2. The ventral border beneath the 

cheek teeth is convex.

The postcrania from locality D854 were compared 

with those of some specimens of Antilocapra and 

Capromeryx, and with some illustrations of Paracosoryx and Ramoceras. Some differences are noted 

below.

The proximal radius (specimen D854-5) resembles 

that of the much larger Antilocapra. The bicipital rugosity, however, is less pronounced than in Antilocapra.

FIGURE 17. Meryceros sp., from locality D854, Citadel Plateau 

15-minute quadrangle, Moffat County, Colo. A, buccal view of left 

dentary with incisor (I 2?), premolars (P2_4), and molars (Mj_3); 

specimen D854-3. B, side view of medial phalanx; D854-15. C, 

side view of proximal phalanx; D854-13. D, anterior view of left 

astragalus; D854-10. E, mesial view of left calcaneum; D854-11. 

F, lingual view of left Mx, D854-1.
42 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

TABLE 16. Dental and skeletal measurements of Meryceros sp. from locality D854, Citadel 

Plateau 15-minute quadrangle, Moffat County, Colorado, and comparison with M. hookwayi 

from Nevada

[Measurements in millimeters; leaders ( ) indicate no data. LACM-CIT, Los Angeles County Museum; collection formerly

at the California Institute of Technology]

Meryceros sp. M. hookwayi*

D854-1 through D854-15 LACM-CIT LACM-CIT

712 1257

Mx APxTR. ...... ..................... 12.3x10.2

I 2 APxTR... ......................... 4.1x3.0

P 2 APxTR............................ 5.4x3.0 6. Ox

P 3 APxTR............................ 7.0x3.5 6.9

P 4 APxTR............................ 8.5x3.9 7.8x 8.9x

Mj APxTR............................ 8.5x6.3 9.2x

M2 APxTR............................ 9.6x6.9 10. 4x 11. Ox

M3 APxTR............................ 16.7x6.4 15. Ox 14. Ox

P. 2 . 4 ............................... 20.8

35.6

P 2 _M3 ............................... 57.8 59.6 56.8

Depth of jaw below P 2 .. ............. 14.5 

Depth jaw below anterior lobe M3 .... 19.3 

Proximal radius TR width.. .......... 21.0 

Proximal tibia AP length. ........... 34.4 

Distal tibia APxTR widths........... 15.5x20.3

Proximal metatarsal APxTR widths.. .. 17.7x17.3 

Naviculocuboid APxTR widths.. ....... 19.3x19.2

Astragalus:

lateral length. ................... 24.1 

medial length..................... 22.1 

minimum length. ................... 19.2 

distal trochlear TR....... ........ 15.5 

Calcaneum:

maximum length. ................... 54.8 

maximum AP length................. 19.4 --

tuberosity APxTR.. ................ 13.7x12.0

Proximal phalanx (manus?):

length............................ 35.1

proximal APxTR.................... 10.8x10.1

distal APxTR...................... 7.4x8.5

Proximal phalanx (pes?) proximal 

APxTR....................... n=2; O.R.=9.1-9.4 (AP)

8.6-8.8 (TR); X=9.2x8.7

Medial phalanx:

length. ........................... 18.0 

proximal APxTR.. .................. 8.6x7.6 

distal APxTR...................... 8.7x6.1

iMeasurements taken from Furlong (1935, table 1).
ORDER ARTIODACTYLA OWEN 43

FIGURE 18. Meryceros sp. and Merycodontinae sp., from localities 

D854 and D856, Citadel Plateau 15-minute quadrangle, Moffat 

County, Colo. A, Meryceros sp., occlusal view of left dentary, showing premolars and molars (P2-M3 ); specimen D854-3. B, 

Merycodontinae sp., occlusal view of right dentary, showing 

MI 3 ; specimen D856-87.

Proximally, the tibia (specimen D854-6) has the 

medial condyle and the tubercles of the spine preserved. 

Medial to the popliteal notch, the attachment tubercle 

for the posterior cruciate ligament is less pronounced 

on specimen D854-6 than in Antilocapra. Distally, the 

groove for the flexor digitalis longus is deeper and more 

sharply defined, and the anteromedial distal process 

between the astragalar articular grooves is narrower in 

specimen D854-7 than in Antilocapra. Laterally, the 

fibular articular facets are more strongly developed on 

Antilocapra. The anterior fibular facet on specimen 

D854-7 is saddle-shaped, being concave from side to 

side and convex fore-aft. In Antilocapra it is strongly 

convex, forming a rollerlike surface. The posterior 

fibular facet is shallower and the fibular groove is 

shallow and less troughlike than in Antilocapra.

On the proximal fragment of the metatarsal (specimen 

D854-9), the ectomesocuneiform facet is less 

posteromesially expanded than in Antilocapra. The entocuneiform facet is more triangular owing to greater 

posterior extension in Antilocapra; it is rounder owing 

to less posterior extension in Capromeryx. The posterior 

groove bounding the cuboidal facet is less deep in 

specimen D854-9 than it is in Antilocapra.

On the naviculocuboid (specimen D854-10), the 

metatarsal 4 facet lies more distally from the entocuneiform facet than in Antilocapra. The latter facet 

on specimen D854-10 is relatively longer than on 

Paracosoryx and Capromeryx and resembles that of Antilocapra. The proximoposterior tuberosity on the 

medial side of the naviculocuboid is smaller and less 

rugose than in Antilocapra. The medial calcanear facet

resembles that of Antilocapra and Capromeryx in that 

it is more strongly developed than in Paracosoryx.

The anterodorsal tuberosity of the calcaneum 

(specimen D854-11) resembles that of Paracosoryx and 

Capromeryx in that it is less strongly developed than 

in Antilocapra. The sustentacular process is slightly less 

prominent than in Antilocapra. The astragalar facet 

anterior to the cuboidal facet is much smaller on 

specimen D854-11 than on Antilocapra.

The proximal phalanx is more slender on the locality 

D854 fossil antilocaprid than it is on Antilocapra and 

Ramoceros. The proximolateral articular surface is 

smaller and more distally displaced in relation to the 

proximomedial articular surface than in Antilocapra. 

The plantar tuberosities are less prominent than in Antilocapra. The medial phalanx is more slender and 

smaller than in Antilocapra.

Comparisons and discussion. Because the horn core 

is lacking, dental criteria must be used to establish 

generic identification. Henshaw noted that merycodont 

evolutionary trends include size increase of the M3 , 

lengthening of the post-symphyseal diastema (1942, 

p. 161) and reduction in the size of the premolars (Henshaw, 1940, p. 26). These characteristics can be determined from most illustrations of antilocaprid dentitions. 

In their analysis of Merycodus necatus and Cosoryx furcatus, Skinner and Taylor (1967, p. 44) mentioned reduction of premolars and crown height of molars as 

diagnostic.

The antilocaprid from locality D854 is distinguished 

from all genera in the subfamily Antilocaprinae by 

possessing less hypsodont molars. The absence of a 

posterior fourth lobe on the M3 supports placement of 

the locality D854 antilocaprid within the Merycodontinae, although the presence of the M3 fourth lobe is 

somewhat variable in the Antilocaprinae (Skinner, 1942; 

Webb, 1973).

The dentition of Ramoceros osborni (Matthew, 1904) 

is about 12 percent shorter than that of specimen 

D854-3, based on F:AM 31636 (Frick, 1937, fig. 43). The 

premolars are slightly shorter and the molars less 

anteroposteriorly expanded than in specimen D854-3.

Based on F:AM 31019 and F:AM 31020 (Frick, 1937, 

fig. 35a), specimen D854-3 differs from Merriamoceros 

in having a dentition 15 percent longer. The premolar 

row is 10 percent shorter, the P3 paraconid is less 

lingually inflected, and the M3 is less expanded in 

Merriamoceros.

The premolars of both Cosoryx furcatus Leidy, 1869, 

and Cosoryjc cerroensis Frick, 1937, are highly reduced 

in relation to those of specimen D854-3, and the M3 is 

higher crowned (AMNH 9825, Frick, 1937, fig. 45).

Merycodus sabulonis Matthew and Cook, 1909, is 

distinguished from specimen D854-3 by having more 

reduced premolars, and the base of the M3 is less
44 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

anteroposteriorly expanded than in specimen D854-3.

The premolars of Cosoryx (Paracosoryx) alticornis 

Frick, 1937, (Frick, 1937, fig. 47), are about 20 percent 

shorter, the molars are slightly shorter, and the M3 is 

more anteroposteriorly expanded at the base than in 

specimen D854-3. The premolars of C. (P.) furlongi 

Frick, 1937, (illustrated in Furlong, 1927, pi. 26, fig. 1) 

are about 10 percent shorter than on specimen D854-3. 

The dentition of C. (P.) loxoceros (Furlong, 1935) is 

about 20 percent shorter than that of specimen D854-3. 

C (P.) wilsoni Frick, 1937, has premolars only slightly 

more reduced than those of specimen D854-3, and the 

length of the M3 is nearly the same. However, the jaw 

length anterior to the P3 is much less in C. (P.) wilsoni, 

resulting in an abbreviated look.

The premolars of Meryceros crucensis Frick, 1937, are 

reduced compared with those of specimen D854-3, and 

the P4 possesses a posterobuccal sulcus. M. nenzelensis Frick, 1937, is similar to specimen D854-3 in having nonreduced premolars. However, the base of the 

M3 is longer in P. nenzelensis and there is a posterior 

stylid present on the third lobe of the M3 (Frick, 1937, 

fig. 44, F:AM 31902). The premolars are more reduced 

and the cheek tooth row is shorter in Meryceros warreni (Leidy, 1858). The P2 -M3 length of specimen 

D854-3 is greater than in M. joraki Frick, 1937. 

However, in Frick's many illustrations (1937, figs. 46 

and 47) of this species, some have unreduced premolars 

and short M3, and others have reduced premolars and 

long M3 . These latter specimens also possess posterior 

stylids on the M3 . The length of the cheek tooth row 

of specimen D854-3 is very close to that of M. hookwayi 

(Furlong, 1935). The premolars of M. hookwayi are 

relatively unreduced and the M3 length is only slightly less than in specimen D854-3; this is due to its being less worn. The posterobuccal sulcus of the premolars 

is deeper on M. hookwayi than on specimen D854-3. 

The P4 of M. hookwayi differs from that of specimen 

D854-3 in possessing a small fossettid posterior to the 

anterolingual sulcus; this fossettid is formed by a 

lingual enamel connection between the metaconid and 

paraconid, resulting in a P4 morphologically similar to 

that of Antilocapra. The anterior buccal and lingual 

stylids of the M2 and M3 appear to be slightly stronger 

on M. hookwayi. M. hookwayi is similar to specimen 

D854-3 in that it lacks a stylid on the M3 posterior 

lobe. M. crucianus Frick, 1937, has a dentition much 

smaller than that of specimen D854-3. Meryceros sp., 

reported by Cassiliano (1980, p. 48), has shorter 

premolars relative to the molars than in specimen 

D854-3.

On the basis of tooth size, nonreduction of premolars 

relative to molars, and M3 crown height, specimen 

D854-3 most closely resembles species of Meryceros.

The molariform P4 with small fossettid on M. hookwayi is a derived characteristic, which nonetheless is 

merycodontine in development. According to Frick 

(1937, p. 22-23), "the worn to slightly worn P4 

of* * *merycodonts differs from that of Texoceros and 

Ilingoceros in the tendency to development of the 

metaconid rather than the metastylid portion of the 

median accessory column. In the P4 of the Recent 

pronghorn, both metaconid and metastylid are 

developed, enclosing anterior and posterior fossettes." 

The P4 fossettid on M. hookwayi is anteriorly placed, 

forming from the union of the paraconid and the 

anterior (metaconid) portion of the median accessory 

column. If M. hookwayi is correctly referred to 

Meryceros (the type of M. hookwayi is a ramus; the horn 

cores from the same locality were referred to that 

species on the basis of size), then the genus Meryceros 

contains forms both with and without a P4 fossettid. 

Likewise, the presence of the M3 posterior stylid, a 

derived characteristic, is variable in Meryceros. The individual morphological characteristics of the teeth of 

antilocaprids are here considered less important taxonomically than are the size and relative proportions 

of the teeth. Upon this basis, specimen D854-3 falls 

within the range of variation of Meryceros, as this genus 

was constituted by Frick (1937).

Merycodontinae sp.

Figures 16B, 18B

Referred material. D856-87, right ramus with M1-3.

Description. There is little that is distinctive about 

these teeth. The M3 possesses a minute posterior stylid. Dental measurements (in mm) for this fossil are as 

follows: M, APXTR, 7.8X5.6; M2 APXTR, 8.8X6.3; 

M3 APXTR, 13.6X6.0; M:_3 AP, 36.4.

Comparisons and discussion. Measurements were 

taken on M. warreni F:AM 31276 from the Devil's 

Gulch Member of Skinner and others (1968) of the 

Valentine Formation (late Barstovian) of Lugn (1938), 

andonM warrenijohnsoni N.S.M. 1.23.7.34 andN.S.M. 

2.3.8.34 (these last two illustrated in Frick, 1937, fig. 

28b). The M1 3 is 10-12 percent shorter on these 

specimens than on specimen D856-87. Frick, unfortunately, did not give measurements or illustrations of 

the dentitions of M. crucensis. In size, specimen 

D856-87 falls close to the largest specimens of M joraki 

illustrated by Frick (1937, fig. 47). M. joraki includes 

specimens with the M1-3 length from 3 to 26 percent 

shorter than on specimen D856-87; this large range suggests that more than one species could be included in 

M. joraki. The largest specimen of M. joraki differs from 

D856 in that it has a posterior stylid on the M3 third 

lobe. M. nenzelensis F:AM 31902, illustrated by Frick
MAMMALIAN AGE AND CORRELATION 45

(1937, fig. 44), is slightly larger than specimen D856-87 

and has a posterior stylid on the M3 . The dentition of 

M. crucianus is too small for that of specimen D856-87. 

The M13 length is close to that given for M. hookwayi 

(Furlong, 1935, table 1), and specimen D856-87 

resembles M. hookwayi in that it lacks a posterior stylid 

on the M3. Specimen D856-87 differs from Meryceros 

sp. D854-3 in that the basal part of the M3 is less 

anteroposteriorly expanded. This difference, however, 

is no greater than that seen on specimens of Cosoryx 

furcatus (Frick, 1937, fig. 45, especially F:AM 31274 and 

AMNH 9825).

Unfortunately, specimen D856-87 cannot be excluded 

from Cosoryx, especially the two specimens just noted. 

Although Meryceros and Cosoryx are distinguished 

from each other by the relative reduction of the 

premolars, they cannot be so distinguished by their 

molars alone. The crown height of specimen D856-87 

also overlaps with that of C furcatus (Frick, 1937, fig. 

45, F:AM 31724). Specimens of Cosoryx (Paracosoryx) 

alticornis (Frick, 1937, fig. 47, F:AM 31041 and 31043) 

also cannot be distinguished from specimen D856-87. 

The molars of F:AM 30944, a specimen which Frick 

(1937, fig. 41) could not allocate toRamoceros, Cosoryx, 

or Meryceros, are virtually indistinguishable from those 

of specimen D856-87.

This lower dentition may belong to the same animal 

as the horn core, that is, to Meryceros warreni. The 10 

percent difference in the M1-3 length with that of M 

warreni may be within the range of variation of that 

species, especially if the 23 percent range of variation 

seen inM. joraki is truly due to individuals of just that 

one species. However, considering the diversity of 

merycodonts in the Barstovian and Clarendonian, there 

are no compelling reasons why more than one species 

cannot be found in the same quarry. For the present, 

the best procedure is to recognize this specimen as 

generically indeterminate.

MAMMALIAN AGE AND CORRELATION

Locality D857 has yielded fossils indicative of the 

Hemingfordian land mammal age. The horse from this 

locality most closely resembles Parahippus leonensis 

from the Thomas Farm and other Hemingfordian 

localities of Florida. The presence of cement on the teeth 

helps establish a maximum age for the rocks of this 

locality. In Nebraska, the type specimen of Parahippus 

nebrascensis, a form without cement, was collected from 

the upper Arikareean Marsland Formation of Schultz 

(1938). Galusha (1975, p. 66) noted that some Parahippus teeth from the superjacent Runningwater Formation of Cook (1965) "have a light coating of cement;

most do not." The Runningwater Formation is considered early Hemingfordian (Tedford, 1981, p. 1015). 

Galusha (1975, p. 58) noted that a skeleton of Parahippus cf. P. vellicans was found in the Box Butte Formation of Galusha (1975), which overlies the Runningwater 

Formation, and is late Hemingfordian in age (Skinner 

and others, 1977, fig. 10). He stated that this specimen 

has "a fairly strong covering of cement on both the 

premolars and molars," which is thicker than that on 

horse teeth from the Thomas Farm fauna of Florida or 

the Garvin Gulley fauna of Texas. Thus, the presence 

of cement suggests that the D857 horse is at least postMarsland in age.

Patton (1969, p. 210) judged that the Thomas Farm 

fauna is slightly younger than the Garvin Gulley fauna, 

which he tentatively correlated with the Runningwater 

fauna. The stage of evolution of the equids suggests 

that the Thomas Farm is no younger than the Box 

Butte fauna and may be slightly older; there are no 

Merychippus recognized from the Thomas Farm, while 

the earliest Merychippus in Nebraska occurs in the Box 

Butte Formation, and the Box Butte Parahippus has 

slightly more cement on the teeth than the Thomas 

Farm Parahippus (Galusha, 1975, p. 59). Tedford and 

Hunter (1984) noted a near-species identity of horses 

in the Thomas Farm and correlative Florida faunas with 

horses from the youngest part of the Runningwater 

Formation.

Thus, the presence of P. leonensis suggests an early 

to late Hemingfordian age for locality D857. However, 

as indicated previously, the Browns Park horse is not 

all that different from Parahippus avus. In the Great 

Plains and Great Basin, Parahippus persisted into the 

early Barstovian. The type specimens of Parahippus 

avus and P. brevidens are part of the Mascall fauna of 

Oregon, and the type specimen of P. coloradensis is 

probably from the Eubanks local fauna of northeastern 

Colorado. Other early Barstovian occurrences of 

Parahippus include the Virgin Valley, High Rock Canyon, and Camp Creek faunas of Nevada, the Sucker 

Creek, Skull Spring, and Beatty Buttes faunas of 

Oregon, and the North Coalinga fauna of California. 

Thus, the minimum age for the fauna from locality D857 

is early Barstovian, based only on the occurrence of 

Parahippus.

The age of the fauna from locality D857 can possibly 

be further refined by the presence of Blickomylus 

galushai. This species is found in both the Chamisa 

Mesa Member of Galusha (1966) and the Canada Pilares 

Member of Gawne (1981b) of the Zia Sand. The occurrences of B. galushai in the Chamisa Mesa and Canada 

Pilares Members are considered early Hemingfordian 

by Gawne (1981a, p. 1007), and early to early late Hemingfordian by Tedford (1981, fig. 3). Frick and Taylor
46 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

(1968, fig. 14) show Blickomylus as Hemingfordian in 

age. B. galushai also occurs in the Split Rock local fauna 

(Munthe, 1979). Munthe considered the Split Rock local 

fauna to be most similar to the fauna from the late Hemingfordian Box Butte Formation of Galusha (1975), 

though also showing strong similarities to the late Hemingfordian fauna from the Sheep Creek Formation.

The evidence as outlined above indicates that the 

fauna from locality D857 is early to late Hemingfordian 

in age. Although the presence ofParahippus might also 

suggest an early Barstovian age, because Blickomylus 

has not yet been found in Barstovian faunas, we restrict 

the age of locality D857 to the Hemingfordian.

The fossils from the other localities in the Browns 

Park Formation indicate an age younger than Hemingfordian. The canid, Tomarctus temerarius, has been 

reported only from late Barstovian faunas, and is part 

of the Niobrara River fauna (Webb, 1969) which was collected from the Crookston Bridge Member of the Valentine Formation (Skinner and others, 1968, p. 404). 

Tomarctus paula is from the late Barstovian Tonopah 

fauna of Nevada. Early Barstovian assemblages commonly include the large form, Tomarctus rurestris. 

Reported Clarendonian occurrences of Tomarctus include large species, such as T. euthos or T. robustus. 

An exception to this is the canid, of indeterminate genus 

and species, reported by Wilson (1960) from the Lapara 

Creek fauna of Texas. As noted, this canid is similar 

to specimen D856-1.

The stage of evolution of the Browns Park Megahippus can be matched with forms of Barstovian age. 

Specimen D856-9 is similar in size to Hypohippus affinis. As noted, M. mckennai, from near the base of the 

upper member of the Barstow Formation (Tedford and 

Alf, 1962, p. 114), is also near the size ofH. affinis. The 

lingual cingulum on the Mx of specimen D856-9 is less 

well developed than on Megahippus cf. M. matthewi 

from the Clarendonian Cedar Mountain fauna from 

Nevada, and less well developed than on the type specimen of M. matthewi from the latest Barstovian Burge 

fauna of Nebraska. The dP: on D856-9 is larger than 

on latest Barstovian and early Clarendonian Megahippus. As noted, the Browns Park specimen was matched 

with one in the Frick collection from the Pojoaque 

Member of the Tesuque Formation.

The limb bones of cf. Protolabis are not useful for correlation because the genus Protolabis extends from the 

Hemingfordian through the Clarendonian. The genus 

Michenia extends from the late Arikareean to the 

Clarendonian; the relative reduction and simplification 

of the P2 and P3 , and the suppression of the Plf suggest apost-Marsland (post-Arikareean) stage of evolution for the Michenia from the Browns Park. The 

early-middle Clarendonian Michenia yavapaiensis from

Milk Creek, Ariz., has a P2 that is variably present, 

suppressed, or lost; when present it averages about 

5 mm in length, which is close to that of the Michenia, 

D856-13.

The large Procamelus sp. or Aepycamelus sp. of the 

Browns Park Formation seems to represent a late 

Barstovian stage of evolution. The size of the limb 

bones and the crown height of the M: match a few 

American Museum specimens from the Pojoaque 

Member of the Tesuque Formation, that is, specimens 

of late Barstovian and possibly early Clarendonian age 

(Tedford, 1981, fig. 2; Barghoorn, 1981, p. 1040). The 

large Browns Park camel is also matched in these 

characteristics by Procamelus from the latest Barstovian Burge fauna. Clarendonian Procamelus from the 

Clarendon fauna of Texas have higher crowned M/s 

than those from the Burge fauna; hence, the Browns 

Park camel probably represents a pre-Clarendonian 

stage of evolution.

Meryceros warreni is apparently limited to late 

Barstovian faunas. The type specimen is from the composite fauna from the valley of the Niobrara River (see 

Webb, 1969, for discussion), and may have been from 

the lower part of the Valentine Formation, in which the 

M. warreni-type horn is characteristic (Skinner and 

Taylor, 1967, p. 45). The type of M. warreni johnsoni 

is from the Crookston Bridge Member of the Valentine 

Formation (Skinner and Johnson, 1984). Galbreath 

(1953, p. 35) reported its probable occurrence in the late 

Barstovian Kennesaw fauna of northeastern Colorado, 

apparently basing this judgment on Frick's (1937, 

p. 367) reported occurrences in the Horse and Mastodon 

Quarries in the Pawnee Creek Formation of Galbreath 

(1953). In an unpublished correlation chart, R. H. Tedford (written commun., 1975) showed the Horse and 

Mastodon Quarries to be slightly older than the Kennesaw fauna, but younger than the early Barstovian 

Eubanks fauna. Forsten (1970, p. 50) reported the 

presence of Meryceros warreni-type horns from the Trail 

Creek fauna of Wyoming (but refers them to Merycodus 

cf. necatus, as she considers M. warreni a synonym 

of M. necatus). The Trail Creek fauna, considered 

Barstovian by Voorhies (1965) and Forsten (1970), 

was assigned a Valentinian4 age by Cassiliano (1980, 

p. 64), who noted it contained genera and species also 

found in each of the three members of the Valentine

4Valentinian is an informal land mammal age used by some paleontologists and 

stratigraphers, and is based on faunas from the Valentine Formation of Nebraska. Previously 

it had been used as roughly equivalent to a combined late Barstovian (in part) and early 

Clarendonian time. However, with the placement of the Burge fauna in the latest Barstovian, the Valentinian falls entirely within the late Barstovian. For discussions of the term 

Valentinian, see Skinner and others (1968); Webb (1969); Cassiliano (1980); and Skinner and 

Johnson (1984).
MAMMALIAN AGE AND CORRELATION 47

Formation. Mercyceros warreni is present in American 

Museum collections from the late Barstovian Devil's 

Gulch Horse Quarry of Nebraska.

Meryceros crucensis is from the late Barstovian and 

possibly early Clarendonian Pojoaque Member of the 

Tesuque Formation. M. joraki, the other short-shafted 

Meryceros, is from the upper member of the Barstow 

Formation (Frick's "First Division" (1937, p. 367)). A 

longer-shafted form, Meryceros cf. M. major, has been 

reported from the Clarendonian Ogallala-Wolf Creek 

fauna of South Dakota, and Merriam (1916, fig. 42) illustrated a probable long-shafted Meryceros from the 

Clarendonian Cedar Mountain fauna of Nevada. Some 

of Voorhies' (1969) material from the Verdigre fauna 

may be Meryceros. No Meryceros has been reported 

from early Barstovian faunas. Short-shafted Meryceros 

does not seem to have persisted later than the earliest 

Clarendonian, and is most common in late Barstovian 

faunas.

The Meryceros from locality D854 is probably the 

same age as the fossils from localities D855 and D856. 

As noted in the discussion of specimen D854-3, the 

reduction of the premolars relative to the molars, and 

the total length of the cheek tooth row, is similar to that 

of M. hook wayi from the late Barstovian Tonopah 

fauna. Slight morphological differences in the teeth prevent the Browns Park specimen from being assigned 

to M. hookwayi.

The fauna! evidence indicates most strongly a late 

Barstovian (post-Lower Snake Creek to Burge) age for 

the fossils from localities D854, D855, and D856 (Cedar 

Springs Draw local fauna), although an early Clarendonian age is not entirely excluded. The primitive 

Megahippus may indicate a pre-latest Barstovian (preBurge) age for the fauna. The Cedar Springs Draw local 

fauna shares Meryceros warreni with the Niobrara 

River fauna (that is, the fauna from the Crookston 

Bridge Member of the Valentine Formation (latest 

Barstovian)). In addition, these two faunas share Protolabis, and the two large camels, Aepycamelus and Procamelus, are both present in the Niobrara River fauna. 

The D856 locality shares Meryceros warreni with the 

Trail Creek fauna (Valentinian); in addition, Forsten 

(1970, p. 49) gave measurements for a deciduous 

Aepycamelus sp. (called (l)Alticamelus sp. by Forsten, 

and corrected to Aepycamelus by Cassiliano, 1980) 

which are similar to those for the large Browns Park 

camel. Tomarctus cf. T. paula is close to Tomarctus 

paula from Tonopah (late Barstovian). Tonopah also has 

Aepycamelus^) stocki. Meryceros, however, is represented at Tonopah by the longer shafted M. hookwayi. 

Except for the occurrences in the Browns Park Formation, M. warreni has been reported only east of the 

Rocky Mountains. The quarries D854, D855 and D856

have a high generic similarity to the Barstow fauna, but 

none of the species are demonstrably the same. The 

strongest similarity, however, may be to the fauna from 

the Pojoaque Member (late Barstovian to possibly early 

Clarendonian) of the Tesuque Formation, which shares 

Megahippus and possibly Aepycamelus sp. Meryceros 

crucensis occurs in the Pojoaque Member of the 

Tesuque Formation; as defined by Frick (1937), M. 

crucensis includes forms with horn cores nearly identical to those of M. warreni. Both Protolabis and 

Michenia occur in the Pojoaque Member of the Tesuque 

Formation.

There is little similarity between the Browns Park 

fossils described in this study and those described by 

Peterson (1924, 1928). Peterson's 1928 Browns Park 

fauna consisted of the following forms:

Bassariscops willistoni

Ticholeptusl

Camelidae gen. et sp. indet.

Camelidae near Stenomylus

Merycodusl sp.?

Aphelops ceratorhinus

Chalicotheroidea

Serridentinus fricki

These fossils were found at several localities throughout the formation, and at least one was collected from 

another formation. The camelid near Stenomylus is considered an indeterminate antilocaprine by Frick and 

Taylor (1968, p. 7, footnote). Skinner (1968, p. 17) 

pointed out that the presumed chalicothere is actually 

a Teleodus from the Duchesne River Formation near 

Vernal, Utah. McGrew (1951, p. 56) considered the supposed Ticholeptus to be an indeterminate oreodont. The 

other camelid and Merycodusl sp. consist of unillustrated postcrania. The Bassariscops was found "one 

mile south and west of Sunbeam, Moffat County, Colorado" (Peterson, 1924, p. 300), whereas the Aphelops 

and Gomphotherium (= "Serridentinus" = "Trilophodon") were found about 22 miles farther west at 

Douglas Mountain, near Greystone, Colo.

Peterson (1928, p. 88) judged the Browns Park 

assemblage to be similar in age to faunas from "the 

Pawnee Creek of Colorado, the Madison Valley, probably the Flint Creek and Deep River of Montana; the 

Santa Fe of New Mexico; the Mascall of Oregon, and 

the Virgin Valley of Nevada." McGrew (1951, p. 56) considered the Browns Park Formation to be middle 

Miocene (Hemingfordian). However, this age was based 

partly on the presence of the supposed chalicothere. J. 

LeRoy Kay (oral commun. related in Untermann and 

Untermann, 1954, p. 186) considered Bassariscops and 

Aphelops to indicate a late Miocene (Barstovian) age. 

The only record of Bassariscops willistoni is from the 

Browns Park Formation. Frailey (1979, p. 134-140) has
48 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

described a second species of Bassariscops, B. achoros, 

from the Arikareean Buda local fauna of Florida. The 

temporal relationship of the two species relative to each 

other are unknown, but the Florida occurrence suggests 

that the Browns Park Bassariscops may be as old as 

Arikareean.

The type and referred material of Aphelops 

ceratorhinus are from the Lower Madison Valley, Montana (Douglass, 1903; 1908). Matthew (1932) considered 

Aphelops montanus Douglass, 1903, from the Flint 

Creek local fauna, Montana, synonymous with A. 

ceratorhinus. According to Black (1961, p. 75), the Flint 

Creek local fauna is probably middle to late Barstovian 

and probably equivalent to part of the Lower Madison 

Valley fauna.

Osborn (1936, p. 312) considered "Trilophodon" fricki 

to be one of the most primitive gomphotheres in North 

America, and assigned a middle to late Miocene age to 

Peterson's fauna. Tobien (1973, p. 223) synonymized 

Gomphotherium fricki with Gomphotherium productum 

from the Pojoaque Member of the Tesuque Formation, 

and (1973, p. 218) considered the Browns Park specimen 

to be one of the earliest representatives of North 

American mastodonts. Gomphotheres do not occur in 

North America until the late Barstovian, when they appear as an immigrant group (Tedford, 1981, p. 1012).

Given that Gomphotherium productum is an early, 

primitive North American gomphothere, and considering the stratigraphic position of the Douglas Mountain 

fauna, Peterson's Douglas Mountain fauna may be 

slightly older than the Cedar Springs Draw fauna. 

Peterson's locality at Douglas Mountain appears to be 

about midway stratigraphically in the formation. Peterson (1928, p. 93) measured a section of 224.3 m from 

the base of the formation to the top of his fossil horizon, 

called by him the "Weller Horizon." The fossils of the 

Cedar Springs Draw fauna were collected from near the 

top of the formation in the white crossbedded sandstone 

unit. As mentioned previously, 510 m of Browns Park 

was drilled in the Texaco No. 1 State-K well, where the 

white, cross-bedded unit is exposed on the surface; core 

holes drilled in the white, crossbedded unit near 

localities D855 and D856 record a total depth for the 

Browns Park of 490-535 m.

ISOTOPIC AGE

An ash near the D855 and D856 quarries gave a 

zircon fission-track age of 11.3±0.8 m.y. This ash (field 

no. 72G74) is about 1 mile northwest of D856, in the 

NW1/4SE1/4 sec. 25, T. 6 N., R. 97 W. (fig. 10). The ash 

is exposed only at a single outcrop and cannot be traced 

south toward the D855 and D856 fossil localities owing

TABLE 17. Fission-track age data for zircon microphenocrysts from 

the upper part of the Browns Park Formation, sample no. 72G74

[t, track; Ps, spontaneous track density; Pi, induced track density; F, neutron fluence; n, 

neutron; cm, centimeter; yrs, years. ± is one sigma. Number of tracks counted in 

parentheses]

Sample 

no.

72G74

PsxlO6 

t/cm2

1.88 

(189)

PixlO 7 

t/cm2

1.03 

(900)

FxlO 15 

n/cm2

1.01

Age x 106 

yrs

11.3±0.8

to plant cover. The topographic position of the ash projected into the level of the D855 quarry; thus, the ash 

lies about 15m topographically below the D856 quarry. 

Table 17 gives fission-track age data for this ash.

This 11.3±0.8 m.y. age falls within the time range of 

latest Barstovian and early Clarendonian (Tedford, 

1981, fig. 2). This age is similar to those determined by 

Evernden and others (1964, p. 164) for the Fish Lake 

Valley, Cedar Mountain, and Avawatz faunas. As possible evidence for a close faunal relationship of D856 to 

Fish Lake Valley is the fact that the postcrania of the 

D856 cf. Protolabis are inseparable from the postcrania 

of Protolabis coartatus, which occurs at the Fish Lake 

Valley fauna and the slightly older Cedar Mountain 

fauna. The slightly younger Avawatz Mountain fauna 

(Henshaw, 1940) has a Michenia present and also P. 

coartatus-sized postcrania.

REFERENCES CITED

Adams, John, and Patton, Jean, 1979, Sebkha-dune deposition in the 

Lyons Formation (Permian), northern Front Range, Colorado: 

Mountain Geologist, v. 16, no. 2, p. 47-57.

Ahlbrandt, T. S., and Andrews, Sarah, 1978, Distinctive sedimentary 

features of cold-climate eolian deposits, North Park, Colorado: 

Palaeogeography, Palaeoclimatology, Palaeoecology, v. 25, 

p. 327-351.

Ahlbrandt, T. S., and Fryberger, S. G., 1980, Eolian deposits in the 

Nebraska Sand Hills: U.S. Geological Survey Professional Paper 

1120-A, p. 1-24.

____1981, Sedimentary features and significance of inter dune 

deposits, in Ethridge, F. G., and Flores, R. M., eds., Recent and 

ancient nonmarine depositional environments; models for exploration: Society of Economic Paleontologists and Mineralogists 

Special Publication No. 31, p. 293-314.

Alien, J. R. L., 1965, A review of the origin and characteristics of recent alluvial sediments: Sedimentology, v. 5, no. 2, p. 91-191.

Andrews, Peter, Meyer, G. E., Pilbeam, D. R., Van Couvering, J. A., 

and Van Couvering, J. A. H., 1981, The Miocene fossil beds of 

Maboko Island, Kenya: Geology, age, taphonomy and paleontology: Journal of Human Evolution, v. 10, p. 35-48.

Barghoorn, S., 1981, Magnetic-polarity stratigraphy of the Miocene 

type Tesuque Formation, Santa Fe Group, in the Espanola Valley, 

New Mexico: Geological Society of America Bulletin, pt. I, v. 92, 

no. 12, p. 1027-1041.
REFERENCES CITED 49

Behrensmeyer, A. K., 1975, The taphonomy and paleoecology of PlioPleistocene vertebrate assemblages east of Lake Rudolf, Kenya: 

Bulletin of the Museum of Comparative Zoology, Harvard University, v. 146, no. 10, p. 473-578.

____1978, Taphonomic and ecologic information from bone weathering: Paleobiology, v. 4, no. 2, p. 150-162.

Behrensmeyer, A. K., and Dechant-Boaz, D. E., 1980, The Recent 

bones of Amboseli National Park, Kenya, in relation to East 

African paleoecology, in Behrensmeyer, A. K., and Hill, A. P., eds., 

Fossils in the making, vertebrate taphonomy and paleoecology: 

University of Chicago Press, p. 72-92.

Berggren, W.A., Kent, D.V., Flynn, J.J., and Van Couvering, J.A., 

1985, Cenozoic geochronology: Geological Society of America 

Bulletin, v. 96, p. 1407-1418.

Bergin, M. J., 1959, Preliminary geologic map of the Maybell-Lay area, 

Moffat County, Colorado: U.S. Geological Survey Open-File 

Report, scale 1:48,000.

Bergin, M. J., and Chisholm, W. A., 1956, Maybell-Lay area, Moffat 

County, Colorado: U.S. Geological Survey Trace Elements Investigations Report 620, p. 190-199.

Berry, C. T., 1938, A Miocene dog from Maryland: Proceedings of the 

U.S. National Museum, v. 85, p. 159-161.

Black, C. C., 1961, Fossil mammals from Montana Part 1, Additions 

to the late Miocene Flint Creek Local Fauna: Annals of the 

Carnegie Museum, v. 36, art. 7, p. 69-76.

Bradley, W. H., 1936, Geomorphology of the north flank of the Uinta 

Mountains: U.S. Geological Survey Professional Paper 185-1, 

p. 163-204.

Brownfield, M. E., and Anderson, Kevin, 1979, Geologic map and 

coal sections of the Lay SE Quadrangle, Moffat County, Colorado: U.S. Geological Survey Open-File Report 79-1680, scale 

1:24,000.

Brownfield, M. E. and Prost, G. L., 1979, Geologic map and coal sections of the Lay Quadrangle, Moffat County, Colorado: U.S. 

Geological Survey Open-File Report 79-1679, scale 1:24,000.

Buffler, R. T., 1967, The Browns Park Formation and its relationship 

to the late Tertiary geologic history of the of the Elkhead region, 

northwestern Colorado-south-central Wyoming: Berkeley, Calif., 

University of California Ph. D. thesis, 175 p.

Cassiliano, Michael, 1980, Stratigraphy and vertebrate paleontology 

of the Horse Creek-Trail Creek area, Laramie County, Wyoming: 

University of Wyoming Contributions to Geology, v. 19, no. 1, 

p. 25-68.

Chisholm, W. A., 1963, Effect of climate and source area location on 

Browns Park Formation petrology [abs.]: American Association 

of Petroleum Geologists Bulletin, v. 47, p. 353.

Chisholm, W. A., Bergin, M. A., and Pritchard, G. E., 1961, Sedimentary petrology and sedimentation of the Miocene Browns Park 

Formation, in Program Abstracts, Annual Meeting of the Society of Economic Paleontologists and Mineralogists, Denver, Colorado, p. 84-85.

Coe, Malcolm, 1980, The role of modern ecological studies in the 

reconstruction of paleoenvironments in Sub-Saharan Africa, in 

Behrensmeyer, A. K., and Hill, A. P., eds., Fossils in the making, 

vertebrate taphonomy and paleoecology: University of Chicago 

Press, p. 55-67.

Condon, Thomas, 1896, Scientific description of two new fossil dogs: 

University of Oregon Bulletin, v. 2, no. 6, p. 10-11.

Cook, H. J., 1965, Runningwater Formation, middle Miocene of 

Nebraska: American Museum Novitates, no. 2227, 8 p.

Cook, H. J., and Macdonald, J. R., 1962, New Carnivora from the 

Miocene and Pliocene of western Nebraska: Journal of Paleontology, v. 36, no. 3, p. 560-567.

Cope, E. D., 1873, Third notice of extinct vertebrata from the Tertiary of the plains: Paleontological Bulletin, v. 16, 8 p.

_1874, Report on the stratigraphy and Pliocene vertebrate 

paleontology of northern Colorado: Bulletin of the U.S. Geological 

Survey and Geographical Survey of the Territories (Hayden), v. 1, 

no. 1, p. 9-28.

_1875, On some new fossil Ungulata: Proceedings of the 

Academy of Natural Sciences of Philadelphia, v. 27, p. 258-262. 

.1877, Report upon the extinct vertebrata obtained in New

Mexico by parties of the expedition of 1874, in Wheeler, G. M.,

Report upon United States geographical surveys west of the one

hundredth meridian, v. 4 Paleontology, pt. 2, 372 p. 

Crews, George, 1963, Geology of a part of northeast Moffat County,

Colorado: Golden, Colo., Colorado School of Mines Masters thesis,

124 p. 

Davidson, Pirie, 1923, Alticamelus alexandrae, a new camel from the

Barstow Upper Miocene of the Mohave Desert: University of

California Publications, Bulletin of the Department of Geological

Sciences, v. 14, no. 12, p. 397-408. 

Douglass, Earl, 1903, New vertebrates from the Montana Tertiary:

Annals of the Carnegie Museum, v. 2, art. 5, p. 145-200. 

____1908, Rhinoceroses from the Oligocene and Miocene deposits

of North Dakota and Montana: Annals of the Carnegie Museum,

v. 4, p. 256-266. 

Downs, Theodore, 1956, The Mascall fauna from the Miocene of

Oregon: University of California Publications in Geological

Sciences, v. 31, no. 5, p. 199-354.

Dyni, J. R., 1968, Geologic map of the Elk Springs quadrangle, Moffat County, Colorado: U.S. Geological Survey Geologic Quadrangle

Map GQ-702, scale 1:62,500. 

Evernden, J. F., Savage, D. E., Curtis, G. H., and James, G. T, 1964,

Potassium-argon dates and the Cenozoic mammalian chronology

of North America: American Journal of Science, v. 262, no. 2,

p. 145-198. 

Forsten, Ann, 1970, The Late Miocene Trail Creek mammalian fauna:

University of Wyoming Contributions to Geology, v. 9, no. 1,

p. 39-51. 

Frailey, David, 1979, The large mammals of the Buda Local Fauna

(Arikareean: Alachua County, Florida): Bulletin of the Florida State

Museum, Biological Sciences, v. 24, no. 2, p. 123-173. 

Frick, Childs, 1937, Horned ruminants of North America: Bulletin of

the American Museum of Natural History, v. 69, 669 p. 

Frick, Childs, and Taylor, B. E., 1968, A generic review of the

stenomyline camels: American Museum Novitates, no. 2353,

51 p. 

____1971, Michenia, a new protolabine (Mammalia, Camelidae) and

a brief review of the early taxonomic history of the genus Protolabis: American Museum Novitates, no. 2444, 24 p. 

Furlong, E. L., 1927, The occurrence and phylogenetic status of

Merycodus from the Mohave Desert Tertiary: University of California Publications, Bulletin of the Department of Geological

Sciences, v. 17, no. 4, p. 145-186. 

____1935, New merycodonts from the Upper Miocene of Nevada:

Carnegie Institute of Washington Publication 453, p. 1-10. 

Galbreath, E. C., 1953, A contribution to the Tertiary geology and

paleontology of northeastern Colorado: University of Kansas

Paleontological Contributions Vertebrata, art. 4, no. 13,

119 p. 

Galusha, Ted, 1966, The Zia Sand Formation, new early to medial

Miocene beds in New Mexico: American Museum Novitates, no.

2271, 12 p. 

___1975, Stratigraphy of the Box Butte Formation, Nebraska:

Bulletin of the American Museum of Natural History, v. 156, art.

1, p. 1-68. 

Galusha, Ted, and Blick, J. C., 1971, Stratigraphy of the Santa Fe

Group, New Mexico: Bulletin of the American Museum of Natural

History, v. 144, art. 1, p. 1-127.
50 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

Gawne, C. E., 1981a, Sedimentology and stratigraphy of the Miocene 

Zia Sand of New Mexico Summary: Geological Society of 

America Bulletin, pt. I, v. 92, p. 999-1007.

____1981b, Sedimentology and stratigraphy of the Miocene Zia Sand 

of New Mexico: Geological Society of America Bulletin, pt. II, 

v. 92, p. 2484-2552.

Gazin, C. L., 1932, A Miocene mammalian fauna from southeastern 

Oregon: Carnegie Institution of Washington Publication 418, 

p. 37-86.

Getty, Robert, 1975, Sisson and Grossman's The anatomy of the 

domestic animals, v. 1: Philadelphia, W. B. Saunders Co., 1211 p.

Gidley, J. W., 1907, Revision of the Miocene and Pliocene Equidae 

of North America: Bulletin of the American Museum of Natural 

History, v. 23, art. 35, p. 865-934.

Glennie, K. W., 1970, Desert sedimentary environments, in Developments in Sedimentology, [v.] 14: Amsterdam, Elsevier Publishing 

Co., 222 p.

Gregory, J. T., 1942, Pliocene vertebrates from Big Spring Canyon, 

South Dakota: University of California Publications, Bulletin of 

the Department of Geological Sciences, v. 26, no. 4, p. 307-446.

Grutt, E. W. Jr., and Whalen, J. F., 1955, Uranium in northern Colorado and southern Wyoming, in Intermountain Association of 

Petroleum Geologists Rocky Mountain Association of Geologists 

Guidebook, 6th Annual Field Conference, Northwest Colorado, 

p. 126-129.

Hancock, E. T., 1925, Geology and coal resources of the Axial and 

Monument Butte Quadrangles, Moffat County, Colorado: U.S. 

Geological Survey Bulletin 757, 134 p.

Hansen, W. R., 1965, Geology of the Flaming Gorge area, UtahColorado-Wyoming: U.S. Geological Survey Professional Paper 

490, 196 p.

Harrison, J. A., 1979, Revision of the Camelinae (Artiodactyla, 

Tylopoda) and description of the new genus Alforjas: University 

of Kansas Paleontological Contributions, paper 95, 20 p.

Haynes, Gary, 1980, Evidence of carnivore gnawing on Pleistocene 

and Recent mammalian bone: Paleobiology, v. 6, no. 3, p. 341-351.

Henshaw, P. C., 1940, A Tertiary mammalian fauna from the Avawatz 

Mountains, San Bernardino County, California: Carnegie Institution of Washington Publication 514, p. 1-30.

____1942, A Tertiary mammalian fauna from the San Antonio 

Mountains near Tonopah, Nevada: Carnegie Institution of 

Washington Publication 530, p. 77-168.

Hill, A. P., 1979, Disarticulation and scattering of mammal skeletons: 

Paleobiology, v. 5, no. 3, p. 261-274.

____1980, Early postmortem damage to the remains of some contemporary East African mammals, in Behrensmeyer, A. K., and 

Hill, A. P., eds., Fossils in the making, vertebrate taphonomy and 

paleoecology: University of Chicago Press, p. 131-155.

Honey, J. G, 1977, The paleontology of the Browns Park Formation 

in the Maybell, Colorado area, and a taphonomic study of two fossil 

quarries in Colorado and Arizona: Tucson, Ariz., University of 

Arizona Masters thesis, 197 p.

Honey, J. G., and Taylor, B. E., 1978, A generic revision of the Protolabidini (Mammalia, Camelidae), with a description of two new 

protolabidines: Bulletin of the American Museum of Natural 

History, v. 161, art. 3, p. 369-425.

Hook, D. L., 1956, Late Cenozoic stratigraphy and structure of a part 

of the Walnut Grove Basin, Yavapai County, Arizona: Tucson, 

Ariz., University of Arizona Masters thesis, 50 p.

Hunt, R. M. Jr., 1978, Depositional setting of a Miocene mammal 

assemblage, Sioux County, Nebraska (U.S.A.): Palaeogeography, 

Palaeoclimatology, Palaeoecology, v. 24, p. 1-52.

Izett, G. A., 1975, Late Cenozoic sedimentation and deformation in 

northern Colorado and adjoining areas: Geological Society of 

America Memoir 144, p. 179-209.

Izett, G. A., Denson, N. M., and Obradovich, J. D., 1970, K-Ar age

of the lower part of the Browns Park Formation, northwestern

Colorado: U.S. Geological Survey Professional Paper 700-C,

p. C150-C152. 

Izett, G. A., Honey, J. G., and Brownfield, M. E., 1985, Geologic map

of the Citadel Plateau quadrangle, Moffat County, Colorado: U.S.

Geological Survey Miscellaneous Geologic Investigations Map

1-1532, scale 1:48,000. 

Leidy, Joseph, 1858, Notice of remains of extinct Vertebrata, from

the valley of the Niobrara River, collected during the exploring

expedition of 1857, in Nebraska, under the command of Lieut.

G. K. Warren, U.S. Top. Engineer, by Dr. F. V. Hayden, Geologist

to the exedition: Proceedings of the Academy of Natural Sciences

of Philadelphia, v. 10, p. 20-29. 

____1869, The extinct mammalian fauna of Dakota and Nebraska,

including an account of some allied forms from other localities,

together with a synopsis of the mammalian remains of North

America, illustrated with 30 plates: Journal of the Academy of

Natural Sciences of Philadelphia, vol. 7, second series, p. 22-472. 

Lewis, G. E., 1968, Stratigraphic paleontology of the Barstow Formation in the Alvord Mountain area, San Bernardino County,

California: U.S. Geological Survey Professional Paper 600-C,

p. C75-C79. 

Lewis, W. S., 1977, Geology of uranium mineralization in the Browns

Park Formation, Carbon County, Wyoming and Moffat County,

Colorado: Golden, Colo., Colorado School of Mines Masters thesis,

85 p. 

Loomis, F. B., 1910, Osteology and affinities of the genus Stenomylus:

American Journal of Science, ser. 4, v. 29, p. 297-323. 

Loring, S. H., and Wood, A. E., 1969, Deciduous premolars of some

North American Tertiary camels (Family Camelidae): Journal of

Paleontology, v. 43, no. 5, p. 1199-1209. 

Love, J. D., 1961, Split Rock Formation (Miocene) and Moonstone

Formation (Pliocene) in Central Wyoming: U.S. Geological Survey

Bulletin 1121-1, p. 11-139. 

Luft, S. J., and Thoen, W. L., 1981, Measured sections of the Browns

Park Formation (Miocene) in Moffat County, Colorado, 1980: U.S.

Geological Survey Open-File Report 81-171, 35 p. 

Lugn, A. L., 1938, The Nebraska State Geological Survey and the

"Valentine Problem": American Journal of Science, ser. 5, v. 36,

p. 220-227. 

Macdonald, J. R., 1949, A new Clarendonian fauna from northeastern

Nevada: University of California Publications, Bulletin of the

Department of Geological Sciences, v. 28, no. 7, p. 173-194. 

Marsh, O. C., 1874, Notice of new equine mammals from the Tertiary

formation: American Journal of Science, ser. 3, v. 7, p. 247-258. 

Matthew, W. D., 1904, A complete skeleton of Merycodus: Bulletin

of the American Museum of Natural History, v. 20, p. 101-129. 

____1907, A Lower Miocene fauna from South Dakota: Bulletin

of the American Museum of Natural History, v. 23, art. 9,

p. 169-219. 

____1909, Faunal lists of the Tertiary Mammalia of the West, in

Osborn, H. F., Cenozoic mammal horizons of western North

America: U.S. Geological Survey Bulletin 361, p. 91-120. 

____1918, Contributions to the Snake Creek fauna: Bulletin of the

American Museum of Natural History, v. 38, art. 7, p. 183-229. 

____1924, Third contribution to the Snake Creek fauna: Bulletin of

the American Museum of Natural History, v. 50, p. 59-210. 

___1932, A review of the rhinoceroses with a description of

Aphelops material from the Pliocene of Texas: University of 

California Publications, Bulletin of the Department of Geological 

Sciences, v. 20, no. 12, p. 411-482.

Matthew, W. D., and Cook, H. J., 1909, A Pliocene fauna from western 

Nebraska: Bulletin of the American Museum of Natural History, 

v. 26, art. 27, p. 361-414.
REFERENCES CITED 51

Matthew, W. D., and Macdonald, J. R., I960, Two new species of Oxydactylus from the middle Miocene Rosebud Formation in western 

South Dakota: American Museum Novitates, no. 2003, 7 p.

Mawby, J. E., 1968, Megahippus and Hypohippus (Perissodactyla, 

Mammalia) from the Esmeralda Formation of Nevada: PaleoBios, 

no. 7, 13 p.

McGrew, P. O., 1935, A new Cynodesmus from the Lower Pliocene 

of Nebraska with notes on the phylogeny of the dogs: University 

of California Publications, Bulletin of the Department of 

Geological Sciences, v. 23, no. 10, p. 305-312.

 1938, The Burge fauna, a Lower Pliocene mammalian assemblage from Nebraska: University of California Publications, 

Bulletin of the Department of Geological Sciences, v. 24, no. 11, 

p. 309-328. 

.1951, Tertiary stratigraphy and paleontology of south-central

Wyoming: Wyoming Geological Association Guidebook, 6th Annual Field Conference, p. 54-57.

McKay, E. J., 1974, Geologic map of the Lone Mountain quadrangle, 

Moffat County, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1144, scale 1:62,500.

McKay, E. J., and Bergin, M. J., 1974, Geologic map of the Maybell 

quadrangle, Moffat County, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1145, scale 1:62,500.

McKee, E. D., 1966, Structures of dunes at White Sands National 

Monument, New Mexico (and a comparison with structures of 

dunes from other selected areas): Sedimentology, v. 7, no. 1, p. 1-69.

____1979, Ancient sandstones considered to be eolian, ch. H of 

McKee, E. D., ed., A study of global sand seas: U.S. Geological 

Survey Professional Paper 1052, p. 187-238.

McKee, E. D., and Bigarella, J. J., 1979, Sedimentary structures in 

dunes (with sections on the Lagoa dune field, Brazil), ch. E of 

McKee, E. D., ed., A study of global sand seas: U.S. Geological 

Survey Professional Paper 1052, p. 83-134.

McKee, E. D., Crosby, E. J., and Berryhill, H. L., 1967, Flood deposits, 

Bijou Creek, Colorado, June 1965: Journal of Sedimentary 

Petrology, v. 37, no. 3, p. 829-851.

McKee, E. D., and Tibbitts, G. C., Jr., 1964, Primary structures of 

a seif dune and associated deposits in Libya: Journal of Sedimentary Petrology, v. 34, p. 5-17.

Merriam, J. C., 1911, Tertiary mammal beds of Virgin Valley and 

Thousand Creek in northwestern Nevada: University of California Publications, Bulletin of the Department of Geological 

Sciences, v. 6, no. 11, p. 199-304.

____1913, Notes on the canid genus Tephrocyon: University of 

California Publications, Bulletin of the Department of Geological 

Sciences, v. 7, no. 18, p. 359-372. 

_1916, Tertiary vertebrate fauna from the Cedar Mountain region

of western Nevada: University of California Publications, Bulletin 

of the Department of Geological Sciences, v. 9, no. 13, p. 161-198.

Munthe, Jens, 1979, Summary of Miocene vertebrate fossils of the 

Granite Mountains Basin, central Wyoming: University of Wyoming Contributions to Geology, v. 18, no. 1, p. 33-46.

Olsen, S. J., 1956, The Caninae of the Thomas Farm Miocene: Breviora, 

no. 66, 12 p.

Osborn, H. F., 1918, Equidae of the Oligocene, Miocene, and Pliocene 

of North America, iconographic type revision: Memoir of the 

American Museum of Natural History (new series), v. 2, pt. 1, 

330 p.

____1936, Proboscidea; a monograph of the discovery, evolution, 

migration and extinction of the mastodonts and elephants of the 

world, vol. 1, Moeritheroidea, Deinotherioidea, Mastodontoidea: 

American Museum of Natural History, New York, 802 p.

Patton, T. H., 1969, Miocene and Pliocene artiodactyls, Texas Gulf 

Coastal Plain: Bulletin of the Florida State Museum, Biological 

Sciences, v. 14, no. 2, p. 115-226.

Peterson, O. A., 1906, The Miocene beds of western Nebraska and 

eastern Wyoming and their vertebrate faunae: Annals of the 

Carnegie Museum, v. 4, art. 3, p. 21-72.

____1924, Discovery of fossil mammals in the Brown's Park Formation of Moffat County, Colorado: Annals of the Carnegie 

Museum, v. 15, art. 12, p. 299-304.

____1928, The Brown's Park Formation: Carnegie Museum Memoir

11, no. 2, p. 87-121.

Powell, J. W., 1876, Report on the geology of the eastern portion of 

the Uinta Mountains and a region of country adjacent thereto: 

U.S. Geological and Geographical Survey of the Territories, v. 7, 

p. 1-218.

Reheis, M. C., 1981, Geologic map and coal resources of the Easton 

Gulch Quadrangle, Moffat County, Colorado: U. S. Geological 

Survey Coal Investigations Map C-87, scale 1:24,000.

Rowley, P. D., Tweto, Ogden, Hansen, W. R., and Carrara, P. E., 1979, 

Geologic map of the Vernal 1°X2° quadrangle, Colorado, Utah 

and Wyoming: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-1163, scale 1:250,000.

Schultz, C. B., 1938, The Miocene of western Nebraska: American Journal of Science, vol. 35, p. 441-444.

Scott, W. B., 1893, The mammals of the Deep River beds: American 

Naturalist, v. 27, p. 659-662.

Sears, J. D., 1924a, Relations of the Browns Park Formation and the 

Bishop Conglomerate, and their role in the origin of the Green 

and Yampa Rivers: Geological Society of America Bulletin, no. 

35, p. 279-304.

____1924b, Geology and oil and gas prospects of part of Moffat 

County, Colorado, and southern Sweetwater County, Wyoming: 

U.S. Geological Survey Bulletin 751-G, p. 269-319.

Sellards, E. H., 1916, Fossil vertebrates from Florida: a new Miocene 

fauna; new Pliocene species; the Pleistocene fauna: Florida State 

Geological Survey, 8th Annual Report, p. 77-119.

Simpson, G. G., 1932, Miocene land mammals from Florida: Florida 

State Geological Survey Bulletin 10, p. 7-41.

Skinner, M. F., 1942, The fauna of Papago Springs Cave, Arizona, 

and a study of Stockoceros; with three new antilocaprines from 

Nebraska and Arizona: Bulletin of the American Museum of 

Natural History, v. 80, ait. 6, p. 143-220.

____1968, A Pliocene chalicothere from Nebraska, and the distribution of chalicotheres in the late Tertiary of North America: 

American Museum Novitates, no. 2346, 24 p.

Skinner, M. F., and Johnson, F. W., 1984, Tertiary stratigraphy and 

the Frick collection of fossil vertebrates from north-central 

Nebraska: Bulletin of the American Museum of Natural History, 

v. 178, art. 3, p. 217-368.

Skinner, M. F., Skinner, S. M., and Goods, R. J., 1968, Cenozoic rocks 

and faunas of Turtle Butte, south-central South Dakota: Bulletin 

of the American Museum of Natural History, v. 138, art. 7, 

p. 379-436.

____1977, Stratigraphy and biostratigraphy of late Cenozoic 

deposits in central Sioux County, western Nebraska: Bulletin of 

the American Museum of Natural History, v. 158, art. 5, 

p. 265-371.

Skinner, M. F., and Taylor, B. E., 1967, A revision of the geology and 

paleontology of the Bijou Hills, South Dakota: America Museum 

Novitates, no. 2300, 53 p.

Stirton, R. A., 1929, Artiodactyla from the fossil beds of Fish Lake 

Valley, Nevada: University of California Publications, Bulletin 

of the Department of Geological Sciences, v. 18, no. 11, p. 291-302.

Tedford, R. H., 1970, Principles and practices of mammalian 

geochronology in North America, in Proceedings of the North 

American Paleontological Convention (Chicago, 1969), pt. F, correlation by fossils: Lawrence, Kansas, Alien Press, Inc., 

p. 666-703.
52 BROWNS PARK FORMATION, MOFFAT COUNTY, COLORADO

 1981, Mammalian biochronology of the late Cenozoic basins of 

New Mexico: Geological Society of America Bulletin, pt. I, v. 92, 

p. 1008-1022.

Tedford, R. H., and Alf, R. M., 1962, A new Megahippus from the 

Barstow Formation, San Bernardino County, California: Southern 

California Academy of Science Bulletin, v. 61, pt. 2, p. 113-122.

Tedford, R. H., and Hunter, M. E., 1984, Miocene marine-nonmarine 

correlations, Atlantic and Gulf Coastal Plains, North America: 

Palaeogeography, Palaeoclimatology, Palaeoecology, v. 47, 

p.129-151.

Tobien, Heinz, 1973, On the evolution of mastodonts (Proboscidea, 

Mammalia) part 1, The bunodont trilophodont groups: Notizblatt 

des Hessischen Landesamte fuer Bodenforschung (Wiesbaden), 

v. 101, p. 202-276.

Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological 

Survey, scale 1:500,000.

Untermann, G. E., and Untermann, B. R., 1954, Geology of Dinosaur 

National Monument and vicinity, Utah-Colorado: Utah Geological 

and Mineralogical Survey Bulletin 42, p. 1-221.

Voorhies, M. R., 1965, The Carnivora of the Trail Creek fauna: University of Wyoming Contributions to Geology, v. 4, no. 1, p. 21-25.

____1969, Taphonomy and population dynamics of an early Pliocene 

vertebrate fauna, Knox County, Nebraska: University of Wyoming Contributions to Geology, Special Paper 1, p. 1-69.

Webb, S. D., 1969, The Burge and Minnechaduza Clarendonian mammalian faunas of north-central Nebraska: University of California Publications in Geological Sciences, v. 78, p. 1-191.

____1973, Pliocene pronghorns of Florida: Journal of Mammalogy, 

v. 54, no. 1, p. 203-221.

Western, David, 1980, Linking the ecology of past and present mammal communities, in Behrensmeyer, A. K., and Hill, A. P., eds., 

Fossils in the making, vertebrate taphonomy and paleoecology: 

University of Chicago Press, p. 41-55.

White, T. E., 1941, Additions to the Miocene fauna of Florida: New 

England Zoological Club Proceedings, v. 18, p. 91-98.

____1942, The Lower Miocene mammal fauna of Florida: Bulletin 

of the Museum of Comparative Zoology, Harvard University, v. 

92, no. 1, p. 1-49.

Williams, G. E., 1971, Flood deposits of the sand-bed ephemeral 

streams of central Australia: Sedimentology, v. 17, p. 1-40.

Wilson, J. A., 1939, A new species of dog from the Miocene of Colorado: Contributions from the Museum of Paleontology, University of Michigan, v. 5, no. 12, p. 315-318.

____1960, Miocene carnivores, Texas Coastal Plain: Journal of 

Paleontology, v. 34, no. 5, p. 983-1000.

Winkler, G. R., 1970, Sedimentology and geomorphic significance of 

the Bishop Conglomerate and the Browns Park Formation, eastern 

Uinta Mountains, Utah, Colorado, and Wyoming: Salt Lake City, 

University of Utah Masters thesis, 115 p.

Wood, H. E., II, Chaney, R. W., Clark, J., Colbert, E. H., Jepsen, G. L., 

Reeside, J. B., Jr., and Stock, C., 1941, Nomenclature and correlation of the North American continental Tertiary: Geological 

Society of America Bulletin, v. 52, no. 1, p. 1-48.

Wortman, J. L., 1898, The extinct camelidae of North American and 

some associated forms: Bulletin of the American Museum of 

Natural History, v. 10, art. 7, p. 93-142.

U.S. GOVERNMENT PRINTING OFFICE: 1988 673-047/86,026 REGION NO. 8