Ammonites and Some Characteristic Bivalves from the 

Upper Cretaceous Frontier Formation, 

Natrona County, Wyoming 

U.S. GEOLOGICAL SURVEY BULLETIN 1917-B 

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Chapter B 

Ammonites and Some Characteristic Bivalves from the 

Upper Cretaceous Frontier Formation,-

Natrona County, Wyoming 

By WILLIAM A. COBBAN 

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern 

U.S. GEOLOGICAL SURVEY BULLETIN 1917 

EVOLUTION OF SEDIMENTARY BASINS-POWDER RIVER BASIN 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-In-Publication Data 

Cobban, William Aubrey, 1916-

Ammonites and some characteristic bivalves from the Upper Cretaceous 

Frontier Formation, Natrona County, Wyoming 1 by William A. Cobban. 

p. cm.-(Evolution of sedimentary basins-Powder River Basin; ch. B) 

(U.S. Geological Survey bulletin; 1917-B) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1917B 

1. Ammonoidea-Colorado-Natrona County. 2. Bivalves, FossilColorado-Natrona County. 3. Paleontology-Cretaceous. 

4. Paleontology-Colorado-Natrona County. 5. Frontier Formation. 

I. Title. II. Series. Ill. Series: U.S. Geological Survey bulletin ; 1917-B. 

QE75.B9 no. 1917-B 

[QE807.A5] 

557.3 s-dc20 

[564' .53'0978793] 9Q-3529 

CIP 
CONTENTS 

Abstract Bl 

Introduction Bl 

Localities of collections B4 

Molluscan record B4 

Zone of Conlinoceras tan-antense B4 

Zones of Acanthoceras granerosense and A. muldoonense B4 

Zone of Acanthoceras bellense B4 

Zone of Acanthoceras amphibolum BS 

Zone of Plesiacanthoceras wyomingense BS 

Zone of Dunveganoceras pondi BS 

Zone of Dunveganoceras problematicum B7 

Zone of Metoicoceras mosbyense B7 

Zone of Sciponoceras gracile B8 

Zone of Bun-oceras clydense B8 

Zone of Neocardioceras juddii B8 

Zones of Nigericeras scotti, Pseudaspidoceras flexuosum, Vascoceras birchbyi, 

and Mammites nodosoides B8 

Zone of Collignoniceras woollgari B9 

Zone of Prionocyclus percarinatus B9 

Zone of Prionocyclus hyatti B9 

Zone of Prionocyclus macombi B9 

Zone of Prionocyclus wyomingensis BlO 

Zone of Scaphites whitfieldi BlO 

Zone of Prionocyclus quadratus BlO 

References cited Bll 

Appendix BlS 

PLATES 

(Plates follow text] 

1-11. Molluscan fossils. 

FIGURES 

1. Map of Wyoming showing location of Natrona County Bl 

2. Chart showing ammonite zonation and correlation of the Frontier Formation 

in Natrona County B2 

3. Map showing outcrops of the Frontier Formation and localities of collections 

of molluscan fossils, Natrona County B3 

TABLE 

1. Localities at which fossils were collected from the Frontier Formation in 

Natrona County, Wyoming B6 

Contents Ill 
CONVERSION FACTORS FOR SOME Sl METRIC AND U.S. UNITS OF MEASURE 

To convert from 

Feet (ft) 

Miles (mi) 

Pounds (lb) 

Degrees Fahrenheit eF) 

IV Contents 

To 

Meters (m) 

Kilometers (km) 

Kilograms (kg) 

Degrees Celsius ec) 

0.3048 

1.609 

0.4536 

Multiply by 

Temp °C=(temp °F-32)/1.8 
EVOLUTION OF SEDIMENTARY BASINS-POWDER RIVER BASIN 

Ammonites and Some Characteristic Bivalves from the 

Upper Cretaceous Frontier Formation, 

Natrona County, Wyoming 

By William A. Cobban 

Abstract 

The Frontier Formation in Natrona County, Wyoming, 

consists of two or three members of mostly shallow-water 

marine shale, siltstone, and sandstone. From oldest to youngest, these units are the Belle Fourche Member, the informal 

member of Emigrant Gap, and the Wall Creek Member. The 

members are separated by disconformities, and locally the 

Wall Creek Member rests on the Belle Fourche Member. 

Molluscan fossils are present in all the members, but only the 

Wall Creek Member contains a great variety of species. 

Ammonites and inoceramid bivalves are most important in 

zoning the Frontier Formation. Seven of the twelve established ammonite zones of middle and late Cenomanian age 

in the Western Interior can be recognized in the Belle 

Fourche Member. Two zones of middle Turonian age are 

present in the member of Emigrant Gap, and three zones of 

late Turonian age are present in the Wall Creek Member. 

INTRODUCTION 

The Frontier Formation in Natrona County, Wyoming (fig. 1 ), consists of mostly shallow-water marine 

sandstone, siltstone, shale, and bentonite and is 255-280 

m thick. It conformably overlies the Mowry Shale and 

conformably underlies the Cody Shale. The Frontier 

consists of either two or three members, which are, from 

oldest to youngest, the Belle Fourche Member of early to 

late Cenomanian age, the informal member of Emigrant 

Gap (formerly known as the unnamed member) of 

middle Turonian age, and the Wall Creek Member of 

late Turonian age (fig. 2). The member of Emigrant Gap, 

which is developed best in the central part of Natrona 

Manuscript approved for publication, Feburary 19, 1990. 

1 wo 108° 106° ,--r-----l----1----: 

: \ "f W Y 0 M I N G """j 

: ~---l l , 

J 

NATRONA I 

I I COUNTY I 

I ~--~ I 

I 

42°L 

I l 

I I 

, I 

L __ L ____ __j_ _____ _L _____ j 

50 100 KILOMETERS 

Figure 1. Map of Wyoming showing location of Natrona 

County. 

County, is bounded above and below by disconformities. 

Where the member is absent, the Wall Creek Member 

rests disconformably on the Belle Fourche Member. 

The Frontier Formation crops out along the flanks 

of several anticlines, domes, and other uplifts in Natrona 

County (fig. 3). A map showing the Frontier outcrops in 

Natrona County was published by Crist and Lowry (1972, 

pl. 2) and reproduced in part later by Lageson (in 

Lageson and others, 1980). 

Many papers have been published on the stratigraphy of the Frontier Formation of Wyoming, and several 

of these reports give details in Natrona County. Papers 

that include maps showing thicknesses of the formation, 

members, and submembers in Natrona County as well as 

Ammonites and Bivalves from the Frontier Formation, Wyoming 81 
NATRONA COUNTY 

STAGE AMMONITE ZONE 

NORTHERN CENTRAL SOUTHEASTERN 

•Prlonocyclus quadratus Wall Creek Wall Creek Wall Creek Member Member a: •scaphites whltfleldi Member w 

a.. r--- ~ ~ ,........ /"" ~ ....... 1----r-- ~ 

,........ 

a.. ~ 

::::) *Prlonocyclus wyomlngensls "-r'--

1'-n_ 

1'--

Prlonocyclus macombl 

z !-----"" --.. ......... 

<( *Prlonocyclus hyattl z w 0 ...J 

a: c Member of Prlonocyclus percarlnatus ::::) c Emigrant Gap 1- ~ !'-" h ~ 

--- ,___........._v--. 

•colllgnonlceras woollgarl Sandstone 

~ ~ ~~ ~ h ~ ~ ,...,....-.._/ ........ 

Mammltes nodosoldes 

a: 

w 

~ Vascoceras blrchbyi 

0 

...J 

Pseudaspldoceras flexuosum 

c:: c:: c:: 

Nlgerlceras scottl 0 0 0 ·.;: ·.;: ·.;: ~ ~ ~ 

E ~ 

~ ~ ~ E E 

*Neocardloceras juddil 0 Sandstone .0 0 LL LL LL 

Q; ~ 

_,...... ~'"'-- .......... Q; ~ Burroceras clydense ·g ·g E a: 0 e e w Lt LL LL ~ ...... --- ~h- a.. *Sclponoceras gracile a.. 

::::) r-- h ~ ~_......_ ~ ....... -- ~ 

r= *Metolcoceras mosbyense 

a: 

<( 

e:. •Dunveganoceras problematicum 

z Sandstone ~ •Dunveganoceras pondl and 

z Q; 

<( .0 shale 

~ *Pieslacanthoceras wyomlngense E 

0 Q) 

z ~ Belle Fourche w Q) Belle Fourche Member u *Acanthoceras amphlbolum ~ 

~ Soap Creek Bentonite Bed Member ::I 

w Acanthoceras bellense 0 

...J LL 

0 ~ c "a) 

~ Acanthoceras muldoonense m Sandstone 

and 

Acanthoceras granerosense shale 

•conllnoceras tarrantense 

Figure 2. Ammonite zonation for the middle Cenomanian through the Turonian in the Western Interior and correlation of the 

Frontier Formation and members in Natrona County. Ammonite zones known to be present in Natrona County are indicated 

by asterisks. 

correlations are given by Masters (1952), Towse (1952), 

Goodell (1962), Crews and others (1976), Weimer 

(1978), Merewether and others (1979), Merewether 

(1983), and Merewether and Cobban (1986a, b). 

Graphic sections and (or) written sections of the Frontier 

Formation in Natrona County have been presented by 

Towse (1952), Goodell (1962), Burgess (1971), Wiegand 

B2 Evolution of Sedimentary Basins-Powder River Basin 

and others (1973), Merewether and Cobban (1973, 

1986a, b), Merewether and others (1979), Merewether 

(1983), Weimer (1983), and Winn (1986). Barlow and 

Haun (1966) defined a "Frontier delta" that extended 

from western Wyoming southeastward into Natrona 

County. Goodell (1962) dealt with the petrology of the 

sandstones of the Frontier, and Bitter (1986) discussed 
,---------~ 107° 

I '---r------ ~1 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

L.., 

I 0 Waltman 

I 0 Powder River 0 Natrona 

I 

43°1-

1 

!16,17 

,~ 

I~ I 

I 

I 

I 

I 

I 

I 

I 

I 

~---~~ ,....-- .....-: ::::v) v I 

? J Lr'Casper 1 

u ) 

<fZ> 0 Freeland 

48·5~ ~ 

\ 

I 

--~-----------------~ 10 20 30 KILOMETERS 

Figure 3. Localities of collections of mqlluscan fossils from the Frontier Formation in Natrona County referred to by number 

in the text and in table 1. Outcrop areas of Frontier Formation (stippled) from Love and Christiansen (1985). 

the phosphatic nodules and cements in the Frontier of 

Natrona County. 

The presence of molluscan fossils in the Frontier 

Formation in Wyoming has been noted in several reports, 

but only four mention a few generic or specific names of 

taxa from Natrona County (Cobban and Reeside, 1952; 

Masters, 1952; Merewether and others, 1979; Cobban 

and Kennedy, 1989). The most detailed treatment of the 

molluscan fossil record is by Merewether and others 

(1979), who illustrated seven species of ammonites from 

Natrona County. 

The present report briefly describes most of tpe 

known species of cephalopods (chiefly ammonites) from 

the Frontier Formation in Natrona County as well as 

some of the more common bivalves. Gastropods are 

scarce in the Frontier, and other invertebrate fossils, such 

Ammonites and Bivalves from the Frontier Formation, Wyoming 83 
as arthropods, are almost unknown. Fossil collections 

referred to in this report are either those reported in the 

literature or those reported here for the first time. 

Fossils described in this report are kept in the 

National Museum of Natural History in Washington, 

D.C., and all have USNM catalog numbers. Plaster casts 

of a few of the specimens are in the reference collection 

of the U.S. Geological Survey at the Federal Center, 

Denver, Colorado. R.E. Burkholder, who recently retired 

from the U.S. Geological Survey, made the photographs. 

LOCALITIES OF COLLECTIONS 

Localities at which the specimens described or 

illustrated in this report were collected are shown in 

figure 3, which also includes other localities mentioned in 

the text. The U.S. Geological Survey Mesozoic locality 

number, names of collectors, year of collection, locality, 

and stratigraphic assignment are given in table 1. 

MOLLUSCAN RECORD 

The Frontier Formation in Natrona County contains molluscan fossils of middle and late Cenomanian 

age and middle and late Turonian age. In the Western 

Interior of the United States, 23 zones of ammonites can 

be recognized from the base of the middle Cenomanian 

to the top of the Turonian. At least 12 of these zones are 

present in the Frontier Formation in Natrona County. 

In the discussions of the fossils and zones, a 

reference is given to the original description of each of 

the fossils, and this reference may be followed by a more 

accessible one that has better illustrations. 

Zone of Conlinoceras tarrantense 

This is the oldest middle Cenomanian zone of 

ammonites known in the Western Interior. Molluscan 

fossils indicative of this zone are present in the Thatcher 

Limestone Member of the Graneros Shale of eastern 

Colorado and northeastern New Mexico, in the Oak 

Canyon Member of the Dakota Sandstone of westcentral New Mexico, in the upper part of the Belle 

Fourche Shale of the Black Hills region of eastern 

Wyoming and western South Dakota, and in the upper 

part of the Belle Fourche Member of the Frontier 

Formation of central and southeastern Wyoming. The 

guide ammonite, Conlinoceras tarrantense (Adkins, 1928, 

p. 241, pl. 28, fig. 3; pl. 29, fig. 1), is a stout, moderately 

sized species that has acanthoceratid inner whorls with 

umbilical, inner and outer ventrolateral tubercles, and 

siphonal ( midventral) tubercles. Tubercles disappear on 

84 Evolution of Sedimentary Basins-Powder River Basin 

the later adult whorls, where the ornament consists of 

alternate long and short ribs that cross the flank and 

venter. For illustrations of this species, the reader is 

referred to Adkins (1928, pl. 28, fig. 3; pl. 29, fig. 1), 

where the species was described as Metacalycoceras? 

tarrantense as well asAcanthoceras wintoni Adkins (1928, 

p. 243, pl. 25, figs. 2, 3). The ammonite described as 

Ca/ycoceras (Conlinoceras) gilberti Cobban and Scott 

(1972, p. 61, pl. 1; pl. 2, figs. 5-9, 13-18; pl. 3, figs. 5-7, 

11; text figs. 23, 24), from the Thatcher Limestone 

Member of the Graneros Shale of eastern Colorado, is a 

closely related form that could be interpreted as a 

subspecies of C. tarrantense. 

Molluscan fossils of this zone have not been 

collected in Natrona County. They are most certainly 

present in the northernmost part of the county inasmuch 

as Borissiakoceras compressum Cobban (1961, p. 747, 

pl. 87, figs. 19-33; pl. 89, figs. 1-9; text figs. 4a-k) and 

Inoceramus eu/essanus Stephenson (1952, p. 65, pl. 44, 

fig. 2), fossils restricted to this zone, have been found a 

few hundred meters north of the Natrona-Johnson 

County line in the NW¥4 sec. 29, T. 41 N., R. 82 W. 

(fig. 2, just north of loc. 2). 

Zones of Acanthoceras granerosense and 

A. muldoorense 

These zones, first recognized in the Graneros Shale 

above the Thatcher Limestone Member in southeastern 

Colorado (Cobban and Scott, 1972, table 2), are present 

in the upper part of the Belle Fourche Member of the 

Frontier Formation in Johnson County just north of 

Natrona County. Neither zone has been recognized in 

Natrona County, but future collecting may prove their 

presence in the upper part of the Belle Fourche Member. 

For illustrations of Acanthoceras granerosense and A. 

muldoonense, the reader is referred to Cobban and Scott 

(1972, pl. 2, figs. 1-4; pl. 4, figs. 1-3; pl. 5, fig. 3; pl. 6, 

figs. 1-3; pl. 7, figs. 1-13; pl. 11, figs. 12, 13; text figs. 27-

30). Illustrations of A. muldoonense from Johnson 

County are given by Cobban (1987, pl. 3, figs. 7-34). 

Zone of Acanthoceras_bellense 

This middle Cenomanian zone is known only from 

near the top of the Belle Fourche Shale on the west side 

of the Black Hills uplift in eastern Wyoming. The guide 

ammonite was originally described from the base of the 

Eagle Ford Group of east-central Texas (Adkins, 1928, 

p. 245, pl. 30, figs. 1, 2). The Wyoming specimens were 

described and illustrated by Cobban (1987, p. 5, pl. 1, 

figs. 18, 19; pl. 2; text fig. 5). 
Zone of Acanthoceras amphibolum 

Characteristic fossils of this zone include the 

ammonite Acanthoceras amphibolum Morrow and the 

bivalves Inoceramus rutherfordi Warren, I. arvanus 

Stephenson, and Ostrea beloiti Logan. These fossils are 

widely distributed in the Western Interior from southcentral Montana to the Mexican border and from western Colorado to central Kansas. 

Acanthoceras amphibolum is a moderate-size 

species that has inner whorls with umbilical, inner and 

outer ventrolateral, and siphonal tubercles. At some 

early growth stage siphonal tubercles may be more 

numerous than the ventrolateral ones, and at a later 

growth stage the siphonal tubercles weaken and are 

replaced by a low siphonal ridge. The inner and outer 

ventrolateral tubercles merge into outward and upward 

directed horns in the adult stage. The generic assignment 

of this species has been a problem. The species was 

originally described as Acanthoceras? amphibolum 

Morrow (1935, p. 470, pl. 49, figs. 1-4, 6; pl. 51, figs. 3, 4; 

text fig. 4 ), and the type lot is from the upper part of the 

Graneros Shale of Kansas. Since Morrow's work, the 

species has been assigned to Euomphaloceras (Stephenson, 1955, p. 63), Paracanthoceras (Haas, 1963, p. 18), 

Plesiacanthoceras (Haas, 1964, p. 610), and Cunningtoniceras (Cobban, 1987, p. 9). New investigations suggest 

that the species is not in the Cunningtoniceras lineage, 

and until further studies are concluded, the species 

amphibolum is probably best left in Acanthoceras in the 

broad sense. 

Acanthoceras amphibolum has been definitely 

identified from only four localities in northwestern Natrona County (fig. 3, Iocs. 7, 8, 10, 15; pl. 3, figs. 6, 7; pl. 4, 

fig. 10). There the species was found in the upper part of 

the Belle Fourche Member along with Inoceramus arvanus Stephenson (1952, p. 65, pl. 12, figs. 6-9) and Ostrea 

beloiti Logan (1899, p. 214, pl. 25, figs. 7, 8). 

Zone of Plesiacanthoceras wyomingense 

This zone, of latest middle Cenomanian age, is 

present over much of the Western Interior from southcentral Montana to northern New Mexico. The guide 

ammonite, Plesiacanthoceras wyomingense (Reagan) 

(Reagan, 1924, p. 181, pl. 19, figs. 1, 2; Cobban, 1987, 

p. 16, pl. 10, figs. 48-82; pis. 11-13), is a large species that 

attains diameters of as much as 350 mm. Earliest whorls 

are acanthoceratid with umbilical, inner and outer ventrolateral, and siphonal tubercles, but the last are lost 

early, and most of the inner whorls have depressed, 

smooth venters bordered by high, clavate ventrolateral 

tubercles (pl. 5, figs. 1, 2). As the shell enlarges, the 

umbilical tubercles become stronger and nodate and 

migrate out onto the lower part of the flank (pl. 1, 

fig. 11). On the last whorl of the adult, the outer 

ventrolateral tubercles weaken and disappear, whereas 

the inner ventrolateral tubercles enlarge into conspicuous horns that are directed outward and a little 

upward. The species has been well illustrated by Haas 

(1963) and Cobban (1987). 

Plesiacanthoceras wyomingense was collected from 

the upper part of the Belle Fourche Member at five 

localities in Natrona County (fig. 3, locs. 16-18, 20, 45). 

The only other fossils found with P. wyomingense in 

Natrona County are the bivalve Inoceramus prefragilis 

Stephenson (1952, p. 64, pl. 12, figs. 10-12; pl. 13, figs. 1, 

2) and the ammonites Hamites sp. and Euca/ycoceras 

templetonense Cobban (1988a, p. 10, pl. 4, figs. 1-13; text 

fig. 8). Examples of the Eucalycoceras are shown in plates 

1 and 4. 

Zone of Dunveganoceras pondi 

This is the oldest of the upper Cenomanian ammonite zones in the Western Interior. The guide ammonite, 

Dunveganoceras pondi Haas (1949, p. 22, pl. 8, figs. 1-5, 

8; pl. 9, figs. 1, 3, 4; pis. 10-14; text figs. 11-13, 16, 17), 

came from the basal part of the Cody Shale near Greybull, Big Horn County, Wyoming. The species is large 

and attains a diameter of as much as 350 mm. Inner 

whorls have umbilical and inner and outer ventrolateral 

tubercles (pl. 6), whereas the outer adult whorl has an 

entirely different appearance. This whorl, which has a 

squarish cross section, is ornamented by widely spaced 

straight ribs of equal lengths that are accentuated at the 

ventrolateral shoulder and then are depressed on 

crossing the venter (pl. 6; pl. 7, fig. 12). Complete adults 

have been shown by Haas (1949, pis. 10, 11) and by 

Merewether and others (1979, pl. 4). 

Dunveganoceras pondi has been found in brownweathering, silty, calcareous concretions in the upper 

part of the Frontier Formation and basal part of the Cody 

Shale in the Bighorn Basin of Wyoming and in similar 

concretions in the upper part of the Belle Fourche 

Member in central Wyoming. Farther east, fragments of 

the species have been found in the basal bed of limestone 

of the Greenhorn Formation in the Black Hills area. 

In Natrona County, D. pondi has been collected 

from brown-weathering, silty concretions in the upper 

part of the Belle Fourche Member at 11 localities. 

Calycoceras (Proeucalycoceras) canitaurinum (Haas) 

(Haas, 1949, p. 9, pis. 1-3; pl. 4, figs. 1, 2, 4; text figs. 

1-4), a large, robust ammonite characterized by adult 

whorls with broad, flat ribs of alternate lengths, is a 

common associate. A moderately sized specimen is 

shown in plate 3. Other fossils found with D. pondi in 

Natrona County include the bivalves Inoceramus prefragilis Stephenson (1952, p. 15, pl. 19, figs. 1, 2, 4) and 

Ammonites and Bivalves from the Frontier Formation, Wyoming 85 
Table 1. Localities at which fossils were collected from,the Frontier Formation in Natrona County, Wyoming 

[Localities shown by number on figure 3; USGS numbers are Mesozoic locality numbers] 

Locality USGS 

number number 

D6962 

2 DI3041 

3 D13043 

4 D9835 

5 D9833 

6 D10664 

7 D12959 

8 D13042 

9 D9831 

10 D8471 

11 D8470 

12 D7385 

13 D7383 

14 D7382 

15 D12963 

16 D8917 

17 D8918 

18 09324 

19 D9325 

20 09319 

21 09787 

22 D9877 

23 D9878 

24 09766 

25 09749 

26 D9748 

27 D9747 

28 012547 

29 D9337 

30 D9359 

31 D9746 

32 D9118 

33 D12645 

34 010522 

35 09358 

Collector, year of collection, description of locality, and stratigraphic assignment 

W.A. Cobban, 1969, 1970. Gierse Ranch, SW1/4 sec. 30, T. 41 N., R. 80 W. From a 2-m-thick, orangebrown-weathering, concretionary sandstone bed in upper part of Belle Fourche Member. 

W.A. Cobban, 1989. North of Willow Creek in the SW1/4 sec. 29, T. 41 N., R. 82 W. Float from a 

calcareous, sandstone concretion from below the Wall Creek Member. 

W.A. Cobban, 1989. SE1/4NE1/4SW1/4 sec. 29, T. 41 N., R. 82 W. From sandy concretions about 25m 

below sandstone cliff of Wall Creek Member. 

E.A. Merewether, 1975. NE1/4 sec. 32, T. 40 N., R. 82 W. From sandstone concretions 2.4-2.7 m above 

third ledge-forming sandstone below top of Frontier Formation. 

E.A. Merewether, 1975. NW1/4 sec. 33, T. 40 N., R. 82 W. From second sandstone ledge below top of 

Wall Creek Member. 

W.A. Cobban, 1978. NE1/4 sec. 18, T. 39 N., R. 82 W. From large sandstone concretions in Wall Creek 

Member. 

M.C. Huff, 1988. NE1/4 sec. 34, T. 39 N., R. 83 W. From sandstone bed overlying Soap Creek Bentonite 

Bed of Belle Fourche Member. 

W.A. Cobban, 1989. NE1/4NE1/4NE1/4 sec. 34, T. 39 N., R. 83 W. From sandstone bed overlying Soap 

Creek Bentonite Bed of Belle Fourche Member. 

E.A. Merewether, 1975. SE1/4 sec. 5, T. 38 N., R. 83 W. From Belle Fourche Member. 

E.A. Mcrcwether, 1972. NE1/4 sec. 4, T. 38 N., R. 85 W. From Belle Fourche Member. 

E.A. Merewether, 1972. NEl/4 sec. 20, T. 38 N., R. 85 W. From Belle Fourche Member. 

W.A. Cobban, 1970. Lester Trail in the SEl/4 sec. 20, T. 38 N., R. 85 W. From gray limestone 

concretions in Belle Fourche Member. 

W.A. Cobban, 1970. SW1/4 sec. 21, T. 38 N., R. 85 W. From sandstone unit in upper part of Belle 

Fourche Member. 

W.A. Cobban, 1970. NWl/4 sec. 28, T. 38 N., R. 85 W. From basal 0.6 m of shale that overlies a 

prominent bluff-forming sandstone unit in upper part of Belle Fourche Member. 

E.A. Merewether, 1988. SWl/4 sec. 28, T. 38 N., R. 85 W. From concretionary siltstone in Belle Fourche 

Member. 

E.A. Merewether, 1973. NW1/4 sec. 19, T.34 N., R. 88 W. From upper part of Belle Fourche Member. 

E.A. Merewether, 1973. SW1/4 sec. 19, T. 34 N., R. 88 W. From upper part of Belle Fourche Member. 

E.A. Merewether and W.A. Cobban, 1974. Northeast of Jameson Ranch in the SE1/4 sec. 2, T. 33 N., R. 

87 W. From brown and dusky-red ferruginous, silty concretions in Belle Fourche Member. 

E.A. Merewether and W.A. Cobban, 1974. SE1/4 sec. 2, T. 33 N., R. 88 W. From coarse-grained, friable 

sandstone in member of Emigrant Gap. 

W.A. Cobban, 1974. NW1/4 sec. 6, T. 32 N., R. 86 W. From dusky-red, ferruginous concretions beneath a 

thick, gray, bentonitic unit in Belle Fourche Member. 

E.A. Merewether, 1975. North side of Pine Mountain in the SW1/4 sec. 18, T. 35 N., R. 83 W. From about 

12m below top of Wall Creek Member. 

W.A. Cobban, 1976. NE1/4 sec. 34, T. 33 N., R. 82 W. From small, brown-weathering limestone 

concretions 7.5-9 m above coarse-grained sandstone of member of Emigrant Gap. 

E.A. Merewether and W.A. Cobban, 1976. NE1/4 sec. 34, T. 33 N., R. 82 W. From brown-weathering 

limestone concretions 12m above coarse-grained sandstone of member of Emigrant Gap. 

E.A. Merewether, 1975. SW1/4 sec. 35, T. 33 N., R. 82 W. From float from brown-weathering limestone 

concretions of member of Emigrant Gap below Wall Creek Member. 

E.A. Merewether, 1975. SWl/4 sec. 19, T. 34 N., R. 81 W. From member of Emigrant Gap. 

E.A. Merewether, E.T. Cavanaugh, and W .A. Cobban, 1975. SWl/4 sec. 19, T. 34 N., R. 81 W. From 

pebbly sandstone bed in upper part of Belle Fourche Member. 

E.A. Merewether, E.T. Cavanaugh, and W.A. Cobban, 1975. NW1/4 sec. 30, T. 34 N., R. 81 W. From 

Belle Fourche Member. 

E.A. Merewether and W.A. Cobban, 1984. NE1/4 sec. 25, T. 34 N., .R. 82 W. From Belle Fourche 

Member. 

E.A. Merewether and w:A. Cobban, 1974. Emigrant Gap Ridge in the NW1/4 sec. 32, T. 34 N., R. 81 W. 

From pebbly sandstone bed in upper part of Belle Fourche Member. 

E.T. Cavanaugh, 1975. NE1/4 sec. 32, T. 34 N., R. 81 W. From top of Wall Creek Member. 

E.A. Merewether, E.T. _Cavanaugh, and W.A. Cobban, 1975. SE1/4 sec. 32, T. 34 N., R. 81 W. From 

upper part of member of Emigrant Gap. 

E.A. Merewether and W.A. Cobban, 1974. Emigrant Gap Ridge in the NE1/4 sec. 4, T. 33 N., R. 81 W. 

From calcareous, sandstone concretions in uppermost part of Wall Creek Member. 

W.A. Cobban, 1985. N1/2 sec. 4, T. 33 N., R. 81 W. From upper few meters of member of Emigrant Gap. 

E.A. Merewether, S.C. Hook, and W.A. Cobban, 1978. NW1/4 sec. 4, T. 33 N., R. 81 W. From limestone 

concretions in gray sandy shale unit in upper part of member of Emigrant Gap. 

E.T. Cavanaugh, 1975. SE1/4 sec. 5, T. 33 N., R. 81 W. From top of Wall Creek Member. 

B6 Evolution of Sedimentary Basins-Powder River Basin 
Table 1. Continued 

locality USGS 

number number 

36 09333 

37 09742 

38 09335 

39 09365 

40 09116 

41 09357 

42 09764 

43 03117 

44 08914 

45 09125 

46 011796 

47 011793 

48 09119 

49 09120 

50 09121 

Collector, year of collection, description of locality, and stratigraphic assignment 

W.A. Cobban, 1974. Emigrant Gap Ridge in the SE1/4 sec. 4, T. 33 N., R. 81 W. From very fine grained 

shaly sandstone in middle part of member of Emigrant Gap. 

E.A. Merewether, E.T. Cavanaugh, and W.A. Cobban, 1975. SWl/4 sec. 3, T. 33 N., R. 81 W. From upper 

part of Wall Creek Member. 

E.A. Merewether, 1974. SE1/4 sec. 4, T. 33 N., R. 81 W. From brown-weathering very fme grained 

sandstone concretions in a sandstone bed in the Belle Fourche Member. 

E.T. Cavanaugh, 1975. SE1/4 sec. 4, T. 33 N., R. 81 W. From a 15-m-thick sandstone bed in the Belle 

Fourche Member. 

E.A. Merewether and W.A. Cobban, 1974. SE1/4 sec. 3, T. 33 N., R. 81 W. From sandstone concretions 

near top of Wall Creek Member. 

E.T. Cavanaugh, 1975. NW1/4 sec. 11, T. 33 N., R. 81 W. From Wall Creek Member. 

E.A. Merewether, 1975. NE1/4 sec. 9, T. 32 N., R. 81 W. From top of Wall Creek Member. 

D.L. Eicher, 1961. NW1/4 sec. 12, T. 30 N., R. 83 W. From gray limestone concretions in Belle Fourche 

Member 6 m below base of member of Emigrant Gap. 

E.A. Merewether, W.J. Kennedy, and W.A. Cobban, 1973. NE1/4 sec. 31, T. 30 N., R. 82 W. From 

brown-weathering, sandy, limestone concretions at top of a soft sandstone bed in Belle Fourche 

Member. 

E.A. Merewether and W.A. Cobban, 09125. NE1/4 sec. 31, T. 30 N., R. 82 W. From a 4.5-m-thick bed of 

soft sandstone that overlies a unit of dark-gray shale with several beds of bentonite in the Belle Fourche 

Member. 

E.A. Merewether, 1976. NW1/4 sec. 32, T. 30 N., R. 81 W. From upper part of Wall Creek Member. 

E.A. Merewether and W.A. Cobban, 1976. NW1/4 sec. 32, T. 30 N., R. 81 W. From a brown-weathering, 

sandy, limestone concretion in upper part of Belle Fourche Member. 

E.A. Merewether and W.A. Cobban, 1974. SW1/4 sec. 36, T. 32 N., R. 78 W. From a 6-m-thick unit of 

carbonaceous, sandy shale and flaggy sandstone between ledge-forming sandstone units in the Wall 

Creek Member. 

E.A. Merewether and W.A. Cobban, 1974. SW1/4 sec. 36, T. 32 N., R. 78 W. From basal part of a 14-mthick, ledge-forming sandstone unit that underlies 09119 in the Wall Creek Member. 

E.A. Merewether and W.A. Cobban, 1974. SW1/4 sec. 36, T. 32 N., R. 78 W. From a 6-m-thick, ledgeforming sandstone unit that overlies 09119 in the Wall Creek Member. 

Plicatula sp. and the ammonites Calycoceras newboldi 

wyomingense Cobban (1988b, p. 5, pl. 3, fig. 5; pl. 4, 

figs. 2, 3; text figs. 3, 4), Tarrantoceras flexicostatum 

Cobban (1988a, p. 7, pl. 2, figs. 23-26; text fig. 5) (pl. 9, 

fig. 25), Eucalycoceras templetonense Cobban (1988a, 

p. 10, pl. 4; text fig. 8), Metoicoceras praecox Haas (1949, 

p. 15, pis. 5-7; text figs. 5-9), and Hamites sp. 

in the upper part of the Belle Fourche Member at three 

localities in Natrona County (fig. 3, locs. 26, 29, 47). This 

subspecies differs from the typical form in that ribs are 

more numerous and tend to be of equal lengths (pl. 8, 

fig. 3). 

Fossils found with D. problematicum natronense in 

Natrona County are Inoceramus prefragilis Stephenson, 

Moremanoceras costatum Cobban, Hoo~, and Kennedy 

(1989, p. 19, figs. 19, 64A-D, G, H), Calycoceras aff. 

canitaurinum (Haas) (Cobban, 1988b, p. 7, pl. 5; pl. 6, 

figs. 1, 2), Neocardioceras uptonense Cobban (1988a, 

p. 20, pl. 10, figs. 36-70; text fig. 17), Metoicoceras 

frontierense Cobban (1988b, p. 13, pl. 3, figs. 1-3; pl. 13, 

figs. 1, 2; pl. 14; text fig. 12) (pl. 3, figs. 8, 9; pl. 9, figs. 26, 

27), and Hamites sp. 

Zone of Dunveganoceras problematicum 

This zone is known only from the west side of the 

Powder River Basin in central and north-central Wyoming. Dunveganoceras problematicum Cobban (1988b, 

p. 9, pl. 2, figs. 4-6; pl. 3, fig. 4; pl. 8, figs. 1, 2; pis. 10-12; 

pl. 13, fig. 3; text figs. 8-11) is a large, moderately 

compressed ammonite that has very involute inner 

whorls with weak, closely spaced ribs and umbilical 

bullae and stronger inner and outer ventrolateral tuber.:. 

cles enclosing a sulcate venter. The outer adult whorl is 

much different; it is fairly evolute and has an ogival cross 

section and ornament of ribs only (pl. 8, fig. 3). 

A subspecies, D. problematicum natronense Cobban (1988b, p. 11, pl. 3, fig. 4; pls.11, 12; pl.13, fig. 3; text 

figs.10, 11), has been found in a conglomeratic sandstone 

Zone of Metoicoceras mosbyense 

Molluscan fossils representing this middle upper 

Cenomanian zone are widely distributed over the Western Interior from northern Montana to southwestern 

New Mexico. Metoicpceras mosbyense is a fairly large, 

compressed ammonite that has very involute inner 

whorls ornamented by prim~ry and secondary ribs with 

Ammonites and Blvalv~s from the Frontier Formation, Wyoming B7 
umbilical bullae and inner and outer ventrolateral 

tubercles (pl. 2, figs.1-3). Adult whorls are more evolute 

and have broad, flattened ribs and no tubercles (pl. 2, 

figs. 8, 9). The species was first described from the Mosby 

Sandstone Member of the Belle Fourche Shale of eastcentral Montana (Cobban, 1953a, p. 48, pl. 6, figs. 1-14; 

pl. 7, figs. 1-3). 

Metoicoceras mosbyense has been collected from 

near the top of the Belle Fourche Member of the 

Fronti,er Formation at 26 localities in Natrona County. 

Most fossils are from concretions of calcareous siltstone 

and from beds of sandstone. Fossils found with M. 

mosbyense are the bivalves Inoceramus pictus Sowerby 

(1829, p. 215, pl. 604, fig. 1) (pl. 1, fig. 3), I. ginterensis 

Pergament (1966, p. 50, pl. 25, fig. 5; pl. 26, figs. 1, 2; 

pl. 27, figs. 1, 2; pl. 28, figs. 1, 2; pl. 29, fig. 1) (pl. 10, 

figs. 10-12), and Pycnodonte aff. kellumi (Jones) (Jones, 

1938, p. 107, pl. 3, figs. 3-5) (pl. 3, figs. 1-4), and the 

ammonites Calycoceras obrieni Young (1957, p. 1171, 

pl. 150, figs. 1-4; text figs. 1f, h), Dunveganoceras conditum Haas (1951, p. 5, figs. 2-9), D. alberlense (Warren) 

(Warren, 1930, p. 21, pl. 1, figs. 1, 2), Neocardioceras 

minutum Cobban (Cobban, 1988a, p. 23, pl. 10, 

figs. 1-35; text fig. 20), and Hamites salebrosus Cobban, 

Hook, and Kennedy (1989, p. 57, fig. 95BB, EE-11) 

(pl. 3, figs. 12, 13). Dunveganoceras conditum is a large, 

evolute ammonite that has inner whorls with umbilical 

bullae and conspicuous inner and outer ventrolateral 

tubercles (pl. 8, figs. 1, 2) and outer adult whorl with 

rounded section and prominent ribs of equal lengths 

crossing the flanks and venter (pl. 11 ). Dunveganoceras 

alberlense, which seems to occur a little below D. conditum, has similar inner whorls, but the outer adult whorl 

has an ogival section and sparser ribs. 

Zone of Sciponoceras gracile 

Rocks that represent this upper Cenomanian zone 

are known in Natrona County'only near the southern 

edge of the county (fig. 3, Joe. 43). Elsewhere in the 

county, rocks of this age apparently were removed before 

deposition of the member of Emigrant Gap or the Wall 

Creek Member, both of which have disconformable basal 

contacts (Merewether and Cobban, 1986a). 

The guide fossil, Sciponoceras gracile (Shumard) 

(Shumard, 1860, p. 596), is a small, straight ammonite 

that has a subcircular cross section, low angle of taper, 

and ornament of weak ventral ribs and fewer but stronger 

constrictions (pl. 9, figs. 4-15). The species has a wide 

geographic distribution that extends from Montana to 

southwestern New Mexico and to eastern Texas and 

western Europe. 

In the southern part of Natrona County, near 

Alcova, limestone concretions in the Belle Fourche 

Member 6 m below the disconformable base of the Wall 

B8 Evolution of Sedimentary Basins-Powder River Basin 

Creek Member containS. gracile (pl. 9, figs. 4-15) and 

the bivalves Inoceramus ginterensis Pergament (pl. 10, 

fig. 10) and Phelopteria sp. and the ammonites Placenticeras (Karamaites) cumminsi Cragin (1893, p. 237), 

Borissiakoceras orbicu/atum Stephenson (1955, p. 64, 

pl. 6, figs. 1-4) (pl. 9, figs. 23, 24), Euomphaloceras 

septemseriatum (Cragin) (Cragin, 1893, p. 240; Cobban 

and Scott, 1972, p. 72, pl. 12, figs. 5-27), and Allocrioceras annulatum (Shumard) (Shumard, 1860, p. 595) 

(pl. 9, figs. 1-3). 

Zone of Burroceras clydense 

This upper Cenomanian zone is known only in 

southwestern New Mexico. The guide fossil, Burroceras 

clydense, and associated ammonites were recently described by Cobban and others (1989). 

Zone of Neocardioceras j uddi i 

The ornate ammonite Neocardioceras juddii (Barrois and de Guerne) (Barrois and de Guerne, 1878, p. 46, 

pl. 1, figs. 1a, b, 2a, b), originally described from France, 

is widely distributed in the Western Interior. This fairly 

small, moderately evolute species has numerous, narrow 

primary and secondary ribs that cross the venter and bear 

small inner and outer ventrolateral and siphonal tubercles (Cobban and others, 1989, p. 31, figs. 33, 75F-DD, 

11-MM). The longer (primary) ribs arise from thin 

umbilical bullae. 

The zone of N. juddii is present in a thin sandy unit 

in the Frontier Formation below the Wall Creek Member 

at two localities in northern Natrona County (fig. 3, locs. 

3, 4). This sandy unit contains some coarse-grained 

sandstone; small, black-coated chert pebbles; and lightgray to yellow or orange weathering, tabular calcareous 

concretions with cone-in-cone structure. The known 

fauna consists of solitary corals; the bivalves Phelopteria 

sp. and Inoceramus cf. tenuistriatus Nagao and Matsumoto (1939, p. 272, pl. 24, fig. 8; pl. 26, figs. 1-4); small 

gastropods; and the ammonites Burroceras irregulare 

Cobban, Hook, and Kennedy (1989, p. 38, figs. 39, 

80S-V), Euomphaloceras costatum Cobban, Hook, and 

Kennedy (1989, p. 37, figs. 37, 77S-EE, 78A-H), Sciponoceras gracile (Shumard), and Yezoites sp. (Kennedy, 

1988, pl. 24). 

Zones of Nigericeras scotti, 

Pseudaspidoceras flexuosum, Vascoceras 

tirchbyi, and Mammites nodosoides 

These zones of latest Cenomanian and early Turonian age seem to be absent in Natrona County owing to 

erosion at the disconformities at either the base of the 
member of Emigrant Gap or the base of the Wall Creek 

Member (Merewether and Cobban, 1986a). Nigericeras 

scotti Cobban (1971, p. 18, pl. 9, figs. 1-4; pl. 18, figs.l-9; 

text figs. 15-17), of latest Cenomanian age, is known only 

from southeastern Colorado, northeastern New Mexico, 

and possibly southwestern New Mexico. Pseudaspidoceras flexuosum Powell (1963, p. 318, pl. 32, figs. 1, 9, 10; 

text figs. 2a-c, f, g), originally described from TransPecos Texas, has been found in southwestern New 

Mexico, eastern Arizona, and southeastern Colorado but 

not farther north. Vascoceras birchbyi Cobban and Scott 

(1972, p. 85, pl. 22; pl. 23, figs. 1-13; pl. 24, figs. 1-12; 

pl. 25; pl. 26, figs. 5-8, 11, 12;' pl. 27, figs. 1-6; text 

figs. 43-47) is known from central Kansas, eastern 

Colorado, central Utah, and northeastern and southwestern New Mexico. Mammites nodosoides (Schluter) 

(Schluter, 1871, p. 19, pl. 8, figs. 1-4; Cobban and others, 

1989, p. 41, figs. 42, 900-H, M, N) has been found at 

many localities from southern New Mexico to southwestern Wyoming, but not farther north. 

Zone of Collignoniceras woollgari 

Collignoniceras woollgari (Mantell) is an ornate, 

somewhat evolute ammonite characterized by narrow, 

forwardly slanting ribs that bear umbilical bullae, inner 

and outer ventrolateral tubercles, and elongated siphonal 

tubercles (pl. 5, figs. 6-13). Most specimens are small, 

although the species is known to attain a fairly large size. 

Collignoniceras wool/gari (Mantell) was originally described from the Middle Chalk of England (Mantell, 

1822, p. 197, pl. 21, fig. 16; pl. 22, fig. 7). The species has 

a wide geographic distribution in rocks of early middle 

Turonian age; specimens have been recorded from 

Europe, Texas, the Western Interior of the United States 

and Canada, California, Oregon, Japan, and northern 

Australia. 

Two chronologie subspecies have been recognized 

in the Western Interior (Cobban and Hook, 1979). The 

older, C. woollgari woollgari, has in the adult stage 

secondary ribs as well as more siphonal tubercles than 

ventrolateral ones. The younger subspecies, C. woollgari 

regulare (Haas) (Haas, 1946, p.154, pl.16, figs.l-17; text 

figs. 10-12, 59-74, 78, 80, 81, 83), has more uniformly 

sized ribs and equal numbers of ventrolateral and siphonal tubercles. 

Both forms of C. wool/gari have been collected 

from the member of Emigrant Gap of the Frontier 

Formation at 23 localities in an east-west belt across 

central Natrona County. Specimens occur either in fairly 

coarse, crossbedded sandstone or in silty, calcareous 

concretions, where they may be abundant (pl. 5, fig. 9). 

Other molluscan fossils are not common but include the 

bivalves Inoceramus fragilis Hall and Meek (1856, p. 388, 

pl. 2, figs. 6a, b; Cobban, 1983, p. 3, pl. 1, fig. 2) and/. 

hercynicus Petrascheck (1903, p. 156, pl. 8, figs. 1-3; text 

fig. 1) (pl. 1, fig. 6); the ammonites Tragodesmoceras sp., 

Baculites yokoyamai Tokunaga and Shimizu (1926, 

p. 195, pl. 22, figs. 5a, b; pl. 26, fig. 11) (pl. 9, figs. 16-22), 

and Allocrioceras billinghursti Klinger (1976, p. 32, pl. 9, 

figs. 2a, b; text fig. 7b; Cobban and Kennedy, 1989, p. 173, 

figs. la-f); and the belemnite Actinocamax sp. (J eletzky, 

1961). 

A single specimen of C. woollgari was recently 

collected from a calcareous, sandy concretion from an 

unnamed unit below the Wall Creek Member at the 

northern edge of Natrona County (fig. 3, loc. 2). 

Zone of Prionocyclus percarinatus 

Prionocyclus is an ornate, somewhat evolute 

ammonite that has a finely notched keel, hence, the name 

prionos (Greek, saw) and kyklos (Greek, circle) or circular saw. The various species of Prionocyclus are 

important in the zonation of the middle and upper 

Turonian (fig. 1). 

Prionocyclus percarinatus (Hall and Meek), originally described as Ammonites percarinatus Hall and 

Meek (1856, p. 396, pl. 4, figs. 2a, b), is a common species 

in the Carlile Shale in an east-west trending belt that 

extends from the southern end of the Black Hills to 

southwestern Minnesota. The types and other specimens 

were illustrated by Cobban (1983, pl. 5, figs. 26-37). 

Prionocyclus percarinatus has not been found in 

Natrona County. 

Zone of Prionocycl us hyatti 

Prionocyclus hyatti (Stanton), of latest middle 

Turonian age, has both primary ribs with umbilical bullae 

and inner and outer ventrolateral tubercles, and shorter 

. and weaker secondary ribs with only ventrolateral tubercles (pl. 4, figs. 8, 9; pl. 5, figs. 3-5). The species was first 

described as Prionotropis hyatti Stanton (1893, p. 176, 

pl. 42, figs. 5-8) from rocks now assigned to the Codell 

Sandstone Member of the Carlile Shale of south-central 

Colorado. Prionocyclus hyatti has been found in silty, 

calcareous concretions in the member of Emigrant Gap 

at only three localities in the Casper area (fig. 3, 

locs. 22-24). Associated fossils are Inoceramus howelli 

White (1876, p. 114; 1879, p. 284, pl. 4, figs. la-c) (pl. 10, 

fig. 9) and the ammonites Placenticeras (Placenticeras) 

pseudoplacenta Hyatt (1903, p. 216) andPteroscaphites n. 

sp. 

Zone of Prionocyclus macoma 

This zone of early late Turonian age is unknown in 

Natrona County owing to the erosional contact at the 

Ammonites and Bivalves from the Frontier Formation, Wyoming 89 
base of the Wall Creek Member (fig. 1). The guide 

ammonite, Prionocyclus macombi Meek (1876a, p. 132, 

pl. 2, figs. 3a-d; Stanton, 1893, p. 172, pl. 41, figs. 1-5), is 

a slender species that has densely ribbed early whorls and 

almost smooth later whorls. The species is abundant in 

the basal beds of the Juana Lopez Member of the 

Mancos Shale in southwestern Colorado and northwestern New Mexico. Prionocyclus macombi has been 

found in the basal part of the Wall Creek Member in 

Carbon County south of Natrona County. 

Zone of Prionocyclus wyomingensis 

Prionocyclus wyomingensis Meek (1876b, p. 452) is 

a robust, moderately evolute, keeled species that has 

narrow ribs of irregular height of which the higher or 

stronger ones bear umbilical tubercles (double occasionally) as well as inner and outer ventrolateral tubercles of which the outer ones are very weak. The species 

was not illustrated by Meek, ~ut White (1883, pl. 15, 

fig. la-e) showed good sketches of the types. White's 

illustrations have been reproduced in several publications (for example, Stanton, 1893, pl. 40; Grabau and 

Shimer, 1910, figs. 1510a-d). Molluscan fossils usually 

found with P. wyomingensis are Inoceramus dimidius 

White (1877, p. 181, pl. 16, figs. 2a-d; Stanton, 1893, 

p. 78, pl. 10, figs. 5, 6), Lopha /ugubris (Conrad) (Conrad, 

1857, p. 156, pl. 10, figs. Sa, b; Stanton, 1893, pl. 4), and 

Scaphites warreni Meek and Hayden (1860, p. 177; Cobban, 1951, pl. 3, figs. 8-21). 

Prionocyc/us wyomingensis has not been found in 

Natrona County, but the bivalve Lopha lugubris (pl. 2, 

figs. 5-7) was discovered in the basal part of the Wall 

Creek Member of the · Frontier Formation in the 

southeastern part of the county (fig. 3, loc. 49). 

Zone of Scaphites whitfieldi 

The ammonite Scaphites whitfieldi Cobban (1951, 

p. 24, pl. 4, figs. 30-40; pi. 5, figs. 2-4 only) is typical of 

the scapliites in which the adult body chamber becomes 

partly straightened and uncoiled. Ornament of the body 

chamber consists· of closely and evenly spaced primary 

and secondary ribs. 

Scaphites whitfieldi is widely distributed in the 

Western Interior from southern ·Montana to southern 

New Mexico. Associated fossils are usually Inoceramus 

perplexus Whitfield (1880, p. 392, pl. 8, fig. 3; pl. 10, 

figs. 4, 5) (pl. 1, fig. 8; pl. 10, fig. 1 ), Prionocyclus 

novimexicanus (Marcou) (Marcou, 1858, p. 36, pl. 1, 

figs. 2, 2a), Prionocyclus wyomingensis var. elegans of 

Haas (1946, p. 200, pl. 19, figs. 1-7, 11-14; pl. 20, fig. 4; 

pl. 21, figs. 1-3, 5; pl. 22, figs. 1, 2; text figs. 98-104), and 

B10 Evolution of·Sedlmentary Basins-Powder River Basin 

Carota? cf. biplicata Stephenson (a gastropod identified 

by N.F. Sohl). The occurrence of I. perplexus in the Wall 

Creek Member at 12 localities in northern and 

southeastern Natrona County suggests that the zone of 

S. whitfieldi is probably present in the lower part of the 

member over much of the county. Prionocyclus novimexicanus was found with I. perplexus at seven of these 

localities. Other fossils are scarce but include an 

occasional cardiinid bivalve such as Pleuriocardia sp. 

(pl._ 3, fig. 5). 

Zone of Prionocyclus quadratus 

· The ammonite, Prionocyc/us quadratus Cobban 

(1953b, p. 354, pl. 48, figs. 1-8), is rather uncommon in 

the Western Interior. Many specimens referred to it are 

probably P. gennari (Reuss) (Reuss, 1845, p. 22, pl. 7, 

figs. lOa-c). Prionocyc/us quadratus has inner whorls that 

have closely spaced primary and secondary ribs of which 

the primary ones bear umbilical and inner and outer 

ventrolateral tubercles. Adult whorls have stout, squarish 

whorl sections and ornament of primary ribs with umbilical and inner ventrolateral tubercles as well as an 

incipient midflank tubercle. Prionocyclus gennari has 

more slender whorls and lacks the incipient midflank 

tubercle. 

The zone of P. quadratus, of latest Turonian age, is 

the most fossiliferous part of the Frontier Formation in 

Natrona County. Collections were made from the Wail 

Creek Member at 32localities scattered over much of the 

county. Small echinoids were found at three places (fig. 3, 

locs. 32, 37, 41). The largest collection of molluscan 

fossils came from calcareous sandstone concretions in 

the uppermost sandstone unit of the Wall Creek Member 

at the Emigrant Gap anticline (fig. 3, loc. 32). Here the 

following species were collected: 

Bivalves 

Nucula sp. 

Nuculana sp. 

Solemya obscura Stanton 

Modiolus sp. 

Pinna sp. 

Phelopteria sp. 

Inoceramus incertus Jimbo 

L cuvierifonnis Pergament 

Rhynchostreon sp. 

Syncyc/onema sp. 

Crassatel/a hendricksoni Henderson 

Pleuriocardia (Dochmocardia) aff. pauperculum 

(Meek) 

Tel/ina? isonema Meek 

Cymbophora utahensis (Meek) 

C. emmonsi (Meek) 

Latemula sp. 

Parapholas sphenoideus White 
Gastropods . 

Gyrodes depressa Meek 

G. conradi Meek 

Euspira? sp. 

Anisomyon apicalis Sidwell 

Ammonites 

Placenticeras (P.) pseudoplacenta Hyatt 

(pl. 4, figs. 13, 14) 

Prionocyclus gennari (Reuss) (pl. 7, figs. 1-11) 

Baculites yokoyamai Tokunaga and Shimizu 

B. aff. sweetgrassensis Cobban (pl. 3, figs. 10, 11) 

Eubostrychoceras matsumotoi Cobban (pl. 9, 

figs. 28-35) 

Scaphites c01vensis Cobban (pl. 4, figs. 3-7) 

Nautiloid 

Eutrephoceras sp. (pl. 4, figs. 11, 12) 

A variety of inoceramid species is present in the 

Wall Creek Member. The most abundant species, Inoceramus incertus Jimbo (1894, p. 43, pl. 8, fig. 7; Scott and 

others, 1986, fig. 6g), is a well-ornamented form that has 

two orders of concentric folds (pl. 10, figs. 2-5). Another 

abundant species, I. longealatus Troger (1967, p. 95, 

pl. 10, fig. 2; Scott and others, 1986, figs. a-e, i), has a 

long, straight hingeline that forms part of a conspicuous 

wing (pl. 10, figs. 6, 7). Other species include I. apicalis 

Woods (1912, p. 319, pl. 53, figs. 4-6) (pl. 1, figs. 1, 2), I. 

cuvieriformis Pergament (1971, p. 52, pl. 6, figs.1, 2; pl. 7, 

fig. 1) (pl. 10, fig. 8), and other forms (pl. 1, fig. 9). 

Ammonites are fairly common in the Wall Creek 

Member, and illustrations of several species are shown in 

plates 3, 4, 7, and 9. 

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Ammonites and Bivalves from the Frontier Formation, Wyoming 813 

f:r U.S. GOVERNMENT PAINTING OFFICE: 199Q-573-049/26,011 REGION NO.8 

PLATES 1-11 

Contact photographs of the plates in this report are available, at cost, 

from U.S. Geological Survey Photographic Library, Federal Center, 

Denver, Colorado 80225 
PLATE 1 

[All figures natural size] 

Figures 1, 2. Inoceramus cf. apicalis Woods (p. B11). 

USNM 443794, from the Wall Creek Member of the Frontier Formation at locality D9764 (text fig. 3, loc. 42). 

3. Inoceramus pictus Sowerby (p. B8). 

USNM 443795, from the Belle Fourche Member of the Frontier Formation at locality D6962 (text fig. 3, loc. 1). 

4, 5. Rhynchostreon sp. 

USNM 443796, from the Belle Fourche Member at locality D9335 (text fig. 3, loc. 38). 

6. Inoceramus hercynicus Petrascheck (p. B9). 

Latex cast ofUSNM 443797, from the member of Emigrant Gap of the Frontier Formation at locality D9749 (text 

fig. 3, loc. 25). 

7. Eucalycoceras templetonense Cobban (p. BS). 

USNM 443798, from the Belle Fourche Member at locality D89f8 (text fig. 3, Joe. 17). 

8. Inoceramus perplexus Whitfield (p. B10). 

USNM 443799, from the Wall Creek Member at locality D9121 (text fig. 3, Joe. 50). 

9. Inoceramus sp. (p. B11). 

Latex cast of USNM 443800, from the Wall Creek Member at locality D11796 (text fig. 3, loc. 46). 

10, 11. Plesiacanthoceras wyomingense (Reagan) (p. BS). 

USNM 443801, from the Belle Fourche Member at locality D9324 (text fig. 3, Joe. 18). 
U.S. GEOLOGICAL SURVEY BULLETIN 1917-B PLATE 1 

FOSSILS FROM THE FRONTIER FORMATION 
PLATE2 

[All figures natural size] 

Figures 1-3, 8, 9. Metoicoceras mosbyense Cobban (p. B8). 

1-3. USNM 443802, from the Belle Fourche Member of the Frontier Formation at locality D7383 (text 

fig. 3, loc. 13). 

8, 9. USNM 443803, from the Belle Fourche Member at locality D8470 (text fig. 3, loc. 11). 

4. Rhynchostreon sp. 

USNM 443804, from the Belle Fourche Member at locality D9365 (text fig. 3, loc. 39). 

5-7. Lopha lugubris (Conrad) (p. B10). 

5. USNM 443805, from the Wall Creek Member of the Frontier Formation at locality D9120 (text 

fig. 3, loc. 49). 

6, 7. USNM 443806, from the Wall Creek Member at locality D9120 (text fig. 3, loc. 49) 
PLATE 3 

(All figures natural size] 

Figures 1-4. Pycnodonte aff. kellumi (Jones) (p. B8). 

1, 2. USNM 443807, from the Belle Fourche Member of the Frontier Formation at locality 07382 (text fig. 

3, Joe. 14). 

3, 4. USNM 443808, from the Belle Fourche Member at locality 07385 (text fig. 3, loc. 12). 

5. Pleuriocardia sp. (p. B10). 

Latex cast of USNM 443809, from the Wall Creek Member of the Frontier Formation at locality 09119 (text 

fig. 3, loc. 48). 

6, 7. Acanthoceras amphibolum Morrow (p. B5). 

USNM 443810, from the Belle Fourche Member at locality D8471 (text fig. 3, loc. 10). 

8, 9. Metoicoceras frontierense Cobban (p. B7). 

Holotype USNM 376927, from the Belle Fourche Member at locality 22809 in the NE~ sec. 1, T. 43 N, R. 83 W., 

Johnson County, Wyo. 

10, 11. Baculites aff. sweetgrassensis Cobban (p. B11). 

USNM 443811, from the Wall Creek Member at locality D9116 (text fig. 3, Joe. 40). 

12, 13. Hamites salebrosus Cobban, Hook, and Kennedy (p. B8). 

USNM 443812, from the Belle Fourche Member at locality D6962 (text fig. 3, Joe. 1). 

14, 15. Calycoceras canitaurinum (Haas) (p. B5). 

USNM 443813, from the Belle Fourche Member at locality 08914 (text fig. 3, Joe. 44). 
U.S. GEOLOGICAL SURVEY 

BULLETIN 1917- B PLATE 3 

12 13 14 1 5 

FOSSILS FROM THE FRONTIER FORMATION 
PLATE 4 

[AU figures natural size] 

Figures 1, 2. Eucalycoceras templetonense Cobban (p. B5). 

USNM 443814, from the Belle Fourche Member of the Frontier Formation at locality D12547 (text fig. 3, Joe. 28). 

3-7. Scaphites corvensis Cobban (p. Bll). 

3-5. USNM 443815, from the Wall Creek Member of the Frontier Formation at locality D9118 (text fig. 3, 

Joe. 32). 

6, 7. USNM 443816, from the Wall Creek Member at locality D9358 (text fig. 3, Joe. 35). 

8, 9. Prionocyclus hyatti (Stanton) (p. B9). 

USNM 443818, from the member of Emigrant Gap of the Frontier Formation at locality D9766 (text fig. 3, 

Joe. 24). 

10. Acanthoceras amphibolum Morrow (p. B5). 

USNM 443817, from the Belle Fourche Member at locality D8471 (text fig. 3, Joe. 10). 

11, 12. Eutrephoceras sp. (p. Bll). 

USNM 443819, from the Wall Creek Member at locality D9116 (text fig. 3, loc. 40). 

13, 14. Placenticeras (P.) pseudoplacenta Hyatt (p. Bll). 

USNM 443820, from the Wall Creek Member at locality D9358 (text fig. 3, loc. 35). 
U.S. GEOLOGICAL SU RVEY BULLETIN 1917- B PLATE 4 

3 

6 

1 1 

12 13 14 

FOSSILS FROM THE FRONTIER FORMATION 
PLATE 5 

[All figures natural size] 

Figures 1, 2. Plesiacanthoceras wyomingense (Reagan) (p. BS). 

USNM 443821, from the Belle Fourche Member of the Frontier Formation at locality D9324 (text fig. 3, loc. 18). 

3-5. Prionocyclus hyatti (Stanton) (p. B9). 

USNM 443822, from the member of Emigrant Gap of the Frontier Formation at locality D9877 (text fig. 3, 

loc. 22). 

6-13. Collignoniceras woollgari (Mantell) (p. B9). 

6. USNM 443823, from the member of Emigrant Gap at locality D9746 (text fig. 3, loc. 31). 

7, 8. USNM 443824, from the member of Emigrant Gap at locality D10522 (text fig. 3, loc. 34). 

9. USNM 443825, from the member of Emigrant Gap at locality D9746 (text fig. 3, loc. 31). 

10. USNM 443826, from the member of Emigrant Gap at locality D9325 (text fig. 3, loc. 19). 

11. Latex cast of USNM 443827, from the member of Emigrant Gap at locality D9333 (text 

fig. 3, loc. 36). 

12, 13. USNM 443828, from the member of Emigrant Gap at locality D12645 (text fig. 3, loc. 33). 
US GEOLOGICAL SURVEY 

BULLETIN 1917-B PLATE 5 

5 

3 

2 

8 

12 

FOSSILS FROM THE FRONTIER FORMATION 
PLATE 6 

[Figure natural size] 

Dunveganoceras pondi Haas (p. BS) 

USNM 443829, from the Belle Fourche Member of the Frontier Formation at locality D9747 (text fig. 3, loc. 27). See 

plate 7, figure 12, for another view. 
U.S. GEOLOGICAL SURVEY BULLETIN 1917-B PLATE 6 

DUNVEGANOCERAS POND/ 
PLATE 7 

[All figures natural size] 

Figures 1-11. Prionocyclus gennari (Reuss) (p. B11). 

USNM 443830--443835, from the Wall Creek Member of the Frontier Formation at locality D9118 (text fig. 3, 

loc. 32). Figure 7 is a latex cast. 

12. Dunveganoceras pondi Haas (p. BS). 

USNM 443829, from the Belle Fourche Member of the Frontier Formation at locality D9747 (text fig. 3, loc. 27). 

See plate 6 for lateral view. 
BULLETIN 1917-B PLATE 7 

U.S. GEOLOGICAL SURVEY 

7 

11 

PRIONOCYCLUS AND DUNVEGANOCERAS 
PLATE 8 

(All figures natural size] 

Figures 1, 2. Dunveganoceras conditum Haas (p. B8). 

USNM 443836, from the Belle Fourche Member of the Frontier Formation at locality 08470 (text fig. 3, loc. 11). 

3. Dunveganoceras problematicum natronense Cobban (p. B7). 

Holotype USNM 376921, from the Belle Fourche Member at locality 09337 (text fig. 3, loc. 29). 
U.S. GEOLOGICAL SURVEY BULLETIN 1917-B PLATE 8 

3 

DUNVEGANOCERAS 
PLATE 9 

[All figures natural size] 

Figures 1-3. Allocrioceras annulatum (Shumard) (p. B8). 

USNM 443837, from the Belle Fourche Member of the Frontier Formation at locality D3117 (text fig. 3, loc. 43). 

4-15. Sciponoceras gracile (Shumard) (p. B8). 

From the Belle Fourche Member at locality D3117 (text fig. 3, loc. 43). 

4-6. USNM 443838. 

7-9. USNM 443839. 

10-12. USNM 443840. 

13-15. USNM 443841. 

16--22. Baculites yokoyamai Tokunaga and Shimizu (p. B9). 

16--18. USNM 443842, from the Wall Creek Member of the Frontier Formation at locality D9118 (text fig. 

3, loc. 32). 

19, 20. USNM 443843, from the Wall Creek Member at locality D9742 (text fig. 3, loc. 37). 

21, 22. USNM 443844, from the Wall Creek Member at locality D9118 (text fig. 3, loc. 32). Specimen has a 

complete adult aperture. 

23, 24. Borissiakoceras orbiculatum Stephenson (p. B8). 

USNM 443845, from the Belle Fourche Member at locality D3117 (text fig. 3, loc. 43). 

25. Tan-antoceras flexicostatum Cobban (p. B7). 

USNM 443846, from the Belle Fourche Member at locality D8714 (text fig. 3, Joe. 44). 

26, 27. Metoicoceras frontierense Cobban (p. B7). 

USNM 443847, from the Belle Fourche Member at locality D9337 (text fig. 3, loc. 29). 

28-35. Eubostrychoceras matsumotoi Cobban (p. B11 ). 

28, 29. Paratype USNM 395816, from the Wall Creek Member at locality D9118 (text fig. 3, loc. 32). 

30-33. Paratype USNM 395810, from the Wall Creek Member at locality D9118 (text fig. 3, loc. 32). 

34, 35. Holotype USNM 395808, from the Wall Creek Member at locality D9359 (text fig. 3, loc. 30). 
U.S. GEOLOGICAL SU RVEY BULLETIN 1917-B PLATE 9 

2 3 4 5 6 

7 8 9 

10 1 1 12 

13 14 15 

21 

16 17 18 

19 20 

22 

28 

25 

27 

31 

33 35 

FOSSILS FROM THE FRONTIER FORMATION 
PLATE 10 

(All figures natural size] 

Figure 1. Inoceramus perplexus Whitfield (p. B10). 

USNM 443848, from the Wall Creek Member of the Frontier Formation at locality D9833 (text fig. 3, loc. 5). 

2-5. Inoceramus incertus Jimbo (p. Bll). 

From the Wall Creek Member at locality D9118 (text fig. 3, Joe. 32). 

2. USNM 443849. 

3. USNM 443850. 

4. USNM 443851. 

5. USNM 443852. 

6, 7. Inoceramus longealatus Troger (p. B9, Bll) 

6. USNM 443853, from the Wall Creek Member at locality D9787 (text fig. 3, loc. 21). 

7. USNM 443854, from the Wall Creek Member at locality D9116 (text fig. 3, loc. 40). 

8. Inoceramus cuvierifonnis Pergament (p. Bll) 

USNM 443855, from the Wall Creek Member at locality D9116 (text fig. 3, loc. 40). 

9. Inoceramus howelli White (p. B9). 

USNM 443856, from the member of Emigrant Gap of the Frontier Formation at locality D9878 (text fig. 3, loc. 23). 

10-12. Inoceramus ginterensis Pergament (p. B8). 

10. USNM 443857, from the Belle Fourche Member of the Frontier Formation at locality D3117 (text fig. 3, 

loc. 43). 

11. USNM 443858, from the Belle Fourche Member at locality D8470 (text fig. 3, loc. 11). 

12. USNM 443859, from the Belle Fourche Member at locality D6962 (text fig. 3, loc. 1). 
U.S. GEOLOGICAL SURVEY BULLETIN 191 7-B PLATE 10 

1 1 

INOCERAMUS 
PLATE 11 

[Figure natural size] 

Dunveganoceras conditum Haas (p. B8). 

USNM 443860, from the Belle Fourche Member of the Frontier Formation at locality D9831 (text fig. 3, loc. 9). 
U.S. GEOLOGICAL SURVEY BULLETIN 1917- B PLATE 11 

DUNVEGANOCERAS 

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