Cranial Morphology 

of Some Oligocene

/

Artiodactyla

GEOLOGICAL SURVEY PROFESSIONAL PAPER 243-H
Cranial Morphology 

of Some Oligocene 

Artiodactyla

By FRANK C. WHITMORE, JR.

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 1952, PAGES 117-160

GEOLOGICAL SURVEY PROFESSIONAL PAPER 243-H

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1953
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 35 cents (paper cover)
CONTENTS

Page

Abstract_--_-------___________-__________________ 117

Introduction _______________________________________ 117

Acknowledgments_ ____________________________ 117

Cranial morphology of some Oligocene Artiodactyla____- 118

Cranial morphology of Merycoidodon ______________ 118

Bones of the skull___.__________________ 118

Occipital.__________________________ 118

Basisphenoid_ ________________________ 120

Alisphenoid_ __________________________ 123

Presphenoid_ _________________________ 123

Orbitosphenoid— _ ______________________ 124

Ethmoid and turbinals________________ 124

Vomer__________________________ 126

Preinterparietal and interparietal— _______ 126

Parietal.. ______________________ 126

Frontal—_____________________ 126

SquamosaL ____________________________ 127

Tympanic region. __________________________ 131

Pars petrosa of the periotic bone________ 131

Middle ear and surrounding structures- - _ _ 132

External auditory meatus______________ 135

Pars mastoidea of the periotic bone _______ 135

Pneumatic sinuses__________________________ 136

Circulatory system of the skull-______________ 137

Arteries _______________________________ 137

Veins and venous sinuses._______________ 138

Dorsal sinus system_________________ 138

Basilar sinus system_______________ 139

Subsphenoid veins and sinus venosus

ossis sphenoidalis_________________ 140

Nerves of the skull region__________________ 141

Cranial morphology of Poebrotherium______________ 141

Bones of the skull_________________________ 141

Tympanic region.__________________________ 144

Pars petrosa of the periotic bone________ 144

Middle ear and surrounding structures. _ _ _ 145

External auditory meatus______________ 146

Pars mastoidea of the periotic bone _______ 146

Pneumatic sinuses__________________________ 146

Circulatory system of the skull— _____________ 147

Arteries.,_____________________________ 147

Cranial morphology of some Oligocene Artiodactyla—Con. 

Cranial morphology of Poebrotherium—Continued 

Circulatory system of the skull—Continued 

Veins and venous sinuses____----___---_-

Dorsal sinus system_______-___-_-.__

Basilar sinus system——____________

Sinus venosus ossis sphenoidalis- _ 

Nerves of the skull region.— ____-----------_

Cranial morphology of Leptomeryx. _______________

Bones of the skull_________________________

Tympanic region____________--_------_--_--

Pars petrosa of the periotic bone_____.__._

Middle ear and surrounding structures 

External auditory meatus— _____________

Pars mastoidea of the periotic bone.______

Pneumatic sinuses__________________________

Circulatory system of the skull____---____-_--

Arteries ______________-____---___------

Veins____________________---___----__

Dorsal sinus system_________________

Basilar sinus system———____________

Sinus venosus ossis sphenoidalis-_____

Morphological conclusions______-_____---_---_----_--

Bones of the skull_____________________________

Primitive characteristics_____________________

Characteristics of doubtful significance__ ______

Tympanic region. ______________________________

Primitive characteristics_____________________

Characteristics of doubtful significance. _______

Pneumatic sinuses._____________________________

Primitive characteristics.____________________

Arteries.-________--_____-----_--------------_-

Primitive characteristics _____________________

Veins and venous sinuses--__-_--_-------_-------

Primitive characteristics.____________________

Anatomical peculiarities indicating habits, evolutionary trends, relationships.----------------------

Phylogenetic conclusions___________________________

Tylopoda._______________________-_--_--__---

Mery coidodontidae. _____________ _______________

Leptomerycinae. ______________---___-----_----_

Summary of phylogenetic conclusions _____________

Selected bibliography.-________-_---_---------_-----_

Index.______________________---___------_--_-----_

ILLUSTRATIONS

FIGURE 14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

Merycoidodon culbertsonii. Lateral view of skull__-____________________-_---_____---_-_-

M. culbertsonii. Ventral view of basis cranii.__________________________________________

M. culbertsonii. Internal view of basis cranii, from above________________-_-___-__------

M. culbertsonii. Ventral view of basis cranii, with restoration of blood vessels..____________

Frontal sections of the skulls of M. culbertsonii and Dicotyles labiatus... ______-_____--__-_-

M. culbertsonii. Thick section of skull, viewed posteriorly into cerebellar fossa.____________

Thick section of skull in region of auditory bulla___________.______--_---

Thick section of skull at level of post-glenoid processes._._________---_---

Thick section of skull, looking anteriorly into olfactory lobes_______--__-_-

Thick section of skull, looking anteriorly through nasal cavity___________

Same as figure 1, with nasal cavity and pneumatic sinuses outlined.. ______-

Poebrotherium wilsoni. Lateral view of skull____________________-___-_____-_______-_---

P. wilsoni. Ventral view of basis cranii.______________________________________________

P. wilsoni. Thick section of skull, looking posteriorly from level of vagina processus hyoidei. 

P. wilsoni. Thick section of skull, looking anteriorly into olfactory lobes__________________

Leptomeryx evansi. Lateral view of skull_________________________-__-_-______-___-_-_-

L. evansi. Ventral view of basis cranii______________________________._________________

L. evansi. Thick section of skull, looking anteriorly from region of bulla._____-___-__-_-__-

M. culbertsonii. 

M. culbertsonii. 

M. culbertsonii. 

M. culbertsonii. 

M. culbertsonii.

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CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA

By FRANK C. WHITMOKB, JR.

ABSTRACT

A study of the cranial morphology of three Oligocene Artiodactyla (Merycoidodon, Poebrotherium, and Leptomeryx), 

based largely on serial sections, reveals many cranial characteristics found only in the most primitive Recent mammals.

The Oligocene genera had several cranial veins which indicate 

that these artiodactyls were not far removed from a primitive 

insectivore-like ancestor. They had a well-developed internal 

carotid artery in contrast to the Recent Artiodactyla, very few 

of which retain this structure. The cranial circulation pattern 

of the Oligocene forms shows an evolutionary tendency in the 

Artiodactyla to abandon endocranial for extra-cranial paths.

Other primitive characteristics are the lateral partitions of 

the pituitary fossa (representing the primitive side wall of the 

skull) and large temporal venous sinus in Merycoidodon, and 

the simple pneumatic sinus system in all three genera.

The selenodont Artiodactyla exhibit an evolutioaary trend 

toward the amastoid skull pattern now found only in bunodont 

families.

The subarcuate fossa of the pars petrosa of the periotic bone 

in the Tylopoda has a peculiar shape, constant in the suborder 

and differing from the condition in other selenodont Artiodactyla. This is added evidence that the Tylopoda must have 

separated from other artiodactyl lines of evolution no later than 

the middle Eocene. The simpler subarcuate fossa of Merycoidodon and Leptomeryx indicates their alliaace with the suborder 

Pecora.

On the basis of cranial structure, the family Hypertragulidae 

is divided into the subfamilies Hypertragulinae and Leptomerycinae, both of which existed from the early Oligocene through 

the early Miocene.

INTRODUCTION

The Artiodactyla, or "even-toed ungulates", form 

one of the largest and most varied of the orders of mammals. They are today near the acme of their development ; their distribution is world-wide and they include 

in their numbers the pigs, peccaries, hippopotami, 

camels, deer, giraffes, cattle, sheep, goats, antelope and 

the tiny chevrotains of Asia and Africa.

An examination of the Artiodactyla reveals several 

characteristics in common: hoofs, a more or less herbivorous diet, and a tendency toward a foot structure 

in which the weight rests mainly upon two toes of each 

foot. This mesaxonic type of foot, whose axis runs 

between the third and fourth toes, has given rise to the 

name of the order. Two other morphological peculiarities possessed by all the Artiodactyla are not externally 

visible but are very important. The first of these is 

the absence of the third trochanter of the femur, an

eminence on the posterior side of the bone which selves 

as a muscle attachment in some mammals. The second 

and even more useful criterion is the presence upon the 

astragalus of a distal roller surface (for articulation 

with the navicular and cuboid bones) in addition tc the 

proximal roller surface present in other mammals.

Many years of study by zoologists have clarified the 

relationships among the many living artiodactyl 

groups, diverse as they are; but paleontology har revealed a tangle of problems in the many extinct artiodactyl genera. The relationships of the extinct 

genera with each other and with modern types are often 

only a matter of conjecture; their ancestry is unknown. 

Therefore many uncertainties exist concerning the 

stratigraphic and paleoecologic significance of many 

extinct genera and species. Their morphology if, of 

course, less well known than that of the modern Artiodactyla. It is the purpose of this study to examir e in 

detail the cranial anatomy of some of these extinct 

genera, because endocranial characteristics are p^obably nonadaptive, that is, unlikely to be influenced by 

the environment, and therefore useful in determining 

the taxonomic position of groups of animals. 

These studies were made with the aid of serial sections, 

a technique used by several workers in the study of 

fossil skulls (Darrah, 1936; Dunkle, 1940; Gral *m, 

1933; Eomer, 1937; Simpson, 1933, 1936; Sollas, 1916, 

1920; Sollas and Sollas, 1914; Walton, 1928). The 

skull can be oriented in relation to the grinding surface 

to produce sections in any desired direction; the most 

common types of sections used are the sagittal (parallel 

to the plane of bilateral symmetry of the skull) anc1 the 

frontal (at right angles to this plane). The studies 

presented here are based upon frontal sections. The 

sectioning technique used has been described in a previous paper (Olsen and Whitmore, 1944).

ACKNOWLEDGMENTS

The author is especially indebted to Professor A.. S. 

Romer of Harvard University, at whose suggestion this 

research was undertaken, for his assistance and encouragement in every phase of the work.

Many thanks are also due to the late Professor 

Thomas Barbour of the Museum of Comparative Zoology, Harvard University; Dr. Childs Frick, Dr. GK G.

117
118 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

Simpson, and Dr. E. H. Colbert of the American 

Museum of Natural History; Dr. E. C. Olson of the 

University of Chicago; Dr. Glenn L. Jepsen of Princeton University, and Mr. J. Leroy Kay of the Carnegie 

Museum, for the loan of specimens for comparison.

Mr. F. Eussell Olsen, of the Museum of Comparative 

Zoology, Harvard University, has by his skill made 

possible the construction of the apparatus used in this 

work.

Apparatus used in preparing some of the specimens 

for study was purchased by a grant from the Elizabeth 

Thompson Science Fund of Harvard University.

CRANIAL MORPHOLOGY OF SOME OLIGOCENE 

ARTIODACTYLA

Because the serial-section method destroys the specimens used, it was decided to investigate genera of 

which there are abundant specimens in paleontologic 

collections. The White River group of Oligocene age 

has yielded rich collections of mammals for many years, 

and it was found possible to obtain adequate specimens 

on which to base a study of the cranial anatomy of the 

artiodactyls, using three genera representing three different families.

The first of the genera, and that studied in greatest 

detail, is Merycoidodon, a member of the extinct family 

Merycoidodontidae (Oreodontidae), the "ruminating 

hogs" of Joseph Leidy. Merycoidodon is the most 

abundant mammalian genus in Tertiary deposits. The 

second genus studied is Poebrotherium (of the Camelidae) an ancestor of the modern camels; and the third 

is Leptomeryce, of the family Hypertragulidae, which 

are diminutive, hornless, deerlike animals.

These three families comprised most of the selenodont population of the Oligocene epoch of North 

America; besides it was felt that they would reveal 

the stage of cranial evolution reached by the selenodont 

Artiodactyla in Oligocene time.

CRANIAL MORPHOLOGY OF MERYCOIDODON

This study is based primarily upon serial frontal 

sections of a skull of Merycoidodon culbvrtsonii 

(Leidy), M. C. Z. 6450, from the White Kiver group 

(Oligocene) of northeastern Wyoming (figs. 14, 15).

Sections were made at 2mm intervals through most 

of the skull. Posterior to the foramen ovale (fig. 15), 

an interval of 0.5mm was used because of tie complexity of the structures in this region.

In the following description, special attention is paid 

to those parts of the cranial bones which do not appear 

upon the surface of the skull, to the relations among 

those bones, and to the impressions left upon them by 

blood vessels and nerves. The external characteristics 

of the skull, as shown in figures 14 and 15, furnish 

points of reference in locating the features of the 

internal anatomy.

The brains of Merycoidodon, and also those of Poebrotherium and Leptomeryso, have already been described in the basis of natural casts (Black, 1921; 

Tilney, 1931; Moodie, 1922; Bruce, 1883), and are not 

discussed in the present study.

BONES OP THE SKULL

OCCIPITAL

The occipital bone contains, or is bounded by, several foramina for the passage of blood vessels and

EF PT

PRO

5 INCHES

FIGURE 14. — Merycoidodon culbertsonii (Leidy). Lateral view, about X Modified after W. B. Scott. Part of zygomatic arch removed.

AS, alisphenoid bone.

ASC, anterior opening of sinus canal.

EF, ethmoidal foramen.

FLA, foramen lacerum anterius.

FM, mastoid foramen.

FPP, postparietal foramen.

PR, frontal bone.

F8A, anterior subsphenoid foramen.

IF, infraorbital foramen.

L, lacrimal bone.

LG, lambdoid crest.

MAE, external auditory meatus.

MX, maxilla.

N, nasal bone.

NG, nuchal crest.

OG, occipital bone.

OS, orbitosphenoid bone. 

PA, parietal bone. 

PAL, palatine bone. 

PMX, premaxilla. 

PPO, paroccipital process. 

PT, pterygoid bone. 

SQ, squamosal bone.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 119

nerves. The mastoid foramen (fig. 14, FM) is the exit 

of a canal which, passing posteriorly through the occipital bone, carries the mastoid emissary vein. This 

vessel is one of several draining blood from the great 

system of sinuses of the squamosal bone and the neighboring surface of the dura mater of the brain; it drains 

blood from the transverse venous sinus of the dura 

mater to the occipital vein.

Another vessel draining venous blood from the 

cranium, in this instance from the base of the brain,

ran in the petro-basilar canal (figs. 15,21, PEG). This 

vessel was a large one, the inferior petrosal sinus. TTie 

venous blood flowed posteriorly into it from, the cavernous sinus of the base of the brain. The sinus left the 

petro-basilar canal and extended posteriorly in a groove 

near the lateral edge of the basioccipital. Before leaving the petro-basilar canal, however, the inferior petrosal sinus gave off a branch, the condyloid vein, which 

extended posteriorly in a groove on the endocranial side 

of the basioccipital. It left the skull through the

AS

CF

occ

I INCH

FIGURE 15.—Mervcoidodon culbertsonii (Leidy), M. C. Z. 5808. Ventral view of basts cranii. Zygomatfc arch missing.

AS, alisphenoid bone.

B, tympanic bulla.

BO, basioccipital bone.

BS, basisphenold bone.

OF, condylar foramen.

OPO, opening of craniopharyngeal

canal.

FLA, foramen lacerum anterins. 

FLM. foramen lacerum medius.

FLP, foramen lacerum posterius.

FO, foramen ovale.

F8A, anterior subsphenoid foramina.

FSM, stylomastoid foramen.

FG, nssura glaseri.

OGO, occipital condyles.

OTT, ostium tympanicum tubae.

PEG, petrobasilar canal.

PET, pars petrosa of periotic bone.

PN, palatonarial border.

PS. presphenoid bone.

PTP, posttympanic process of squa

mosal bone. 

8Q, squamosal bone (zygomatic arch

broken). 

TG, opening of temporal canal.
120 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 

FLM FO

TVS

I INCH

FIGURE 16.—Merycoidodon culbertsonii (Leidy), M. C. Z. 6447. Internal view of basis cranii, viewed from above,

posteriorly and to the right.

A8G, ridge containing anterior semicircular canal. 

OF, condylar foramen. 

CR, chiasma ridge. 

D, diploe in squamosal bone. 

EG, ethmoidal crest. 

FLA, foramen lacerum anterius. 

FLM, foramen lacerum medius. 

FLP, foramen lacerum posterius. 

FO, foramen ovale.

FSP, posterior subsphenoid foramen.

IOA, groove for internal carotid artery.

MAI, Internal auditory meatus.

MM A, middle meningeal artery.

MY, myelencephalic base.

OL, olfactory lobe.

POP, posterior clinoid processes

(broken).

PET, pars petrosa of periotic. 

PF, pituitary fossa.

PL, floor of pyriform lobe.

PSG, posterior opening of sinus canal.

8F, subarcuate fossa.

TVS, temporal venous sinus.

VCC, groove for vena collaterals

cerebri.

II, opening of canal for optic nerver. 

V etc., groove for nerves III, IV, "

(1, 2) and VI.

condylar foramen (figs. 15,16, OF) and extended posteriorly as the occipital vein. The twelfth cranial nerve 

also found its exit through the condylar foramen.

A few millimeters antero-lateral to the condylar 

foramen is the foramen lacerum posterius (fig. 15, 

FLP) , from which emerged the inferior cerebral vein, 

one of the posterior branches of the inferior petrosal 

sinus, and a tributary of the internal jugular vein (fig. 

17).

The ninth, tenth and eleventh cranial nerves also 

passed through the foramen lacerum posterius.

BASISPHENOID

The basisphenoid bone is much thicker dorsoventrally than is the basioccipital. The result of this 

is that, while the basicranial axis is horizontal when 

viewed from below, the brain itself shows a sharp 

ventral bend at the level of the posterior clinoid processes (fig. 16, PCP) , which arise from the posterior end 

of the pituitary fossa. Thus the bases of the pyriform 

lobes of the brain and of the pituitary fossa (fig. 16, 

PZ, PF) are in essentially the same plane, while the 

myelencephalic base dips posteriorly at an angle of 

about 20° from the plane of the base of the cerebral 

hemispheres. This is a marked difference from any 

modern ungulates, in which the base of the brain slants

gently backward in essentially one plane from the optic 

nerves to the foramen magnum.

The foramen lacerum medius, whose anterior border 

is formed by the basisphenoid, transmitted blood both 

to and from the cranial cavity. The internrl carotid 

artery, the chief blood vessel nourishing the brain, 

entered the cranium here after having passed through 

the auditory bulla on its way from the common carotid 

artery. Inside the skull, this artery traveled anteriorly 

in a groove in the basisphenoid bone, immediately lateral to the pituitary fossa (fig. 22, IOA) . At the anterior end of the basisphenoid, at the level of th^ chiasma 

ridge (fig. 16, CR)^ the artery branched into smaller 

arteries that traversed the surface of the cerebral 

hemispheres. The grooves for these arteries were observed in the dissected braincase shown in figure 16, 

but not in the sections, because they lie abcut in the 

plane of sectioning. Posterior to the pituitary fossa 

is a groove in which ran the posterior communicating 

artery, which connected the internal carotid s.rteries of 

both sides.

In the same groove as the internal carotid artery 

(fig. 16, 1C A) ran the cavernous sinus, a venous vessel. 

The cavernous sinus received venous blood from the 

region of the face via the orbital venous plerus. This 

blood entered the cranium through the foramen lacerum
CRANIAL MORPHOLOGY OF SOME OLIGOCBNE ARTIODACTYLA 121

svos

MMA

ICV

IJV

I INCH

FIGUEB 17.—Merycoldodon cullertsonii (Leidy), M. C. Z. 5808. Veatral view of basis cranii, with restoration of blood vessels.

0V, condyloid vein.

EGA, external carotid artery.

EJV, external jugular vein.

FLM, foramen lacerum medius.

FLP, foramen lacerum posterius.

FO, foramen ovale.

FSA, anterior subsphenoid foramen.

FSP, posterior subsphenoid foramen.

I A, infraorbital artery. 

IOA. internal carotid artery. 

IGv, inferior cerebral vein. 

IJV, internal jugular vein. 

IMA, internal maxillary artery. 

IPS, inferior petrosal sinus. 

MMA, middle meningeal artery. 

OF, ophthalmic vein.

PBO, petrobasilar canal.

PTC, opening of posterior branch of

temporal canal. 

SOVj superior cerebral vein. 

8PA, sphenopalatine artery. 

S80, subsphenoid canal. 

SVOS, sinus venosus ossis sphenoidalis. 

TO, foramen jugulare spurium.

anterius (figs. 15, 16, FLA). After making its exit 

through the foramen lacerum medius, it passed posteriorly into the inferior petrosal sinus (see above). 

Posterior to the pituitary fossa, the cavernous sinuses 

of the two sides were connected by the intercavernous 

sinus, running in the same groove as the intercarotid 

artery.

238435—53———2

The pituitary fossa (sella turcica), which is surrounded on three sides by grooves for blood vessels, is 

an extremely shallow depression in the cranial surface 

of the basisphenoid bone (fig. 16, PF). It thus resembles that of the modern Tylopoda and differs from 

that of the Bovidae, which is very deep. This lack of 

excavation of the basiphenoid bone is compensated,
122 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

however, by the presence of a vertical lamina of bone 

upon either side of the sella turcica. These laminae 

are of delicate construction and cannot be found in 

fragmentary skulls or natural brain-casts. They are 

well shown, however, by serial sections (fig. 22, LOP). 

At their highest point they are 4 mm. high. Their 

antero-posterior length is about 10 mm. Posteriorly 

they decrease gradually in height until the level of the 

braincase floor is reached; anteriorly they are continuous with the posterior root of the orbitosphenoid bone 

(fig. 22, POS). No structures of this sort are found in 

living Artiodactyla; the nearest parallel to them occurs 

in the embryonic stages in Lepus, in which the posterior 

clinoid processes extend along the sides of the sella 

turcica in the same way (Voit, 1909, p. 549). Voit 

agrees with Gaupp (1902,1906) that these lateral laminae represent the taenia interclinoidalis of the chondrocranium, and thus are a remnant of the primitive lateral 

wall of the braincase. This thesis is supported by 

deBeer (1937, p. 388).

In many recent mammals (Camis, among others) the 

taenia interclinoidalis is represented by the anterior and 

posterior clinoid processes. The condition observed in 

Merycoidodon, in which these two vestigial processes 

are joined, is an earlier stage in the reduction of the 

primitive lateral wall of the skull.

Just lateral to the groove for the internal carotid 

artery and the cavernous sinus is a deeper groove, which 

runs from the region of the foramen ovale to the 

foramen lacerum anterius. It served as a course for the 

forward passage of the oculomotor (III), trochlear 

(IV), abducens (VI), ophthalmic (Vi), and maxillary 

(V2 ) nerves, which originate on the base of the brain 

posterior to the pituitary fossa. The third branch of 

the trigeminal nerve passed out of the skull through 

the foramen ovale (fig. 15, FO).

Within the foramen lacerum medius, and perforating its anterior (basisphenoid) wall, careful examination reveals a small foramen, which we shall term the 

posterior subsphenoid foramen (fig. 16, FSP). The 

serial sections (fig. 22) show that these foramina open 

into canals (SSG) that extend forward and medially 

in the body of the basisphenoid bone about 6 mm. medial to the foramen ovale (fig. 17, SSC). Below and 

behind the pituitary fossa (directly ventral to the posterior clinoid processes) and about 6 mm. from the 

foramen lacerum medius the two canals meet in the 

midline to form a cavity in the thickest part of the body 

of the basisphenoid (fig. 17, SVOS). This cavity is 

approximately 6 mm. long, 8 mm. wide, and 3 mm. deep 

in its deepest part.

From its anterior corners spring two canals which 

diverge sharply and, about 6 mm from the cavity,

emerge into the infratemporal fossa, one on eith er side 

of the basisphenoid bone, through a foramen s;tuated 

from 2 to 5 mm posterior to the foramen lacenm anterius. In some specimens one of the canals divides 

again just before its emergence into the infrateraporal 

fossa, with the result that two foramina are present 

between the foramen lacerum anterius and the foramen 

ovale.

These canals have never before been observed in 

any Artiodactyla, and they are beyond doubt absent 

in all living members of the order. A large cav: ty was 

noted in the basisphenoid bone of one sagittally sectioned skull of Sus scrofa, but it was in no way connected with the exterior. The canals were first observed in serial sections of the skull of Merycoidodon 

ouZbertsonii; then the basisphenoid bones of thre°, other 

specimens were dissected, and the canals were found 

in all (fig. 17 SSC}. Skulls of M&rycoidodon gracilis, 

the smallest species of the genus, were too rare, in the 

Museum of Comparative Zoology collections to be 

dissected; examination of all available specimens, however, showed the presence of foramina which are undoubtedly the anterior openings of these canal?. External examination of other skulls of Merycwdodontidae in the Museum collections revealed the presence 

of the anterior foramina in F^rotoreodon (late Eocene), 

their doubtful presence in one of five specimens examined of Oyvlopidius (Miocene) and their undoubted 

absence in Merycoides (early and middle Miocene), 

Promerycochoerus (late Oligocene and early Miocene), 

Mesoreodon (early Miocene) and Eporeod&n (middle 

Oligocene to early Miocene). Conclusions reached on 

the basis of such superficial examination are, of course, 

questionable. The evidence in this case, however, suggests that such canals are present in the basisphenoid 

bone only in the more primitive Merycoidodontidae.

Because of the absence of such structures in modern 

artiodactyls and the lack of knowledge of such details 

in fossil forms, it was necessary to seek their explanation in the cranial anatomy of other modern mammals. 

It was then found that similar canals in the body of the 

basisphenoid bone occur in Lepus (Arai, 1907), in some 

Eodentia (Bovero, 1905) and in Marsupialia and Insectivora (Gregory, 1910). The close correspondence 

between the bony canals in these groups and those 

observed in Merycoidodon makes it apparent tl at they 

are for the transmission of veins.

The anterior foramen (Arai called it "foramen 

venosum", but anterior subsphenoid foramen seems a 

more specific term) receives a vein from the orbital 

venous plexus. The blood travels posteriorly in the 

subsphenoid canals (a term used by both Arai and 

Bovero) until it reaches the sinus venosus ossis sphenoidalis (again a term introduced by Arai), which
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ABTIODACTYLA 123

occupies the above-mentioned cavity in the body of the 

basisphenoid. Posterior to this the subsphenoid canals 

resume their course postero-laterally, debouching 

through the posterior subsphenoid foramen (fig. 17, 

FSP). They empty their venous blood into the posterior end of the cavernous sinus, whence it flows posteriorly into the inferior petrosal sinus.

The subsphenoid canals will be further discussed 

below, in connection with the venous circulation in 

Merycoidodon.

The foramen rotundum is absent in Merycoidodon. 

As mentioned above, the maxillary nerve undoubtedly 

found its exit through the foramen lacerum anterius, 

its usual course in the absence of the foramen rotundum.

Thorpe (1931), in discussing the osteology of Eporeodon, states that the foramen rotundum is always present in Merycoidodon ; this he points out as an expression 

of the more primitive skull structure of the latter genus. 

Thorpe's foramen rotundum of Merycoidodon is actually the anterior subsphenoid foramen, mentioned 

above. Identification of the foramen rotundum as a 

primitive feature is probably unwise in view of Gregory's observations (1910), which seem to indicate that 

such a foramen is easily formed secondarily by the 

formation of a cartilage bar separating the maxillary 

nerve from the other nerves that leave the cranium 

through the foramen lacerum anterius. This fact does 

not, however, invalidate the general rule that the presence of many separate foramina in the basicranial 

region is a primitive condition.

ALISPHENOID

The alisphenoid bone in Merycoidodon has its usual 

mammalian form: it is a plate of bone projecting dorsolaterally from the basisphenoid and forming the median 

wall of the inferotemporal fossa (fig. 14, AS). It 

ossifies from a center distinct from the basisphenoid, 

but in Merycoidodon it is usually partly, sometimes 

entirely, fused with it.

The anterior end of the auditory bulla marks the 

posterior termination of the surface exposure of the 

alisphenoid bone. The serial sections, however, show 

that a slim, dorso-ventrally flattened process of the 

alisphenoid projects posteriorly between the squamosal 

and petrosal bones (fig. 21, AS). It is suturally attached laterally to the squamosal. At the beginning of 

its posterior course it serves as a roof for the eustachian 

tube; farther backward it lies dorsal to the anterior 

portion of the bulla and forms a small part of its roof, 

separating it from the petrosal.

This backward-projecting process of the alisphenoid 

is the pars ossea tubae (van Kampen 1905, p. 370). 

Joined with it and forming the lateral bony wall of 

the tube is the styliform process of the tympanic bone.

The anterior terminus of the pars ossea tubae is at. the 

point of entry of the eustachian tube into the pharrnx; 

its posterior end is marked by the ostium tympanicum 

tubae, the gap in the anteromedial wall of the tympanic 

cavity whereby the eustachian tube enters the middle 

ear.

The presence of the pars ossea tubae is typical of 

the ungulates (van Kampen 1905, p. 370). Merycoidodon differs from modern Artiodactyla in that the pars 

ossea does not form a complete bony tube around the 

eustachian tube. Examination of any modern artiodactyl, such as Ovi$, explains this difference: in modern 

Artiodactyla the ventral wall of the bony tube is formed 

by the expanded medial wall of the tympanic bnlla, 

which has come into contact with the basisphenoid 

bone. This is also true of most merycoidodont genera 

in and after the late Oligocene. Merycoidodor-, in 

which the bullae are not as yet greatly inflated, may be 

regarded as primitive in this respect.

Within the pars ossea tubae of the alisphenoid, near 

its medial border and extending antero-posteriorly, the 

serial sections (fig. 21, VCL) revealed a tiny canal, extending from the foramen lacerum medius to the tympanic cavity. This probably carried a vestigial vein, 

no longer of any importance in drainage of the blood 

from the cranium. It will be further discussed in 

connection with the venous circulation.

Extending antero-posteriorly in a gently curved 

course along the ventro-lateral side of the pyriform 

lobes (fig. 16, VOO) is a shallow groove in the endocranial surface of the alisphenoid bone, which ends 

anteriorly in a small foramen (fig. 16, PSG) at the 

transverse ridge separating the pyriform lobe from 

the tuberculum olfactorium. The serial sections (fig. 

22) show that this foramen is the posterior openir? of 

a canal (SC) running posteriorly between the alisphenoid and orbitosphenoid from a foramen or the 

anterior border of the surface exposure of the alisphenoid bone (fig. 14, ASG). This canal and the 

groove which is its continuation are clearly homologous 

with the sinus canal found by W. K. Parker in the 

Insectivora (1886). It carries a posterior branch from 

the orbital venous sinus to join the vena collateralis 

cerebri. The latter travels around the pyriform lobe 

and enters the temporal venous sinus system, which 

drains blood to the external jugular vein. These veins 

will be further considered below.

PRESPHENOID

The presphenoid bone (fig. 15, PS) contains a large 

cavity, the posterior end of the spheno-palatine sinus, 

an air cavity which is in direct connection with the 

nasal passage. This sinus extends posteriorly beneath
124 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

the olfactory lobes, reaching as far back as the chiasma 

ridge (fig. 16, OR).

ORBITOSPHENOID

Within the skull, the anterior portions of the orbitosphenoid bones possess horizontal, flat, medially projecting processes, which meet ventrally at the midline 

to form the floor of the olfactory fossa of the braincase (fig. 22, OL}. Anteriorly these plates are separated, and the ethmoidal crest of the mesethmoid bone 

(fig. 22, EC] rises between them in the midline. This 

crest serves as a septum that partly separates the olfactory lobes. Anteriorly, the orbitosphenoid walls of the 

olfactory fossa are bounded by the cribriform plate 

of the mesethmoid bone.

ETHMOID AND TTJRBINALS

The two chief parts of the ethmoid bone, the cribriform plate and the mesethmoid bone, can be observed 

in serial sections. There also can be seen traces of the 

three turbinal bones, the ethmoturbinals, nasoturbinals, 

and maxilloturbinals.

Posteriorly the mesethmoid projects backward into 

the sphenopalatine sinus below the olfactory lobes. 

Such a backward extension appears to be common (see 

Paulli, 1900) in all but the smallest mammals.

In the posterior part of the nasal cavity proper (fig. 

23), the mesethmoid forms a complete vertical septum 

about 1 mm thick. Dorsally it is fused to the frontal 

bone; ventrally it rests upon the vomer, a very small 

bone in this part of the nasal cavity.

About 8 mm anterior to the plane of section in (fig. 

23), the mesethmoid divides in about its ventral third. 

From the point of division a thin plate descends laterally on either side, joining the horizontal plate of 

the maxilla at an angle of approximately 45 degrees. 

The passage, triangular in cross section, is termed by 

Paulli (1900, p. 199) the "pneumatic part of the nasal 

septum." It leads posteriorly directly to the posterior 

nares, and is the most direct route by which inspired 

air can travel from the nostrils to the pharynx. A 

homologous formation can be found in some living 

mammals, but in many individuals it is hard to detect 

because of the extremely intricate infolding which has 

taken place in the mesethmoid walls of the passage, 

rendering it difficult to distinguish in cross section from 

other parts of the nasal cavity.

In living Mammalia, the Dicotylidae (peccaries) 

show the nearest approach to the structure of the pneumatic part of the nasal septum as observed in Merycoidodon (fig. 18). As figure 18 shows, however, even 

the Dicotylidae exhibit a considerably more complex 

structure in this region than Merycoidodon. Dicotyles 

possesses a maxillary extension (I') of the frontal sinus.

B

1 INCH

FIGURE 18.—Frontal sections, at the level of the first molar, of the 

skulls of Merycoidodon culbertsonii (Leidy) (M. C. Z. 5450) and 

Dicotyles labiatus (after Paulli, 1900).

A, pneumatic part of nasal septum. 

B, sinus in horizontal plate of maxilla. 

0, lateral nasal cavity. 

I', maxillary extension of frontal sinus. 

8, bony nasal septum.

This is typical of the living Non-Kuminantia, and is 

lacking in Merycoidodon.

The two cavities of the nasal passage proper are 

found both in Merycoidodon and in Dicotyles, and are 

labelled A and C in figure 18. The cavity contained 

in the pneumatic part of the nasal septum (A) is in 

Dicotyles, in cross-section, about three times th<? size 

of that in Merycoidodon, due partly to the lateral expansion of its lateral wall, partly to the fact that this
CRANIAL MORPHOLOGY OF SOME OLIGOCBNE ARTIODACTYLA 125

wall springs from a higher point on the nasal septum, 

or mesethmoid ($). This expansion of the pneumatic 

part of the septum in Dicotyles takes place at the expense of the lateral nasal cavity (<7), which contains 

the maxilloturbinals, the nasoturbinals and, posterior 

to the section shown in figure 18, the ethmoturbinals.

The pneumatic part of the nasal septum in Merycoidodon may, from its comparative simplicity, be regarded as a primitive stage in the inflation of this 

septum.

Anteriorly, the walls of the pneumatic portion of 

the nasal septum diverge and are continuous with the 

maxilloturbinals.

Still farther anteriorly the mesethmoid is extremely 

heavily ossified, especially in the area midway between 

the palate and the roof of the nasal cavity.

In Merycoidodon culbertsomi, the mesethmoid forms 

a complete, strong bony nasal septum as far forward 

as the level of the first premolar, and continues for 

about 6 mm. anterior to this as a partial bony septum 

attached dorsally to the nasal bones. Thus it forms a 

vertical median septum in the nasal cavity for a distance of 72 mm. in a skull whose anteroposterior length 

is 190 mm. Comparison with modern osteological material and a search of the literature reveal that very 

few mammals have a comparable degree of ossification 

of the mesethmoid. Such ossification was present in 

the extinct Ehinocerotidae, but has been greatly reduced 

in their living descendants. The order Pinnipedia also 

shows as much mesethmoid ossification (Flower, 1885). 

Dicotyles possesses a more completely ossified nasal 

septum even than Merycoidodon. Of 25 peccary skulls 

examined, all the adult specimens possessed a bony 

nasal septum reaching at least as far forward as the 

canine teeth, that is, within 25 mm. of the end of the 

snout. In Sins, the bony nasal septum also terminates 

anteriorly at the level of the canine tooth; however, 

the snout extends much farther in advance of the canines than it does in Dicotyles.

The conclusion forcibly suggested by these observations is that Merycoidodon was, like Dicotyles, an animal of rooting habits. This theory is borne out by 

the fact that the strong nuchal crest of Merycoidodon 

indicates such powerful cervical musculature as is possessed by Dicotyles and by Sus. The vegetation of the 

Oligocene epoch seems also to have been such as would 

allow this mode of life, for it is well known that the 

early Tertiary flora was primarily lush, and that the 

increasingly dry climate of the West in Miocene and 

Pliocene time increased the area occupied by grasslands 

(Scott, 193T; Chaney and Elias, 1938).

After the Miocene epoch, it is probable that the 

Merycoidodontidae abandoned their rooting habits, for

the nasals are greatly reduced in many genera, eliminating an important snout support. Also, the general 

snout structure in many later Merycoidodontidae is 

much weaker than in Merycoidodon itself.

The ethmoturbinal bones are extremely delicate, and 

it is surprising that in the Merycoidodon skull sectioned 

for this study they should have been preserved at all. 

They are attached not only to the orbital wall of the 

frontal bone, but also to the frontal where it forms the 

walls of the supraorbital extension of the nasal cs vity 

(the anterior continuation of the frontal pneumatic 

sinus). This dorsal extension of the turbinals occurs 

in all mammals possessing large hollow supraorbital 

processes of the frontal bone.

Examination of the serial sections reveals that Merycoidodon has five ethmoturbinals. Their broken condition makes it impossible to determine whether they 

are ectoturbinals or endoturbinals except in one of the 

sections, which shows a strong scroll with a heavy base, 

easily identifiable as an endoturbinal. This is situated 

at the base of the orbital wing of the frontal bone, the 

farthest ventral position which can be occupied by an 

ethmorturbinal.

The number of ethmoturbinals (five) possessed by 

Merycoidodon is typical of the Ruminantia (Paulli, 

1900) as opposed to the Non-Ruminantia, which have 

six to eight. A striking difference from the modern 

Ruminantia is, however, the fact that in Merycoidodon 

the strongest ethmoturbinal is the fifth, or basal one, 

whereas in all modern ruminants the second ethmoturbinal is far stronger.

The only modern mammals showing a strong: development of the farthest ventral ethmoturbinal are 

the Carnivora, which, however, have only four ethmoturbinals.

The serial sections show almost no trace of the nasoturbinal bone, a scroll derived from the ethmoid 

cartilage and, in most mammals, fused to the frontal 

and nasal bones along the whole length of the roof of 

the nasal cavity. The only evidence of the presence 

of this bone is in two successive serial sections (fig. 18), 

in which the roots of the nasoturbinals are seen springing dorsally between the dorsal maxilloturbinals and 

the median nasal septum. Probably the nasoturbinal 

scroll itself was too delicate to be preserved.

The maxilloturbinal bones are by far the best developed of the turbinals in Merycoidodon. Ther are 

attached not only to the maxillae but also to the frontal 

bones, and extend from the level of the lacrimal fossa 

to the anterior nasal opening, a distance of 60 mm. 

(fig. 23). They lie anterior to the ethmoturbinalf and 

below the nasoturbinals. The maxilloturbinals are not 

equally well developed throughout their extent, but
126 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

reach their maximum size in the region of the lacrimal 

fossa and at the anterior end of the nasal cavity.

The maxilloturbinals may be subdivided into several 

parts. Beginning posteriorly, the first of these to 

appear are the dorsal maxilloturbinals which, at the 

level of the lacrimal fossa (fig. 23), spring from the 

nasal septum. Four millimeters farther forward the 

dorsal maxilloturbinals are fused to the frontal bones. 

Thus, for a short distance (and probably for a longer 

space in life, when the partitions were extended by 

cartilage) the nasal passage is divided into four passages. Of these, the ventral one (pneumatic part of 

the nasal septum) was the most direct air passage. The 

dorsal pasage was most closely concerned with the 

sense of smell (an important function in Merycoidodon, 

which was macrosmatic; see Black, 1920). The lateral 

passages, one on either side, were connected through 

large apertures with the pneumatic sinuses of the maxillary bones, whose mucous membrane lining, together 

with that of the maxilloturbinals, probably played some 

part in warming the inhaled air.

Anterior to the region just described, at the level 

of the anterior part of the lacrimal fossa, appear the 

ventral maxilloturbinals, which are attached at their 

lower ends to the horizontal plate of the maxilla, directly above the palatine canal. They form, on either 

side, a scroll that is rolled in a lateral direction and is 

not attached to the mesethmoid bone.

Anteriorly these scrolls divide to form a small dorsal 

scroll in addition to the large lateral one. The bodies 

of the ventral maxilloturbinals are strong laminae, as 

much as 2 mm. thick. The scrolls, on the other hand, 

are delicate, and do not extend as far forward as the 

stronger laminae which support them.

The bones which we shall term here anterior maxilloturbinals are, in most living mammals, the only maxilloturbinals present. They are the scroll-shaped bones 

visible in the dried skull as one looks into the anterior 

nares.

The above observations on the turbinal bones make 

it plain that, in Merycoidodon, they are far more 

strongly ossified than in any living mammals except, 

perhaps, Dicotyles. This exceptional degree of ossification is undoubtedly a result of the heavy ossification of 

the mesethmoid cartilage which, in turn, is correlated 

with the probable rooting functions of the animal.

VOMER

The vomer is a long, slim, trough-shaped bone, which 

runs antero-posteriorly along the mid-line of the floor 

of the nasal cavity. Its concave side is dorsal, and in 

it rests the base of the vertical nasal septum.

In the anterior portion of the nasal cavity, the nasal 

septum rests between two upturned flanges, one springing from the medial border of each of the palatine 

processes of the maxillae. These flanges are in contact 

along the floor of the nasal cavity, and diverge upward, 

forming a V-shaped trough like that of the vomer.

This condition is not found in living Artiodetyla, but 

occurs in the Carnivora.

PREINTERPARIETAL AND INTERPARIETAL

A preinterparietal bone is present in Merycoidodon. 

It is very small and does not appear on the surface of 

the skull, being covered by the parietal. In the specimen sectioned it occupies the dorsal wall of the braincase for 6 mm. on either side of the midlin^, just 

posterior to the fronto-parietal suture. This lx»ne is 

not found in adult Artiodactyla, but Wilhelm (1924) 

notes its presence in the young of Bos, and it probably 

occurs in youth in many other genera. In spite of its 

possession of the preinterparietal, the skull of Meryooidodon examined here is an adult, as is shown by the 

molar teeth.

The interparietal bone, which also appears in the 

young of modern Artiodactyla, is absent as a seperate 

ossification in the skull here studied.

Not much importance can be attached to the presence 

or absence of the preinterpartietal and interp arietal 

bones. Their independent existence is characteristic of 

an early ontogenetic stage in the ossification of the dermal bones of the skull roof, and it is natural that the 

time of their fusion with the neighboring parietal 

should be subject to individual variation.

PARIETAL

The parietal bone (fig. 14, PA) forms most of the 

roof of the braincase.

The parietal walls of the braincase are very thick 

and filled with diploe.

Toward the posterior end of the parietal, tetween 

the cerebrum and the cerebellum, the diploe reaches 

its maximum thickness of about 14 mm. Into this 

diploe there extends, on either side of the skull, a cavity 

that is the dorsal extension of the temporal venous 

sinus (fig. 21, SVT), a large blood vessel which is 

further discussed below.

Within the cranial cavity, the parietal surface directly below the sagittal crest is excavated by an antero-posterior groove that contained the sagittal venous 

sinus, one of the vessels draining blood posteriorily 

from the brain and from the face (fig. 22, SS).

FRONTAL

The frontal bones (fig. 14, FR), by reason cf their 

heavy supraorbital extensions and their postorbital 

processes, are strongly developed in Meryooidodon as 

in most members of the family Merycoidodontidae.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 127

Anterior to the temporal ridges, the frontals form 

an almost flat surface between the orbits. On this 

surface, only a few millimeters on either side of the 

sagittal suture, are the supraorbital foramina. These 

are the dorsal openings of the supraorbital canals, 

which open into the dorsal side of the frontal pneumatic 

sinus. A continuation of the same canal leads from 

the ventral side of the supraorbital extension of the 

frontal sinus, through the lower wall of the supraorbital 

process, and into the orbit through a foramen in its 

roof.

As in modern Artiodactyla, the supraorbital canal 

carried the frontal vein from the orbital plexus (or 

orbital venous sinus) upward through the frontal pneumatic sinus and out through the supraorbital foramen.

On the suture between the orbital wing of the frontal 

and the orbitosphenoid bone is situated the ethmoidal 

foramen (fig. 14, EF}. This leads into a canal that 

runs posteriorly in the orbital wing as far as the cribriform plate. It carries the ethmoidal artery, a branch 

of the ophthalmic artery.

The ethmoidal foramen and canal are present in all 

modern Artiodactyla.

Anterior to the postorbital constriction are the 

farthest posterior extensions of the frontal pneumatic 

sinus. In cross section, these extensions appear as two 

small cavities, about 3 mm in maximum diameter, one 

situated at either lateral limit of the diploic portion 

of the bone.

In a section just anterior to the posterior nares, the 

frontal sinuses are considerably larger. Here each is 

incompletely divided into two parts: a medial cavity, 

still situated at the lateral edge of the diploe, and a lateral one occupying the supraorbital process. Both these 

cavities were separated from the olfactory lobes only 

by a very thin wall of bone, which in this section has 

been broken through. That the two were originally 

separated by bone was determined by cleaning the matrix from the inside of the braincase of the skull from 

which figure 16 was made.

A cross-section of the posterior part of the nasal 

cavity shows the right frontal sinus, which here occupies the supraorbital process, widely open into the nasal 

cavity proper.

A narrow extension of the frontal sinus reaches into 

the anterior processes of the frontals that thrust forward between the nasal and the lacrimal bones. In 

most sections this is a mere excavation of the frontal 

wall of the nasal cavity; in one section, however, it 

is separated from the nasal cavity by a thin wall. This 

suggests that the part of the frontal sinus immediately 

posterior to this section may also have been so separated.

SQUAMOSAL

The squamosal bones (fig. 14, SQ) form a large portion of the lateral walls of the braincase; they are tlick 

and heavy, and their relations to the neighboring skull 

elements are in places rather complex.

The posttympanic process of the squamosal forms the 

entine posterior wall of the bony external auditory 

meatlus (fig. 15, FTP). In this region the squamosal 

extends medially a distance equivalent to the lengtl of 

the external auditory meatus (fig. 19). Its medial surface is the continuation of the inner squamosal surface 

which, farther dorsally, forms part of the braincase 

wall. Medial to the posttympanic process, however, 

this surface is in close contact with the lateral wall of 

the pars petrosa of the periotic bone (figs. 15,16, PET).

An anterior extension of the posttympanic process 

forms the medial part of the ventral wall of the external 

auditory meatus. The lateral end of this wall (the cnly 

part readily visible from the exterior of the skull) is 

formed by the tympanic portion of the meatus.

In the modern Giraffidae and Cervidae, the postt7mpanic process forms a part of the posterior wall of the 

external auditory meatus; but in no modern artiodactyl 

does it participate in the formation of the ventral wall, 

as in Mev^ycoidodon. This participation of the squamosal in the ventral wall of the meatus must not be 

confused with the meatus auditorious spurius, prerent 

in some modern and a few fossil Artiodactyla, in wl ich 

the posttympanic and postgleiioid processes meet and 

fuse below the external auditory meatus of the tympanic 

bone. The meatus spurius simply covers the wall of 

the true auditory meatus; it is not a part of it. It is 

present in the Suidae and some Bovidae, and is very 

nearly complete in the Hippopotamidae.

The serial sections show the squamosal bone to be 

thick and filled with diploic tissue, especially in the 

lower part of its cranial portion (fig. 22) and, to a 

lesser extent, in the zygomatic process.

In the diploic bone of the cranial wall of the squamosal, dorso-medial to the postglenoid process, lies 

a cavity, 6 mm long, 6 mm wide, and 15 mm high (fig. 

21, SVT). It tapers anteriorly to a blind end, and is 

surrounded on all but one side by the squamosal. Hhe 

only other bone abutting upon it is the pars petrosr. of 

the periotic, part of whose lateral side covers a large 

gap in the medial wall of the cavity.

This cavity has two openings, one into the cranium 

and another to the exterior of the skull. The former 

is a gap between the squamosal and the apex of the 

pars petrosa of the periotic (fig. 20, FJSP). The latter 

is in the form of a canal that leaves the base of the 

cavity near its anterior end and extends posteroventrally through the squamosal bone, just medial to the
128 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

SF

FSM

I INCH

FIGURE 19.—Merycoidodon cuttertsonii (Leidy). X 1%. Thick section of skull constructed from section MtMJl, looking 

posteriorly into cerebellar fossa from level of auditory bulla.

AASC 

MTS

I INCH

FIGURE 20.—Merycoidodon culbertsonii (Leidy). X 1%. Thick section of skull from sections M33-38, In .region of auditory 

bulla, looking anteriorly.

AA80, ampulla of anterior semicircular canal.

A.C, area cochleae.

ANF, area nervus facialis.

AS, alisphenoid.

BO, basioccipital.

BS, basisphenoid.

CER. cerebellum.

CT, crista tympanica.

CV, crista vestibuli.

DMT, dorsal maxilloturbinal.

DP, diploe of parietal bone.

Symbols used in figures 19-28

DP8, groove for dorsal petrosal sinus.

D8, diploe of squamosal bone.

EC, ethmoidal crest.

ENT, endoturbinal.

ET, endotympanic.

FC, fenestra cochleae.

FGA, anterior carotid foramen.

PG, flssura glaseri.

FJSP, foramen jugulare spurium primitivum.

FLA, foramen lacerum auterius.

FM, foramen magnum.

FR, frontal.

F8, facial sulcus.

FSM, stylomastoid foramen.

FV, fenestra vestibuli.

HOF, hiatus canalis facialis.

HSC, horizontal semicircular canal.

If, incus?

I AM, internal auditory meatus.

IGA, groove for internal carotid arterj-.

IT, incisura tympanica.

LGP, lateral clinoid processus.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 129

SH

I INCH

FiGURE'21.—Merycoidodon culbertsonii (Leidy). X 1%. Thick section of skull from sections M42-54, at level of postglenoid processes. Looking anteriorly.

I INCH

PA

FIGURE 22.—Merycoidodon culbertsonii (Leidy). X !%• Thick section of skull from sections M57-77, looking anteriorly 

into olfactory lobes.

If, pars mastoidea.

M2, second molar.

ME, mesethmoid.

MF, margo fissurae of squamosal.

MTS, margo tympanic! of squamosal.

MX, maxilla.

JVC, nasal cavity.

O, orbit.

OL, olfactory lobes.

P, promontorlum.

PA, parietal.

PAL, palatine.

PBC, petrobasilar canal.

238485—58——8

Symbols used in figures 19-23—Continued

PC, pars cochlearis of pars petrosa. 

PCP, posterior clinoid process. 

PET, pars petrosa. 

PF, pituitary fossa. 

PFM, processus folii of malleus. 

PME, pneumatic part of mesethmoid. 

PO8, posterior root of orbitosphenoid. 

PVP, pars restibularis of pars petrosa. 

RE, recessus epitympanicus. 

8G, sinus canal. 

8F, subarcuate fossa. 

SH, hypotympanic sinus.

8MS, superficies meatus of squamosal.

88, sagittal sinus.

88C, subsphenoid canal.

8VT, sinus venosus temporally

T, ectotympanic.

TC, temporal canal.

TT, tegmen tympani.

V, vomer.

VOL, vena capitis lateralis.

VPH, vagina processns hyoidei.

V etc., groove for cranial nerves.
130 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

ENT

M2

I INCH

FIGURE 23.—Merycoidodon culbertaonU (Leidy). X 1%. Thick section of skull from sections M86-120, looking anteriorly 

through nasal cavity.

postglenoid process (fig. 20, TO). The external opening of this canal is through the foramen jugulare spurium (TC, fig. 15) between the tympanic bulla and 

the postglenoid process.

The positions of the outlets of this cavity lead to 

the conclusion that it must play a part in the venous 

circulation of the skull, in close connection with the 

large venous sinuses of the posterior portion of the 

brain. The canal linking it with the exterior is, from 

its course and the position of its exit, the temporal 

canal. This canal, to some degree present in all mammals, carries the superior cerebral vein to its junction 

with the external jugular vein. The above-mentioned 

cavity in the squamosal is simply an enlargement of the 

temporal canal to form a venous sinus similar to those 

lining the cranial cavity, but surrounded by bone. The 

term sinus venosus temporalis is here applied to it. 

Its opening into the cranial cavity is therefore the foramen jugulare spurium primitivum (van Kampen, 1905, 

p. 382). This corresponds to the foramen through 

which, in the embryological stages, the superior 

cerebral vein left the chondrocranium.

The sinus venosus temporalis is found in no living 

mammals, to the best of the author's knowledge. The 

only mention of its presence in fossil skulls is by Simpson (1936) who reports the presence of an identical 

cavity in the squamosal bone of Oldfieldthomasia, a 

primitive notoungulate from the Eocene of Argentina. 

Merycoidodon and Oldfteldthotnasia are both primitive mammals, with small brains relative to the size

of the skulls and to the area of insertion necessary for 

their temporal muscles. It is therefore possible that the 

presence of the sinus venosus temporalis indicates that 

the small size of the brain and concomitant thickness 

of the diploic portion of the squamosal bone allowed the 

superior cerebral vein to attain a large size. £ince the 

exit of the temporal canal (fig. 15, TO) is small, the 

sinus venosus temporalis must have served as a reservoir for venous blood, rather than as a main drainage 

vessel. Blood probably flowed both into and out of it 

through the diploic veins. In modern mammals, with 

their large brains, the dorsal cerebral vein and the 

temporal canal are of relatively minor importance. 

The lateral growth of the brain prevents any large 

veins from taking a dorso-ventral course around it; 

therefore all venous drainage of the braincase is along 

its roof (as the mastoid emissary vein) or along its 

base (as the inferior petrosal sinus).

Another ramification of the cranial venous system is 

indicated by the presence of a canal running posterolaterally from the temporal canal, between the squamosal and tympanic bones. It has its exit just anterior 

to the external auditory meatus, through the p ostsquamosal foramen of Cope (1880). This foramen was not 

observed by Cope in Merycoidodon; it is very small in 

this genus and absent in some specimens.

The postsquamosal foramen is reported by Cope to 

occur in the Hippopotamidae, Giraffidae, C^rvidae, 

Antilopidae and Capridae among the Artiodoc^yla.

Still another canal of the cranial venous system ex-
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 131

tends backward along the suture between the parietal 

and squamosal bones from the frontal plane of the 

external auditory meatus to the plane of the condylar 

foramen), where it emerges upon the surface of the 

skull through the postparietal foramen of Cope (1880), 

(fig. 14, FPP). These foramina (there may be two or 

three on either side of the skull) are present in all the 

Merycoidodontidae but in few living Artiodactyla (the 

Antilocapridae, Cervidae, Antilopinae and Caprinae). 

Other aspects of the squamosal bone will be considered in the discussion of the tympanic region.

TYMPANIC REGION

For purposes of discussion, this complex region will 

be divided into four parts: (a) the pars petrosa of the 

periotic bone; (b) the middle ear and surrounding 

structures, including the auditory bulla; (c) the external auditory meatus; and (d) the pars mastoidea of 

the periotic bone.

PARS PETROSA OF THE PERIOTIC BONE

This bone, contained entirely within the cranial cavity (fig. 16, PET), is composed of two parts. The 

dorso-lateral portion is the pars vestibularis (fig. 20, 

PVP). It contains the semi-circular canals, and is 

situated mainly posterior, as well as dorsal to the pars 

cochlearis (fig."20, POP).

As has been pointed out, the pars petrosa is in contact laterally with the inner face of the ventral plate 

of the squamosal, and ventro-medially with the lateral 

border of the basioccipital. It reaches its greatest size 

posteriorly; anteriorly its dorsal-surface slopes to its 

termination at a point lateral to the foramen lacerum 

medius. Just posterior to this point, the pars petrosa 

gives off a slim process which mounts dorso-laterally 

along the cranial wall of the squamosal. This is the 

processus perioticus superior. Its position in Merycoidodon is nearer that of modern mammals than that 

of the Eocene OldfieldtJiomj/asia (Simpson, 1936) in 

which the process projects laterally along the floor of 

the temporal venous sinus. In Meryeoidodon, this 

process is separated from the sinus by about 5 mm. of 

diploic bone.

Posterior to the processus perioticus superior a 

groove extends backward along the dorsal crest of the 

pars petrosa (fig. 21, DPS). It is about 3 mm. long, 

and in life carried the dorsal petrosal sinus, which 

joined the transverse venous sinus at the foramen 

jugulare spurium primitivum, thus emptying blood 

from the midbrain region into the temporal sinus and 

canal.

Posterior to the above-mentioned groove (figs. 16,19, 

SF) , is a depression, the subarcuate fossa, in the dorsal

surface of the pars petrosa. In its center is a small, 

deep depression.

The subarcuate fossa must have lodged the lobuhis 

petrosus of the flocculus of the cerebellum, and small 

blood vessels probably passed ventrally from the flocculus into the pars petrosa. Davidson Black (1920, 

p. 309) notes the similarity of the deep subarcuate forsa 

of the Merycoidodontidae to that of the Carnivcra 

(such a fossa is also present in Rodentia and many 

Primates; Bolk et al., 1936, vol. 2, p. 89). Black contrasts this condition with that of the living Artiodactyla, in which, he says, the subarcuate fossa is shallow or absent because of the absence of the lobulus 

petrosus of the cerebellum. This is true, so far as can 

be determined by examination of the literature and of 

dried skulls of recent Artiodactyla, except in the 

suborder Tylopoda, which have a very large lobulus 

petrosus. This condition will be discussed further in 

the section on Poebrotherwm.

The subarcuate fossa of Merycoidodon is smaller and 

much simpler than that of the living Tylopoda or of 

the contemporary Poebrotherium, and resembles very 

closely the fossa of the Carnivora. The presence of 

this fossa seems to indicate that Merycoidodon, like the 

Carnivora, Rodentia, and some Primates, possessed1 a 

motor reflex center that is present only in the Tylopoda 

among modern Artiodactyla.

On the medial side of the parts petrosa is a large 

opening, the internal auditory meatus (fig. 16, MAI). 

Its course is directly lateral through about two-thirds 

the thickness of the pars petrosa. At its fundus, the 

internal auditory meatus divides into two parts (fig. 

19). The dorsal branch is the area nervus facialis 

(ANF), into which passes the seventh cranial nerve on 

its way to the facial canal. The more ventral of the 

two branches is the area cochleae (AC) , through which 

the duct of the auditory nerve passes to the cochlea.

Backward for a short distance from the internal auditory meatus, the facial canal lies in the pars petrosa. 

It then passes out of the pars petrosa, through the ap^rtura tympanica canalis facialis, into the cavity of the 

middle ear, along whose roof it travels as the facial 

sulcus (fig. 19, £F).

From the fundus of the internal auditory meatus 

extends anteriorly a very narrow canal, the hiatus 

canalis facialis (fig. 21, EOF). It issues into the cranial cavity through a foramen on the anterior slope of 

the pars petrosa. It carries the nervus facialis sup°,rficialis major, an anterior branch of the facial nerve, 

which extends anteriorly as the nerve of the pterygoid 

canal to the sphenopalatine plexus and ganglia. It is 

the equivalent of the palatine nerve of lower vertebrates.

The facial sulcus, a groove on the ventro-lateral side
132 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

of the pars petrosa (fig. 19, /SF) , is the posterior continuation of the facial canal. It extends posteriorly 

about 4 mm., attaining such a depth (about 3 mm.) that 

it might be called a recessus epitympanicus. Regardless of the terminology employed, however, this sulcus 

almost certainly carried the facial nerve, as it ends posteriorly directly above the stylomastoid foramen, the 

exit of the facial from the tympanic cavity (fig. 19, 

FSM).

The central cavity of the pars vestibularis of the pars 

petrosa (fig. 20, PVP) is the vestibule (fig. 19, F). 

This cavity has in its ventro-lateral wall an opening, 

the f enestra vestibuli or f enestra ovalis, through which 

it communicates with the cavity of the middle ear.

In the anteroventral wall of the vestibule is a small 

circular depression, the recessus sphericus. This, in 

life, contained the saccule of the inner ear.

The posterior end of the vestibule is just anterior to 

the level of the stylomastoid foramen (fig. 15, FSM). 

Ventrally, a horizontal crest projecting laterally from 

the body of the pars petrosa separates the vestibule 

from the fenestra cochleae, which lies below it. This 

crest is the crista vestibuli (fig. 19, CV).

Within the vestibule are several openings, leading to 

other parts of the cavity of the internal ear. One of 

these, near the posterior termination of the vestibule 

and on its medial side, is the opening of the aquaeductus vestibuli, a bony canal running postero-laterally 

and opening upon the medial surface of the pars 

petrosa. This canal transmits the endolymphatic duct, 

which ends, in modern mammals, in a cul-de-sac between the layers of the dura mater.

Nothing need be said here concerning the semicircular canals. As is shown by the work of Turkewitsch (1933), the only differences that they exhibit 

within a mammalian order are those of dimensions, 

and such measurements must be based upon a large 

suite of specimens.

The lateral side of the pars vestibularis of the pars 

petrosa is an eminence, the prominentia canalis lateralis, which is closely applied to the squamosal bone. 

Its lower edge forms the tegmen tympani (fig. 20, TT).

The tegmen tympani, as the serial sections show, is 

simply a projection of the pars vestibularis over the 

tympanic cavity. In Merycoidodon this projection is 

far greater than in living Artiodactyla, with the result 

that the tegmen forms almost the entire roof of the 

tympanic cavity, whereas in modern artiodactyls it 

rarely forms more than half.

When the pars petrosa of Merycoidodon is compared 

with that of Ovis, which is typical of modern Kuminantia, it becomes apparent that the tegmen tympani 

of the former is borne relatively much lower on the

lateral side of the bone. This is a reflection of the fact 

that, in Merycoidodon, the pars vestibularis of the pars 

petrosa is a more important element in the side wall 

of the braincase than in modern Kuminantia. The same 

comparison seems to hold with the Non-Kuminantia.

The total antero-posterior length of the tegmen tympani is approximately six millimeters; it extends posteriorly from the level of the foramen jugulare spurium 

(figs. 15, 20, TO) to that of the stylomastoid foramen 

(fig. 15, FSM). The tegmen tympani is absent in the 

anterior half of the roof of the tympanic cavit7, which 

is formed by the squamosal (fig. 21). This i« a rare 

condition in modern mammals, being found, according 

to van Kampen (1905, p. 345) only in the Marsupialia 

and Chiroptera.

Medial to the tegmen tympani, directly anterior to 

the facial sulcus, and below the hiatus canalis facialis, 

lies a short, deep groove in the pars petrosa (fig. 20). 

This is the sulcus in which is inserted the tensor tympani muscle.

From the posterior side of the fenestra cochleae (fig. 

19, FC) a small canal runs posteromedially through 

the bone. This is the aquae-ductus cochleae which, like 

the aquae-ductus vestibuli, formed a path for interflow 

of lymph between the inner ear and the subarachnoid 

space between the arachnoidea and the pia mater of 

the brain.

The lower part of the lateral wall of the pars petrosa, 

which forms most of the medial wall of the middle ear 

cavity, bulges somewhat into that cavity. The bulge, 

which is caused by the first coil of the cochlea, i^1 known 

as the promontorium (fig. 20, P). The promontorium 

in Merycoidodon is conspicuous, but not so much so 

as in Oldfieldtho'inasia (Simpson, 1936). No data are 

available as to the relative degrees of prominence of 

the structure in modern mammals.

The cochlea, as revealed in frontal section, appears 

to possess 2% spiral turns, the same number as in 

modern ungulates.

MIDDLE EAR AND SURROUNDING STRUCTUr.ES

Of the various cavities (usually termed "sinuses" 

and "recesses") of the middle ear, the one underlying 

the auditory ossicles is the largest in Merycoidodon. 

This cavity occupies the hollow auditory bulla (fig. 

15, #), and is thus surrounded on all but th«, dorsal 

side by the inflated tympanic bone. It is known as the 

hypotyinpanic sinus (fig. 20, SH). The upper boundary of the main part of the hypotympanic sinus lies 

at the level of the chain of auditory ossicles. Anteriorly, the sinus extends about 4 mm beyonc1 the ossicles (fig. 21, $#), and for a short distance is entirely 

surrounded by the tympanic bone; behind this point 

the tympanic is incomplete dorsally and the horizontal
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 133

part of the squamosal forms the roof of the sinus. The 

dorsal gap in the tympanic bulla is the "tympanicumdefekt" (Bondy, 1907). It is larger in Merycoidodon 

than in modern Artiodactyla.

In spite of the fact that Merycoidodon has extremely 

small auditory bullae, the hypotympanic sinus is rather 

large. This is, of course, because the bulla is entirely 

hollow, with no filling of cancellous bony tissue such 

as occurs in many Artiodactyla, including the later 

merycoidodonts. The sinus is shown at its maximum 

size in figure 21 (SH).

In the Mammalia, the middle ear cavity may extend 

into two cavities dorsal to the auditory ossicles, known 

as the epitympanic recess and the epitympanic sinus.

The epitympanic recess is the cavity lying immediately dorsal to the auditory ossicles. It is enclosed in the ventral wall of the skull (usually between 

the pars petrosa and squamosal) and composes the 

dorsal portion of the tympanic cavity proper.

The epitympanic sinus, on the other hand, is an accessory cavity of the tympanic cavity. Where present 

it lies in the squamosal or the pars mastoidea, and opens 

into the epitympanic recess.

The epitympanic recess in Merycoidodon is a cavity 

about 2 mm. deep and 3 mm. long, situated dorsal to the 

deepest portion of the hypotympanic sinus (fig. 21, 

RE). Its roof consists of three elements. The middle 

element is the slanting lateral surface of the pars 

petrosa of the periotic, and the lateral element is the 

f acies epitympanica of the squamosal bone. This portion of the squamosal lies medial to the postglenoid 

process and forms the floor of the temporal venous 

sinus.

The facies epitympanica is in contact with the pars 

petrosa; between them lies a small bone whose identity 

is in doubt, but which is probably a piece broken from 

the tegmen tympani of the pars petrosa. This is the 

third element of the roof of the epitympanic recess.

The living Artiodactyla in which the epitympanic 

recess most closely resembles that of Merycoidodon are 

the Cervidae and Camelidae. In both these families 

the recess is small, and roofed over as much by the pars 

petrosa as by the squamosal, but in the Suidae and 

Bovidae it is roofed mainly by the squamosal. The 

recess in the Tragulidae resembles that of Merycoidodon except that the tympanic bone, in addition to the 

squamosal, forms its lateral boundary. This is because 

the "tympanicumdefekt" in the Tragulidae is smaller.

The epitympanic sinus has less functional importance than the recess, and is correspondingly more 

varied. In many mammals it is absent, but in others, 

such as the Notoungulata (Simpson, 1936) it attains 

enormous size.

In Merycoidodon, the epitympanic sinus is absent, 

the part of the squamosal above the epitympanic recess 

being a solid plate containing no cavities. This is al^o 

true, so far as could be determined, of all the living 

Artiodactyla.

In addition to the large cavities of the middle er.r, 

there is in Merycoidodon a very small cavity, the 

incisura tympanica (fig. 20, IT), which lies just dorsal 

to the tympanic membrane. Its lateral wall is formed 

by the facies epitympanica of the squamosal, and its 

roof by an overhanging process from that bone. Ventrally it is bounded by the tympanic bone. In t^e 

fossil it is open medially into the epitymanic recess, 

but in life this opening was covered by a membrane 

(the membrana shrapnelli or pars flaccida), a nonfunctional part of the tympanic membrane. Its lower 

border is the dorsal part of the sulcus tympanicus, the 

part of the tympanic ring in which the tympanic membrane was held and stretched taut.

The position of the incisura tympanica in Merycoidodon is the same as that in the Suidae. Data concerning its condition in the Ruminantia are incomplete, 

but it is known to be large in the Cervidae, and absent 

in the Camelidae and Tragulidae.

The most striking external feature of the ear region 

(although smaller in Merycoidodon than in most mammals) is the auditory bulla (fig. 15, B). The most anterior part of the bulla is, as in all other animals with 

small or medium-sized bullae, the styliform process. 

This slim anteriorly projecting extension is applied 

closely to the lower surface of the squamosal and, in 

Merycoidodon, branches at its anterior end into a 

medial and a lateral arm, each about 1 mm. long. It 

formed the lateral wall of the ostium tympanicum 

tubae (see above), and probably, as in living Artiodactyla, provided the origin for the levator veli musoK

In the living Cervidae and Bovidae the stylifora 

process is present and probably performs the same 

functions as in Merycoidodon. In the living Camelidae the anterior wall of the bulla forms the lateral 

covering of the ostium tympanicum tubae, and there 

is only a suggestion of a styliform process. No process 

at all is present in Tragulus or the Suidae.

Just posterior to the styliform process, the bulla of 

Merycoidodon is slightly notched for the reception of 

the eustachian tube, which passes medial to it.

The bulla in Merycoidodon is a low inflated hemisphere of bone situated between the postglenoid process 

and the basioccipital. Its longest dimension is tl °- 

antero-posterior one, and it does not project so frr 

ventrally as does the postglenoid process. Laterally it 

overlaps ventrally the facies epitympanica of the squamosal; medially it is separated from the basioccipital
134 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

by a broad groove that in life held the inferior petrosal 

venous sinus (fig. 15, PEG}. Posteriorly, the bulla 

narrows to a ridge, which is suturally attached to the 

antero-lateral side of the paroccipital process of the occipital bone (fig. 14:). Here it forms a part of the 

medial wall of the articulation for the tympanohyal 

bone. In contrast with some modern Artiodactyla, the 

bulla is not fused to the pars petrosa of the periotic, 

and there is a gap between the two through which the 

internal carotid artery probably passed (fig. 21, FCA ).

The bulla of Merycoidodon is smaller in proportion 

to skull size than that of any modern artiodactyl, 

although the Cervidae and Qiraffidae have a small 

bulla. In most of the living genera there is a tendency to develop a large bulla, that is laterally 

compressed and reaches ventrally well below the postglenoid process.

The presence of a gap between bulla and basioccipital 

is correlated with the size of the bulla, and occurs 

among modern Artiodactyla only in the Cervidae and 

Giraffidae. No such direct explanation can be found, 

however, for the presence or absence of fusion of the 

bulla tq the pars petrosa of the periotic. The two are 

independent of each other, as in Merycoidodon^ in the 

Suidae, Hippopotamidae, Camelidae, and Caprinae. 

In the Suidae alone of these the two bones are in contact. They are fused together in the Bovidae and in 

some Cervidae. The most obvious explanation of these 

various conditions in living Artiodactyla is the degree 

of development of the internal carotid artery. In living Artiodactyla this is present in the late embryological stages but degenerates and becomes small or 

disappears entirely in the adult. Thus the presence of 

a gap between bulla and pars petrosa and its size, 

if present, may depend upon the ontogenetic stage at 

which the internal carotid disappeared or ceased to 

function. This is borne out by the fact that in Suidae, 

where a gap is present, the artery is functional, though 

greatly reduced. This is also true in Catnelus.

One of the chief problems concerning the auditory 

bulla has been that of its composition. It was first 

thought to be formed by one bone, the tympanic (more 

properly ectotympanic). Later it was discovered that 

another bone, the entotympanic, sometimes participates 

in the formation of the bulla. This is a neomorph in 

mammals, and ossifies relatively late in ontogeny. As 

its name indicates, it forms, if present, the medial side 

of the bulla. More and more instances of the presence 

of the entotympanic in the wall of the bulla are being 

brought to light (van Kampen, 1905; van der Klaauw, 

1931).

It has been the consensus of opinion that in the 

Artiodactyla (with the very doubtful exception of the

Suidae: Parker, 1886) the bulla is composed entirely 

of the ectotympanic.

The composition of the bulla remains largely a matter 

of opinion, as stated above, because any elements forming it are in almost every case completely fused together in the adult stage. In a few, however, a suture 

is present in the adult, indicating the presence of both 

ectotympanic and entotympanic. Such a suture was 

found in the sectioned skull of Merycoidodon (f sj. 20, T 

and fig. 21, ET) and in 1 of 12 other Merycoidodon 

skulls examined. It thus seems possible that tin is genus 

is an exception to the previously accepted rule as to 

the composition of the bulla in the Artiodactyla.

The auditory bulla is bordered by several openings in 

the skull. Among these is the fissura glaseri (figs. 15 

and 21, FG) , through which the chorda tympani branch 

of the facial nerve passes from the tympanic cavity on 

its way to anastomose with the mandibular branch of 

the trigeminal nerve (see Goodrich, 1930, p. 462). Into 

the inner end of the fissura glaseri projected the processus folii of the malleus, a remnant of Meckel's 

cartilage.

The fissura of Merycoidodon is completely surrounded by the ectotympanic bone except for a space 

of about 2 mm on its antero-lateral side where the 

squamosal participates in its wall. This surface of the 

squamosal (fig. 21, MF] is known as the margo fissurae. 

This condition differs from that in living mammals 

in which the entire lateral wall of the fissura glaseri 

is formed by the margo fissurae of the squamosal.

Between the ventral crest of the external auditory 

meatus and the posterior crest of the tympanic bone, 

which is suturally attached to the paroccipital process, 

lies the vagina processus hyoidei, the pit in which is 

lodged the cranial end of the tympanohyal bone. It 

lies posterior to the inflated part of the bulla, and is 

separated by a low ridge of the occipital bone from 

the stylomastoid foramen, which lies posterior and 

slightly lateral to it (fig. 15, SM; the vagina processus 

hyoidei is not labelled in this figure).

The vagina is seen, in section, to be an excavation in 

the thick portion of the ectotympanic bone that forms 

a part of the external auditory meatus (fig. 1£, VPH).

In the modern Artiodactyla, as in Merycoidodon, the 

vagina processus hyoidei is well separated from the 

stylomastoid foramen. In the living families their 

swollen bullae grow around the tympanohyal, making 

the vagina deeper and more conspicuous. In some 

families this posterior inflation of the bullae \s so great 

that the tympanohyal springs from the side of the 

bulla instead of behind it (Tragulidae, Bpvida^). The 

inflated bullae may entirely surround the tympanohyal, 

forming a pit. This is true of the Tragulidae, Giraffidae, Camelidae, and some of the Bovidae.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA J35

The stylomastoid foramen (figs. 15 and 19, FSM] lies 

just posterior to the vagina processus hyoidei. It opens 

into the extreme posterior end of the tympanic cavity, 

separated from the foramen lacerum posterius by the 

paroccipital process. Merycoidodon thus resembles 

the living Ruminantia and the early Tertiary bunodonts (presumably Non-Ruminantia) (Colbert, 1938). 

It differs from the living Non-Ruminantia, in which 

the stylomastoid foramen is confluent with the foramen lacerum posterius.

The facial nerve leaves the tympanic cavity through 

the inner opening of the stylomastoid canal. This is the 

foramen stylomastoideum primitivum. Here the chorda 

tympani leaves the facial nerve and goes forward into 

the tympanic cavity.

EXTERNAL AUDITORY MEATUS

This bony tube (fig. 14, MAE\ fig. 15), which connects 

the middle ear with the exterior of the skull, has already 

been discussed in part under the consideration of the 

squamosal bone, which forms its dorsal, posterior and 

part of its ventral wall.

The remainder of the meatus is formed by the ectotympanic bone; in other words, it is a direct lateral 

extension of the auditory bulla. The meatus is much 

more heavily ossified than the bulla. Its course from 

the exterior is inward and slightly downward for most 

of its length. At the medial border of the postglenoid 

process, it turns slightly anteriorly and enters the 

bulla.

At its entrance into the bulla, the ectotympanic wall 

of the meatus projects about 5 mm. into the tympanic 

cavity (fig. 20, CT). This, projection is called the 

crista tympanica of the meatus (Bondy, 1907; Simpson, 

1936). The tympanic cavity extends laterally both 

anterior and posterior to the crista; such an extension 

is known as a recessus meatus. This is also present in 

all living Artiodactyla.

The medial termination of the external auditory 

meatus is an almost complete ring (it is open for a few 

millimeters dorsally) upon which was stretched the 

tympanic membrane (fig. 20, CT). This semi-ring 

represents the earliest point of ossification of the ectotympanic. The dorsal gap in the tympanic ring is 

filled by the margo tympanici of the squamosal, a medially projecting process of the superficies meatus, which 

forms the dorsal wall of the external auditory meatus 

(fig. 20, MTS, SMS}. This is in contrast with most 

living genera of Ruminantia and also with Hippopotamus (van Kampen, 1905, p. 592). In these genera, the 

entire external auditory meatus is formed by the tympanic bone. To the margo tympanici was attached the 

dorsal edge of the tympanic membrane.

The orientation of the external auditory meatus

(pointing backward and slightly upward) agrees with 

that of the Ruminantia and differs sharply from that 

of the Non-Riuninantia.

PARS MASTOIBEA OF THE PERIOTIC BONE

This is the posterior part of the ossification of the 

otic capsule, and is distinguished by its cancellous 

structure from the pars petrosa, which is rock-like in 

its density.

The pars mastoidea is exposed on the occipital surface of the skull as a plate of bone (M, fig. 14) lateral 

to the occipital bone and medial to the lambdoid cr?-st. 

This plate terminates ventrally between the posttympanic and paroccipital processes as the very small m astoid process, quite insignificant as an area of murole 

attachment. Medially, this process is in contact with 

the paroccipital process.

Sections show that the mastoid process is in contact 

with the posttympanic process not only posteriorly, but 

also medially. A small flange (M, fig. 19) projects 

anteriorly from the mastoid, between the posttympanic 

process and the pars petrosa, and forms part of the 

lateral wall of the stylomastoid foramen.

Because the pars mastoidea is exposed at the surface 

of the skull, Merycoidodon is one of the "mastoid Artiodactyla," one of the two divisions of the order suggested by Helga Pearson (1927). In the other division, 

the "amastoid Artiodactyla," the pars mastoidea is covered by a flange of the squamosal, which overlaps upon 

the occipital plate.

Miss Pearson limits her two groups as follows (1927, 

p. 458) :

Choeropotamus, Cebochoerus, Hippopotamus, the Anthrncotheriidae, Mixtotherium, Entelodon, and the Suidae appear to 

belong to one group of Artiodactyla, while the Dichobunidae, 

Dacrytherium, the Anoplotheriidae, Tapirulus, Amphimeryaa, the 

Cainotheriidae, together with the remaining post-Eocene families, form another. These may conveniently be termed the 

"amastoid" and the "mastoid" Artiodactyla respectively.

In most living Ruminantia, as in Meryeoidodon, the 

pars mastoidea is well exposed upon the occipital surface. However, in the Giraffidae and the Camelidae, 

the posttympanic process touches the paroccipital 

process, and covers the ventral surface of the p^.rs 

mastoidea.

For the purpose of comparison with Merycoidodon, 

the mastoid regions of a series of Merycoidodontidae 

were examined at the American Museum of Natural 

History through the courtesy of Dr. C. Bertrgnd 

Schultz and Mr. Charles H. Falkenbach. This series 

of skulls, culminating with the genus Brachycrus, of 

late Miocene and Pliocene time, exhibits a progressive 

tendency of the external auditory meatus to migrate 

upward and backward between the squamosal and the
136 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

5 INCHES

FIGURE 24.—Merycoidodon culbertsonii, same as figure 14, but with nasal cavity and pneumatic sinuses outlined. X %•

Parallel lines slanting to right: nasal cavity. 

Parallel lines slanting to left: pneumatic sinuses. 

F8, frontal sinus.

MS, maxillary sinus.

NG. nasal cavity.

SPS, sphenopalatine sinus.

exoccipital. These two bones in Brachycrus show the 

beginning of a tendency to grow over the auditory tube; 

the continuation of this trend, had the Merycoidodontidae survived beyond the Pliocene epoch, would have 

resulted in the amastoid condition typical of the nonRuminantia.

Thus we see, within one well-defined family, an almost complete transition from one to the other of Miss 

Pearson's two artiodactyl types. The possibility of 

such a transition is suggested by Miss Pearson in her 

statement (1927, p. 459) that the mastoid Artiodactyla 

show a tendency toward reduction of the size of the 

pars mastoidea.

These observations prove that the mastoid condition is a primitive one relative to the amastoid. All 

Artiodactyla are evolving more or less in the direction 

of the amastoid condition; the Non-Ruminantia attained it in Eocene time, the Merycoidodontidae approached it in Pliocene time. The Ruminantia, 

however, show at present only a slight tendency toward 

it. The chief difference, then, between the mastoid and 

amastoid Artiodactyla is in the relative speed with 

which the mastoid region is evolving.

PNEUMATIC SINUSES

These sinuses have been described individually under 

the discussion of the bones that contain them. Here 

they will be discussed as a system, and compared with 

the systems of other Artiodactyla.

The pneumatic sinus system of Merycoidodon is 

shown in figure 24. It consists of three sinuses, each 

opening directly into the nasal cavity (NC].

The sphenopalatine sinus (SPS] projects the farthest 

posteriorly of the three. It is a long, narrow sinus 

whose posterior end is contained in the presphenoid 

bone and whose anterior part is surrounded by the palatine and the orbitosphenoid. It extends posteriorly 

below the anterior part of the cerebrum.

The development of the sphenopalatine sinus as seen 

here is typical of large mammals. In the smaller ones 

the development of the orbits and braincase, which are 

very large in proportion to their skull size, have restricted the growth of the presphenoid bone. Thus the 

sphenoid portion of the sphenopalatine sinus is reduced 

or absent.

The frontal sinus (FS] terminates posteriorly in the 

frontal bone. It is the only pneumatic sinus cortained 

in the roofing bones of the skull.

The maxillary sinus is the largest of the three, and 

is most widely open into the nasal cavity. Its posterior 

termination is a blind end in the zygomatic process of 

the maxilla.

The extent of the pneumatic sinus system is to a large 

degree determined by skull size. The raison d'etre of 

the pneumatic sinuses seems to be to effect an adjustment between the space necessary to house tH soft 

parts within the skull and the external area necessary 

for muscle attachment, lodgment of the teeth, etc. The 

larger the skull, the greater is likely to be the disparity 

between the space thus required within and outside 

the skull.

Among the Artiodactyla, the sinus system of Merycoidodon most closely resembles that of Ovis ; the skulls 

of M. cuZbertsonii and of O. aries are about the same 

size. The chief difference is that Ovis lacks the sphenopalatine sinus, which is well developed in Merycoidodon. Also, the frontal sinus in Ovis is larger and, 

unlike that of Merycoidodon, communicates around the 

orbit with the maxillary sinus.

The small size of the cerebrum in Merycoidodon 

explains the presence and size of the sphenopalatine 

sinus; except for this, the sinus system of the genus
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 137

is less extensive than in most modern Artiodactyla of 

comparable size. This is the more astonishing when 

one considers that the small brain is surrounded by a 

great thickness of parietal and squamosal bone that 

might be expected to be pneumatized, as it is in the 

Suidae which have a parietal pneumatic sinus extending ventrally into the squamosal and thence into the 

zygomatic arch.

The Suidae also possess a well developed sinus in the 

horizontal plate of the maxilla (see Dictyles, fig. 18), 

which is absent in M'erycoidodon.

Of the Ruminantia, only the Cervidae, Tragulidae, 

and Camelidae have a less extensive pneumatic sinus 

system than Merycoidodon (Aubert, 1929). The first 

two of these families possess only a maxillary sinus; 

the last, frontal and sphenopalatine sinuses but no 

maxillary.

The sinus system of Merycoidodon is not a highly 

evolved one; the sinuses connect only with the nasal 

cavity, and have no connections with each other (compare the extremely complicated systems of the Giraffidae and of most Bovidae; Paulli, 1900).

The sinus pattern of Merycoidodon most closely resembles that of the Camelidae among the Artiodactyla, 

and bears least resemblance of all to that of the living 

Non-Ruminantia. However, the skulls of the latter 

are highly specialized; and unfortunately the arrangement of sinuses in their fossil relatives is unknown.

It is probably best to regard the sinus arrangement 

of Merycoidodon as primitive. It has no peculiarities 

to indicate the beginning of a trend to any one of the 

bizarre modifications of the system in late Tertiary 

Artiodactyla.

CIRCULATORY SYSTEM OF THE SKTJUL

In describing the bones of the skull, mention was 

made of all grooves, foramina, and canals that give 

evidence of the cranial circulation in Merycoidodon. 

This circulation will here be compared with that of 

other artiodactyls, and of various other mammals.

ARTERIES

The arterial system of Merycoidodon appears to have 

been in all but one respect the same as that of living 

Artiodactyla, both ruminant and non-ruminant (fig. 

17). The difference was in the size and course of the 

internal carotid artery. This vessel is present and 

functional, but very small, in the Suidae (Sisson and 

Grossman, 1938) and the Camelidae (Lesbre, 1903); 

in the Tragulidae (Tandler, 1899) the internal carotid 

is better developed than in all other living ungulates. 

It is generally absent as a rule in other living Artiodactyla (Hofmann, 1900), but has been reported in 

Cervus tarandus, and probably exists in other genera,

238435—53———4

as an essentially non-functional vestige (Tandler, 1899, 

p. 703).

The course of the internal carotid artery in Merycoidodon is very well indicated, .and shows the artery 

to have been a large one (fig. 17, 1CA ). Undoubtedly 

it branched from the common carotid artery, as it does 

in living mammals, and extended dorso-laterally to the 

base of the skull. There the first evidence of its 

presence in Merycoidodon is seen.

A canal that passes dorso-laterally between the posterior extension of the tympanic and the pars petrosa 

of the periotic is identified as the posterior carotid 

canal. Its opening to the exterior is through the foramen lacerum posterius, and it is through this foramen 

that the internal carotid artery enters the skull (figs. 

15-17, FLP).

After entering the foramen lacerum posterius, the 

internal carotid diverged from the other vessels that 

passed through the foramen, all of which ran medial, 

instead of lateral, to the pars petrosa. It then passed 

through the posterior carotid canal, and turned anteriorly into the tympanic cavity.

Although the internal carotid can be definitely traced 

into the tympanic cavity, there is no evidence ai- to 

its course through the cavity. Further proof that it 

crossed this region is, however, furnished by the large 

antero-medial opening in the bulla, just behind the 

ostium tympanicum tubae (fig. 15, OTT), which is 

obviously the anterior carotid foramen. Here the 

artery left the tympanic cavity, lay for a short distance 

between the bulla and basisphenoid, and entered the 

cranium through the foramen lacerum medius (fig. 17, 

FLM). It lay on the floor of the cranium in a groove 

(fig. 16) whose size indicates that the internal carotid 

artery was an important vessel in Merycoidodon. It 

did not divide to form a rete mirabile, or network of 

small arteries, but extended undiminished to the chiarma 

ridge, where it divided into the anterior and middle 

cerebral arteries. Posterior to the sella turcica, the 

internal carotid arteries of the two sides were connected by the posterior intercarotid artery (fig. 16). 

There is no groove in the base of the braincase to 

indicate the presence of an anterior intercarotid artery.

A comparison of the basis cranii of Merycoidodon 

with that of its probable ancestor, Protoreodon of the 

late Eocene, gives a clue as to the primitive position 

of the internal carotid artery in the Merycoidodontidae. 

In Protoreodon (M. C. Z. 5334), the posterior carotid 

canal opens on the base of the skull through a foranen 

independent of the foramen lacerum posterius, 3 mm 

anterior to it on the medial side of the posterior part 

of the bulla. This arrangement is the same as that in 

the living Lepus (Gregory, 1910, p. 329), the Creo-
138 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

donta (Matthew, 1910, p. 297), all Carnivora, and 

some Nptoungulata (van der Klaauw, 1931). The anterior carotid foramen, present in Merycoidodon and 

in Protoreodon, and situated in the antero-medial wall 

of the bulla, is also reminiscent of these mammals.

In the few modern Artiodactyla that have an internal 

carotid artery, its path differs considerably from that 

in Merycoidodon, in which it traverses the tympanic 

cavity. The internal carotid in the non-Ruminantia 

describes a loop ventral to the auditory bulla, so that 

the first contact of the artery with the skull is at its 

entry into the foramen lacerum medius (Sisson and 

Grossman, 1938; van Kampen, 1905).

In the living Camelidae, the internal carotid artery 

is in a completely closed bony canal whose laterial wall 

is formed by the medial wall of the bulla, its medial 

wall by the basioccipital.

The internal carotid in the Tragulidae also lies along 

the medial wall of the bulla. In Tragulus it is enclosed in a canal formed entirely by the tympanic bone; 

in the other genera it simply lies in a shallow groove 

(van Kampen, 1905).

The above comparison of Merycoidodon with the living Artiodactyla shows that there are found, within 

this order, two of the three possible courses of the 

internal carotid through the tympanic region. The 

third course, with one branch of the artery passing 

through the bulla and another lying medial to it, is 

found only in the Muridae (van der Klaauw, 1931, 

p. 180).

Within the cranium, the groove for the internal 

carotid (fig. 16, 10A) is evidence for the statement that 

this artery was much larger than in living Artiodactyla. 

It also differed from the latter in the absence of a rete 

mirabile, as has been pointed out by Black (1921) on 

the basis of his study of natural endocranial casts. 

Thus the intracranial portion of the internal carotid in 

Merycoidodon resembles that of the Carnivora (see 

Ellenberger and Baum, 1891, p. 373), but differs from 

the latter, in the absence of the anterior inter-carotid 

artery.

VEINS AND VENOUS SINUSES

The system of vessels draining the blood from the 

cranium of Merycoidodon (fig. 17) was considerably 

more complicated than in present-day Artiodactyla, as 

it contained several additional vessels.

As is usual in the Mammalia, an important part in 

the head drainage in Merycoidodon was played by the 

venous sinuses, which are enclosed by dense membranes 

and usually lie in bony grooves. They consist of two 

groups, the dorsal and basilar sinus systems, and their 

attendant veins. A third venous system is that of the 

subsphenoid canals of the basisphenoid bone, a venous

arrangement here observed for the first time in the 

Artiodactyla.

DORSAL SINUS SYSTEM

The vessels of this group lie on the dorsal and dorsolateral sides of the brain. The most anterior of them 

is the sagittal sinus, a median vessel lying directly 

above the longitudinal fissure of the brain ard just 

below the sagittal crest (fig. 22, SS). Its length, as 

indicated by the groove it occupied in the roof of the 

braincase, was 4 mm.

No impressions in the braincase indicate the anterior 

and posterior connections of the sagittal sinus. We 

may assume, from the anatomy of modern Artiodactyla, 

that the sagittal sinus of Merycoidodon collected venous 

blood from the numerous small veins of the surface of 

the cerebral hemispheres, and transmitted it posteriorly 

into the transverse sinus, between the cerebrum and 

cerebellum.

Ventro-lateral to the sagittal sinus, on the side walls 

of the pyriform lobes, lies the vena collateralis cerebri, 

which is the only connection in Merycoidodon between 

the veins of the face and orbit and the dorsal sinus 

system. The presence of this vein is indicated by an 

antero-posterior groove in the side wall of the braincase 

(fig. 16, Ftftf).

The vena collateralis cerebri entered the cranium 

through the sinus canal (fig. 16, PSC], which carried 

it from its origin in the orbital venous plexus. The 

groove for the vena collateralis cerebri extends backward as far as the posterior part of the cerebrum where, 

presumably, it joined the transverse sinus near its entry 

into the temporal canal.

A vein almost identical with that indicated in Merycoidodon is found in many modern Insectivora. Gregory (1910, p. 248) reports it in Solenodon, Microgale, 

and Erinaceus; and Shindo (1915) notes its presence 

in Talpa europaea. In the last, however, th^s vena 

collateralis cerebri enters the cranium through tte optic 

foramen rather than an independent canal. According 

to Shindo, this vein is a remnant of the anterior cerebral 

vein of the early embryonic stages. In some living 

mammals, the back part of the vena collateralis cerebri 

is present, but the anterior connection with the orbital 

plexus never develops. See Lepus (Shindo, 1915, p. 

370).

In the possession of this vein in its entirety, Merycoidodon differs from all but the most primitive of 

living mammals.

The chief path by which the venous blood of the 

dorsal sinus system left the cranial cavity was the 

temporal canal (see p. 130). The blood passed from 

the transverse sinus into at least three veins, the supe-
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 139

rior cerebral, the vein of the postparietal canal, and 

the mastoid emissary.

The superior cerebral vein entered the temporal 

canal through a foramen between the pars petrosa and 

the endocranial surface of the squamosal. This 

foramen opened into the sinus venosus temporalis (fig. 

20, SVT), which lies almost entirely in the squamosal. 

Besides receiving blood from the superior cerebral vein, 

this sinus undoubtedly collected it from the tiny veins 

filling the thick diploe of the squamosal. From the 

sinus the superior cerebral vein passed ventroposteriorly into the temporal canal (fig. 20, TC; fig. 21, 

FJSP), leading it through the foramen jugulare 

spurium (figs. 15, 17, TO) to join the external jugular 

vein (fig. 17, EJV).

The foramen jugulare spurium is very small considering the size of the sinus venosus temporalis, which 

it drains but this disparity can be explained. Probably the temporal sinus served chiefly as a reservoir, in 

other words, the pressure of the blood therein was low. 

Besides, a posterior branch of the temporal canal (see 

below) relieved the foramen jugulare spurium of the 

necessity of transmitting all the blood in the temporal 

sinus.

Although it has not been reported in any living 

mammals, the sinus venosus temporalis cannot be regarded as a primitive structure. It is, however, an 

expression of the small size and correspondingly primitive condition of the cerebrum of M' erycoidodon.

The temporal canal is present in the Ruminantia, but 

absent in the Non-Ruminantia.

The posterior branch of the temporal canal begins a 

few millimeters above the foramen jugulare spurium. 

Thence it extends postero-laterally to its exit from the 

skull through a foramen between the external auditory 

meatus and the postglenoid process (fig. 17, PTC), 

After leaving the skull, this vein also joined the external jugular. The vein homologous with this one 

is the only external orifice of the temporal canal in 

the modern Artiodactyla and the living Equidae and 

Carnivora (Dennstedt, 1903).

Another vessel that drained the transverse sinus of 

Merycoidodon was the vein of the post-parietal canal, 

which extended backward beneath the parieto-squamosal suture. Just before its exit from the skull, this 

vein in some cases divided into two or more small 

branches. It is represented in Merycoidodon by the 

post-parietal foramina (fig. 14, FPP). This vein is 

present in many modern, mammals, including some 

Artiodactyla (see above).

Some distance ventral to the postparietal canal is the 

passage for another vein, the mastoid emissary. This 

leads from the temporal canal back to the mastoid foramen on the occipital surface of the skull. It probably 

drained posteriorly into the vertebral or the deep cervical vein and thence to the anterior vena cava.. The 

mastoid emissary vein is present in living Ruminantia; 

it is absent in the Non-Ruminantia, probably because 

the overlapping of the squamosal bone upon the occipital surface caused a rerouting of the blood by 

covering the area where the mastoid foramen lay.

The dorsal petrosal sinus is the last member of the 

dorsal sinus system of which traces are found in the 

skull of Merycoidodon. This vessel was, as in living 

mammals, a tributary of the superior cerebral vein, 

which it joined at its entrance to the sinus vencsus 

temporalis. The dorsal petrosal sinus gathered venous 

blood from the lower part of the cerebral hemispheres 

and, before joining the superior cerebral vein, coursed 

posteriorly in a groove in the dorsal side of the pars 

petrosa. Judging from the depth of this groove, the 

sinus was rather small (fig. 21, DPS).

BASILAB SINUS SYSTEM

These vessels in Merycoidodon, as in modern forms, 

were much more closely connected with the anteroposterior drainage of the head than was the dorsal 

system. There is no evidence that the two systems were 

connected in any direct manner.

The basilar sinus system, judging from the condition 

in living mammals, received blood from the nasal cavity 

and maxillary region by way of the orbital venous 

plexus. From this plexus a large vein extended backward through the foramen lacerum anterius (fig. 15, 

FLA). Within the cranium it entered the cavernous 

sinus, which lay posteriorly in a groove on the cranial 

floor (fig. 16, 1C A).

Behind the pituitary fossa (fig. 16, PF), the cavernous sinuses of both sides were connected by the posterior 

intercavernous'sinus. In this respect Merycoidodon 

differs from Sus (Dennstedt, 1903) which has no posterior intercavernous sinus.

Merycoidodon had no anterior intercavernous sirus. 

In this it differs from Bos but resembles the Caprinae 

(Dennstedt, 1903).

The cavernous sinus debouched from the braincase 

through the foramen lacerum medius (fig. 15, FLM). 

Here it was continuous posteriorly with the inferior 

petrosal sinus, a large vessel that was contained in a 

groove formed dorsally by the pars petrosa and medially by the basioccipital bone. This groove is the petrobasilar canal (figs. 14,16, PBO).

The internal carotid artery of Merycoidodon passed 

lateral to the pars petrosa, and the inferior petrcsal 

sinus ventro-medial to it (fig. 17). As well as co^ld 

be determined from study of the skull of Camelus (for
140 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

the literature is vague on this point) these two vessels 

in that genus run side by side in the petrobasilar canal. 

This is also true of the living Perissodactyla (Sisson 

and Grossman, 1938).

In the posterior third of its course (fig. 15) the petrobasilar groove in Merycoidodon lies medial to the pars 

petrosa, and is impressed in the basioccipital only. Beyond this groove, the extracranial part of the inferior 

petrosal sinus probably emptied into the inferior cerebral vein, and thence to the internal jugular (fig. 17).

The serial sections revealed a minute canal extending 

posteriorly in the backward extension of the alisphenoid 

bone above the auditory bulla (fig. 21', VCLf}. This 

canal probably carried a vestigial vein, so small as to 

be non-functional, which branched from the cavernous 

sinus at the foramen lacerum medius and passed into 

the dorsal side of the tympanic cavity. This course 

suggests that it may be a remnant of the embryonic 

lateral head vein: it runs medial (although posterior) 

to the alisphenoid bone, which is the homologue of the 

lamina ascendens of the embryonic ala temporalis. 

The ala temporalis, in turn, is homologised by Broom 

(1907) with the reptilian epipterygoid. Furthermore, 

the position of this small vein dorsal to the auditory 

ossicles is equivalent to a location dorsal to the jaw 

articulation of the Reptilia, where the lateral head 

vein lay.

To return to the functional portion of the basilar 

sinus system, we find that the inferior petrosal sinus, 

slightly anterior to the beginning of its extracranial 

portion, emitted a branch,, the condyloid vein, which 

continued posteriorly within the skull in a groove at the 

lateral side of the myelencephalic base. It found its 

exit from the skull through the condylar foramen (figs. 

15, 16, OF; fig. 17 CV) and joined the occipital vein, 

which probably transmitted the blood to the deep cervical or the vertebral vein. This constituted the second 

of the two outlets of the basilar sinus system.

SUBSPHENOID VEINS AND THE SINUS VENOSUS OSSIS SPHENOIDALI8

The evidence for the presence of this most unusual 

set of veins, hitherto unrecorded in Artiodactyla or in 

most other mammalian orders, consists of the welldeveloped canals by means of which they passed 

through the basisphenoid bone of Merycoidodon (see 

p. 122, and fig. 17).

A survey of anatomical literature proves beyond a 

doubt that the cavity in the basisphenoid bone (fig. 17, 

8VOS] contained a venous sinus, and that the canals 

leading from it to the surface of the skull (fig. 17, SSG} 

conveyed blood into it from the orbital region and out 

of it posteriorly.

Such a venous cavity in the basisphenoid was first

reported by W. Krause (1868; cited by Arai, 1907), 

who observed it in the human foetus and in 10 percent 

of new-born infants studied. Arai (1907) found the 

same structure in the adult Lepus cuniculus, and named 

it the sinus venosus ossis sphenoidalis (this rather cumbersome title is necessary to differentiate it from the 

sphenoidal pneumatic sinus). In Lepus, the upper 

wall of this sinus is open into the pituitary f osra, and 

the membranous wall of the venous sinus is in contact 

with the dura mater which surrounds the pituitary 

body. In Lepus, the venous sinus receives blood from 

the veins of the dura mater. In Meryooidodor, there 

is no such connection between the sphenoidal venous 

sinus and the pituitary fossa. This venous sinus has 

been regarded by most anatomists (Arai, 1907; Waldeyer, 1907; Bovero, 1905) as a remnant of Rathke's 

pocket. Voit (1909), however, feels that it is too far 

posterior and must be a secondary structure. As the 

sinus is found in the embryos of many mammals that 

lack it in the adult stage, the first hypothesis seems the 

more likely.

As in Merycoidodon, Lepus has one or more canals 

leading into the sphenoidal venous sinus from the exterior of the basisphenoid bone. These are designated 

by Arai (1907) the foramina venosa but, since tl ^y are 

the openings of what he calls the subsphenoid canals, 

they are here given the more specific title cf subsphenoid foramina.

A functional sphenoidal venous sinus with it" associated subsphenoid canals is found in the adult s*age in 

relatively few mammals. These are the Sciuromorpha 

and Myomorpha (Bovero, 1905), the Mars'ipialia 

(Gregory, 1910, p. 431) and the Lagomorpha (Arai, 

1907).

The manner of circulation of the venous blood of 

the head in the above-mentioned groups indicates the 

part which the sphenoidal venous sinus of Merifcoidodon played in its cranial circulation.

Anteriorly the subsphenoid canal must have received, as in these groups, a branch of the ophthalmic 

coming from the orbital plexus. Posteriorly, the 

passed out through a foramen in the anterior wall 

of the .foramen lacerum medius (fig. 16, FSP]. Thence 

the blood undoubtedly flowed into the posterior portion 

of the cavernous sinus, just anterior to the confluence 

of the latter with the inferior petrosal sinus.

It is apparent from the skulls of Merycoidodon which 

were dissected that the vessels of the sphenoidal sinus 

system were larger than the cavernous sinus and, therefore, that the sphenoidal vessels were among tte most 

important paths by which the blood of the facial region 

in Merycoidodon was drained posteriorly. We have 

no way, of course, of knowing the size in this genus of
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 141

the external maxillary vein, which is the chief posteriorly directed vein leaving the facial region of living 

Artiodactyla.

The facts here set forth show that, despite the size 

of the temporal canal, the preponderant venous outlet 

of the skull of Merycoidodon was the internal jugular 

vein rather than the external jugular as in' modern 

Artiodactyla (Bolk, Groppert etc., 1936). This fact is 

-proved by the large size of the sinus venosus ossis 

sphenoidalis and of the inferior petrosal sinus, and by 

the small size of the opening of the temporal canal, 

which led to the external jugular vein (fig. 17).

The internal jugular, a remnant of the lateral head 

vein, is the more primitive means of drainage. In this 

respect, Ovw and Bos are mammals of the most advanced type. In these genera, the external jugular vein 

drains the skull, and the last vestige of the lateral head 

vein is lost.

In Meryco-idodon, a large part of the blood from the 

face must have run posteriorly through the subsphenoid 

canals. In the more advanced Artiodactyla, the mechanical advantages of an extracranial route apparently prevailed; the subsphenoid canals degenerated, 

and the external maxillary vein carried the blood which 

they had once transmitted. This change in circulation 

took place much earlier in evolution than the equivalent 

one in the arterial system.

NERVES OF THE SKTJUL REGION

The nerves supplying the cranium of Meryooidodon, 

as described above from grooves and foramina in the 

skull bones, do not differ in any important characteristics from those of living Artiodactyla.

CRANIAL MORPHOLOGY OF POEBROTHERIUM

Poebrotherhwi is the genus which represented, in 

White Kiver (Oligocene) time, the ancestral line leading to the modern camels and llamas. Like the other

extinct camel-like mammals, it is included with the 

living genera in the suborder Tylopoda.

For this study, serial frontal sections were made of 

a skull of Poebrotheriutm wilsoni Leidy, from Oligocene 

rocks exposed near James Creek, in the Hat Crsek 

Basin, Sioux County, Nebraska. It was collected in 

1922 by a party from the Walker Museum, University 

of Chicago. Study of the serial sections was aided by 

comparison with four other skulls of P. wilsoni, and 

with one of P. eximium Hay, a larger species. No skulls 

of P. labiatum Cope, the only other species of the genus, 

were available for study; but reference has been made 

to descriptions and plates of the skull of this species 

(Scott, 1891,1940).

The anatomy of the skull of Poebrotheriwn will not 

be described here in as great detail as was that of Merycoidodon; it will suffice to point out some significant 

similarities and differences between the two, especially 

those made available for the first time by the use of 

serial sections.

BOXES OF THE SKULL,

As in Merycoidodon, the basioccipital, exoccipitals 

and supraoccipital are fused in Poebr other-turn into an 

occipital bone (fig. 25, OC}. The paroccipital process 

of this bone (fig. 25, PPO} is suturally attached for its 

entire length to the auditory bulla. This is due chiefly 

to the great inflation of the latter. Another result of 

this inflation is that the basioccipital is in contact with 

the medial wall of the bulla (fig. 26), there being no 

gap between them as in Merycoidodon.

Within the braincase, the course of the condyloid vein 

is indicated by an antero-posterior groove near the 

lateral edge of the basioccipital. This is present also 

in Merycoidodon.

The most striking characteristic of the basispheiioid 

bone (fig. 26, BS] is that, as in Merycoidodon, it contains a large sinus venosus ossis sphenoidalis, with its

2 INCHES

FIGURE 25.—PoeT>rotherium wilsoni. Lateral view of skull. Modified after W. B. Scott. X

F8M, stylomastold foramen.

OO, occipital bone.

PA, parietal bone.

PM, pars mastoidea of periotic bone.

PPO, paroccipital process of exoccipital. 

SQ, squamosal bone.

TEAM, expanded external auditory meatus. 

TY, tympanic bone.
142 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

PPO

FIGURE 26.—Poebrotherium wilsoni, ventral view of basis cranii. Composite, drawn from Princeton Museum no. 12692 and M. C. Z. 5998, 

X %.

B, auditory bull a.

BO, basiocciptal bone.

BS, basisphenoid bone.

OF, condylar foramen.

FLM, foramen lacerum medius.

FLP, foramen lacerum posterius.

FOj foramen ovale.

F8A, anterior subsphenoid foramen.

FSM, stylomastoid foramen.

G, groove separating bulla from external auditory meatus.

OS, glenoid surface of squamosal bone.

IM, inflated part of external auditory meatus.

MAE, external auditory meatus.

PPO, paroccipital process of exoccipital.

tributary subsphenoid canals (fig. 28, SVOS: cf. fig. 26, 

F8A). The sphenoidal venous sinus is larger than in 

Merycoidodon because of the greater dorso-ventral 

dimension of the basisphenoid. The sinus becomes 

narrower anteriorly, where the portions of the basisphenoid lateral to the sinus are filled with large-celled 

cancellous tissue. The sphenoidal venous sinus terminates anteriorly below the anterior end of the pituitary 

fossa; this differs from the condition in Merycoidodon^ 

in which the anterior end of the sinus lies below the 

posterior end of the fossa.

Within the cranial cavity, the basisphenoid is excavated by the pituitary fossa, which is shallow, as in 

Merycoidodon and the living Tylopoda (Lesbre, 1903). 

The anterior end of the fossa is marked by an eminence 

which rises about 2 mm. above the floor of the braincase. 

At its posterior end are the posterior clinoid processes, 

whose bases are about 3 mm. apart. They are very 

delicate, and arch toward each other, almost touching 

at the midline of the skull.

Along either side of the pituitary fossa lie the grooves 

for the cavernous sinus and internal carotid artery and, 

lateral to these, the grooves for cranial nerves III, IV, 

VI, and the first two branches of nerve V. These pathways are separated by a very low ridge. Lateral to the 

groove which carries the cranial nerves is a higher, 

sharp ridge.

The alisphenoid bone of Poebrotherium differs from 

that of Merycoidodon in that it lacks a posteriorly 

projecting plate above the ostium tympanicum tubae. 

This function is taken over by the squamosal (see below). The posterior portion of the alisphenoid is in 

sutural contact with this horizontal plate of the squamosal, but does not reach backward as far as the 

ostium.

Within the cranium, a few millimeters medial to the 

alisphenoid-squamosal suture, a deep groove run? anteriorly from the region of the foramen lacerum medius 

to the foramen ovale (fig. 26, FO; hidden by the auditory bulla). Judging from its course, this must have 

borne the third (mandibular) branch of cranial nerve V.

The alisphenoid of Poebrotherium further differs 

from that of Merycoidodon in that the cavity for the 

sinus venosus ossis sphenoidalis extends laterally into 

it, as in the living Leporidae (Arai, 1907).

The deep, narrow presphenoid bone is of nearly the 

same shape as that of Merycoidodon. It is filled with 

cancellous tissue of much finer texture than that present 

elsewhere in the Poebrotheriuni skull.

At its anterior end the presphenoid is no narrower 

than in Merycoidodon, but, in contrast to this genus, 

it contains no pneumatic sinus.

On the external surface of the skull of Poebrotherium, the orbitosphenoid bone is in about the sam^ position as in Merycoidodon. Within the cranium it shows 

an interesting difference in that its posterior roof projects inward beneath the cerebrum as a curved, dorsally 

convex plate roofing the cranial nerves (IV, V±, V2, VI), 

which lay in a groove lateral to the pituitary f os^-a. In 

the anterior part of their intracranial course, therefore, 

these nerves were surrounded on all but the medi al side 

by bone (fig. 28, FLA). This plate of the orbitosphenoid is probably a secondary ossification rather than a 

homologue of the lateral lamina of the sella turcica in 

Merycoidodon.

Where it forms part of the anterolateral wall of the 

cerebrum, the orbitosphenoid reaches its maximum 

thickness of 5 mm. This is the thickest part of the 

braincase wall, which as a whole is distinguished from 

that of Merycoidodon by its light construction. The 

thickened portion of the orbitosphenoid is solidly ossified, in contrast to the other bones of the Poebrotherium 

skull, which tend to be filled with large-celled cancellous tissue.

In the skull of Poebrotheriunn which was serially sectioned, almost no trace remained of the ethmoid bone. 

A small fragment of the bony nasal septum appears 

approximately in place in one section; it is only 1 mm 

thick.

The only evidence concerning the characteristics of
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ABTIODACTYLA 143

the turbinals is a sharp ridge projecting ventrally into 

the nasal cavity from each nasal bone. These projections run the entire length of the nasals, which are very 

long in this genus; they undoubtedly served as roots 

for the naso-turbinal bone or cartilage. It may be inferred, then, that these scrolls at least were well developed in Poebrotherium. Similar ridges are present 

in the living Camelm. They support a cartilaginous 

nasoturbinal scroll.

The nasal septum and turbinal scrolls were probably 

much less heavily ossified in Poebrotheriivm, than in 

Merycoidodon.

The vomer was missing in the sectioned skull of 

Poebrotheriwm.

There is no separate ossification of the preinterparietal or interparietal in this skull.

The parietal bones are very long, and form almost 

the entire roof of the braincase. Posteriorly, the parietals conform much more closely to the shape of the 

brain (fig. 27) than do those of Merycoidodon. The 

vermis cerebelli of Poebrothermm was higher than that 

of Merycoidodon, projecting above the dorsal plane of 

the cerebrum, and very narrow. Its sides were nearly

vertical. These observations agree with the deductions 

drawn by Bruce (1883) and Scott (1940) on the Hsis 

of studies of damaged natural endocranial casts.

The posterior portions of the parietals are filled with 

cliploe for about 10 mm on either side of the sagittal 

crest (fig. 27). The diploic layer is thin, however, and 

in this region the maximum thickness of the parietals 

is only 5 mm.

Over the junction of the cerebrum and cerebellum, 

the parietals become thicker, and their diploic cells 

increase in size to an average diameter of 2 mm. Here 

the diploe spreads farther laterally than over the cerebellum, extending about 18 mm ventrolaterally from the 

sagittal crest.

- At their anterior termination, the parietals are again 

simple bony plates, devoid of cancellous tissue. This 

is true also of the posterior part of the frontal tone, 

dorsal to the anterior part of the cerebrum, which has 

an average thickness of only 2 mm.

The descending plate of the frontal, which forms 

part of the side wall of the braincase, is heavier than 

the dorsal plate and, in its posterior portion, consists 

of solid non-diploic bone like that of the orHtoPAR

TH

FIGURE 27.—PoebrotJierium wilsoni Leidy. Thick section of skull, prepared from sections P 1-17 

viewed posteriorly from level of vagina processus hyoidei. Natural size.

B, bulla.

BO. basioccipital.

FM, foramen magnum.

ICC. internal carotid canal.

IT. incisura tympanica

PAR, parietal.

PET, pars petrosa.

RE, recessus epitympanicus.

8F, subarcuate fossa.

SH, hypotympanic sinus.

SM, superficies meatus,

SQ, squamosal.

SVT, sinus venosus temporalis.

TO, temporal canal.

TH, tympanohyal.
144 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

sphenoid, which adjoins it. Farther anteriorly the 

large diploic cells of the supraorbital process are continued ventrally into the lateral wall of the olfactory 

bulbs.

The supraorbital process of the frontal is very thick 

dorsoventrally, in marked contrast to the brain-case 

portion of the same bone. It is filled with large-celled 

dipolic tissue. Between these processes, and dorsal to 

the anterior moiety of the olfactory bulbs, the horizontal plate of the frontal reaches its greatest thickness 

(5 mm) and is entirely cancellous in structure.

The olfactory lobes are somewhat larger than those 

of Merycoidodo'n, which has so often been cited as an 

extremely macrosmatic genus.

Comparison of the olfactory lobes of Poebrotherium 

with those of the living Caanelus (Lesbre, 1903) indicates that, even considering the greater size of the neopallium of Camelus, the lobes of smell in Poebrotherium 

are nearly twice as large for the size of the brain.

The squamosal bone of Poebrotherium differs from 

that of Merycoidodon in that it is diploic only along 

its base (medial to the temporal crest and, farther forward, to the root of the zygoma). Above this the 

greater portion of the squamosal plate is very thin and 

conforms closely to the shape of the cerebrum (fig. 27). 

This bone is relatively much thicker in living Tylopoda.

The horizontal ventral plate of the squamosal is even 

more intimately involved in the structure of the middle 

ear in Poebrotherium, than in M erycoidodon. The term 

"horizontal plate" is not applicable here in a strict sense, 

for parts of this structure are at a sharp angle with 

the horizontal.

A part of the ventral squamosal plate, the superficies 

meatus (fig. 27, SM] forms the roof of the external 

auditory meatus. It is a very thin plate of bone, lying 

in an almost vertical attitude due to the steep slant 

of the tube of the meatus. In Merycoidodon the superficies meatus is horizontal and about three times as 

thick as in Poebrothermm.

The superficies meatus extends ventromedially into 

the tympanic cavity, terminating 2 mm lateral to the 

promontorium, and dividing the front part of the tympanic cavity into dorsal and ventral cavities.

In the living Tylopoda, there is no such medially 

projecting plate of the superficies meatus (van Kampen, 

1905). In Merycoidodon it projects medially in a horizontal direction and its medial end is in contact with 

the pars petrosa above the tympanic cavity.

A more truly horizontal part of the ventral plate 

of the squamosal is that which projects medially above 

the ostium tympanicum tubae. To its ventral side, 

lateral to the ostium, is attached the anterior part of 

the auditory bulla. Over the anterior end of the

ostium, the squamosal plate meets the alisphenoid, and 

also extends behind it, where a gap between the squamosal and the basioccipital forms the foramen lacerum 

medius.

The squamosal therefore plays the part in Poebrotheriwm which, in M erycoidodon, is taken by the posteriorly projecting plate of the alisphenoid, which 

partially roofs the tympanic cavity.

The temporal canal, bearing the superior cerebral 

vein, is not contained, as in Merycoidodon, entirely 

within the squamosal, but is bounded only dorsolaterally by it (fig. 27, TO). This canal is further 

discussed below.

Like Merycoidodon^ Poebrotherium, possessed an 

anterior branch of the temporal canal; it ran ertirely 

within the squamosal, however, and not between the 

squamosal and the tympanic. The vein which occupied 

this canal reached the surface just medial to the root of 

the zygoma through two foramina, the supraglenoid 

foramina of Cope ( 1880) . Examination of other skulls 

of the genus reveals that either one or two supragi'enoid 

foramina may be present and that the number may 

differ on the two sides of the same skull. The srme is 

true of the living Tylopoda.

No supraglenoid foramen is present in Merycoidodon.

Poebrotheriwn possesses a maxillary pneumatic sinus 

along the medial side of the cheek-tooth row and dorsal 

to it, and projecting posterior to the third molar into 

the tuber maxillare (fig. 28). The sinus is almost 

square in frontal section, with a height of about 15 mm. 

Anteriorly it reaches as far as the fourth premclar, a 

distance of about 33 mm. The maxillary sinus arcends 

into the vertical plate of the maxilla, ventral to the 

lacrimal fossa. Probably the entrance of this sinus 

into the nasal cavity lay in the medial wall of this extension; this could not be definitely determined, however, because of the broken condition of the bone in this 

area.

The ventrally projecting vertical plate of the palatine bone is much heavier than in M erycoidodon. It is 

directed ventro-laterally and is in contact medially 

with the pterygoid bone, a slimmer plate that is oriented 

vertically. This union of the two bones forms a bifurcated process such as is also typical of living Tylopoda 

(fig. 2

TYMPANIC REGION

PARS PETROSA OF THE PERIOTIC BONE

An interesting feature of this bone in Poebrotherkim 

is the extremely large subarcuate fossa that occupies its 

entire dorsal side. The front part of the fossa (f g. 27, 

&F) is a simple depression in the pars petrosa, such as 

was observed in Merycoidodon, but behind this it is a 

cavity whose floor is the dorsal side of the pars vestib-
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 145

PT

FIGURE 28.—PoebrotAeri«m wilsoni Leidy. Thick section of skull, prepared from sections P 37-81, 

viewed anteriorly into olfactory lobes. Skull somewhat distorted. Natural size.

A8, alisphenoid.

B8, basisphenoid.

FLA, foramen lacerum anterius.

FR, frontal.

MS, maxillary sinus.

MX, maxilla.

MS, third molar.

OF, optic foramen.

ularis, and whose side walls and roof are formed by 

the bony arch which housed the posterior semicircular 

canal (fig. 27, F). Its posterior wall is a part of the 

dorsal portion of the pars mastoidea of the periotic.

This fossa was the receptacle of the lobulus petrosus 

of the cerebellum, mentioned above in the discussion of 

Merycoidodon ; the lobulus of Poebrotheriwm must have 

been at least three times this size.

The modern Lagomorpha have an identical fossa. 

Krause (1884, p. 182) described it, and designated it the 

fossa mastoidea. A very similar structure is reported 

by Elliot Smith (1903, p. 428) to exist in all Primates 

except Simia, Anthropopithecus and Homo.

For comparison with modern descendants of Poebrotherium, the writer examined the skulls of Tylopoda 

in the osteological collection of the Museum of Comparative Zoology at Harvard University. The roof 

of the braincase had been removed from only two skulls, 

specimens of Lama hwmachus (M. C. Z. 1746, M. C. Z. 

29878); and in both skulls the fossa mastoidea was 

identical with that of Poebrotherium. No mention of 

this fossa has been made in the literature but it seems 

safe to conclude that it is typical of modern Tylopoda.

OL, olfactory lobes.

08, orbitosphenoid.

PAL, palatine.

PT, pterygoid.

8C, sinus canal.

SQ, squamosal. 880, subsphenoid canal. 8V08, sinus venosus ossis sphenoiaalis.

Further examination of skulls of present-day Artiodactyla reveals that no others possess the fossa mastoidea. Its presence in the living Tylopoda contradicts 

the statement of Black (1921, p. 310) and Bolk (1906) 

that no modern ungulates possess the lobulus petrosus 

of the cerebellum.

MIDDLE EAR AND SURROUNDING STRUCTURES

The auditory bulla of Poebrotheriwm (fig. 26, B) is 

typical of Tylopoda—extremely large and filled with 

cancellous bone.

The relationships of the bulla to its surrounding 

structures differ from those of Merycoidodon largely 

because of its greater size, and agree correspondingly 

with those of the living Tylopoda.

The bulla projects posteriorly along the lateral side 

of the par occipital process, to which it is attached. Ventrally, it overlaps the anterior part of the process. 

Dorsally, the main part of the bulla is in contact with 

the pars petrosa and the squamosal. It projects ruich 

farther anteriorly than in most mammals. The anterior part of the bulla in PoebrotJienwrn is well inflated 

and entirely cancellous; it is laterally compressed to-
146 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

ward its dorsal side, and serial sections show its dorsal 

margin to be in. contact with the suture between the 

alisphenoid and squamosal.

As in the living Tylopoda, the bulla of Poebrotherium is in contact medially with the pars petrosa but 

is not fused with it (fig. 27, PET, B). No gap exists 

between the two as in Merycoidodon.

Despite the great size of the Poebrotherium bulla, the 

hypotympanic sinus is no larger than that of Merycoidodon (fig. 27, JSH). It is of relatively the same 

size and shape as that of Lama and Camelus (van 

Kampen, 1905, p. 594).

The ostium tympanicum tubae is longer than that 

of Merycoidodon, because of the greater anterior extension of the bulla. It is closed below by the contact 

of the bulla with the basisphenoid, and above by the 

horizontal plate of the squamosal bone. The living 

Tylopoda differ from Poebrotherium in that the ostium 

tympanicum tubae is roofed by the alisphenoid (van 

Kampen, 1905) as in Merycoidodon.

In Poebrotherium^ the tympanic bone forms almost 

all of the medial wall in addition to the ventral and 

part of the lateral wall of the tympanic cavity. The 

pars petrosa is thus limited almost entirely to the roof 

of the cavity (fig. 14). The cavity is wider and 

shallower than that of Merycoidodon.

The epitympanic recess of Poebrothwiurn, instead of 

lying directly above the tympanic cavity as in Merycoidodon and Camelus (van Kampen, 1905), extends 

dorsolaterally above the external auditory meatus. It 

is very small, and is bounded laterally by the tympanic 

and squamosal, and dorso-medially by the pars petrosa 

of the periotic (fig. 27, RE}.

Just behind the epitympanic recess and also dorsomedial to the external auditory meatus, lies the incisura 

tympanica (fig. 27, IT). As in other mammals, both 

its dorsal and ventral sides are formed by the squamosal bone; it is much larger than in most mammals, 

however. Comparison of the diameter of the medial 

end of the external auditory meatus with that of the 

incisura, reveals that, in Poebrotherium, the pars tensa 

of the tympanic membrane was very small, and the 

pars flaccida very large. The incisura tympanica is 

absent in the living Tylopoda (van Kampen, 1905).

The vagina processus hyoidei is almost completely 

surrounded by the auditory bulla. In the vagina, sections show the tympanohyal (fig. 27, TH) which, as 

Scott (1940, p. 621) suggested, is not fused to the tympanic. This fusion is present in the adult modern 

Tylopoda (van Kampen, 1905, p. 598), in which the 

bulla completely encloses the vagina to form a true pit.

The stylomastoid foramen of Poebrotherium is the 

lateral opening of a nearly horizontal canal leading

from the tympanic cavity to the posterolateral side of 

the bulla (fig. 26, FSM). Dorsally, this cr.nal is 

bounded by the squamosal; it differs from the stylomastoid canal in Merycoidodon in that its ventral wall is 

formed by the laterally swollen bulla rather than by 

the paroccipital process of the exoccipital. Th^ canal 

in Camelus is formed in the same way.

EXTERNAL AUDITORY MEATUS

At first glance the porus acusticus externus of Poebrotherium appears to open directly out of the tympanic 

bulla, with no intervening tubular meatus (fig. 26, 

MAE), but what appears to be the lateral portion of 

the auditory bulla is actually the inflated meatus (fig. 

13, IM).

The external auditory meatus in Poebrotherium, is 

surrounded anteriorly, vent rally, and posteriorly by the 

inflated tympanic bone, and dorsally by the superficies 

meatus of the squamosal, but in living Tylopoia, the 

meatus roof is formed by a thin plate of the tympanic, 

so that the meatus is a tube formed by this one bone. 

In both Poebrotherium and its living relatives^the tube 

of the meatus slants sharply dorsolaterally from the 

tympanic cavity.

PARS MASTOIDEA OF THE PERIOTIC BONE

This bone is well exposed upon the lateral surface 

of the skull (fig. 25, PM) , the exposed area being larger 

than in Merycoidod&n. In Poebrotherium, the pars 

mastoidea is bounded anteroventrally by the tympanic 

portion of the auditory bulla, rather than by ths squamosal, which in Merycoidodon overlaps its lower part 

(figs. 14, 25). In Camelus also (van Kampen, 1905) 

the squamosal covers the ventral part of the mastoid 

exposure. This appears to be another instance, like 

that cited above in the family Merycoidodontidae, of 

progressive covering, in the course of evolution of a 

group, of the pars mastoidea by neighboring elements.

PNEUMATIC SINUSES

The sinus system of Poebrotherium is less expensive 

than that of Merycoidodon, as no sphenoidal sinus is 

present. The frontal sinus is much smaller thr.n that 

of Merycoidodon: instead of occupying the entire supraorbital process of the frontal bone, it is four d only 

in the anteriormost part of this process. Its anteroposterior length is only about 4 mm., and it opens into 

the nasal cavity dorsal to the lacrimal fossa.

The maxillary sinus (fig. 28, MS) is considerably 

longer, but narrower, than that of Merycoidodon. Its 

greater extent as a cavity independent of the nasal cavity is due to the greater thickness in Poebrotherium of 

the palatine plate of the maxilla.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 147

In Camelus (Aubert, 1929), in contrast to Poe- 

~broiherium, the maxillary sinus is absent and the 

sphenoidal sinus is well developed. This discrepancy 

in a supposed descendant of Poebrotherium (Scott, 

1891) is easily explained: The development of larger 

cheek teeth would occupy the space in the maxilla 

formerly filled by the sinus, and the almost threefold 

increase in skull size would allow room for a sinus 

ventral to the olfactory lobes (compare Merycoidodon).

CIRCULATORY SYSTEM OF THE SKULL

ARTERIES

The internal carotid artery did not, as in Meryooidodon, traverse the tympanic cavity; instead it lay in a 

bony canal formed by the tympanic bulla and the 

basioccipdtal and basisphenoid (fig. 27, ICC) .

As Scott (1891) first pointed out, the internal carotid 

entered this canal through a foramen anterior to the 

foramen lacerum posterius. The farthest posterior appearance of the carotid canal is, therefore, just anterior 

to this foramen and at the level of the vagina processus 

hyoidei. From this point the canal extends forward 

4 mm. to an opening medial to the ostium tympanicum 

tubae. After leaving the canal, the artery entered the 

cranium through the foramen lacerum medius, which 

lies dorsal to the anterior end of the bulla (fig. 26, 

FLM).

The carotid canal in Poebrotherium differs from that 

of Camelus only in its less superficial position (because 

of the more swollen bulla of the former) and in the 

separation of its posterior foramen from the foramen 

lacerum posterius. In Poebrotherium^ the size of the 

carotid canal indicates that the carotid artery was 

already, as it is in Cam-elus, relatively unimportant in 

supplying cranial blood, although still functional.

VEINS AND VENOUS SINUSES 

DORSAL SINUS SYSTEM

Poebrotherium, like Merycoidodon, differed from 

living Artiodactyla in the possession of a venous tributary that passed from the orbital plexus through the 

sinus canal (fig. 15, SC) to the vena collateralis cerebri.

The superior cerebral vein in this genus ran ventrolaterally in a space between the ascending plate of the 

subarcuate fossa (near the anterior end of the pars 

petrosa) and the descending plate of the squamosal 

(fig. 27). At the base of the latter plate it entered a 

small (10 mm. in diameter), subspherical sinus venosus 

temporalis (fig. 27, SVT), whose medial and ventral 

walls were formed by the tegmen tympani of the pars 

petrosa and the dorsal side of the tympanic bulla. The 

size of this sinus is in distinct contrast to that of the 

homologous sinus in Merycoidodon.

The temporal venous sinus opens into the temporal 

canal, which, after coursing ventro-laterally, op?.ns 

upon the surface of the skull in a foramen between the 

auditory bulla and the squamosal bone. This foramen 

is difficult to find because it is overhung by the roo* of 

the zygoma.

The anterior branch of the temporal canal was mentioned in the discussion of the squamosal bone.

BASILAB SINUS SYSTEM

The depth of the grooves that extend antero-posteriorly alongside the pituitary fossa indicates that the 

cavernous sinus was a relatively small vessel. So, too, 

was the inferior petrosal sinus, if it was present at all. 

The carotid canal (fig. 27, ICC) is so small that it cc'ild 

not have contained two functional blood vessels. The 

inferior petrosal sinus must, then, have been ex^racranial in its course, or absent. No detailed account of 

the venous system of Cannelus is known to the author, 

and it was impossible to determine whether this sinus 

is present in living Tylopoda.

SINUS VENOS0S OSSIS SPHENOIDALIS

This venous sinus in Poebrotherium, with its tributary subsphenoid canals, differed from that of Merycoidodon only in that it extended laterally into the 

root of the alisphenoid bone (fig. 28, SVOS, AS). In 

figure 28 the anterior subsphenoid canals (SSC) are 

seen, extending transversely in the plane of the section 

from their openings at the sides of the basispheroid 

bone.

The antero-posterior length of the sphenoidal verous 

sinus is about 5 mm.

Lesbre's illustration of a sagitally sectioned skull of 

Camelus (1903, p. 19) shows that the basisphenoid contains no cavities of any sort.

The above observations suggest that in Poebrotherium the dorsal sinus system was the main patl of 

drainage of blood from the cranial cavity, and the 

venous blood from the facial region ran posteriorly 

through the sphenoidal venous sinus and extracranial 

paths, rather than through the cranium.

NERVES OF THE SKULL REGION

The arrangement of the cranial nerves of Poebrotherium differs from that of Camelus (Lesbre, 1903) only 

in the position of the optic nerves. These nerves lie 

very close together throughout their length, and the 

optic foramina (fig. 28, OF) are separated only by 

the thin medial bony plate formed by the two orfrtosphenoids. The great size of the orbits causes tHse 

bones to be closely pressed together in the midline of 

the skull—a condition common in small mammals, SMch
148 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

as the Lagomorpha, Tragulidae and Hypertragulidae. 

Because of the larger size of the skull, the optic foramina of the living Tylopoda are separated by the 

sphenoidal pneumatic sinus, which lies between the 

orbitosphenoid plates.

CRANIAL MORPHOLOGY OF LEPTOMERYX

Leptomeryx is a member of the family Hypertragulidae, which is well represented in the fauna from the 

White River group. The members of this family bear 

a very close resemblance to the living Tragulidae and 

their relationships have long been a subject of 

controversy.

Serial sections of the skulls of the Hypertragulidae 

do not yield as much information as those of larger 

mammals, because their delicate bones retain fewer 

impressions of the endocranial soft parts.

Because well-preserved Leptomeryx skulls are rare, 

one skull was divided sagittally, the left half serially 

sectioned at 0.5 mm intervals, and the right half kept 

intact for reference.

The skull on which this study is chiefly based is one 

of Leptomeryx evansi Leidy, M. C. Z. 6566 (figs. 29, 

30). It was collected by S. W. Garman in 1880, in the 

Bad Lands of northeastern Wyoming, and was given 

to the Museum of Comparative Zoology by Alexander 

Agassiz.

BONES OF THE SKULL

The supraoccipital portion of the coossified occipital 

bone is, unlike that of the Tragulidae, extended posteriorly into a nuchal crest, which is filled with diploe 

the size of whose cells resembles those of the Camelidae 

more than those of Merycoidodon.

The proportion of the occipital surface of th*. skull 

formed by the supraoccipital is less than in any other 

artiodactyl family, because the pars mastoidea is exposed on the occipital rather than the lateral surface of 

the skull (fig. 30, M).

The basisphenoid bone, like those of Merycoidodon 

and Poebrotherium, contains a cavity for the, sinus 

venosus ossis sphenoidalis.

The sagittally sectioned skull reveals that th?, postclinoid processes are very high (3.5 mm) in proportion 

to the size of the skull, and that the pituitary fossa, as 

in the Camelidae and Merycoidodon, is shallow.

The alisphenoid bone has a posterior process that 

lies dorsal, not only to the Eustachian tube but also, 

behind this, dorsal to the tympanic cavity itself. This 

process thus reaches farther back than in Merycoidlodon, 

where it lies dorsal only to the ostium tympr.nicum 

tubae. In Tragulus the alisphenoid seems, insofar as 

could be determined from external examination of 

skulls in toto, to have the same relationships as in 

Leptomeryx.

The subsphenoid canals, leading to the sinus venosus 

ossis sphenoidalis, lie for the most part in the alisphenoid, and their foramina open on its surface (fig. 30, 

FSA).

Considerable portions of the turbinal ossifications of 

the ethmoid bone were preserved in this specimen. In 

the back part of the nasal cavity are the roots of three 

turbinals thick enough to be tentatively termed endoturbinals. There may well have been more than three 

of these scrolls in life, but the three preserved apparently occupied more than two-thirds of the posterior 

part of the nasal cavity; it is therefore improbable that

PRO

I INCH

FIGURE 29.—Leptomeryx evansi, lateral view of skull. Modified after W. B. Scott. Natural size.

FPP, post-parietal foramen. 

FSA, anterior subsphenoid foramen. 

M, pars mastoidea of periotic bone. 

10, occipital bone. 

PPO, paroccipital process of exoccipital bone.

SQ, squamosal bone.

TO, foramen jugulare spurium.

TT, tympanic bone.

VPH, vagina processus hyoidei.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 149

.OC

M

I INCH

FIGURE 30.—Leptomeryx evansi, ventral view of basis cranii. M. C. Z. 

6566. X 2.

B, auditory bulla.

CF, condylar foramina.

FLA, foramen lacerum anterius.

FLPt foramen lacerum posterius.

FO, foramen ovale.

F8A, anterior subsphenoid foramen.

FSM, stylomastoid foramen.

M, pars mastoidea of periotic bone.

OC, occiptal bone.

PBC, petrobasilar canal.

SQ, squamosal bone.

TO, foramen jugulare spurium.

Leptomeryse had the six endoturbinal scrolls of the 

modern Artiodactyla (including the Tragulidae).

Many delicate fragments of ectoturbinals can also 

be seen, but the number of these cannot be estimated.

The nasoturbinals (a single scroll) and the maxilloturbinals (a bifurcated scroll) are identical with those 

of the Tragulidae, being more heavily ossified than the 

ethmoturbinals.

The external surface of the parietal bone closely reflects the shape of the brain. It is cancellous only 

where it fills the gap between cerebrum and cerebellum; 

elsewhere it is little more than 1 mm thick.

Like the parietal, the part of the squamosal participating in the braincase wall is very thin and nondiploic.

The horizontal plate of the squamosal is, relative to 

the size of the vertical plate, much less well developed 

in Leptomeryss than in most other Artiodactyla. The 

facies epitympanica (fig. 31, FE) is a plate only 2 mm.

wide, roofing the lateral portion of the tympanic cavHy. 

There is no superficies meatus. The posterior horizontal process of the alisphenoid forms a larger area 

of the roof of the auditory bulla than in Merycoidodon.

TYMPANIC REGION 

PARS PETROSA OF THE PERIOTIC BONE

This bone possesses several peculiarities -which are 

not shared by the pars petrosa of most living Artiodactyla : unfortunately it was impossible to determine, 

either from the literature or from dried skulls, whether 

these are shared by the Tragulidae.

The ascending plate forming the lateral border of 

the subarcuate fossa ia, in Leptomeryw, extraordinarily 

high. It reaches dorsally a distance of 7 mm. from the 

plane of the temporal ridge (almost one-half the height 

of the braincase), and serves as the medial wall of the 

proximal part of the temporal canal (fig. 31', APP).

Ventromedially, another process of the pars petr->sa 

projects between the basisphenoid bone and the auditory bulla (fig. 31, VPP) . Lull reports this feature in 

Allomerym planiceps (1922). In most mammals the 

pars cochlearis lies in or near this postion; in Leptomerym it lies dorsal to it, well within the braincase.

PAR

FIQUKB 31.—Leptomeryx evansi Leidy. X 2. Thick section from sections L 14-36, viewed anteriorly from region of bulla.

APP, ascending process of pars petrosa.

AS, alisphenoid.

B, bulla.

BS. basisphenoid.

FE, facies epitympanica of squamosal.

F8, facial sulcus.

ICC, internal carotid canal.

PAR, parietal.

PET, pars petrosa.

PM, pars mastoidea.

8F, subarcuate fossa.

SO, squamosal.

TO, temporal canal.

TT, tegmen tympani.

VPP, ventral process of pars petrosa.
150 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

The subarcuate fossa of Leptomeryx (fig. 31, SF) 

resembles that of Merycoidodon and the carnivores in 

being a definite but shallow depression in the dorsal 

wall of the pars petrosa. It is divided into two subequal portions by a low antero-posterior ridge, which 

may indicate that the fossa contained two cerebellar 

gyri. The subarcuate fossa bears no resemblance to 

that of the Camelidae.

The sulcus facialis of Leptomeryx lies in the extreme 

lateral side of the roof of the tympanic cavity (fig. 31, 

FS). It is separated from the squamosal by a ventrally 

projecting plate of the pars petrosa, the lower end of 

which is inserted in the stylomastoid foramen and extends to within a few millimeters of the ventral surface 

of the skull.

MIDDLE EAR AND SURROUNDING STRUCTURES

The epitympanic recess in Leptomeryw is, considering the size of the middle ear, a fairly long cavity (fig. 

31). Its anterior part is covered chiefly by the tegmen 

tympani of the pars petrosa; laterally it is bounded by 

the squamosal and the tympanic. Farther anteriorly it 

is roofed entirely by the tympanic.

The epitympanic recess of Tragulus (van der 

Klaauw, 1931) is like that of Leptomeryx.

As in all Artiodactyla, no tympanic sinus is present.

The auditory bulla of Leptomeryx (fig. 17, B) is 

hollow, unlike that of most tragulid species. It is 

only moderately inflated, extending ventrally to the 

level of the basioccipital. It is in contact medially 

with the petrosal, a ventral process of which separates 

it from the basioccipital; dorsally with the posterior 

process of the alisphenoid (fig. 31) and laterally with 

the squamosal. It is fused to none of these elements.

The tympanic gap ("tympanicumdefekt" of Bondy, 

1907), or dorsal opening in the inflated auditory bulla, 

is small in Leptomeryx. The tympanic bone arches 

dorsal to the lateral part of the epitympanic recess, so 

that the tympanic gap is only half as wide as in Merycoidodon and PoebrotJierium. In Leptomeryx^ the gap 

is closed partly by the pars petrosa, as is usual, and 

partly by the posterior process of the alisphenoid.

The tympanic gap does not extend to the anterior 

part of the bulla (fig. 31).

The incisura tympanica is absent in Leptomeryx as 

in the Tragulidae. In the former, the position usually 

occupied by the incisura, dorsal to the medial end of 

the external auditory meatus, is covered within the 

tympanic cavity by the ventro-lateral plate of the pars 

petrosa which serves as the lateral wall of the facial 

sulcus. Leptomeryx thus could not have possessed a 

pars flaccida of the tympanic membrane.

The stylomastoid foramen of Leptomeryx is, like

that of the Tragulidae, situated between the paroccipital process, the pars mastoidea, the posttympanic 

process, and the tympanohyal bone (fig. 30, FSM). 

The tympanohyal separates the foramen from th^ bulla.

EXTERNAL AUDITORY MEATUS

The meatus is short, with a diameter almost ss great 

as that of the bulla and twice as large as that of the 

Tragulidae. It slants posterodorsally. Its posterior 

wall is formed by the posttympanic process of tto squamosal; the tympanic occupies the other three vails of 

the tube. In the Tragulidae, however, the tympanic 

forms the entire meatus.

FARS MASTOIDEA OF THE FERIOTIC BONE

As figures 29 and 30 (M) show, the pars mastoidea 

is extensively exposed upon the occipital surface of the 

skull as in the Carnivora, rather than on its lateral 

surface as in all other mastoid Artiodactyla, including the Tragulidae, and Hypertragulus. Thi^1 characteristic of Leptomeryx is probably a primitive one; 

in this respect Leptomeryx resembles the hypothetical 

ancestor of the amastoid Artiodactyla postulated by 

Helga Pearson (1927, p. 456, 457).

The pars mastoidea is large, relative to that of Merycoidodon and Poebrotheriwn (fig. 31, PM). Here, in 

the back part of the bone, is a large cavity which, having no direct outlet, must be diploic in nature. In the 

front part, the mastoid cells are smaller.

As in the Tragulidae (Milne-Edwards, 186*), the 

mastoid process is separated from the bulla by the 

posttympanic process of the squamosal (fig. 2S).

PNEUMATIC SINUSES

The maxillary pneumatic sinus is the only one present 

in Leptomeryx. This is also true of the Tragulidae 

(Aubert, 1929), and seems to be typical of small mammals. In Leptomeryx it lies above the cheek tocth row, 

and extends antero-posteriorly about 5 mm.

CIRCULATORY SYSTEM OF THE SKUU. 

ARTERIES

No separate posterior carotid foramen is viable on 

the basis cranii of Leptomeryx (fig. 30), but the serial 

sections indicate that here, as in Poebrotheriw^ Protoreodon and a few groups of living mammals, the 

internal carotid artery entered the carotid canal 

through an opening anterior to the foramen lacerum 

posterius.

A cross-section of the posterior portion of the carotid 

canal in Leptomeryx is seen in figure 31 (ICC), which 

shows a canal between the dorsomedial tymparic wall 

of the tympanic cavity and the ventral side of the
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTTLA Ifl

posterior process of the alisphenoid bone. Apparently the artery lies for a short distance in the posterior third of the petrobasilar canal (fig. 30, PBC), 

then enters the carotid canal near the dorsal edge of 

the medial wall of the bulla. The artery extends dorsal 

to the bulla for only about 2 mm, leaves the canal 

through the anterior carotid foramen, enters the cranium through the foramen lacerum medius, and extends 

anteriorly in a groove in the floor of the basis cranii. 

The internal carotid artery in Leptomeryx was much 

less superficial in position than that of the Tragulidae, 

in which as a rule, it lies in a simple groove in the 

medial wall of the bulla.

VEINS

DORSAL SINUS STSTEM

There is no indication in the Leptomeryx skull of 

the details of the dorsal cranial vessels anterior to the 

transverse sinus, but several of the distributaries of 

the sinus are well marked.

There was no temporal venous sinus in Leptomeryx, 

and the temporal canal, bearing the superior cerebral 

vein, was similar to that of the living Artiodactyla. 

The superior cerebral vein left the brain cavity high 

up on its lateral wall, extended ventrally in the temporal canal (fig. 31, TO), between the mastoid and 

squamosal bones, and left the skull through a foramen 

just behind the external auditory meatus (fig. 30, TO}.

As in most Artiodactyla, an anterior branch of the 

temporal canal passed anteriorly in the temporal crest 

to a foramen on its dorsal side in the region of the root 

of the zygoma.

The transverse sinus of Leptomeryx was also drained 

by a vein that occupied the post-parietal canal, which 

extends posteriorly in the parietal bone and opens upon 

the dorsal surface of the braincase through the postparietal foramen (fig. 29, FPP).

BASILAE SINUS STSTEM

Other than the groove in the cranial floor for the 

cavernous sinus, and the petrobasilar canal for the 

inferior petrosal sinus (fig. 30, PBC), there is no evidence as to the condition of this system in Leptomeryx.

SINUS VENOSUS OSSIS SPHENOIDALIS

This sinus in Leptomeryx was about 3 mm long. It 

differed from those of Merycoidodon and Poebrotherium chiefly in that the associated subsphenoid canals 

extended laterally into the alisphenoid bone (fig. 30, 

FSA).

MORPHOLOGICAL CONCLUSIONS

The studies upon which this paper is based have 

yielded a number of new facts concerning the endocranial anatomy of Oligocene Artiodactyla. Some of

these observations add to the existing information concerning evolutionary trends in this order; others, because of the lack of sufficient data for comparison, can 

only be recorded in hopes that further studies, bridging 

the gap between the Oligocene and Recent epochs, will 

give them significance.

The characteristics observed may be divided into 

three categories :

(a) Primitive characteristics.

(b) Characteristics whose significance is doubtful, 

but which are noted because they have never before be^-n 

observed.

(c) Anatomical peculiarities indicating the habits of 

the genus involved, evolutionary tendencies within the 

artiodactyl group to which it belongs, or its relationships to other artiodactyl groups.

The characteristics to which the first two categories 

may be applied are listed under the headings usod 

above for subdividing the morphological discussions, 

then the diagnostic anatomical features are discussed 

individually.

BONES OF THE SKULL

PRIMITIVE CHARACTERISTICS

1. In Merycoidodon, the pituitary body was contained between two thin, longitudinally oriented, bony 

plates. These represent the taenia interclinoidalis, t\e 

primitive lateral wall of the braincase.

2. In Merycoidodon and Poebrotherium the superficies meatus of the squamosal is wide and heavily os^ified as compared to that of Leptomeryx and the living 

Artiodactyla. The reduction of the superficies in living 

genera is concomitant with the shortening and lightening of the bony meatus.

3. Merycoidodon is unique among the Artiodactyla 

in that the posttympanic process of the squamosal participates in forming the ventral wall of the exterral 

auditory meatus.

CHARACTERISTICS OF DOUBTFUL SIGNIFICANCE

1. The mesethmoid bone of Merycoidodon is 

heavily ossified midway between the cribriform plate 

and the anterior nares. This suggests that the mesethmoid and presphenoid bones in this genus ossified separately. If this were the case, this genus would be 

sharply differentiated from the other Artiodactyla. It 

is probable, however, that the thickness of the mesethmoid is simply a part of the heavy ossification of the 

entire nasal region in Merycoidodon.

2. The posterior horizontal process of the alisphenoid 

of Leptomeryx extends far enough posteriorly to close 

the anterior part of the tympanic gap. This is apparently most unusual among Artiodactyla, but comparative data are incomplete.
152 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

TYMPANIC REGION 

PRIMITIVE CHARACTERISTICS

1. The tympanohyal bone of Merycoidodon, Poebrotherium, and Leptomeryx was inserted loosely into 

the vagina processus hyoidei. This is in contrast to the 

living Artiodactyla, in which the tympanohyal is fused 

to the tympanic in the fundus of the vagina.

2. The tympanic gap of Merycoidodon is larger than 

that of living Artiodactyla or of Leptomeryx. This is 

regarded as primitive because it more closely approximates the primitive ring-form of the tympanic (van 

der Klaauw, 1931) than does the condition in. which 

the bulla is partly closed dorsally.

The size of the tympanic gap determines the composition of the roof of the epitympanic recess. If it is 

small, as in Leptomeryso and most living Artiodactyla, 

the tympanic itself forms most of the roof. If it is 

large, as in Merycoidodon and the Tylopoda, the pars 

petrosa is the important element involved.

CHARACTERISTICS OF DOUBTFUL SIGNIFICANCE

1. In Merycoidodon, the fissura glaseri is almost entirely surrounded by the tympanic bone. This is not 

the case in any living artiodactyl (van Kampen, 1905), 

or in Poebrotheriwn or Leptomeryx. It may, at least 

until further information becomes available, be regarded as a structure characteristic of Merycoidodon.

2. An entotympanic bone is present in Merycoidodon. 

Because of the sparseness of knowledge concerning this 

bone, even in Recent Artiodactyla, speculation on this 

fact is useless.

3. The meaning of the presence or absence of the 

incisura tympanica is often difficult of interpretation. 

It is missing in Leptomeryx and the Tragulidae, probably because the relatively large size of the tympanic 

annulus, together with the great reduction of the horizontal plate of the squamosal, leaves no room for such 

a structure. The lack of the incisura in these genera 

is due to the small size of their skulls.

The condition in the Tylopoda is, however, puzzling. 

The incisura tympanica is missing in the living members of the suborder (van Kampen, 1905), but well 

developed in Poebrotherium. Only one explanation 

can at present account for this discrepancy. In the 

living Tylopoda a portion of the tympanic bone forms 

the roof of the external auditory meatus, and thus the 

superficies meatus of the squamosal, which performed 

this function in Poebrotheriivm, is excluded from its 

participation in the attachment of the tympanic membrane. This membrane in modern Tylopoda is, therefore, attached throughout its circumference to a ring 

of bone; in Poebrotherium, due to a gap in this ring, it 

was possible for a dorsal portion (the pars flaccida) of

the membrane to be attached to a process of the squamosal below which lay the incisura tympanica.

This evolutionary trend in the Tylopoda conforms 

with the already mentioned tendency in all mammals 

toward closure of the tympanic gap.

4. Leptomeryx differs from all other mammals observed by the writer or mentioned in the literature in 

the extreme height of the ascending plate of the pars 

petrosa, which serves as the lateral border of the subarcuate fossa. In this genus, this plate composes 

almost the entire medial wall of the temporrl canal. 

It probably appears unusually high simply because of 

the small size of the skull. This supposition is borne 

out by the large size of the pars petrosa as a v^hole in 

proportion to the size of the Leptomeryso skull. It is 

a recognized fact that, in small mammals, the organs of 

sense occupy, comparatively, a greater space than in 

larger ones.

5. A characteristic which, in the present state of 

knowledge, distinguishes Leptomeryx from all other 

Artiodactyla is the great ventral extent of that plate 

of the pars petrosa which forms the lateral wall of the 

facial sulcus. No functional reason for this condition 

is evident, nor is it a stage in any known evolutionary 

trend in skull development. It is not present in the 

Tragulidae, so that it would not seem necessarily associated with small skull size. It is therefore her** tentatively suggested that this peculiarity ma7 be a 

hypertragulid (or perhaps only a leptomerycid) 

characteristic.

PNEUMATIC SINUSES 

PRIMITIVE CHARACTERISTICS

In their small extent and number, and in the lack of 

any interconnections between them, the pneumatic 

sinuses of the three Oligocene genera here discussed are 

primitive. This is most strikingly apparent in Merycoidodon^ whose thick cranial bones were capable of 

housing a much more complicated sinus system than 

the animal possessed. This bears out Paulli's statement (1900) that the complicated sinus system of 

modern type did not appear until late in the Miocene 

epoch.

ABTEBIES 

PRIMITIVE CHARACTERISTICS

1. In the three Oligocene genera studied, evidence 

was found that the internal carotid artery was functional, a primitive condition in the Artiodactyla. The 

artery was relatively much smaller in Poebrotherium 

than in Merycoidodon or Leptomeryx.

2. The absence in Merycoidodon of a rete mirabile of 

the intracranial part of the internal carotid artery is,, 

in all probability, also primitive.
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 153

VEINS AND VENOUS SINUSES 

PRIMITIVE CHARACTERISTICS

1. The sinus venosus ossis sphenoidalis and the subsphenoid canals, possessed by all three of the Oligocene 

genera studied here, were part of an archaic type of 

venous cranial circulation. This conclusion is based 

upon their absence in living Artiodactyla as opposed to 

their apparently widespread occurrence in Oligocene 

time, and also upon their presence in primitive living 

mammals: the Insectivora and Marsupialia, as well as 

the Lagomorpha and some Rodentia. The conclusion 

that these blood vessels are primitive in character is 

further borne out by their absence in the Merycoidodontidae after the late Oligocene.

2. The vein of the sinus canal, which was possessed 

by Merycoidodon and Poel>rotherium, was another 

primitive artiodactyl characteristic. The same evidence contributes to this conclusion as was cited in the 

case of the sinus venosus ossis sphenoidalis.

3. The presence of a temporal venous sinus in Merycoidodon, and to a far lesser extent in Poebrotheri/u/m^ 

is due to the fact that the small size of the brain relative 

to skull size leaves space available for the sinus.

4. The vestigial vena capitis lateralis, observed in 

Merycoidodon^ was probably merely a relic from the 

embryonic stage, various manifestations of which are 

often encountered in adult modern Mammalia.

ANATOMICAL PECULIARITIES INDICATING HABITS, 

EVOLUTIONARY TRENDS, OR RELATIONSHIPS

Mesethmoid ossification in Merycoidodon.—As has 

been discussed at length above, the extremely strong 

bony nasal septum of this genus resembles closely that 

of the peccary, and indicates similar rooting habits.

The thickness and extensive development of the turbinal bones of this genus were undoubtedly the result of 

the unusual degree of ossification of the septum.

Tendency of Selenodontia to approach the amastoid 

condition.—Comparison of Poebrotherium with the living Tylopoda, and especially of Merycoidodon with the 

later Merycoidodontidae, reveals a progressive and distinct reduction in the area of the pars mastoidea exposed 

upon the surface of the skull. This does not diminish 

the value of the mastoid-amastoid distinction drawn 

by Pearson (1927); it simply points out that the mastoid and amastoid Artiodactyla differ in this respect 

only in the degree of advancement of a trend that is 

present in both.

Absence of the epitympanic sinus.—In this characteristic, the three Oligocene genera examined resemble the 

living Artiodactyla. In mammals as a whole, the absence of the epitympanic sinus is also the rule rather 

than the exception.

Course of the internal carotid artery.—In Merycoiiodon, this artery courses through the auditory bulla as 

in the Carnivora; in this it differs from all other Artiodactyla in which the internal carotid is known. Probably this is true of all the Merycoidodontidae.

The other two Oligocene genera studied possessed a 

carotid canal medial to the bulla, as in the living Tylopoda and Tragulidae.

In the study of various phylogenetic lines of the 

Artiodactyla there was observed a tendency to posterior 

migration of the posterior carotid foramen: in oth^r 

words, the length of the carotid canal progressively increased. This backward migration culminates when 

the posterior carotid foramen merges with the foramen 

lacerum posterius.

Migration of the posterior carotid foramen was completed early in the evolution of the Merycoidodontidr^, 

much later in the Camelidae.

Tendency toward reduction of the internal carotid 

artery in the Artiodactyla.—The internal carotid, absent or non-functional in almost all living Artiodactyla, was present and well developed in some Oligocene 

members of the order, according to the observations 

recorded here. It was strong in Merycoidodon ard 

Leptomeryw; in Poebrotherium, oddly enough, it was 

relatively no larger than in the living Oamelus. This 

suggests that, in the Oligocene ancestors of the Tylopoda, the reduction in size of this artery may have 

progressed further than in Merycoidodon and Leptomeryx, which apparently represented more archaic 

sidelines of artiodactyl phylogeny.

Internal jugular vein replaced by external jugular as 

dominant efferent cranial vessel.—In Merycoidodon especially, and to a less degree in Poebrotherwm, most of 

the blood from the facial region and the brainca^e 

found its exit from the skull by way of the basilar 

sinus system and the sphenoidal venous sinus, ard 

thence to the internal jugular vein. In the course of 

evolution to the modern artiodactyl condition, the sph<vnoidal venous sinus disappeared and the ventral petrosal sinus lost some of its tributaries. The result was 

that blood from the face adopted a course outside tl^ 

cranium, while most • of the endocranial blood was 

drained off through the superior cerebral vein. Both 

these vessels are tributary to the external jugular vein.

The lobulus petrosus of the cerebellum.—This lobule 

of the brain is absent in all but a few living Artioda<v 

tyla. In two of these, Oreamnos and Sus, it is very 

small. In the modern Tylopoda, on the other hand, it is 

well developed, and is enclosed dorsally by the arch 

of the posterior semicircular canal. This is also tJie 

case in the Oligocene Poebrotherium. In Leptomer^'ss 

and Merycoidodon of the Oligocene the lobulus was
154 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

present and very large; it resembled that of the living 

Carnivora in being housed in a deep fossa in the dorsal 

side of the pars petrosa. It was not arched over by 

the semicircular canal as in the Tylopoda.

The strong similarity, amounting almost to identity, 

between the lobulus petrosus of the Tylopoda and those 

of the Lagomorpha and some Rodentia is difficult to 

explain, especially as the function of this lobule is 

not definitely known. The consensus of opinion seems 

to be that it functions in maintaining proper static and 

kinetic conditions in a fluid medium, since it is highly 

developed in the Pinnipedia. It may be that this type 

of lobule in the Lagomorpha serves in maintaining 

equilibrium in jumping, and that in the Tylopoda it 

is a relic of ancestral leaping habits in the Artiodactyla, 

such as are suggested by the double rollers possessed by 

the artiodactyl astragalus.

PHYLOGENETIC CONCLUSIONS

Study of the cranial anatomy of Merycoidodon, 

Leptomeryx and Poebrotheriwm reveals that these three 

genera (and, presumably, most other early Oligocene 

Artiodactyla) possessed structures which are found 

today only in the Marsupialia, Insectivora, Lagomorpha, and some Kodentia. These characteristics are 

evidence of the fact that Oligocene artiodactyls were 

nearer in structure to their primitive, insectivorelike 

ancestors than might be judged from their superficial 

appearance. Investigation of the cranial anatomy of 

Artiodactyla from the Wasatch, Bridger, and Uinta 

formations is likely to give further information concerning the steps involved in the gradual loss of primitive mammalian cranial characteristics by the artiodactyls. It may even yield specific information concerning 

the stem group from which the Artiodactyla sprang, 

presumably in the Paleocene.

The crania of the Oligocene Artiodactyla also possess 

some details of structure which add to the body of 

knowledge from which, in turn, can be derived a partial 

understanding of the interrelationships of the various 

artiodactyl families and suborders. The conclusions 

resulting from this evidence are discussed below.

TYLOPODA

Poebrotheriwn was an advanced artiodactyl by early 

Oligocene standards, as is shown particularly by the 

pattern of its cranial circulation. Its relatively advanced state of evolution facilitates comparison with 

living Tylopoda and the establishment of Poebrotherium as one of an almost direct line leading to the modern genera.

One of the most striking pieces of evidence linking 

Poebrotherium with the living Tylopoda is its peculiarly constructed subarcuate fossa, which is identical 

in the genera of Oligocene and modern times and occurs 

in no other Artiodactyla, living or fossil. T" ?. great 

antiquity and unique character of this structure is 

strong evidence of the long independent existence of 

the suborder, which must have separated from the other 

artiodactyl lines in the middle Eocene at the latest.

Most cranial features of Poebrotheriwm,, su?-h as its 

inflated, cancellous bullae, carotid canal and the structure of the middle ear region, bear out the ophiion of 

previous authors that this genus was a direct ancestor 

of modern Tylopoda.

The living Tylopoda, as is shown by their many resemblances to their Oligocene relative, are no longer 

advanced members of the, artiodactyl fauna. This reflects the fact, evident in pdstcranial as well as in cranial 

structures, that tylopod evolution was rapid in the early 

Tertiary and proceeded at a much more leisurely rate 

thereafter.

MEBYCOIDODONTIDAE

Merycoidodon has the greatest number of primitive 

cranial features of the Oligocene genera examined; it 

may therefore be regarded as an archaic member of the 

artiodactyl fauna of the White River group. This 

agrees with the degree of advancement observed in 

other merycoidodont genera as compared to that of 

their contemporaries.

The structure of the subarcuate fossa in Merycoidodon is also important taxonomically. Although well 

developed, it is smaller than that of Poebrcth&rium, 

and is formed in an entirely different manner. This 

fact supports the majority of paleontologists in denying Riitimeyer's and Scott's hypothesis that the Merycoidodontidae are closely related to the Tylopoda.

The subarcuate fossa of Merycoidodon differs only in 

size from that of Oremnnos (one of the Capridae); in 

other living Pecora this structure has disappeared. 

This evidence is not cited to prove Oreamnos a close 

relative of Merycoidodon; rather, it is cited as proof 

that the Pecora once possessed a subarcuate fossa of the 

type found in Merycoidodon, and that this fossa has 

been, since the Oligocene, reduced and finally lost.

It may therefore be concluded on the basis of evidence 

now available that the.Merycoidodontidae belong near 

the base of the phylogenetic tree of the suborder Pecora.

Before the above conclusion can be regarded as final, 

much further investigation will be necessary. The 

problem of the origin and relationships of the Merycoidodontidae apparently will depend for its solution 

upon the unearthing of more details concerning the 

morphology of other primitive selenodonts and bunoselenodonts of the lower Tertiary. Particularly im-
CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 155

portant in this respect are the Anthracotlieriidae, 

Caenotheriidae, Anoplotheriidae, and Homacodontidae. It is possible that research into the cranial 

anatomy of these primitive families will establish their 

taxonomic positions more definitely.

LEPTOMEBYCINAE

The cranial morphology of Leptomeryw and related 

genera strongly supports including them in a subfamily 

of the Hypertragulidae, equal in status to the subfamily 

Hypertragulinae. The diagnostic cranial features of 

the Leptomerycinae are the exposure of the pars mastoidea upon the surface of the occipital plate (a feature 

otherwise unknown in Selenodontia) and the presence 

of a ventral process of the pars petrosa separating the 

basioccipital bone from the auditory bulla. Both these 

characteristics are found in Allomery-%, of the John 

Day formation of early Miocene age, a fact that proves 

that the leptomerycine line survived as long or longer 

than that of the Hypertragulinae.

The simple subarcuate fossa of Leptotneryx reinforces other cranial evidence in suggesting Pecoran 

affinities. Again the theories of Rutimeyer and Scott 

are contradicted, for these authors believed the Hypertragulidae, like the Merycoidodontidae, to have been 

closely related to the Tylopoda.

No conclusive evidence has been adduced concerning 

the relationship between the Hypertragulidae and the 

Tragulidae. Many similarities exist between the extinct family and the living one, but probably most of 

these are due to small skull size.

SUMMARY OF PHYLOGENETIC CONCLUSIONS

1. The early Oligocene Artiodactyla were not far removed from an ancestral group resembling the 

Insectivora.

2. The Tylopoda represent an evolutionary line that 

has been independent of all other Artiodactyla 

since the middle Eocene, at least.

3. The Hypertragulidae are members of the suborder 

Pecora; the Merycoidodontidae belong close to 

the base of the phylogenetic tree of this suborder.

4. The Hypertragulidae consist of two subfamilies, the 

Hypertragulinae and Leptomerycinae, both of 

which survived from early Oligocene into early 

Miocene time.

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INDEX

Page 

Acknowledgments..______.._____------__---------- — ---------- 117

alisphenoid bone________-._--_________-_-___-..._------------- 123

Attomeryx.... ________.__.______..-_____.-._-———....-— 155

planiceps...........——_______-.__-------- — --- ——.-.--__.- 149

amastoid condition, significance of tendency toward---.__-.-- — ----------- 153

Amphimeryx........ -———__._______.-._—————-——————--- 135

anatomical peculiarities indicating habits, evolutionary trends, or relationships-------—....---— — ..- — —.-_.- — — — ---.-. 153-154

Anthropopithecui..--..... _-------------------__.-_-_- — __-__-_-_-_-- 145

ones, Ovis—....................................-.——— — .————-- 136

arteriesoftheskull-...——..-- —_——.—-.---_-.-- — -- 137-138,147,150-151

basilar sinus system...-----__....__.-_..-.---_____-__..___._--- 139-140,147,151

basiphenoid bone_-_----.-_-.-___-_______._-__..----_-_.-----__-.--. 120-123

Bibliography._________...--______.______.-_______-... 155

bonesoftheskull—---—---_ — _--..-_--- — .-- 118-131,141-144,148-149,151 

Bos.-... — — ...-—_ — --- — .—-—---. — -.._-.------------- 141

Braehyerug.................................................................. 135,136

Camdut.................................................... 134,139,143,144,146,147

Omis—----—-——- — -——— — -— — ------------------------- 122

carotid artery, significance of course, reduction______-_.__-____-_-----.----- 153

Cebochoerus....................-_._____..--.-...-.-.-------------.------ 135

characteristics of doubtful significance-_..-_.-__-.-._....-_-------.--. 151,152

Choeropotamus...................................______------- — ---— 135

circulatory system of skull.........................__..__.. 137-141,147,150-151

conclusions, phylogenetic.___________________..—————— --- 155 

culbertsonii, Merycoidodon—........................................ 118,122,125,126

Cfydopidius..—.............................................................. 122

Dacrytherium................................................................. 135

Dicoteles............................................................ 124,125,126,137

dorsal sinus system__......___._...___.__——.——-— 138-139,147,151

Entelodon..................................................................... 135

epitympanic sinus, significance of absence..._____________.._-. 153 

Eporeodon.................................................................. 122,123

Erinaceus............______________.________.____.--.... 138

ethmoid bone___.________________ _______._.___—. 124-126 

evansi, Leptomeryx........................... _____------------—- ——.. 148

eximium, Poebrotherium....................................................... 141

external auditory meatus.____._______.__.--...____ 135,146,150

frontal bone._______________________._.-.__.....__ 126-127 

gracilis, Merycoidodon____.______.______... — .—.___-__ 122

Hippopotamus.—.........................................................—.. 135

Homo.........................................................----- —— ------ 145

huanachus, Lama............................................................. 145

Hypertragulus..... .———— —.__.___._„..————..— — .-.—-....— 150

Interparietal bone___--------____------______._-.-..-----.. 126

Introduction..___.______________________-_.._- 117

Jugular vein, significance of changes in__....___......................... 153

labiatum, Poebrotherium.............................................-......... 141

Lama......................................................................... 146

huanachus.........................................___.--___---_._ 145

Leptomerycinae, evolution...______....____..--__-.__..-. 155

Leptomeryx............................................................. 118,148-155

evansi................................... _____.-._..-—.————.. 148

evolution..--.-_-.-__..___---.___-____.______._- 155 

Lepus.............................................................. 122,137,138,140

cuniculus...................................................... ———— — 140

lobulus petrosus, significance of development..----- —-----------------—-.-. 153

Merycoides.............................................................------- 122

Merycoidodon..................................... .______—__—_ 118-155

culbertsonii.—.................................................. 118,122,125,136

evolution.-._-_-.-__--------------_________—---------- 154-155

gracilis........................................... ...................——.. 122

Merycoidodontidae, evolution__________-_____--.____. 154-155 

mesethmoid ossification, significance_.______._____--__-__. 153 

Mesoreodon............................................-..__——_- — --... 122

Microgale..................................................................... 138

midd^ ear and surrounding structures___-----_____. 132-135,145-146,150

Mixtotherium................................................................. 135

morphological conclusions-____________-__-___—.-——_ 151-154

nerves of skull region.-_.____________________——__. 141,147-148

occipital bone......._----_----_.__.____....._.................. 118-120

Oldfieldthomasia.-—-—.-.............................................. 130,131,132

orbitosphenoid bone__._________—_——__——.———-———----- 124

Oreamnos............. .-..——._-——— ——————————————— ——— —... 154

Ovis.................................................................... 132,136,141

aries.........................................__-_...__--.—--.---- 136

parietal bone...____..__..__________.___..__—_--------- 126

pars mastoidea of periotic bone..-_.-._____________.__. 135,146,150 

pars petrosa of per iotic bone---------_....___.....--.__.. 131-132,144,149--150

Pearson, quoted____.__________________.___—_——_.— 135 

preinterparietal bone. -__.._...._——__..__—.———...————— 126

periotic bone, pars petrosa of________________.__—-.-— 119,144-145 

Phylogenetic conclusions....-___________________-_------- 154-155

planicepa, Attomeryx—........................................................ 149

pneumatic sinuses___. ______________________. 136-137,146-147,150 

Poebrotkerium.............................................. 118,131,141-148,150-155

evolution_________________.___________.—_——.... 154

eximium.................................................................. 141

labiatum............ ....................................... ———————.. 141

wilioni—................................................................. 141

presphenoid bone______......—__... __—__._..—_....... 123-124

primitive characteristics, artery.....-..._...__—...——————————.„ 152

bones of skull___. ____________________________ - 151 

pneumatic sinuses.. __.__.___._______._._—-————.. 152 

tympanic region..-.__________________.__—_.-.._.. 162 

veins and venous sinuses..__....„—.———...——-——.————— 168

Promerycockoerus............................................................. 122

Protoreodon—...................................................... 122,137,138, ISO

scrofa,Stts.............................................................—..... 122

Selected bibliography_.......___...._____________.-..---....... 156

Simia... .........................——..——————————————-------——... 146

sinus venosus ossis sphenoidalis__________________-. 140-141,147,151 

Solenodon............... ______.__...__„__...__„.._—__. 138

squamosal bone____.__________.__________________ 127-131 

subsphenoid veins.________..___.___.__....__—_——_ 140-141

Sus........................................................................... 139

scrofa..................................................................... 122

Talpa europaea............................................................... 138

Tapirulus.................................................................... 135

Tragulus........................................................... 133,138,148,150

turbinal bones- ____. ______________________________ 124-126 

Tylopoda, evolution_-_.—__......—_——__——__—..——--... 154

tympanic region...—...................................... 131-135,144-146,149-150

veins and venous sinuses..__-________________. 138-141,147,151,153 

vomer-..__..--._____.._...._——_——__——_...--..___.. 126

wilsoni, Peebrotherium... ___._____________________-.-.. 141

159

Shorter Contributions 

to General Geology 

1952

GEOLOGICAL SURVEY PROFESSIONAL PAPER 243

This professional paper was printed 

as chapters A-H, inclusive

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTO.N : 1953
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director
CONTENTS

[The letters in parentheses preceding the titles are those used to designate the papers for separate publication]

(A) Ostracodes from the upper part of the Sundance formation of South Dakota, Wyoming, and southern Montana, by

Frederick M. Swain and James A. Peterson (published in December, 1952)...................................... 1

(B) Tertiary stratigraphy of South Carolina, by C. Wythe Cooke and F. Stearns MacNeil (published in August, 1952)...... 19

(C) Probable Reklaw age of a ferruginous conglomerate in eastern Texas, by Lloyd William Stephenson (published in March,

1953)................................................................................................... 31

(D) Cenomanian ammonite fauna from the Mosby sandstone of central Montana, by William A. Cobban (published in

August, 1953)........................................................................................... 45

(E) Mollusks from the Pepper shale member of the Woodbine formation, McLennan County, Texas, by Lloyd William

Stephenson (published in May, 1953)....................................................................... 57

(F) Conodonts of the Barnett formation of Texas, by Wilbert H. Hass (published in August, 1953)...................... 69

(G) Auditory region of North American fossil Felidae: Its significance in phylogeny, by Jean Hough (published in April, 1953).. 95 

(H) Cranial morphology of some Oligocene Artiodactyla, by Frank Whitmore (published in August, 1953)................ 117

ILLUSTRATIONS

PLATES 1, 2. Cytherellidae, Cypridae, and Cytheridae....................................................... Following 18

3. Map showing areal distribution of Wilcox and lower part of Claiborne groups.......................... In pocket

4. Specimens and outcrops of ferruginous conglomerate, sand, and sandstone............................. Facing 34

5. Outcrops of quartzitic sandstone and f ossiliferous ferruginous sandstone............................... Facing 35

6-9. Metoicoceras ................................................................................ Following 56

10-12. Dunveganoceras.............................................................................. Following 56

13. Molluscan fossils, mainly from the Pepper shale................................................. Following 68

14-16. Barnett formation conodonts................................................................. Following 94

FIGURE 1. Index map showing localities from which ostracodes were obtained........................................ 2

2. Correlation of Tertiary formations of South Carolina..................................................... 20

3. Whorl sections of Dunveganoceras. ..................................................................... 53

4. Map showing localities at which conodonts of the Barnett formation were collected........................... 75

5. Hoplophoneus primaevus oreodontis ..................................................................... 98

6, 7. Smilodon californicus ................................................................................. 101

8. Dinictisfelina ....................................................................................... 104

9. Dinictis cyclops. ..................................................................................... 105

10. Viverricida indica rasse ............................................................................... 108

11. Felis catus. .......................................................................................... 108

12. Cam's dingo ......................................................................................... 110

13. Vulpes velox ......................................................................................... Ill

14. Merycoidodon cidbertsonii, lateral view.................................................................. 118

15. Merycoidodon culbertsonii, ventral view of basis cranii..................................................... 119

16. Merycoidodon culbertsonii, internal view of basis cranii.................................................... 120

17. Merycoidodon culbertsonii, ventral view of basis cranii with blood vessels restored. ............................ 121

18. Frontal sections of skulls of M. culbertsonii and Dicoteles labiatus. .......................................... 124

19, 20. Merycoidodon culbertosnii, thick sections of the skull...................................................... 128

21, 22. Merycoidodon culbertsonii, thick sections of the skull. ..................................................... 129

23. Merycoidodon culbertsonii, thick section of the skull. ..................................................... 130

24. Merycoidodon culbertsonii, thick section of the skull, with nasal cavity and pneumatic sinuses outlined. ......... 136

25. Poebrotherium wilsoni, lateral view of the skull........................................................... 141

26. Poebrotherium wilsoni, ventral view of basis cranii........................................................ 142

27. Poebrotherium wilsoni, thick section of the skull. ......................................................... 143

28. Poebrotherium wilsoni, thick section of skull. ............................................................ 145

29. Leptomeryx evansi, lateral view of skull.................................................................. 148

30. Leptomeryx evansi, ventral view of basis cranii........................................................... 149

31. Leptomeryx evansi, thick section........................................................................ 149

in