Cranial Morphology of Some / 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 Cranial morphology of some Oligocene Artiodactyla—Con. Page Abstract_--_------______-______117 Cranial morphology of Poebrotherium—Continued Introduction ______117 Circulatory system of the —Continued Acknowledgments______117 Veins and venous sinuses____----___---_- 147 Cranial morphology of some Oligocene Artiodactyla____- 118 Dorsal sinus system______-___-_-.__ 147 Cranial morphology of ______118 Basilar sinus system——______147 of the skull___.______118 Sinus venosus ossis sphenoidalis- _ 147 Occipital.______118 Nerves of the skull region.— ____------_ 147 Basisphenoid______120 Cranial morphology of Leptomeryx. ______148 Alisphenoid______123 Bones of the skull______148 Presphenoid______123 Tympanic region______--_------_--_-- 149 Orbitosphenoid— ______124 Pars petrosa of the periotic bone_____.__._ 149 Ethmoid and turbinals______124 Middle and surrounding structures 150- Vomer______126 External auditory — ______150 Preinterparietal and interparietal— ______126 Pars mastoidea of the periotic .______150 Parietal.. ______126 Pneumatic sinuses______150 Frontal—______126 Circulatory system of the skull____---____-_-- 150 SquamosaL ______127 ______-____---___------150 Tympanic region. ______131 Veins______---___----__ 151 Pars petrosa of the periotic bone______131 Dorsal sinus system______151 and surrounding structures- - _ _ 132 Basilar sinus system———______151 External auditory meatus______135 Sinus venosus ossis sphenoidalis-_____ 151 Pars mastoidea of the periotic bone ______135 Morphological conclusions______-_____---_---_----_-- 151 Pneumatic sinuses______136 Bones of the skull______151 Circulatory system of the skull-______137 Primitive characteristics______151 Arteries ______137 Characteristics of doubtful significance______151 152 Veins and venous sinuses.______138 Tympanic region. ______Primitive characteristics______152 Dorsal sinus system______138 Characteristics of doubtful significance. ______152 Basilar sinus system______139 Pneumatic sinuses.______152 Subsphenoid veins and sinus venosus Primitive characteristics.______152 ossis sphenoidalis______140 Arteries.-______--_____-----_------_- 152 Nerves of the skull region______141 Primitive characteristics ______152 Cranial morphology of Poebrotherium______141 Veins and venous sinuses--__-_--_------_------153 Bones of the skull______141 Primitive characteristics.______153 Tympanic region.______144 Anatomical peculiarities indicating habits, evolution­ Pars petrosa of the periotic bone______144 ary trends, relationships.------153 154 Middle ear and surrounding structures. _ _ _ 145 Phylogenetic conclusions______Tylopoda.______-_--_--__--- 154 External auditory meatus______146 Mery coidodontidae. ______154 Pars mastoidea of the periotic bone ______146 Leptomerycinae. ______---___-----_----_ 155 Pneumatic sinuses______146 Summary of phylogenetic conclusions ______155 Circulatory system of the skull— ______147 Selected bibliography.-______-_---_------_-----_ 155 Arteries.,______147 Index.______---___------_--_-----_ 159

ILLUSTRATIONS Page FIGURE 14. Merycoidodon culbertsonii. Lateral view of skull__-______-_---_____---_-_- 118 15. M. culbertsonii. Ventral view of basis cranii.______119 16. M. culbertsonii. Internal view of basis cranii, from above______-_-___-__------120 17. M. culbertsonii. Ventral view of basis cranii, with restoration of blood vessels..______121 18. Frontal sections of the of M. culbertsonii and Dicotyles labiatus... ______-_____--__-_- 124 19. M. culbertsonii. Thick section of skull, viewed posteriorly into cerebellar fossa.______128 20. M. culbertsonii. Thick section of skull in region of auditory bulla______.______--_--- 128 21. M. culbertsonii. Thick section of skull at level of post-glenoid processes._.______---_--- 129 22. M. culbertsonii. Thick section of skull, looking anteriorly into olfactory lobes______--__-_- 129 23. M. culbertsonii. Thick section of skull, looking anteriorly through nasal cavity______130 24. M. culbertsonii. Same as figure 1, with and pneumatic sinuses outlined.. ______- 136 25. Poebrotherium wilsoni. Lateral view of skull______-___-_____-______-_--- 141 26. P. wilsoni. Ventral view of basis cranii.______142 27. P. wilsoni. Thick section of skull, looking posteriorly from level of vagina processus hyoidei. 143 28. P. wilsoni. Thick section of skull, looking anteriorly into olfactory lobes______145 29. Leptomeryx evansi. Lateral view of skull______-__-_-______-___-_-_- 148 30. L. evansi. Ventral view of basis cranii______.______149 31. L. evansi. Thick section of skull, looking anteriorly from region of bulla._____-___-__-_-__- 140

CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA

By FRANK C. WHITMOKB, JR.

ABSTRACT eminence on the posterior side of the bone which selves A study of the cranial morphology of three Oligocene Artio- as a muscle attachment in some . The second dactyla (Merycoidodon, Poebrotherium, and Leptomeryx), and even more useful criterion is the presence upon the based largely on serial sections, reveals many cranial charac­ astragalus of a distal roller surface (for articulation teristics found only in the most primitive Recent mammals. with the navicular and cuboid bones) in addition tc the The Oligocene genera had several cranial veins which indicate that these artiodactyls were not far removed from a primitive proximal roller surface present in other mammals. insectivore-like ancestor. They had a well-developed internal Many years of study by zoologists have clarified the carotid in contrast to the Recent Artiodactyla, very few relationships among the many living artiodactyl of which retain this structure. The cranial circulation pattern groups, diverse as they are; but har re­ of the Oligocene forms shows an evolutionary tendency in the vealed a tangle of problems in the many extinct artio­ Artiodactyla to abandon endocranial for extra-cranial paths. Other primitive characteristics are the lateral partitions of dactyl genera. The relationships of the extinct the pituitary fossa (representing the primitive side wall of the genera with each other and with modern types are often skull) and large temporal venous sinus in Merycoidodon, and only a matter of conjecture; their ancestry is unknown. the simple pneumatic sinus system in all three genera. Therefore many uncertainties exist concerning the The selenodont Artiodactyla exhibit an evolutioaary trend stratigraphic and paleoecologic significance of many toward the amastoid skull pattern now found only in bunodont families. extinct genera and . Their morphology if, of The subarcuate fossa of the pars petrosa of the periotic bone course, less well known than that of the modern Artio­ in the has a peculiar shape, constant in the suborder dactyla. It is the purpose of this study to examir e in and differing from the condition in other selenodont Artio­ detail the cranial anatomy of some of these extinct dactyla. This is added evidence that the Tylopoda must have genera, because endocranial characteristics are p^ob- separated from other artiodactyl lines of evolution no later than the middle . The simpler subarcuate fossa of Merycoido­ ably nonadaptive, that is, unlikely to be influenced by don and Leptomeryx indicates their alliaace with the suborder the environment, and therefore useful in determining . the taxonomic position of groups of . On the basis of cranial structure, the family These studies were made with the aid of serial sections, is divided into the subfamilies Hypertragulinae and Leptomery- a technique used by several workers in the study of cinae, both of which existed from the early Oligocene through the early . fossil skulls (Darrah, 1936; Dunkle, 1940; Gral *m, 1933; Eomer, 1937; Simpson, 1933, 1936; Sollas, 1916, INTRODUCTION 1920; Sollas and Sollas, 1914; Walton, 1928). The The Artiodactyla, or "even-toed ", form skull can be oriented in relation to the grinding surface one of the largest and most varied of the orders of mam­ to produce sections in any desired direction; the most mals. They are today near the acme of their develop­ common types of sections used are the sagittal (parallel ment ; their distribution is world-wide and they include to the plane of bilateral symmetry of the skull) anc1 the in their numbers the , , hippopotami, frontal (at right angles to this plane). The studies , , giraffes, , , , antelope and presented here are based upon frontal sections. The the tiny of Asia and Africa. sectioning technique used has been described in a previ­ An examination of the Artiodactyla reveals several ous paper (Olsen and Whitmore, 1944). characteristics in common: hoofs, a more or less herbiv­ ACKNOWLEDGMENTS orous diet, and a tendency toward a foot structure in which the weight rests mainly upon two toes of each The author is especially indebted to Professor A.. S. foot. This mesaxonic type of foot, whose axis runs Romer of Harvard University, at whose suggestion this between the third and fourth toes, has given rise to the research was undertaken, for his assistance and en­ name of the . Two other morphological peculiari­ couragement in every phase of the work. ties possessed by all the Artiodactyla are not externally Many thanks are also due to the late Professor visible but are very important. The first of these is Thomas Barbour of the Museum of Comparative Zool­ the absence of the third trochanter of the femur, an ogy, 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 These three families comprised most of the seleno- Museum of Natural History; Dr. E. C. Olson of the dont population of the Oligocene epoch of North University of Chicago; Dr. Glenn L. Jepsen of Prince- America; besides it was felt that they would reveal ton University, and Mr. J. Leroy Kay of the Carnegie the stage of cranial evolution reached by the selenodont Museum, for the loan of specimens for comparison. Artiodactyla in Oligocene time. Mr. F. Eussell Olsen, of the Museum of Comparative Zoology, Harvard University, has by his skill made CRANIAL MORPHOLOGY OF MERYCOIDODON possible the construction of the apparatus used in this This study is based primarily upon serial frontal work. sections of a skull of Merycoidodon culbvrtsonii Apparatus used in preparing some of the specimens (Leidy), M. C. Z. 6450, from the White Kiver group for study was purchased by a grant from the Elizabeth (Oligocene) of northeastern Wyoming (figs. 14, 15). Thompson Science Fund of Harvard University. Sections were made at 2mm intervals through most of the skull. Posterior to the (fig. 15), CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA an interval of 0.5mm was used because of tie com­ plexity of the structures in this region. Because the serial-section method destroys the speci­ In the following description, special attention is paid mens used, it was decided to investigate genera of to those parts of the cranial bones which do not appear which there are abundant specimens in paleontologic upon the surface of the skull, to the relations among collections. The White River group of Oligocene age those bones, and to the impressions left upon them by has yielded rich collections of mammals for many years, blood vessels and nerves. The external characteristics and it was found possible to obtain adequate specimens of the skull, as shown in figures 14 and 15, furnish on which to base a study of the cranial anatomy of the points of reference in locating the features of the artiodactyls, using three genera representing three dif­ internal anatomy. ferent families. The brains of Merycoidodon, and also those of Poe- The first of the genera, and that studied in greatest brotherium and Leptomeryso, have already been de­ detail, is Merycoidodon, a member of the extinct family scribed in the basis of natural casts (Black, 1921; Merycoidodontidae (Oreodontidae), the "ruminating Tilney, 1931; Moodie, 1922; Bruce, 1883), and are not hogs" of Joseph Leidy. Merycoidodon is the most discussed in the present study. abundant mammalian in deposits. The second genus studied is Poebrotherium (of the Came- BONES OP THE SKULL lidae) an ancestor of the modern camels; and the third OCCIPITAL is Leptomeryce, of the family Hypertragulidae, which The contains, or is bounded by, sev­ are diminutive, hornless, deerlike animals. eral 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. IF, . OS, orbitosphenoid bone. ASC, anterior opening of sinus . L, lacrimal bone. PA, . EF, ethmoidal foramen. LG, lambdoid crest. PAL, palatine bone. FLA, foramen lacerum anterius. MAE, external auditory meatus. PMX, premaxilla. FM, . MX, maxilla. PPO, paroccipital process. FPP, postparietal foramen. N, nasal bone. PT, pterygoid bone. PR, . NG, nuchal crest. SQ, squamosal bone. F8A, anterior subsphenoid foramen. OG, occipital bone. CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 119 nerves. The mastoid foramen (fig. 14, FM) is the exit ran in the petro-basilar canal (figs. 15,21, PEG). This of a canal which, passing posteriorly through the occi­ vessel was a large one, the inferior petrosal sinus. TTie pital bone, carries the mastoid emissary vein. This venous blood flowed posteriorly into it from, the cavern­ vessel is one of several draining blood from the great ous sinus of the base of the brain. The sinus left the system of sinuses of the squamosal bone and the neigh­ petro-basilar canal and extended posteriorly in a groove boring surface of the dura mater of the brain; it drains near the lateral edge of the basioccipital. Before leav­ blood from the transverse venous sinus of the dura ing the petro-basilar canal, however, the inferior petro­ mater to the occipital vein. sal sinus gave off a branch, the condyloid vein, which Another vessel draining venous blood from the extended posteriorly in a groove on the endocranial side cranium, in this instance from the base of the brain, 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. FLP, foramen lacerum posterius. PN, palatonarial border. B, tympanic bulla. FO, foramen ovale. PS. presphenoid bone. BO, basioccipital bone. F8A, anterior subsphenoid foramina. PTP, posttympanic process of squa BS, basisphenold bone. FSM, . mosal bone. OF, condylar foramen. FG, nssura glaseri. 8Q, squamosal bone (zygomatic arch OPO, opening of craniopharyngeal OGO, . broken). canal. OTT, ostium tympanicum tubae. TG, opening of temporal canal. FLA, foramen lacerum anterins. PEG, petrobasilar canal. FLM. foramen lacerum medius. PET, pars petrosa of periotic bone. 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 semi­ FSP, posterior subsphenoid foramen. PL, floor of pyriform lobe. circular canal. IOA, groove for internal carotid artery. PSG, posterior opening of sinus canal. OF, condylar foramen. MAI, . 8F, subarcuate fossa. CR, chiasma ridge. MM A, middle meningeal artery. TVS, temporal venous sinus. D, diploe in squamosal bone. MY, myelencephalic base. VCC, groove for vena collaterals EG, ethmoidal crest. OL, olfactory lobe. cerebri. FLA, foramen lacerum anterius. POP, posterior clinoid processes II, opening of canal for optic nerver. FLM, foramen lacerum medius. (broken). V etc., groove for nerves III, IV, " FLP, foramen lacerum posterius. PET, pars petrosa of periotic. (1, 2) and VI. FO, foramen ovale. PF, pituitary fossa. condylar foramen (figs. 15,16, OF) and extended pos­ gently backward in essentially one plane from the optic teriorly as the occipital vein. The twelfth cranial nerve nerves to the . also found its exit through the condylar foramen. The foramen lacerum medius, whose anterior border A few millimeters antero-lateral to the condylar is formed by the basisphenoid, transmitted blood both foramen is the foramen lacerum posterius (fig. 15, to and from the cranial cavity. The internrl carotid FLP) , from which emerged the inferior cerebral vein, artery, the chief blood vessel nourishing the brain, one of the posterior branches of the inferior petrosal entered the cranium here after having passed through sinus, and a tributary of the internal jugular vein (fig. the auditory bulla on its way from the common carotid 17). artery. Inside the skull, this artery traveled anteriorly The ninth, tenth and eleventh also in a groove in the basisphenoid bone, immediately lat­ passed through the foramen lacerum posterius. eral to the pituitary fossa (fig. 22, IOA) . At the ante­ rior end of the basisphenoid, at the level of th^ chiasma BASISPHENOID ridge (fig. 16, CR)^ the artery branched into smaller The basisphenoid bone is much thicker dorso- arteries that traversed the surface of the cerebral ventrally than is the basioccipital. The result of this hemispheres. The grooves for these arteries were ob­ is that, while the basicranial axis is horizontal when served in the dissected braincase shown in figure 16, viewed from below, the brain itself shows a sharp but not in the sections, because they lie abcut in the ventral bend at the level of the posterior clinoid proc­ plane of sectioning. Posterior to the pituitary fossa esses (fig. 16, PCP) , which arise from the posterior end is a groove in which ran the posterior communicating of the pituitary fossa. Thus the bases of the pyriform artery, which connected the internal carotid s.rteries of lobes of the brain and of the pituitary fossa (fig. 16, both sides. PZ, PF) are in essentially the same plane, while the In the same groove as the internal carotid artery myelencephalic base dips posteriorly at an angle of (fig. 16, 1CA) ran the cavernous sinus, a venous vessel. about 20° from the plane of the base of the cerebral The cavernous sinus received venous blood from the hemispheres. This is a marked difference from any region of the via the orbital venous plerus. This modern ungulates, in which the base of the brain slants 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. IA, infraorbital artery. PBO, petrobasilar canal. EGA, external carotid artery. IOA. internal carotid artery. PTC, opening of posterior branch of EJV, external jugular vein. IGv, inferior cerebral vein. temporal canal. FLM, foramen lacerum medius. IJV, internal jugular vein. SOVj superior cerebral vein. FLP, foramen lacerum posterius. IMA, internal maxillary artery. 8PA, sphenopalatine artery. FO, foramen ovale. IPS, inferior petrosal sinus. S80, subsphenoid canal. FSA, anterior subsphenoid foramen. MMA, middle meningeal artery. SVOS, sinus venosus ossis sphenoidalis. FSP, posterior subsphenoid foramen. OF, ophthalmic vein. TO, foramen jugulare spurium. anterius (figs. 15, 16, FLA). After making its exit The pituitary fossa (), which is sur­ through the foramen lacerum medius, it passed poste­ rounded on three sides by grooves for blood vessels, is riorly into the inferior petrosal sinus (see above). an extremely shallow depression in the cranial surface Posterior to the pituitary fossa, the cavernous sinuses of the basisphenoid bone (fig. 16, PF). It thus re­ of the two sides were connected by the intercavernous sembles that of the modern Tylopoda and differs from sinus, running in the same groove as the intercarotid that of the , which is very deep. This lack of artery. excavation of the basiphenoid bone is compensated, 238435—53———2 122 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 however, by the presence of a vertical lamina of bone emerge into the , one on eith er side upon either side of the sella turcica. These laminae of the basisphenoid bone, through a foramen s;tuated are of delicate construction and cannot be found in from 2 to 5 mm posterior to the foramen lacenm an­ fragmentary skulls or natural brain-casts. They are terius. In some specimens one of the canals divides well shown, however, by serial sections (fig. 22, LOP). again just before its emergence into the infrateraporal At their highest point they are 4 mm. high. Their fossa, with the result that two foramina are present antero-posterior length is about 10 mm. Posteriorly between the foramen lacerum anterius and the foramen they decrease gradually in height until the level of the ovale. braincase floor is reached; anteriorly they are continu­ These canals have never before been observed in ous with the posterior of the orbitosphenoid bone any Artiodactyla, and they are beyond doubt absent (fig. 22, POS). No structures of this sort are found in in all living members of the order. A large cav:ty was living Artiodactyla; the nearest parallel to them occurs noted in the basisphenoid bone of one sagittally sec­ in the embryonic stages in Lepus, in which the posterior tioned skull of Sus scrofa, but it was in no way con­ clinoid processes extend along the sides of the sella nected with the exterior. The canals were first ob­ turcica in the same way (Voit, 1909, p. 549). Voit served in serial sections of the skull of Merycoidodon agrees with Gaupp (1902,1906) that these lateral lam­ ouZbertsonii; then the basisphenoid bones of thre°, other inae represent the taenia interclinoidalis of the chondro- specimens were dissected, and the canals were found cranium, and thus are a remnant of the primitive lateral in all (fig. 17 SSC}. Skulls of M&rycoidodon gracilis, wall of the braincase. This thesis is supported by the smallest species of the genus, were too rare, in the deBeer (1937, p. 388). Museum of Comparative Zoology collections to be In many recent mammals (Camis, among others) the dissected; examination of all available specimens, how­ taenia interclinoidalis is represented by the anterior and ever, showed the presence of foramina which are un­ posterior clinoid processes. The condition observed in doubtedly the anterior openings of these canal?. Ex­ Merycoidodon, in which these two vestigial processes ternal examination of other skulls of Merycwdodon- are joined, is an earlier stage in the reduction of the tidae in the Museum collections revealed the presence primitive lateral wall of the skull. of the anterior foramina in F^rotoreodon (late Eocene), their doubtful presence in one of five specimens ex­ Just lateral to the groove for the internal carotid amined of Oyvlopidius (Miocene) and their undoubted artery and the cavernous sinus is a deeper groove, which absence in Merycoides (early and middle Miocene), runs from the region of the foramen ovale to the Promerycochoerus (late Oligocene and early Miocene), foramen lacerum anterius. It served as a course for the Mesoreodon (early Miocene) and Eporeod&n (middle forward passage of the oculomotor (III), trochlear Oligocene to early Miocene). Conclusions reached on (IV), abducens (VI), ophthalmic (Vi), and maxillary the basis of such superficial examination are, of course, (V2) nerves, which originate on the base of the brain questionable. The evidence in this case, however, sug­ posterior to the pituitary fossa. The third branch of gests that such canals are present in the basisphenoid the trigeminal nerve passed out of the skull through bone only in the more primitive Merycoidodontidae. the foramen ovale (fig. 15, FO). Because of the absence of such structures in modern Within the foramen lacerum medius, and perforat­ artiodactyls and the lack of knowledge of such details ing its anterior (basisphenoid) wall, careful examina­ in fossil forms, it was necessary to seek their explana­ tion reveals a small foramen, which we shall term the tion in the cranial anatomy of other modern mammals. posterior subsphenoid foramen (fig. 16, FSP). The It was then found that similar canals in the body of the serial sections (fig. 22) show that these foramina open basisphenoid bone occur in Lepus (Arai, 1907), in some into canals (SSG) that extend forward and medially Eodentia (Bovero, 1905) and in Marsupialia and In- in the body of the basisphenoid bone about 6 mm. me­ sectivora (Gregory, 1910). The close correspondence dial to the foramen ovale (fig. 17, SSC). Below and between the bony canals in these groups and those behind the pituitary fossa (directly ventral to the poste­ observed in Merycoidodon makes it apparent tl at they rior clinoid processes) and about 6 mm. from the are for the transmission of veins. foramen lacerum medius the two canals meet in the The anterior foramen (Arai called it "foramen midline to form a cavity in the thickest part of the body venosum", but anterior subsphenoid foramen seems a of the basisphenoid (fig. 17, SVOS). This cavity is more specific term) receives a vein from the orbital approximately 6 mm. long, 8 mm. wide, and 3 mm. deep venous plexus. The blood travels posteriorly in the in its deepest part. subsphenoid canals (a term used by both Arai and From its anterior corners spring two canals which Bovero) until it reaches the sinus venosus ossis sphe- diverge sharply and, about 6 mm from the cavity, noidalis (again a term introduced by Arai), which CRANIAL MORPHOLOGY OF SOME OLIGOCENE ABTIODACTYLA 123 occupies the above-mentioned cavity in the body of the The anterior terminus of the pars ossea tubae is at. the basisphenoid. Posterior to this the subsphenoid canals point of entry of the into the pharrnx; resume their course postero-laterally, debouching its posterior end is marked by the ostium tympanicum through the posterior subsphenoid foramen (fig. 17, tubae, the gap in the anteromedial wall of the tympanic FSP). They empty their venous blood into the pos­ cavity whereby the eustachian tube enters the middle terior end of the cavernous sinus, whence it flows pos­ ear. teriorly into the inferior petrosal sinus. The presence of the pars ossea tubae is typical of The subsphenoid canals will be further discussed the ungulates (van Kampen 1905, p. 370). Merycoido­ below, in connection with the venous circulation in don differs from modern Artiodactyla in that the pars Merycoidodon. ossea does not form a complete bony tube around the The is absent in Merycoidodon. eustachian tube. Examination of any modern artio- As mentioned above, the maxillary nerve undoubtedly dactyl, such as Ovi$, explains this difference: in modern found its exit through the foramen lacerum anterius, Artiodactyla the ventral wall of the bony tube is formed its usual course in the absence of the foramen rotundum. by the expanded medial wall of the tympanic bnlla, Thorpe (1931), in discussing the osteology of Epore- which has come into contact with the basisphenoid odon, states that the foramen rotundum is always pres­ bone. This is also true of most merycoidodont genera ent in Merycoidodon ; this he points out as an expression in and after the late Oligocene. Merycoidodor-, in of the more primitive skull structure of the latter genus. which the bullae are not as yet greatly inflated, may be Thorpe's foramen rotundum of Merycoidodon is actu­ regarded as primitive in this respect. ally the anterior subsphenoid foramen, mentioned Within the pars ossea tubae of the alisphenoid, near above. Identification of the foramen rotundum as a its medial border and extending antero-posteriorly, the primitive feature is probably unwise in view of Greg­ serial sections (fig. 21, VCL) revealed a tiny canal, ex­ ory's observations (1910), which seem to indicate that tending from the foramen lacerum medius to the tym­ such a foramen is easily formed secondarily by the panic cavity. This probably carried a vestigial vein, formation of a cartilage bar separating the maxillary no longer of any importance in drainage of the blood nerve from the other nerves that leave the cranium from the cranium. It will be further discussed in through the foramen lacerum anterius. This fact does connection with the venous circulation. not, however, invalidate the general rule that the pres­ Extending antero-posteriorly in a gently curved ence of many separate foramina in the basicranial course along the ventro-lateral side of the pyriform region is a primitive condition. lobes (fig. 16, VOO) is a shallow groove in the endo- cranial surface of the alisphenoid bone, which ends ALISPHENOID anteriorly in a small foramen (fig. 16, PSG) at the The alisphenoid bone in Merycoidodon has its usual transverse ridge separating the pyriform lobe from mammalian form: it is a plate of bone projecting dorso- the tuberculum olfactorium. The serial sections (fig. laterally from the basisphenoid and forming the median 22) show that this foramen is the posterior openir? of wall of the inferotemporal fossa (fig. 14, AS). It a canal (SC) running posteriorly between the ali­ ossifies from a center distinct from the basisphenoid, but in Merycoidodon it is usually partly, sometimes sphenoid and orbitosphenoid from a foramen or the entirely, fused with it. anterior border of the surface exposure of the ali­ The anterior end of the auditory bulla marks the (fig. 14, ASG). This canal and the posterior termination of the surface exposure of the groove which is its continuation are clearly homologous alisphenoid bone. The serial sections, however, show with the sinus canal found by W. K. Parker in the that a slim, dorso-ventrally flattened process of the Insectivora (1886). It carries a posterior branch from alisphenoid projects posteriorly between the squamosal the orbital venous sinus to join the vena collateralis and petrosal bones (fig. 21, AS). It is suturally at­ cerebri. The latter travels around the pyriform lobe tached laterally to the squamosal. At the beginning of and enters the temporal venous sinus system, which its posterior course it serves as a roof for the eustachian drains blood to the external jugular vein. These veins tube; farther backward it lies dorsal to the anterior will be further considered below. portion of the bulla and forms a small part of its roof, separating it from the petrosal. PRESPHENOID This backward-projecting process of the alisphenoid The presphenoid bone (fig. 15, PS) contains a large is the pars ossea tubae (van Kampen 1905, p. 370). cavity, the posterior end of the spheno-palatine sinus, Joined with it and forming the lateral bony wall of an air cavity which is in direct connection with the the tube is the styliform process of the tympanic bone. 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 orbito- sphenoid bones possess horizontal, flat, medially pro­ jecting processes, which meet ventrally at the midline to form the floor of the olfactory fossa of the brain- case (fig. 22, OL}. Anteriorly these plates are sep­ arated, 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 olfac­ tory lobes. Anteriorly, the orbitosphenoid walls of the olfactory fossa are bounded by the of the mesethmoid bone.

ETHMOID AND TTJRBINALS The two chief parts of the , the cribri­ form 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. B 23), the mesethmoid divides in about its ventral third. From the point of division a thin plate descends lat­ erally 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 1 INCH septum." It leads posteriorly directly to the posterior nares, and is the most direct route by which inspired FIGURE 18.—Frontal sections, at the level of the first molar, of the skulls of Merycoidodon culbertsonii (Leidy) (M. C. Z. 5450) and air can travel from the nostrils to the . A Dicotyles labiatus (after Paulli, 1900). homologous formation can be found in some living A, pneumatic part of nasal septum. B, sinus in horizontal plate of maxilla. mammals, but in many individuals it is hard to detect 0, lateral nasal cavity. I', maxillary extension of . because of the extremely intricate infolding which has 8, bony nasal septum. taken place in the mesethmoid walls of the passage, rendering it difficult to distinguish in cross section from This is typical of the living Non-Kuminantia, and is other parts of the nasal cavity. lacking in Merycoidodon. In living Mammalia, the Dicotylidae (peccaries) The two cavities of the nasal passage proper are show the nearest approach to the structure of the pneu­ found both in Merycoidodon and in Dicotyles, and are matic part of the nasal septum as observed in Mery- labelled A and C in figure 18. The cavity contained coidodon (fig. 18). As figure 18 shows, however, even in the pneumatic part of the nasal septum (A) is in the Dicotylidae exhibit a considerably more complex Dicotyles, in cross-section, about three times thbasal one, shows as much mesethmoid ossification (Flower, 1885). whereas in all modern the second ethmotur­ Dicotyles possesses a more completely ossified nasal binal is far stronger. septum even than Merycoidodon. Of 25 skulls The only modern mammals showing a strong: de­ examined, all the adult specimens possessed a bony velopment of the farthest ventral ethmoturbinal are nasal septum reaching at least as far forward as the the Carnivora, which, however, have only four ethmo­ canine teeth, that is, within 25 mm. of the end of the turbinals. snout. In Sins, the bony nasal septum also terminates The serial sections show almost no trace of the naso- anteriorly at the level of the canine tooth; however, turbinal bone, a scroll derived from the ethmoid the snout extends much farther in advance of the ca­ cartilage and, in most mammals, fused to the frontal nines than it does in Dicotyles. and nasal bones along the whole length of the roof of The conclusion forcibly suggested by these observa­ the nasal cavity. The only evidence of the presence tions is that Merycoidodon was, like Dicotyles, an an­ of this bone is in two successive serial sections (fig. 18), imal of rooting habits. This theory is borne out by in which the of the nasoturbinals are seen spring­ the fact that the strong nuchal crest of Merycoidodon ing dorsally between the dorsal maxilloturbinals and indicates such powerful cervical musculature as is pos­ the median nasal septum. Probably the nasoturbinal sessed by Dicotyles and by Sus. The vegetation of the scroll itself was too delicate to be preserved. Oligocene epoch seems also to have been such as would The maxilloturbinal bones are by far the best de­ allow this mode of life, for it is well known that the veloped of the turbinals in Merycoidodon. Ther are early Tertiary flora was primarily lush, and that the attached not only to the maxillae but also to the frontal increasingly dry climate of the West in Miocene and bones, and extend from the level of the lacrimal fossa Pliocene time increased the area occupied by grasslands to the anterior nasal opening, a distance of 60 mm. (Scott, 193T; Chaney and Elias, 1938). (fig. 23). They lie anterior to the ethmoturbinalf and After the Miocene epoch, it is probable that the below the nasoturbinals. The maxilloturbinals are not Merycoidodontidae abandoned their rooting habits, for equally well developed throughout their extent, but 126 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 reach their maximum size in the region of the lacrimal ing from the medial border of each of the palatine fossa and at the anterior end of the nasal cavity. processes of the maxillae. These flanges are in contact The maxilloturbinals may be subdivided into several along the floor of the nasal cavity, and diverge upward, parts. Beginning posteriorly, the first of these to forming a V-shaped trough like that of the vomer. appear are the dorsal maxilloturbinals which, at the This condition is not found in living Artiodetyla, but level of the lacrimal fossa (fig. 23), spring from the occurs in the Carnivora. nasal septum. Four millimeters farther forward the dorsal maxilloturbinals are fused to the frontal bones. PREINTERPARIETAL AND INTERPARIETAL Thus, for a short distance (and probably for a longer A preinterparietal bone is present in Merycoidodon. space in life, when the partitions were extended by It is very small and does not appear on the surface of cartilage) the nasal passage is divided into four pas­ the skull, being covered by the parietal. In the speci­ sages. Of these, the ventral one (pneumatic part of men sectioned it occupies the dorsal wall of the brain- the nasal septum) was the most direct air passage. The case for 6 mm. on either side of the midlin^, just dorsal pasage was most closely concerned with the posterior to the fronto-parietal suture. This lx»ne is sense of smell (an important function in Merycoidodon, not found in adult Artiodactyla, but Wilhelm (1924) which was macrosmatic; see Black, 1920). The lateral notes its presence in the young of , and it probably passages, one on either side, were connected through occurs in youth in many other genera. In spite of its large apertures with the pneumatic sinuses of the max­ possession of the preinterparietal, the skull of Meryooi- illary bones, whose mucous membrane lining, together dodon examined here is an adult, as is shown by the with that of the maxilloturbinals, probably played some molar teeth. part in warming the inhaled air. The interparietal bone, which also appears in the Anterior to the region just described, at the level young of modern Artiodactyla, is absent as a seperate of the anterior part of the lacrimal fossa, appear the ossification in the skull here studied. ventral maxilloturbinals, which are attached at their Not much importance can be attached to the presence lower ends to the horizontal plate of the maxilla, di­ or absence of the preinterpartietal and interp arietal rectly above the palatine canal. They form, on either bones. Their independent existence is characteristic of side, a scroll that is rolled in a lateral direction and is an early ontogenetic stage in the ossification of the der­ not attached to the mesethmoid bone. mal bones of the skull roof, and it is natural that the Anteriorly these scrolls divide to form a small dorsal time of their fusion with the neighboring parietal scroll in addition to the large lateral one. The bodies should be subject to individual variation. of the ventral maxilloturbinals are strong laminae, as PARIETAL much as 2 mm. thick. The scrolls, on the other hand, The parietal bone (fig. 14, PA) forms most of the are delicate, and do not extend as far forward as the roof of the braincase. stronger laminae which support them. The parietal walls of the braincase are very thick The bones which we shall term here anterior maxillo­ and filled with diploe. turbinals are, in most living mammals, the only maxillo­ Toward the posterior end of the parietal, tetween turbinals present. They are the scroll-shaped bones the cerebrum and the cerebellum, the diploe reaches visible in the dried skull as one looks into the anterior its maximum thickness of about 14 mm. Into this nares. diploe there extends, on either side of the skull, a cavity The above observations on the turbinal bones make that is the dorsal extension of the temporal venous it plain that, in Merycoidodon, they are far more sinus (fig. 21, SVT), a large blood vessel which is strongly ossified than in any living mammals except, further discussed below. perhaps, Dicotyles. This exceptional degree of ossifica­ tion is undoubtedly a result of the heavy ossification of Within the cranial cavity, the parietal surface di­ the mesethmoid cartilage which, in turn, is correlated rectly below the sagittal crest is excavated by an an- with the probable rooting functions of the . tero-posterior groove that contained the sagittal venous sinus, one of the vessels draining blood posteriorily VOMER from the brain and from the face (fig. 22, SS). The vomer is a long, slim, trough-shaped bone, which runs antero-posteriorly along the mid-line of the floor FRONTAL of the nasal cavity. Its concave side is dorsal, and in The frontal bones (fig. 14, FR), by reason cf their it rests the base of the vertical nasal septum. heavy supraorbital extensions and their postorbital In the anterior portion of the nasal cavity, the nasal processes, are strongly developed in Meryooidodon as septum rests between two upturned flanges, one spring­ in most members of the family Merycoidodontidae. CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 127 Anterior to the temporal ridges, the frontals form SQUAMOSAL an almost flat surface between the orbits. On this The squamosal bones (fig. 14, SQ) form a large por­ surface, only a few millimeters on either side of the tion of the lateral walls of the braincase; they are tlick sagittal suture, are the supraorbital foramina. These and heavy, and their relations to the neighboring skull are the dorsal openings of the supraorbital canals, elements are in places rather complex. which open into the dorsal side of the frontal pneumatic The posttympanic process of the squamosal forms the sinus. A continuation of the same canal leads from entine posterior wall of the bony external auditory the ventral side of the supraorbital extension of the meatlus (fig. 15, FTP). In this region the squamosal frontal sinus, through the lower wall of the supraorbital extends medially a distance equivalent to the lengtl of process, and into the through a foramen in its the external auditory meatus (fig. 19). Its medial sur­ roof. face is the continuation of the inner squamosal surface As in modern Artiodactyla, the supraorbital canal which, farther dorsally, forms part of the braincase carried the frontal vein from the orbital plexus (or wall. Medial to the posttympanic process, however, orbital venous sinus) upward through the frontal pneu­ this surface is in close contact with the lateral wall of matic sinus and out through the . the pars petrosa of the periotic bone (figs. 15,16, PET). On the suture between the orbital wing of the frontal An anterior extension of the posttympanic process and the orbitosphenoid bone is situated the ethmoidal forms the medial part of the ventral wall of the external foramen (fig. 14, EF}. This leads into a canal that auditory meatus. The lateral end of this wall (the cnly runs posteriorly in the orbital wing as far as the cribri­ part readily visible from the exterior of the skull) is form plate. It carries the ethmoidal artery, a branch formed by the tympanic portion of the meatus. of the ophthalmic artery. In the modern and Cervidae, the postt7m- The ethmoidal foramen and canal are present in all panic process forms a part of the posterior wall of the modern Artiodactyla. external auditory meatus; but in no modern artiodactyl Anterior to the postorbital constriction are the does it participate in the formation of the ventral wall, farthest posterior extensions of the frontal pneumatic as in Mev^ycoidodon. This participation of the squa­ sinus. In cross section, these extensions appear as two mosal in the ventral wall of the meatus must not be small cavities, about 3 mm in maximum diameter, one confused with the meatus auditorious spurius, prerent situated at either lateral limit of the diploic portion in some modern and a few fossil Artiodactyla, in wl ich of the bone. the posttympanic and postgleiioid processes meet and In a section just anterior to the posterior nares, the fuse below the external auditory meatus of the tympanic frontal sinuses are considerably larger. Here each is bone. The meatus spurius simply covers the wall of incompletely divided into two parts: a medial cavity, the true auditory meatus; it is not a part of it. It is still situated at the lateral edge of the diploe, and a lat­ present in the and some Bovidae, and is very eral one occupying the supraorbital process. Both these nearly complete in the . cavities were separated from the olfactory lobes only The serial sections show the squamosal bone to be by a very thin wall of bone, which in this section has thick and filled with diploic tissue, especially in the been broken through. That the two were originally lower part of its cranial portion (fig. 22) and, to a separated by bone was determined by cleaning the mat­ lesser extent, in the . rix from the inside of the braincase of the skull from In the diploic bone of the cranial wall of the squa­ which figure 16 was made. mosal, dorso-medial to the postglenoid process, lies a cavity, 6 mm long, 6 mm wide, and 15 mm high (fig. A cross-section of the posterior part of the nasal 21, SVT). It tapers anteriorly to a blind end, and is cavity shows the right frontal sinus, which here occu­ surrounded on all but one side by the squamosal. Hhe pies the supraorbital process, widely open into the nasal only other bone abutting upon it is the pars petrosr. of cavity proper. the periotic, part of whose lateral side covers a large A narrow extension of the frontal sinus reaches into gap in the medial wall of the cavity. the anterior processes of the frontals that thrust for­ This cavity has two openings, one into the cranium ward between the nasal and the lacrimal bones. In and another to the exterior of the skull. The former most sections this is a mere excavation of the frontal is a gap between the squamosal and the apex of the wall of the nasal cavity; in one section, however, it pars petrosa of the periotic (fig. 20, FJSP). The latter is separated from the nasal cavity by a thin wall. This is in the form of a canal that leaves the base of the suggests that the part of the frontal sinus immediately cavity near its anterior end and extends posteroven- posterior to this section may also have been so separated. trally through the squamosal bone, just medial to the 128 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

FSM SF

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.

Symbols used in figures 19-28 AA80, ampulla of anterior semicircular canal. DP8, groove for dorsal petrosal sinus. FR, frontal. A.C, area cochleae. D8, diploe of squamosal bone. F8, facial sulcus. ANF, area nervus facialis. EC, ethmoidal crest. FSM, stylomastoid foramen. AS, alisphenoid. ENT, endoturbinal. FV, fenestra vestibuli. BO, basioccipital. ET, endotympanic. HOF, hiatus canalis facialis. BS, basisphenoid. FC, fenestra cochleae. HSC, horizontal semicircular canal. CER. cerebellum. FGA, anterior carotid foramen. If, ? CT, crista tympanica. PG, flssura glaseri. I AM, internal auditory meatus. CV, crista vestibuli. FJSP, foramen jugulare spurium primitivum. IGA, groove for internal carotid arterj-. DMT, dorsal maxilloturbinal. FLA, foramen lacerum auterius. IT, incisura tympanica. DP, diploe of parietal bone. FM, foramen magnum. 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 post- glenoid processes. Looking anteriorly.

PA

I INCH FIGURE 22.—Merycoidodon culbertsonii (Leidy). X !%• Thick section of skull from sections M57-77, looking anteriorly into olfactory lobes. Symbols used in figures 19-23—Continued If, pars mastoidea. PC, pars cochlearis of pars petrosa. 8MS, superficies meatus of squamosal. M2, second molar. PCP, posterior clinoid process. ME, mesethmoid. PET, pars petrosa. 88, sagittal sinus. MF, margo fissurae of squamosal. PF, pituitary fossa. 88C, subsphenoid canal. MTS, margo tympanic! of squamosal. PFM, processus folii of . 8VT, sinus venosus temporally MX, maxilla. PME, pneumatic part of mesethmoid. T, ectotympanic. JVC, nasal cavity. PO8, posterior root of orbitosphenoid. TC, temporal canal. O, orbit. PVP, pars restibularis of pars petrosa. TT, tegmen tympani. OL, olfactory lobes. RE, recessus epitympanicus. V, vomer. P, promontorlum. 8G, sinus canal. VOL, vena capitis lateralis. PA, parietal. 8F, subarcuate fossa. VPH, vagina processns hyoidei. PAL, palatine. SH, hypotympanic sinus. V etc., groove for cranial nerves. PBC, petrobasilar canal. 238485—58——8 130 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 open­ of the skulls and to the area of insertion necessary for ing of this canal is through the foramen jugulare spu- their temporal muscles. It is therefore possible that the rium (TC, fig. 15) between the tympanic bulla and presence of the sinus venosus temporalis indicates that the postglenoid process. the small size of the brain and concomitant thickness The positions of the outlets of this cavity lead to of the diploic portion of the squamosal bone allowed the the conclusion that it must play a part in the venous superior cerebral vein to attain a large size. £ince the circulation of the skull, in close connection with the exit of the temporal canal (fig. 15, TO) is small, the large venous sinuses of the posterior portion of the sinus venosus temporalis must have served as a reser­ brain. The canal linking it with the exterior is, from voir for venous blood, rather than as a main drainage its course and the position of its exit, the temporal vessel. Blood probably flowed both into and out of it canal. This canal, to some degree present in all mam­ through the diploic veins. In modern mammals, with mals, carries the superior cerebral vein to its junction their large brains, the dorsal cerebral vein and the with the external jugular vein. The above-mentioned temporal canal are of relatively minor importance. cavity in the squamosal is simply an enlargement of the The lateral growth of the brain prevents any large temporal canal to form a venous sinus similar to those veins from taking a dorso-ventral course around it; lining the cranial cavity, but surrounded by bone. The therefore all venous drainage of the braincase is along term sinus venosus temporalis is here applied to it. its roof (as the mastoid emissary vein) or along its Its opening into the cranial cavity is therefore the fora­ base (as the inferior petrosal sinus). men jugulare spurium primitivum (van Kampen, 1905, Another ramification of the cranial venous system is p. 382). This corresponds to the foramen through indicated by the presence of a canal running postero- which, in the embryological stages, the superior laterally from the temporal canal, between the squa­ cerebral vein left the chondrocranium. mosal and tympanic bones. It has its exit just anterior The sinus venosus temporalis is found in no living to the external auditory meatus, through the p ostsqua- mammals, to the best of the author's knowledge. The mosal foramen of Cope (1880). This foramen was not only mention of its presence in fossil skulls is by Simp- observed by Cope in Merycoidodon; it is very small in son (1936) who reports the presence of an identical this genus and absent in some specimens. cavity in the squamosal bone of Oldfieldthomasia, a The postsquamosal foramen is reported by Cope to primitive notoungulate from the Eocene of . occur in the Hippopotamidae, Giraffidae, C^rvidae, Merycoidodon and Oldfteldthotnasia are both primi­ Antilopidae and Capridae among the Artiodoc^yla. tive mammals, with small brains relative to the size Still another canal of the cranial venous system ex- CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 131 tends backward along the suture between the parietal surface of the pars petrosa. In its center is a small, and squamosal bones from the frontal plane of the deep depression. external auditory meatus to the plane of the condylar The subarcuate fossa must have lodged the lobuhis foramen), where it emerges upon the surface of the petrosus of the flocculus of the cerebellum, and small skull through the postparietal foramen of Cope (1880), blood vessels probably passed ventrally from the floc­ (fig. 14, FPP). These foramina (there may be two or culus into the pars petrosa. Davidson Black (1920, three on either side of the skull) are present in all the p. 309) notes the similarity of the deep subarcuate forsa Merycoidodontidae but in few living Artiodactyla (the of the Merycoidodontidae to that of the Carnivcra , Cervidae, and ). (such a fossa is also present in Rodentia and many Other aspects of the squamosal bone will be con­ Primates; Bolk et al., 1936, vol. 2, p. 89). Black con­ sidered in the discussion of the tympanic region. trasts this condition with that of the living Arti­ odactyla, in which, he says, the subarcuate fossa is shal­ TYMPANIC REGION low or absent because of the absence of the lobulus For purposes of discussion, this complex region will petrosus of the cerebellum. This is true, so far as can be divided into four parts: (a) the pars petrosa of the be determined by examination of the literature and of periotic bone; (b) the middle ear and surrounding dried skulls of recent Artiodactyla, except in the structures, including the auditory bulla; (c) the ex­ suborder Tylopoda, which have a very large lobulus ternal auditory meatus; and (d) the pars mastoidea of petrosus. This condition will be discussed further in the periotic bone. the section on Poebrotherwm. The subarcuate fossa of Merycoidodon is smaller and PARS PETROSA OF THE PERIOTIC BONE much simpler than that of the living Tylopoda or of This bone, contained entirely within the cranial cav­ the contemporary Poebrotherium, and resembles very ity (fig. 16, PET), is composed of two parts. The closely the fossa of the Carnivora. The presence of dorso-lateral portion is the pars vestibularis (fig. 20, this fossa seems to indicate that Merycoidodon, like the PVP). It contains the semi-circular canals, and is Carnivora, Rodentia, and some Primates, possessed1 a situated mainly posterior, as well as dorsal to the pars motor reflex center that is present only in the Tylopoda cochlearis (fig."20, POP). among modern Artiodactyla. As has been pointed out, the pars petrosa is in con­ On the medial side of the parts petrosa is a large tact laterally with the inner face of the ventral plate opening, the internal auditory meatus (fig. 16, MAI). of the squamosal, and ventro-medially with the lateral Its course is directly lateral through about two-thirds border of the basioccipital. It reaches its greatest size the thickness of the pars petrosa. At its fundus, the posteriorly; anteriorly its dorsal-surface slopes to its internal auditory meatus divides into two parts (fig. termination at a point lateral to the foramen lacerum 19). The dorsal branch is the area nervus facialis medius. Just posterior to this point, the pars petrosa (ANF), into which passes the seventh cranial nerve on gives off a slim process which mounts dorso-laterally its way to the . The more ventral of the along the cranial wall of the squamosal. This is the two branches is the area cochleae (AC) , through which processus perioticus superior. Its position in Mery- the duct of the auditory nerve passes to the . coidodon is nearer that of modern mammals than that Backward for a short distance from the internal audi­ of the Eocene OldfieldtJiomj/asia (Simpson, 1936) in tory meatus, the facial canal lies in the pars petrosa. which the process projects laterally along the floor of It then passes out of the pars petrosa, through the ap^r- the temporal venous sinus. In Meryeoidodon, this tura tympanica canalis facialis, into the cavity of the process is separated from the sinus by about 5 mm. of middle ear, along whose roof it travels as the facial diploic bone. sulcus (fig. 19, £F). Posterior to the processus perioticus superior a From the fundus of the internal auditory meatus groove extends backward along the dorsal crest of the extends anteriorly a very narrow canal, the hiatus pars petrosa (fig. 21, DPS). It is about 3 mm. long, canalis facialis (fig. 21, EOF). It issues into the cra­ and in life carried the dorsal petrosal sinus, which nial cavity through a foramen on the anterior slope of joined the transverse venous sinus at the foramen the pars petrosa. It carries the nervus facialis sup°,r- jugulare spurium primitivum, thus emptying blood ficialis major, an anterior branch of the , from the midbrain region into the temporal sinus and which extends anteriorly as the nerve of the . canal to the sphenopalatine plexus and ganglia. It is Posterior to the above-mentioned groove (figs. 16,19, the equivalent of the palatine nerve of lower . SF) , is a depression, the subarcuate fossa, in the dorsal 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 con­ lateral side of the bone. This is a reflection of the fact tinuation of the facial canal. It extends posteriorly that, in Merycoidodon, the pars vestibularis of the pars about 4 mm., attaining such a depth (about 3 mm.) that petrosa is a more important element in the side wall it might be called a recessus epitympanicus. Regard­ of the braincase than in modern Kuminantia. The same less of the terminology employed, however, this sulcus comparison seems to hold with the Non-Kuminantia. almost certainly carried the facial nerve, as it ends pos­ The total antero-posterior length of the tegmen tym­ teriorly directly above the stylomastoid foramen, the pani is approximately six millimeters; it extends pos­ exit of the facial from the (fig. 19, teriorly from the level of the foramen jugulare spurium FSM). (figs. 15, 20, TO) to that of the stylomastoid foramen The central cavity of the pars vestibularis of the pars (fig. 15, FSM). The tegmen tympani is absent in the petrosa (fig. 20, PVP) is the vestibule (fig. 19, F). anterior half of the roof of the tympanic cavit7, which This cavity has in its ventro-lateral wall an opening, is formed by the squamosal (fig. 21). This i« a rare the f enestra vestibuli or f enestra ovalis, through which condition in modern mammals, being found, according it communicates with the cavity of the middle ear. to van Kampen (1905, p. 345) only in the Marsupialia In the anteroventral wall of the vestibule is a small and Chiroptera. circular depression, the recessus sphericus. This, in Medial to the tegmen tympani, directly anterior to life, contained the of the . the facial sulcus, and below the hiatus canalis facialis, The posterior end of the vestibule is just anterior to lies a short, deep groove in the pars petrosa (fig. 20). the level of the stylomastoid foramen (fig. 15, FSM). This is the sulcus in which is inserted the tensor tym­ Ventrally, a horizontal crest projecting laterally from pani muscle. the body of the pars petrosa separates the vestibule From the posterior side of the fenestra cochleae (fig. from the fenestra cochleae, which lies below it. This 19, FC) a small canal runs posteromedially through crest is the crista vestibuli (fig. 19, CV). the bone. This is the aquae-ductus cochleae which, like Within the vestibule are several openings, leading to the aquae-ductus vestibuli, formed a path for interflow other parts of the cavity of the internal ear. One of of lymph between the inner ear and the subarachnoid these, near the posterior termination of the vestibule space between the arachnoidea and the pia mater of and on its medial side, is the opening of the aquae- the brain. ductus vestibuli, a bony canal running postero-laterally The lower part of the lateral wall of the pars petrosa, and opening upon the medial surface of the pars which forms most of the medial wall of the middle ear petrosa. This canal transmits the , cavity, bulges somewhat into that cavity. The bulge, which ends, in modern mammals, in a cul-de-sac be­ which is caused by the first coil of the cochlea, i^1 known tween the layers of the dura mater. as the promontorium (fig. 20, P). The promontorium Nothing need be said here concerning the semicir­ in Merycoidodon is conspicuous, but not so much so cular canals. As is shown by the work of Turke- as in Oldfieldtho'inasia (Simpson, 1936). No data are witsch (1933), the only differences that they exhibit available as to the relative degrees of prominence of within a mammalian order are those of dimensions, the structure in modern mammals. and such measurements must be based upon a large The cochlea, as revealed in frontal section, appears suite of specimens. to possess 2% spiral turns, the same number as in The lateral side of the pars vestibularis of the pars modern ungulates. petrosa is an eminence, the prominentia canalis later- MIDDLE EAR AND SURROUNDING STRUCTUr.ES alis, which is closely applied to the squamosal bone. Its lower edge forms the tegmen tympani (fig. 20, TT). Of the various cavities (usually termed "sinuses" The tegmen tympani, as the serial sections show, is and "recesses") of the middle ear, the one underlying the auditory is the largest in Merycoidodon. simply a projection of the pars vestibularis over the This cavity occupies the hollow auditory bulla (fig. tympanic cavity. In Merycoidodon this projection is 15, #), and is thus surrounded on all but th«, dorsal far greater than in living Artiodactyla, with the result side by the inflated tympanic bone. It is known as the that the tegmen forms almost the entire roof of the hypotyinpanic sinus (fig. 20, SH). The upper bound­ tympanic cavity, whereas in modern artiodactyls it ary of the main part of the hypotympanic sinus lies rarely forms more than half. at the level of the chain of auditory ossicles. An­ When the pars petrosa of Merycoidodon is compared teriorly, the sinus extends about 4 mm beyonc1 the os­ with that of , which is typical of modern Kumi- sicles (fig. 21, $#), and for a short distance is entirely nantia, it becomes apparent that the tegmen tympani surrounded by the tympanic bone; behind this point of the former is borne relatively much lower on the 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 In Merycoidodon, the epitympanic sinus is absent, dorsal gap in the tympanic bulla is the "tympanicum- the part of the squamosal above the defekt" (Bondy, 1907). It is larger in Merycoidodon being a solid plate containing no cavities. This is al^o than in modern Artiodactyla. true, so far as could be determined, of all the living In spite of the fact that Merycoidodon has extremely Artiodactyla. small auditory bullae, the hypotympanic sinus is rather In addition to the large cavities of the middle er.r, large. This is, of course, because the bulla is entirely there is in Merycoidodon a very small cavity, the hollow, with no filling of cancellous bony tissue such incisura tympanica (fig. 20, IT), which lies just dorsal as occurs in many Artiodactyla, including the later to the tympanic membrane. Its lateral wall is formed merycoidodonts. The sinus is shown at its maximum by the facies epitympanica of the squamosal, and its size in figure 21 (SH). roof by an overhanging process from that bone. Ven- In the Mammalia, the middle ear cavity may extend trally it is bounded by the tympanic bone. In t^e into two cavities dorsal to the auditory ossicles, known fossil it is open medially into the epitymanic recess, as the epitympanic recess and the epitympanic sinus. but in life this opening was covered by a membrane The epitympanic recess is the cavity lying im­ (the membrana shrapnelli or pars flaccida), a non­ mediately dorsal to the auditory ossicles. It is en­ functional part of the tympanic membrane. Its lower closed in the ventral wall of the skull (usually between border is the dorsal part of the sulcus tympanicus, the the pars petrosa and squamosal) and composes the part of the tympanic ring in which the tympanic mem­ dorsal portion of the tympanic cavity proper. brane was held and stretched taut. The epitympanic sinus, on the other hand, is an ac­ The position of the incisura tympanica in Mery­ cessory cavity of the tympanic cavity. Where present coidodon is the same as that in the Suidae. Data con­ it lies in the squamosal or the pars mastoidea, and opens cerning its condition in the Ruminantia are incomplete, into the epitympanic recess. but it is known to be large in the Cervidae, and absent The epitympanic recess in Merycoidodon is a cavity in the and Tragulidae. about 2 mm. deep and 3 mm. long, situated dorsal to the The most striking external feature of the ear region deepest portion of the hypotympanic sinus (fig. 21, (although smaller in Merycoidodon than in most mam­ RE). Its roof consists of three elements. The middle mals) is the auditory bulla (fig. 15, B). The most an­ element is the slanting lateral surface of the pars terior part of the bulla is, as in all other animals with petrosa of the periotic, and the lateral element is the small or medium-sized bullae, the styliform process. f acies epitympanica of the squamosal bone. This por­ This slim anteriorly projecting extension is applied tion of the squamosal lies medial to the postglenoid closely to the lower surface of the squamosal and, in process and forms the floor of the temporal venous Merycoidodon, branches at its anterior end into a sinus. medial and a lateral arm, each about 1 mm. long. It The facies epitympanica is in contact with the pars formed the lateral wall of the ostium tympanicum petrosa; between them lies a small bone whose identity tubae (see above), and probably, as in living Artiodac­ is in doubt, but which is probably a piece broken from tyla, provided the origin for the levator veli musoK the tegmen tympani of the pars petrosa. This is the In the living Cervidae and Bovidae the stylifora third element of the roof of the epitympanic recess. process is present and probably performs the same The living Artiodactyla in which the epitympanic functions as in Merycoidodon. In the living Cameli­ recess most closely resembles that of Merycoidodon are dae the anterior wall of the bulla forms the lateral the Cervidae and Camelidae. In both these families covering of the ostium tympanicum tubae, and there the recess is small, and roofed over as much by the pars is only a suggestion of a styliform process. No process petrosa as by the squamosal, but in the Suidae and at all is present in or the Suidae. Bovidae it is roofed mainly by the squamosal. The Just posterior to the styliform process, the bulla of recess in the Tragulidae resembles that of Merycoid­ Merycoidodon is slightly notched for the reception of odon except that the tympanic bone, in addition to the the eustachian tube, which passes medial to it. squamosal, forms its lateral boundary. This is because The bulla in Merycoidodon is a low inflated hemi­ the "tympanicumdefekt" in the Tragulidae is smaller. sphere of bone situated between the postglenoid process The epitympanic sinus has less functional impor­ and the basioccipital. Its longest dimension is tl °- tance than the recess, and is correspondingly more antero-posterior one, and it does not project so frr varied. In many mammals it is absent, but in others, ventrally as does the postglenoid process. Laterally it such as the Notoungulata (Simpson, 1936) it attains overlaps ventrally the facies epitympanica of the squa­ enormous size. mosal; 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 Suidae: Parker, 1886) the bulla is composed entirely venous sinus (fig. 15, PEG}. Posteriorly, the bulla of the ectotympanic. narrows to a ridge, which is suturally attached to the The composition of the bulla remains largely a matter antero-lateral side of the paroccipital process of the oc­ of opinion, as stated above, because any elements form­ cipital bone (fig. 14:). Here it forms a part of the ing it are in almost every case completely fused to­ medial wall of the articulation for the tympanohyal gether in the adult stage. In a few, however, a suture bone. In contrast with some modern Artiodactyla, the is present in the adult, indicating the presence of both bulla is not fused to the pars petrosa of the periotic, ectotympanic and entotympanic. Such a suture was and there is a gap between the two through which the found in the sectioned skull of Merycoidodon (f sj. 20, T internal carotid artery probably passed (fig. 21, FCA ). and fig. 21, ET) and in 1 of 12 other Merycoidodon The bulla of Merycoidodon is smaller in proportion skulls examined. It thus seems possible that tin is genus to skull size than that of any modern artiodactyl, is an exception to the previously accepted rule as to although the Cervidae and Qiraffidae have a small the composition of the bulla in the Artiodactyla. bulla. In most of the living genera there is a tend­ The auditory bulla is bordered by several openings in ency to develop a large bulla, that is laterally the skull. Among these is the fissura glaseri (figs. 15 compressed and reaches ventrally well below the post- and 21, FG) , through which the chorda tympani branch glenoid process. of the facial nerve passes from the tympanic cavity on The presence of a gap between bulla and basioccipital its way to anastomose with the mandibular branch of is correlated with the size of the bulla, and occurs the trigeminal nerve (see Goodrich, 1930, p. 462). Into among modern Artiodactyla only in the Cervidae and the inner end of the fissura glaseri projected the pro- Giraffidae. No such direct explanation can be found, cessus folii of the malleus, a remnant of Meckel's however, for the presence or absence of fusion of the cartilage. bulla tq the pars petrosa of the periotic. The two are The fissura of Merycoidodon is completely sur­ independent of each other, as in Merycoidodon^ in the rounded by the ectotympanic bone except for a space Suidae, Hippopotamidae, Camelidae, and Caprinae. of about 2 mm on its antero-lateral side where the In the Suidae alone of these the two bones are in con­ squamosal participates in its wall. This surface of the tact. They are fused together in the Bovidae and in squamosal (fig. 21, MF] is known as the margo fissurae. some Cervidae. The most obvious explanation of these This condition differs from that in living mammals various conditions in living Artiodactyla is the degree in which the entire lateral wall of the fissura glaseri of development of the internal carotid artery. In liv­ is formed by the margo fissurae of the squamosal. ing Artiodactyla this is present in the late embryo- Between the ventral crest of the external auditory logical stages but degenerates and becomes small or meatus and the posterior crest of the tympanic bone, disappears entirely in the adult. Thus the presence of which is suturally attached to the paroccipital process, a gap between bulla and pars petrosa and its size, lies the vagina processus hyoidei, the pit in which is if present, may depend upon the ontogenetic stage at lodged the cranial end of the tympanohyal bone. It which the internal carotid disappeared or ceased to lies posterior to the inflated part of the bulla, and is function. This is borne out by the fact that in Suidae, separated by a low ridge of the occipital bone from where a gap is present, the artery is functional, though the stylomastoid foramen, which lies posterior and greatly reduced. This is also true in Catnelus. slightly lateral to it (fig. 15, SM; the vagina processus One of the chief problems concerning the auditory hyoidei is not labelled in this figure). bulla has been that of its composition. It was first The vagina is seen, in section, to be an excavation in thought to be formed by one bone, the tympanic (more the thick portion of the ectotympanic bone that forms properly ectotympanic). Later it was discovered that a part of the external auditory meatus (fig. 1£, VPH). another bone, the entotympanic, sometimes participates In the modern Artiodactyla, as in Merycoidodon, the in the formation of the bulla. This is a neomorph in vagina processus hyoidei is well separated from the stylomastoid foramen. In the living families their mammals, and ossifies relatively late in ontogeny. As swollen bullae grow around the tympanohyal, making its name indicates, it forms, if present, the medial side the vagina deeper and more conspicuous. In some of the bulla. More and more instances of the presence families this posterior inflation of the bullae \s so great of the entotympanic in the wall of the bulla are being that the tympanohyal springs from the side of the brought to light (van Kampen, 1905; van der Klaauw, bulla instead of behind it (Tragulidae, Bpvida^). The 1931). inflated bullae may entirely surround the tympanohyal, It has been the consensus of opinion that in the forming a pit. This is true of the Tragulidae, Giraf­ Artiodactyla (with the very doubtful exception of the fidae, Camelidae, and some of the Bovidae. CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA J35 The stylomastoid foramen (figs. 15 and 19, FSM] lies (pointing backward and slightly upward) agrees with just posterior to the vagina processus hyoidei. It opens that of the Ruminantia and differs sharply from that into the extreme posterior end of the tympanic cavity, of the Non-Riuninantia. separated from the foramen lacerum posterius by the paroccipital process. Merycoidodon thus resembles PARS MASTOIBEA OF THE PERIOTIC BONE the living Ruminantia and the early Tertiary buno- This is the posterior part of the ossification of the donts (presumably Non-Ruminantia) (Colbert, 1938). otic capsule, and is distinguished by its cancellous It differs from the living Non-Ruminantia, in which structure from the pars petrosa, which is rock-like in the stylomastoid foramen is confluent with the fora­ its density. men lacerum posterius. The pars mastoidea is exposed on the occipital sur­ The facial nerve leaves the tympanic cavity through face of the skull as a plate of bone (M, fig. 14) lateral the inner opening of the stylomastoid canal. This is the to the occipital bone and medial to the lambdoid cr?-st. foramen stylomastoideum primitivum. Here the chorda This plate terminates ventrally between the posttym- tympani leaves the facial nerve and goes forward into panic and paroccipital processes as the very small m as- the tympanic cavity. toid process, quite insignificant as an area of murole attachment. Medially, this process is in contact with EXTERNAL AUDITORY MEATUS the paroccipital process. This bony tube (fig. 14, MAE\ fig. 15), which connects Sections show that the mastoid process is in contact the middle ear with the exterior of the skull, has already with the posttympanic process not only posteriorly, but been discussed in part under the consideration of the also medially. A small flange (M, fig. 19) projects squamosal bone, which forms its dorsal, posterior and anteriorly from the mastoid, between the posttympanic part of its ventral wall. process and the pars petrosa, and forms part of the The remainder of the meatus is formed by the ecto- lateral wall of the stylomastoid foramen. tympanic bone; in other words, it is a direct lateral Because the pars mastoidea is exposed at the surface extension of the auditory bulla. The meatus is much of the skull, Merycoidodon is one of the "mastoid Artio­ more heavily ossified than the bulla. Its course from dactyla," one of the two divisions of the order sug­ the exterior is inward and slightly downward for most gested by Helga Pearson (1927). In the other division, of its length. At the medial border of the postglenoid the "amastoid Artiodactyla," the pars mastoidea is cov­ process, it turns slightly anteriorly and enters the ered by a flange of the squamosal, which overlaps upon bulla. the occipital plate. At its entrance into the bulla, the ectotympanic wall Miss Pearson limits her two groups as follows (1927, of the meatus projects about 5 mm. into the tympanic p. 458) : cavity (fig. 20, CT). This, projection is called the crista tympanica of the meatus (Bondy, 1907; Simpson, Choeropotamus, Cebochoerus, , the Anthrnco- theriidae, Mixtotherium, Entelodon, and the Suidae appear to 1936). The tympanic cavity extends laterally both belong to one group of Artiodactyla, while the Dichobunidae, anterior and posterior to the crista; such an extension Dacrytherium, the , Tapirulus, Amphimeryaa, the is known as a recessus meatus. This is also present in , together with the remaining post-Eocene fami­ all living Artiodactyla. lies, form another. These may conveniently be termed the The medial termination of the external auditory "amastoid" and the "mastoid" Artiodactyla respectively. meatus is an almost complete ring (it is open for a few In most living Ruminantia, as in Meryeoidodon, the millimeters dorsally) upon which was stretched the pars mastoidea is well exposed upon the occipital sur­ tympanic membrane (fig. 20, CT). This semi-ring face. However, in the Giraffidae and the Camelidae, represents the earliest point of ossification of the ecto­ the posttympanic process touches the paroccipital tympanic. The dorsal gap in the tympanic ring is process, and covers the ventral surface of the p^.rs filled by the margo tympanici of the squamosal, a medi­ mastoidea. ally projecting process of the superficies meatus, which For the purpose of comparison with Merycoidodon, forms the dorsal wall of the external auditory meatus the mastoid regions of a series of Merycoidodontidae (fig. 20, MTS, SMS}. This is in contrast with most were examined at the American Museum of Natural living genera of Ruminantia and also with Hippopota­ History through the courtesy of Dr. C. Bertrgnd mus (van Kampen, 1905, p. 592). In these genera, the Schultz and Mr. Charles H. Falkenbach. This series entire external auditory meatus is formed by the tym­ of skulls, culminating with the genus , of panic bone. To the margo tympanici was attached the late Miocene and Pliocene time, exhibits a progressive dorsal edge of the tympanic membrane. tendency of the external auditory meatus to migrate The orientation of the external auditory meatus 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. MS, maxillary sinus. Parallel lines slanting to left: pneumatic sinuses. NG. nasal cavity. F8, frontal sinus. SPS, sphenopalatine sinus. exoccipital. These two bones in Brachycrus show the tine and the orbitosphenoid. It extends posteriorly beginning of a tendency to grow over the auditory tube; below the anterior part of the cerebrum. the continuation of this trend, had the Merycoidodon- The development of the sphenopalatine sinus as seen tidae survived beyond the Pliocene epoch, would have here is typical of large mammals. In the smaller ones resulted in the amastoid condition typical of the non- the development of the orbits and braincase, which are Ruminantia. very large in proportion to their skull size, have re­ Thus we see, within one well-defined family, an al­ stricted the growth of the presphenoid bone. Thus the most complete transition from one to the other of Miss sphenoid portion of the sphenopalatine sinus is reduced Pearson's two artiodactyl types. The possibility of or absent. such a transition is suggested by Miss Pearson in her The frontal sinus (FS] terminates posteriorly in the statement (1927, p. 459) that the mastoid Artiodactyla frontal bone. It is the only pneumatic sinus cortained show a tendency toward reduction of the size of the in the roofing bones of the skull. pars mastoidea. The maxillary sinus is the largest of the three, and These observations prove that the mastoid condi­ is most widely open into the nasal cavity. Its posterior tion is a primitive one relative to the amastoid. All termination is a blind end in the zygomatic process of Artiodactyla are evolving more or less in the direction the maxilla. of the amastoid condition; the Non-Ruminantia at­ The extent of the pneumatic sinus system is to a large tained it in Eocene time, the Merycoidodontidae ap­ degree determined by skull size. The raison d'etre of proached it in Pliocene time. The Ruminantia, the pneumatic sinuses seems to be to effect an adjust­ however, show at present only a slight tendency toward ment between the space necessary to house tH soft it. The chief difference, then, between the mastoid and parts within the skull and the external area necessary amastoid Artiodactyla is in the relative speed with for muscle attachment, lodgment of the teeth, etc. The which the mastoid region is evolving. larger the skull, the greater is likely to be the disparity between the space thus required within and outside PNEUMATIC SINUSES the skull. These sinuses have been described individually under Among the Artiodactyla, the sinus system of Mery­ the discussion of the bones that contain them. Here coidodon most closely resembles that of Ovis ; the skulls they will be discussed as a system, and compared with of M. cuZbertsonii and of O. aries are about the same the systems of other Artiodactyla. size. The chief difference is that Ovis lacks the spheno­ The pneumatic sinus system of Merycoidodon is palatine sinus, which is well developed in Merycoi­ shown in figure 24. It consists of three sinuses, each dodon. Also, the frontal sinus in Ovis is larger and, opening directly into the nasal cavity (NC]. unlike that of Merycoidodon, communicates around the The sphenopalatine sinus (SPS] projects the farthest orbit with the maxillary sinus. posteriorly of the three. It is a long, narrow sinus The small size of the cerebrum in Merycoidodon whose posterior end is contained in the presphenoid explains the presence and size of the sphenopalatine bone and whose anterior part is surrounded by the pala­ 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 as an essentially non-functional vestige (Tandler, 1899, comparable size. This is the more astonishing when p. 703). one considers that the small brain is surrounded by a The course of the internal carotid artery in Mery­ great thickness of parietal and squamosal bone that coidodon is very well indicated, .and shows the artery might be expected to be pneumatized, as it is in the to have been a large one (fig. 17, 1CA ). Undoubtedly Suidae which have a parietal pneumatic sinus extend­ it branched from the common carotid artery, as it does ing ventrally into the squamosal and thence into the in living mammals, and extended dorso-laterally to the zygomatic arch. base of the skull. There the first evidence of its The Suidae also possess a well developed sinus in the presence in Merycoidodon is seen. horizontal plate of the maxilla (see Dictyles, fig. 18), A canal that passes dorso-laterally between the pos­ which is absent in M'erycoidodon. terior extension of the tympanic and the pars petrosa Of the Ruminantia, only the Cervidae, Tragulidae, of the periotic is identified as the posterior carotid and Camelidae have a less extensive pneumatic sinus canal. Its opening to the exterior is through the fora­ system than Merycoidodon (Aubert, 1929). The first men lacerum posterius, and it is through this foramen two of these families possess only a maxillary sinus; that the internal carotid artery enters the skull (figs. the last, frontal and sphenopalatine sinuses but no 15-17, FLP). maxillary. After entering the foramen lacerum posterius, the The sinus system of Merycoidodon is not a highly internal carotid diverged from the other vessels that evolved one; the sinuses connect only with the nasal passed through the foramen, all of which ran medial, cavity, and have no connections with each other (com­ instead of lateral, to the pars petrosa. It then passed pare the extremely complicated systems of the Giraf- through the posterior , and turned an­ fidae and of most Bovidae; Paulli, 1900). teriorly into the tympanic cavity. The sinus pattern of Merycoidodon most closely re­ Although the internal carotid can be definitely traced sembles that of the Camelidae among the Artiodactyla, into the tympanic cavity, there is no evidence ai- to and bears least resemblance of all to that of the living its course through the cavity. Further proof that it Non-Ruminantia. However, the skulls of the latter crossed this region is, however, furnished by the large are highly specialized; and unfortunately the arrange­ antero-medial opening in the bulla, just behind the ment of sinuses in their fossil relatives is unknown. ostium tympanicum tubae (fig. 15, OTT), which is It is probably best to regard the sinus arrangement obviously the anterior carotid foramen. Here the of Merycoidodon as primitive. It has no peculiarities artery left the tympanic cavity, lay for a short distance to indicate the beginning of a trend to any one of the between the bulla and basisphenoid, and entered the bizarre modifications of the system in late Tertiary cranium through the foramen lacerum medius (fig. 17, Artiodactyla. FLM). It lay on the floor of the cranium in a groove (fig. 16) whose size indicates that the internal carotid CIRCULATORY SYSTEM OF THE SKTJUL artery was an important vessel in Merycoidodon. It In describing the bones of the skull, mention was did not divide to form a rete mirabile, or network of made of all grooves, foramina, and canals that give small arteries, but extended undiminished to the chiarma evidence of the cranial circulation in Merycoidodon. ridge, where it divided into the anterior and middle This circulation will here be compared with that of cerebral arteries. Posterior to the sella turcica, the other artiodactyls, and of various other mammals. internal carotid arteries of the two sides were con­

ARTERIES nected by the posterior intercarotid artery (fig. 16). There is no groove in the base of the braincase to The arterial system of Merycoidodon appears to have indicate the presence of an anterior intercarotid artery. been in all but one respect the same as that of living A comparison of the basis cranii of Merycoidodon Artiodactyla, both and non-ruminant (fig. with that of its probable ancestor, of the 17). The difference was in the size and course of the late Eocene, gives a clue as to the primitive position internal carotid artery. This vessel is present and functional, but very small, in the Suidae (Sisson and of the internal carotid artery in the Merycoidodontidae. Grossman, 1938) and the Camelidae (Lesbre, 1903); In Protoreodon (M. C. Z. 5334), the posterior carotid in the Tragulidae (Tandler, 1899) the internal carotid canal opens on the base of the skull through a foranen is better developed than in all other living ungulates. independent of the foramen lacerum posterius, 3 mm It is generally absent as a rule in other living Artiod­ anterior to it on the medial side of the posterior part actyla (Hofmann, 1900), but has been reported in of the bulla. This arrangement is the same as that in tarandus, and probably exists in other genera, the living Lepus (Gregory, 1910, p. 329), the Creo- 238435—53———4 138 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 donta (Matthew, 1910, p. 297), all Carnivora, and arrangement here observed for the first time in the some Nptoungulata (van der Klaauw, 1931). The an­ Artiodactyla. terior carotid foramen, present in Merycoidodon and DORSAL SINUS SYSTEM in Protoreodon, and situated in the antero-medial wall The vessels of this group lie on the dorsal and dorso- of the bulla, is also reminiscent of these mammals. lateral sides of the brain. The most anterior of them In the few modern Artiodactyla that have an internal is the sagittal sinus, a median vessel lying directly carotid artery, its path differs considerably from that above the longitudinal fissure of the brain ard just in Merycoidodon, in which it traverses the tympanic below the sagittal crest (fig. 22, SS). Its length, as cavity. The internal carotid in the non-Ruminantia indicated by the groove it occupied in the roof of the describes a loop ventral to the auditory bulla, so that braincase, was 4 mm. the first contact of the artery with the skull is at its No impressions in the braincase indicate the anterior entry into the foramen lacerum medius (Sisson and and posterior connections of the sagittal sinus. We Grossman, 1938; van Kampen, 1905). may assume, from the anatomy of modern Artiodactyla, In the living Camelidae, the internal carotid artery that the sagittal sinus of Merycoidodon collected venous is in a completely closed bony canal whose laterial wall blood from the numerous small veins of the surface of is formed by the medial wall of the bulla, its medial wall by the basioccipital. the cerebral hemispheres, and transmitted it posteriorly into the transverse sinus, between the cerebrum and The internal carotid in the Tragulidae also lies along the medial wall of the bulla. In Tragulus it is en­ cerebellum. closed in a canal formed entirely by the tympanic bone; Ventro-lateral to the sagittal sinus, on the side walls in the other genera it simply lies in a shallow groove of the pyriform lobes, lies the vena collateralis cerebri, (van Kampen, 1905). which is the only connection in Merycoidodon between The above comparison of Merycoidodon with the liv­ the veins of the face and orbit and the dorsal sinus ing Artiodactyla shows that there are found, within system. The presence of this vein is indicated by an this order, two of the three possible courses of the antero-posterior groove in the side wall of the braincase internal carotid through the tympanic region. The (fig. 16, Ftftf). third course, with one branch of the artery passing The vena collateralis cerebri entered the cranium through the bulla and another lying medial to it, is through the sinus canal (fig. 16, PSC], which carried found only in the Muridae (van der Klaauw, 1931, it from its origin in the orbital venous plexus. The p. 180). groove for the vena collateralis cerebri extends back­ Within the cranium, the groove for the internal ward as far as the posterior part of the cerebrum where, carotid (fig. 16, 10A) is evidence for the statement that presumably, it joined the transverse sinus near its entry this artery was much larger than in living Artiodactyla. into the temporal canal. It also differed from the latter in the absence of a rete A vein almost identical with that indicated in Mery­ mirabile, as has been pointed out by Black (1921) on coidodon is found in many modern Insectivora. Greg­ the basis of his study of natural endocranial casts. ory (1910, p. 248) reports it in Solenodon, Microgale, Thus the intracranial portion of the internal carotid in and Erinaceus; and Shindo (1915) notes its presence Merycoidodon resembles that of the Carnivora (see in Talpa europaea. In the last, however, th^s vena Ellenberger and Baum, 1891, p. 373), but differs from collateralis cerebri enters the cranium through tte optic the latter, in the absence of the anterior inter-carotid foramen rather than an independent canal. According artery. to Shindo, this vein is a remnant of the anterior cerebral VEINS AND VENOUS SINUSES vein of the early embryonic stages. In some living The system of vessels draining the blood from the mammals, the back part of the vena collateralis cerebri cranium of Merycoidodon (fig. 17) was considerably is present, but the anterior connection with the orbital more complicated than in present-day Artiodactyla, as plexus never develops. See Lepus (Shindo, 1915, p. it contained several additional vessels. 370). As is usual in the Mammalia, an important part in In the possession of this vein in its entirety, Mery­ the head drainage in Merycoidodon was played by the coidodon differs from all but the most primitive of venous sinuses, which are enclosed by dense membranes living mammals. and usually lie in bony grooves. They consist of two The chief path by which the venous blood of the groups, the dorsal and basilar sinus systems, and their dorsal sinus system left the cranial cavity was the attendant veins. A third venous system is that of the temporal canal (see p. 130). The blood passed from subsphenoid canals of the basisphenoid bone, a venous 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 men on the occipital surface of the skull. It probably the mastoid emissary. drained posteriorly into the vertebral or the deep cervi­ The superior cerebral vein entered the temporal cal vein and thence to the anterior vena cava.. The canal through a foramen between the pars petrosa and mastoid emissary vein is present in living Ruminantia; the endocranial surface of the squamosal. This it is absent in the Non-Ruminantia, probably because foramen opened into the sinus venosus temporalis (fig. the overlapping of the squamosal bone upon the oc­ 20, SVT) , which lies almost entirely in the squamosal. cipital surface caused a rerouting of the blood by Besides receiving blood from the superior cerebral vein, covering the area where the mastoid foramen lay. this sinus undoubtedly collected it from the tiny veins The dorsal petrosal sinus is the last member of the filling the thick diploe of the squamosal. From the dorsal sinus system of which traces are found in the sinus the superior cerebral vein passed ventroposte- skull of Merycoidodon. This vessel was, as in living riorly into the temporal canal (fig. 20, TC; fig. 21, mammals, a tributary of the superior cerebral vein, FJSP), leading it through the foramen jugulare which it joined at its entrance to the sinus vencsus spurium (figs. 15, 17, TO) to join the external jugular temporalis. The dorsal petrosal sinus gathered venous vein (fig. 17, EJV). blood from the lower part of the cerebral hemispheres The foramen jugulare spurium is very small con­ and, before joining the superior cerebral vein, coursed sidering the size of the sinus venosus temporalis, which posteriorly in a groove in the dorsal side of the pars it drains but this disparity can be explained. Prob­ petrosa. Judging from the depth of this groove, the ably the temporal sinus served chiefly as a reservoir, in sinus was rather small (fig. 21, DPS). other words, the pressure of the blood therein was low. Besides, a posterior branch of the temporal canal (see BASILAB SINUS SYSTEM below) relieved the foramen jugulare spurium of the These vessels in Merycoidodon, as in modern forms, necessity of transmitting all the blood in the temporal were much more closely connected with the antero- sinus. posterior drainage of the head than was the dorsal Although it has not been reported in any living system. There is no evidence that the two systems were mammals, the sinus venosus temporalis cannot be re­ connected in any direct manner. garded as a primitive structure. It is, however, an The basilar sinus system, judging from the condition expression of the small size and correspondingly primi­ in living mammals, received blood from the nasal cavity tive condition of the cerebrum of M' erycoidodon. and maxillary region by way of the orbital venous The temporal canal is present in the Ruminantia, but plexus. From this plexus a large vein extended back­ absent in the Non-Ruminantia. ward through the foramen lacerum anterius (fig. 15, The posterior branch of the temporal canal begins a FLA). Within the cranium it entered the cavernous few millimeters above the foramen jugulare spurium. sinus, which lay posteriorly in a groove on the cranial Thence it extends postero-laterally to its exit from the floor (fig. 16, 1CA). skull through a foramen between the external auditory Behind the pituitary fossa (fig. 16, PF), the cavern­ meatus and the postglenoid process (fig. 17, PTC), ous sinuses of both sides were connected by the posterior After leaving the skull, this vein also joined the ex­ intercavernous'sinus. In this respect Merycoidodon ternal jugular. The vein homologous with this one differs from Sus (Dennstedt, 1903) which has no poste­ is the only external orifice of the temporal canal in rior intercavernous sinus. the modern Artiodactyla and the living Equidae and Merycoidodon had no anterior intercavernous sirus. Carnivora (Dennstedt, 1903). In this it differs from Bos but resembles the Caprinae Another vessel that drained the transverse sinus of (Dennstedt, 1903). Merycoidodon was the vein of the post-parietal canal, The cavernous sinus debouched from the braincase which extended backward beneath the parieto-squa- through the foramen lacerum medius (fig. 15, FLM). mosal suture. Just before its exit from the skull, this Here it was continuous posteriorly with the inferior vein in some cases divided into two or more small petrosal sinus, a large vessel that was contained in a branches. It is represented in Merycoidodon by the groove formed dorsally by the pars petrosa and medi­ post-parietal foramina (fig. 14, FPP). This vein is ally by the basioccipital bone. This groove is the petro- present in many modern, mammals, including some basilar canal (figs. 14,16, PBO). Artiodactyla (see above). The internal carotid artery of Merycoidodon passed Some distance ventral to the postparietal canal is the lateral to the pars petrosa, and the inferior petrcsal passage for another vein, the mastoid emissary. This sinus ventro-medial to it (fig. 17). As well as co^ld leads from the temporal canal back to the mastoid fora­ 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 reported by W. Krause (1868; cited by Arai, 1907), in that genus run side by side in the petrobasilar canal. who observed it in the human foetus and in 10 percent This is also true of the living Perissodactyla (Sisson of new-born infants studied. Arai (1907) found the and Grossman, 1938). same structure in the adult Lepus cuniculus, and named In the posterior third of its course (fig. 15) the petro­ it the sinus venosus ossis sphenoidalis (this rather cum­ basilar groove in Merycoidodon lies medial to the pars bersome title is necessary to differentiate it from the petrosa, and is impressed in the basioccipital only. Be­ sphenoidal pneumatic sinus). In Lepus, the upper yond this groove, the extracranial part of the inferior wall of this sinus is open into the pituitary f osra, and petrosal sinus probably emptied into the inferior cere­ the membranous wall of the venous sinus is in contact bral vein, and thence to the internal jugular (fig. 17). with the dura mater which surrounds the pituitary The serial sections revealed a minute canal extending body. In Lepus, the venous sinus receives blood from posteriorly in the backward extension of the alisphenoid the veins of the dura mater. In Meryooidodor, there bone above the auditory bulla (fig. 21', VCLf}. This is no such connection between the sphenoidal venous canal probably carried a vestigial vein, so small as to sinus and the pituitary fossa. This venous sinus has be non-functional, which branched from the cavernous been regarded by most anatomists (Arai, 1907; Wal- sinus at the foramen lacerum medius and passed into deyer, 1907; Bovero, 1905) as a remnant of Rathke's the dorsal side of the tympanic cavity. This course pocket. Voit (1909), however, feels that it is too far suggests that it may be a remnant of the embryonic posterior and must be a secondary structure. As the lateral head vein: it runs medial (although posterior) sinus is found in the embryos of many mammals that to the alisphenoid bone, which is the homologue of the lack it in the adult stage, the first hypothesis seems the lamina ascendens of the embryonic ala temporalis. more likely. The ala temporalis, in turn, is homologised by Broom As in Merycoidodon, Lepus has one or more canals (1907) with the reptilian epipterygoid. Furthermore, leading into the sphenoidal venous sinus from the ex­ the position of this small vein dorsal to the auditory terior of the basisphenoid bone. These are designated ossicles is equivalent to a location dorsal to the jaw by Arai (1907) the foramina venosa but, since tl ^y are articulation of the Reptilia, where the lateral head the openings of what he calls the subsphenoid canals, vein lay. they are here given the more specific title cf sub- To return to the functional portion of the basilar sphenoid foramina. A functional sphenoidal venous sinus with it" asso­ sinus system, we find that the inferior petrosal sinus, ciated subsphenoid canals is found in the adult s*age in slightly anterior to the beginning of its extracranial relatively few mammals. These are the Sciuromorpha portion, emitted a branch,, the condyloid vein, which and Myomorpha (Bovero, 1905), the Mars'ipialia continued posteriorly within the skull in a groove at the (Gregory, 1910, p. 431) and the Lagomorpha (Arai, lateral side of the myelencephalic base. It found its 1907). exit from the skull through the condylar foramen (figs. The manner of circulation of the venous blood of 15, 16, OF; fig. 17 CV) and joined the occipital vein, the head in the above-mentioned groups indicates the which probably transmitted the blood to the deep cervi­ part which the sphenoidal venous sinus of Merifcoido- cal or the vertebral vein. This constituted the second don played in its cranial circulation. of the two outlets of the basilar sinus system. Anteriorly the subsphenoid canal must have re­

SUBSPHENOID VEINS AND THE SINUS VENOSUS OSSIS SPHENOIDALI8 ceived, as in these groups, a branch of the ophthalmic coming from the orbital plexus. Posteriorly, the The evidence for the presence of this most unusual passed out through a foramen in the anterior wall set of veins, hitherto unrecorded in Artiodactyla or in most other mammalian orders, consists of the well- of the .foramen lacerum medius (fig. 16, FSP]. Thence developed canals by means of which they passed the blood undoubtedly flowed into the posterior portion through the basisphenoid bone of Merycoidodon (see of the cavernous sinus, just anterior to the confluence p. 122, and fig. 17). of the latter with the inferior petrosal sinus. A survey of anatomical literature proves beyond a It is apparent from the skulls of Merycoidodon which doubt that the cavity in the basisphenoid bone (fig. 17, were dissected that the vessels of the sphenoidal sinus 8VOS] contained a venous sinus, and that the canals system were larger than the cavernous sinus and, there­ leading from it to the surface of the skull (fig. 17, SSG} fore, that the sphenoidal vessels were among tte most conveyed blood into it from the orbital region and out important paths by which the blood of the facial region of it posteriorly. in Merycoidodon was drained posteriorly. We have Such a venous cavity in the basisphenoid was first 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 pos­ extinct -like mammals, it is included with the teriorly directed vein leaving the facial region of living living genera in the suborder Tylopoda. Artiodactyla. For this study, serial frontal sections were made of The facts here set forth show that, despite the size a skull of Poebrotheriutm wilsoni Leidy, from Oligocene of the temporal canal, the preponderant venous outlet rocks exposed near James Creek, in the Hat Crsek of the skull of Merycoidodon was the internal jugular Basin, Sioux County, Nebraska. It was collected in vein rather than the external jugular as in' modern 1922 by a party from the Walker Museum, University Artiodactyla (Bolk, Groppert etc., 1936). This fact is of Chicago. Study of the serial sections was aided by -proved by the large size of the sinus venosus ossis comparison with four other skulls of P. wilsoni, and sphenoidalis and of the inferior petrosal sinus, and by with one of P. eximium Hay, a larger species. No skulls the small size of the opening of the temporal canal, of P. labiatum Cope, the only other species of the genus, which led to the external jugular vein (fig. 17). were available for study; but reference has been made The internal jugular, a remnant of the lateral head to descriptions and plates of the skull of this species vein, is the more primitive means of drainage. In this (Scott, 1891,1940). respect, Ovw and Bos are mammals of the most ad­ The anatomy of the skull of Poebrotheriwn will not vanced type. In these genera, the external jugular vein be described here in as great detail as was that of Mery­ drains the skull, and the last vestige of the lateral head coidodon; it will suffice to point out some significant vein is lost. similarities and differences between the two, especially In Meryco-idodon, a large part of the blood from the those made available for the first time by the use of face must have run posteriorly through the subsphenoid serial sections. canals. In the more advanced Artiodactyla, the me­ BOXES OF THE SKULL, chanical advantages of an extracranial route appar­ ently prevailed; the subsphenoid canals degenerated, As in Merycoidodon, the basioccipital, exoccipitals and the external maxillary vein carried the blood which and supraoccipital are fused in Poebr other-turn into an they had once transmitted. This change in circulation occipital bone (fig. 25, OC}. The paroccipital process took place much earlier in evolution than the equivalent of this bone (fig. 25, PPO} is suturally attached for its one in the arterial system. entire length to the auditory bulla. This is due chiefly to the great inflation of the latter. Another result of NERVES OF THE SKTJUL REGION this inflation is that the basioccipital is in contact with The nerves supplying the cranium of Meryooidodon, the medial wall of the bulla (fig. 26), there being no as described above from grooves and foramina in the gap between them as in Merycoidodon. skull bones, do not differ in any important character­ Within the braincase, the course of the condyloid vein istics from those of living Artiodactyla. is indicated by an antero-posterior groove near the lateral edge of the basioccipital. This is present also CRANIAL MORPHOLOGY OF POEBROTHERIUM in Merycoidodon. Poebrotherhwi is the genus which represented, in The most striking characteristic of the basispheiioid White Kiver (Oligocene) time, the ancestral line lead­ bone (fig. 26, BS] is that, as in Merycoidodon, it con­ ing to the modern camels and . Like the other tains 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. PPO, paroccipital process of exoccipital. OO, occipital bone. SQ, squamosal bone. PA, parietal bone. TEAM, expanded external auditory meatus. PM, pars mastoidea of periotic bone. TY, tympanic bone. 142 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 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 be­ low). The posterior portion of the alisphenoid is in sutural contact with this horizontal plate of the squa­ mosal, but does not reach backward as far as the ostium. Within the cranium, a few millimeters medial to the

PPO alisphenoid-squamosal suture, a deep groove run? ante­ riorly from the region of the foramen lacerum medius to the foramen ovale (fig. 26, FO; hidden by the audi­ tory 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 FIGURE 26.—Poebrotherium wilsoni, ventral view of basis cranii. Com­ posite, drawn from Princeton Museum no. 12692 and M. C. Z. 5998, sinus venosus ossis sphenoidalis extends laterally into X %. it, as in the living Leporidae (Arai, 1907). B, auditory bull a. BO, basiocciptal bone. The deep, narrow presphenoid bone is of nearly the BS, basisphenoid bone. OF, condylar foramen. same shape as that of Merycoidodon. It is filled with FLM, foramen lacerum medius. FLP, foramen lacerum posterius. cancellous tissue of much finer texture than that present FOj foramen ovale. F8A, anterior subsphenoid foramen. elsewhere in the Poebrotheriuni skull. FSM, stylomastoid foramen. G, groove separating bulla from external auditory meatus. At its anterior end the presphenoid is no narrower OS, glenoid surface of squamosal bone. IM, inflated part of external auditory meatus. than in Merycoidodon, but, in contrast to this genus, MAE, external auditory meatus. PPO, paroccipital process of exoccipital. it contains no pneumatic sinus. On the external surface of the skull of Poebrother- tributary subsphenoid canals (fig. 28, SVOS: cf. fig. 26, ium, the orbitosphenoid bone is in about the sam^ posi­ F8A). The sphenoidal venous sinus is larger than in tion as in Merycoidodon. Within the cranium it shows Merycoidodon because of the greater dorso-ventral an interesting difference in that its posterior roof proj­ dimension of the basisphenoid. The sinus becomes ects inward beneath the cerebrum as a curved, dorsally narrower anteriorly, where the portions of the basi­ convex plate roofing the cranial nerves (IV, V±, V2, VI), sphenoid lateral to the sinus are filled with large-celled which lay in a groove lateral to the pituitary f os^-a. In cancellous tissue. The sphenoidal venous sinus termi­ the anterior part of their intracranial course, therefore, nates anteriorly below the anterior end of the pituitary these nerves were surrounded on all but the medi al side fossa; this differs from the condition in Merycoidodon^ by bone (fig. 28, FLA). This plate of the orbitosphe­ in which the anterior end of the sinus lies below the noid is probably a secondary ossification rather than a posterior end of the fossa. homologue of the lateral lamina of the sella turcica in Within the cranial cavity, the basisphenoid is exca­ Merycoidodon. vated by the pituitary fossa, which is shallow, as in Where it forms part of the anterolateral wall of the Merycoidodon and the living Tylopoda (Lesbre, 1903). cerebrum, the orbitosphenoid reaches its maximum The anterior end of the fossa is marked by an eminence thickness of 5 mm. This is the thickest part of the which rises about 2 mm. above the floor of the braincase. braincase wall, which as a whole is distinguished from At its posterior end are the posterior clinoid processes, that of Merycoidodon by its light construction. The whose bases are about 3 mm. apart. They are very thickened portion of the orbitosphenoid is solidly ossi­ delicate, and arch toward each other, almost touching fied, in contrast to the other bones of the Poebrotherium at the midline of the skull. skull, which tend to be filled with large-celled cancel­ Along either side of the pituitary fossa lie the grooves lous tissue. for the cavernous sinus and internal carotid artery and, In the skull of Poebrotheriunn which was serially sec­ lateral to these, the grooves for cranial nerves III, IV, tioned, almost no trace remained of the ethmoid bone. VI, and the first two branches of nerve V. These path­ A small fragment of the bony nasal septum appears ways are separated by a very low ridge. Lateral to the approximately in place in one section; it is only 1 mm groove which carries the cranial nerves is a higher, thick. sharp ridge. The only evidence concerning the characteristics of CRANIAL MORPHOLOGY OF SOME OLIGOCENE ABTIODACTYLA 143 the turbinals is a sharp ridge projecting ventrally into vertical. These observations agree with the deductions the nasal cavity from each nasal bone. These projec­ drawn by Bruce (1883) and Scott (1940) on the Hsis tions run the entire length of the nasals, which are very of studies of damaged natural endocranial casts. long in this genus; they undoubtedly served as roots The posterior portions of the parietals are filled with for the naso-turbinal bone or cartilage. It may be in­ cliploe for about 10 mm on either side of the sagittal ferred, then, that these scrolls at least were well de­ crest (fig. 27). The diploic layer is thin, however, and veloped in Poebrotherium. Similar ridges are present in this region the maximum thickness of the parietals in the living Camelm. They support a cartilaginous is only 5 mm. nasoturbinal scroll. Over the junction of the cerebrum and cerebellum, The nasal septum and turbinal scrolls were probably the parietals become thicker, and their diploic cells much less heavily ossified in Poebrotheriivm, than in increase in size to an average diameter of 2 mm. Here Merycoidodon. the diploe spreads farther laterally than over the cere­ The vomer was missing in the sectioned skull of bellum, extending about 18 mm ventrolaterally from the Poebrotheriwm. sagittal crest. There is no separate ossification of the preinterpa- - At their anterior termination, the parietals are again rietal or interparietal in this skull. simple bony plates, devoid of cancellous tissue. This The parietal bones are very long, and form almost is true also of the posterior part of the frontal tone, the entire roof of the braincase. Posteriorly, the pa- dorsal to the anterior part of the cerebrum, which has rietals conform much more closely to the shape of the an average thickness of only 2 mm. brain (fig. 27) than do those of Merycoidodon. The The descending plate of the frontal, which forms vermis cerebelli of Poebrothermm was higher than that part of the side wall of the braincase, is heavier than of Merycoidodon, projecting above the dorsal plane of the dorsal plate and, in its posterior portion, consists the cerebrum, and very narrow. Its sides were nearly of solid non-diploic bone like that of the orHto-

PAR

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. 8F, subarcuate fossa. FM, foramen magnum. SH, hypotympanic sinus. ICC. internal carotid canal. SM, superficies meatus, IT. incisura tympanica SQ, squamosal. PAR, parietal. SVT, sinus venosus temporalis. PET, pars petrosa. TO, temporal canal. RE, recessus epitympanicus. TH, tympanohyal. 144 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 sphenoid, which adjoins it. Farther anteriorly the ostium, the squamosal plate meets the alisphenoid, and large diploic cells of the supraorbital process are con­ also extends behind it, where a gap between the squa­ tinued ventrally into the lateral wall of the olfactory mosal and the basioccipital forms the foramen lacerum bulbs. medius. The supraorbital process of the frontal is very thick The squamosal therefore plays the part in Poebro- dorsoventrally, in marked contrast to the brain-case theriwm which, in M erycoidodon, is taken by the pos­ portion of the same bone. It is filled with large-celled teriorly projecting plate of the alisphenoid, which dipolic tissue. Between these processes, and dorsal to partially roofs the tympanic cavity. the anterior moiety of the olfactory bulbs, the hori­ The temporal canal, bearing the superior cerebral zontal plate of the frontal reaches its greatest thickness vein, is not contained, as in Merycoidodon, entirely (5 mm) and is entirely cancellous in structure. within the squamosal, but is bounded only dorso- The olfactory lobes are somewhat larger than those laterally by it (fig. 27, TO). This canal is further of Merycoidodo'n, which has so often been cited as an discussed below. extremely macrosmatic genus. Like Merycoidodon^ Poebrotherium, possessed an Comparison of the olfactory lobes of Poebrotherium anterior branch of the temporal canal; it ran ertirely with those of the living Caanelus (Lesbre, 1903) indi­ within the squamosal, however, and not between the cates that, even considering the greater size of the neo- squamosal and the tympanic. The vein which occupied pallium of Camelus, the lobes of smell in Poebrotherium this canal reached the surface just medial to the root of are nearly twice as large for the size of the brain. the zygoma through two foramina, the supraglenoid The squamosal bone of Poebrotherium differs from foramina of Cope ( 1880) . Examination of other skulls that of Merycoidodon in that it is diploic only along of the genus reveals that either one or two supragi'enoid its base (medial to the temporal crest and, farther for­ foramina may be present and that the number may ward, to the root of the zygoma). Above this the differ on the two sides of the same skull. The srme is greater portion of the squamosal plate is very thin and true of the living Tylopoda. conforms closely to the shape of the cerebrum (fig. 27). No supraglenoid foramen is present in Merycoidodon. This bone is relatively much thicker in living Tylopoda. Poebrotheriwn possesses a maxillary pneumatic sinus The horizontal ventral plate of the squamosal is even along the medial side of the cheek-tooth row and dorsal more intimately involved in the structure of the middle to it, and projecting posterior to the third molar into ear in Poebrotherium, than in M erycoidodon. The term the tuber maxillare (fig. 28). The sinus is almost "horizontal plate" is not applicable here in a strict sense, square in frontal section, with a height of about 15 mm. for parts of this structure are at a sharp angle with Anteriorly it reaches as far as the fourth premclar, a the horizontal. distance of about 33 mm. The maxillary sinus arcends A part of the ventral squamosal plate, the superficies into the vertical plate of the maxilla, ventral to the meatus (fig. 27, SM] forms the roof of the external lacrimal fossa. Probably the entrance of this sinus auditory meatus. It is a very thin plate of bone, lying into the nasal cavity lay in the medial wall of this ex­ in an almost vertical attitude due to the steep slant tension; this could not be definitely determined, how­ of the tube of the meatus. In Merycoidodon the su­ ever, because of the broken condition of the bone in this perficies meatus is horizontal and about three times as area. thick as in Poebrothermm. The ventrally projecting vertical plate of the pala­ The superficies meatus extends ventromedially into tine bone is much heavier than in M erycoidodon. It is the tympanic cavity, terminating 2 mm lateral to the directed ventro-laterally and is in contact medially promontorium, and dividing the front part of the tym­ with the pterygoid bone, a slimmer plate that is oriented panic cavity into dorsal and ventral cavities. vertically. This union of the two bones forms a bifur­ In the living Tylopoda, there is no such medially cated process such as is also typical of living Tylopoda projecting plate of the superficies meatus (van Kampen, (fig. 2 1905). In Merycoidodon it projects medially in a hori­ TYMPANIC REGION zontal direction and its medial end is in contact with PARS PETROSA OF THE PERIOTIC BONE the pars petrosa above the tympanic cavity. An interesting feature of this bone in Poebrotherkim A more truly horizontal part of the ventral plate is the extremely large subarcuate fossa that occupies its of the squamosal is that which projects medially above entire dorsal side. The front part of the fossa (f g. 27, the ostium tympanicum tubae. To its ventral side, &F) is a simple depression in the pars petrosa, such as lateral to the ostium, is attached the anterior part of was observed in Merycoidodon, but behind this it is a the auditory bulla. Over the anterior end of the 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. OL, olfactory lobes. B8, basisphenoid. 08, orbitosphenoid. FLA, foramen lacerum anterius. PAL, palatine. FR, frontal. PT, pterygoid. MS, maxillary sinus. 8C, sinus canal. MX, maxilla. SQ, squamosal. MS, third molar. 880, subsphenoid canal. OF, optic foramen. 8V08, sinus venosus ossis sphenoiaalis. ularis, and whose side walls and roof are formed by Further examination of skulls of present-day Artio- the bony arch which housed the posterior semicircular dactyla reveals that no others possess the fossa mas­ canal (fig. 27, F). Its posterior wall is a part of the toidea. Its presence in the living Tylopoda contradicts dorsal portion of the pars mastoidea of the periotic. the statement of Black (1921, p. 310) and Bolk (1906) This fossa was the receptacle of the lobulus petrosus that no modern ungulates possess the lobulus petrosus of the cerebellum, mentioned above in the discussion of of the cerebellum.

Merycoidodon ; the lobulus of Poebrotheriwm must have MIDDLE EAR AND SURROUNDING STRUCTURES been at least three times this size. The modern Lagomorpha have an identical fossa. The auditory bulla of Poebrotheriwm (fig. 26, B) is Krause (1884, p. 182) described it, and designated it the typical of Tylopoda—extremely large and filled with fossa mastoidea. A very similar structure is reported cancellous bone. by Elliot Smith (1903, p. 428) to exist in all Primates The relationships of the bulla to its surrounding except Simia, Anthropopithecus and Homo. structures differ from those of Merycoidodon largely For comparison with modern descendants of Poe- because of its greater size, and agree correspondingly brotherium, the writer examined the skulls of Tylopoda with those of the living Tylopoda. in the osteological collection of the Museum of Com­ The bulla projects posteriorly along the lateral side parative Zoology at Harvard University. The roof of the par occipital process, to which it is attached. Ven- of the braincase had been removed from only two skulls, trally, it overlaps the anterior part of the process. specimens of hwmachus (M. C. Z. 1746, M. C. Z. Dorsally, the main part of the bulla is in contact with 29878); and in both skulls the fossa mastoidea was the pars petrosa and the squamosal. It projects ruich identical with that of Poebrotherium. No mention of farther anteriorly than in most mammals. The an­ this fossa has been made in the literature but it seems terior part of the bulla in PoebrotJienwrn is well inflated safe to conclude that it is typical of modern Tylopoda. 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 from the tympanic cavity to the posterolateral side of margin to be in. contact with the suture between the the bulla (fig. 26, FSM). Dorsally, this cr.nal is alisphenoid and squamosal. bounded by the squamosal; it differs from the stylomas­ As in the living Tylopoda, the bulla of Poebroth­ toid canal in Merycoidodon in that its ventral wall is erium is in contact medially with the pars petrosa but formed by the laterally swollen bulla rather than by is not fused with it (fig. 27, PET, B). No gap exists the paroccipital process of the exoccipital. Th^ canal between the two as in Merycoidodon. in Camelus is formed in the same way. Despite the great size of the Poebrotherium bulla, the hypotympanic sinus is no larger than that of Mery­ EXTERNAL AUDITORY MEATUS coidodon (fig. 27, JSH). It is of relatively the same At first glance the porus acusticus externus of Poe­ size and shape as that of Lama and Camelus (van brotherium appears to open directly out of the tympanic Kampen, 1905, p. 594). bulla, with no intervening tubular meatus (fig. 26, The ostium tympanicum tubae is longer than that MAE), but what appears to be the lateral portion of of Merycoidodon, because of the greater anterior ex­ the auditory bulla is actually the inflated meatus (fig. tension of the bulla. It is closed below by the contact 13, IM). of the bulla with the basisphenoid, and above by the The external auditory meatus in Poebrotherium, is horizontal plate of the squamosal bone. The living surrounded anteriorly, vent rally, and posteriorly by the Tylopoda differ from Poebrotherium in that the ostium inflated tympanic bone, and dorsally by the superficies tympanicum tubae is roofed by the alisphenoid (van meatus of the squamosal, but in living Tylopoia, the Kampen, 1905) as in Merycoidodon. meatus roof is formed by a thin plate of the tympanic, In Poebrotherium^ the tympanic bone forms almost so that the meatus is a tube formed by this one bone. all of the medial wall in addition to the ventral and In both Poebrotherium and its living relatives^the tube part of the lateral wall of the tympanic cavity. The of the meatus slants sharply dorsolaterally from the pars petrosa is thus limited almost entirely to the roof tympanic cavity. of the cavity (fig. 14). The cavity is wider and shallower than that of Merycoidodon. PARS MASTOIDEA OF THE PERIOTIC BONE The epitympanic recess of Poebrothwiurn, instead of This bone is well exposed upon the lateral surface lying directly above the tympanic cavity as in Mery­ of the skull (fig. 25, PM) , the exposed area being larger coidodon and Camelus (van Kampen, 1905), extends than in Merycoidod&n. In Poebrotherium, the pars dorsolaterally above the external auditory meatus. It mastoidea is bounded anteroventrally by the tympanic is very small, and is bounded laterally by the tympanic portion of the auditory bulla, rather than by ths squa­ and squamosal, and dorso-medially by the pars petrosa mosal, which in Merycoidodon overlaps its lower part of the periotic (fig. 27, RE}. (figs. 14, 25). In Camelus also (van Kampen, 1905) Just behind the epitympanic recess and also dorso- the squamosal covers the ventral part of the mastoid medial to the external auditory meatus, lies the incisura exposure. This appears to be another instance, like tympanica (fig. 27, IT). As in other mammals, both that cited above in the family Merycoidodontidae, of its dorsal and ventral sides are formed by the squa­ progressive covering, in the course of evolution of a mosal bone; it is much larger than in most mammals, group, of the pars mastoidea by neighboring elements. however. Comparison of the diameter of the medial end of the external auditory meatus with that of the PNEUMATIC SINUSES incisura, reveals that, in Poebrotherium, the pars tensa The sinus system of Poebrotherium is less expensive of the tympanic membrane was very small, and the than that of Merycoidodon, as no sphenoidal sinus is pars flaccida very large. The incisura tympanica is present. The frontal sinus is much smaller thr.n that absent in the living Tylopoda (van Kampen, 1905). of Merycoidodon: instead of occupying the entire su- The vagina processus hyoidei is almost completely praorbital process of the frontal bone, it is four d only surrounded by the auditory bulla. In the vagina, sec­ in the anteriormost part of this process. Its antero- tions show the tympanohyal (fig. 27, TH) which, as posterior length is only about 4 mm., and it opens into Scott (1940, p. 621) suggested, is not fused to the tym­ the nasal cavity dorsal to the lacrimal fossa. panic. This fusion is present in the adult modern The maxillary sinus (fig. 28, MS) is considerably Tylopoda (van Kampen, 1905, p. 598), in which the longer, but narrower, than that of Merycoidodon. Its bulla completely encloses the vagina to form a true pit. greater extent as a cavity independent of the nasal cav­ The stylomastoid foramen of Poebrotherium is the ity is due to the greater thickness in Poebrotherium of lateral opening of a nearly horizontal canal leading the palatine plate of the maxilla. CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 147 In Camelus (Aubert, 1929), in contrast to Poe- The temporal venous sinus opens into the temporal ~broiherium, the maxillary sinus is absent and the canal, which, after coursing ventro-laterally, op?.ns sphenoidal sinus is well developed. This discrepancy upon the surface of the skull in a foramen between the in a supposed descendant of Poebrotherium (Scott, auditory bulla and the squamosal bone. This foramen 1891) is easily explained: The development of larger is difficult to find because it is overhung by the roo* of cheek teeth would occupy the space in the maxilla the zygoma. formerly filled by the sinus, and the almost threefold The anterior branch of the temporal canal was men­ increase in skull size would allow room for a sinus tioned in the discussion of the squamosal bone. ventral to the olfactory lobes (compare Merycoidodon). BASILAB SINUS SYSTEM CIRCULATORY SYSTEM OF THE SKULL The depth of the grooves that extend antero-pos- ARTERIES teriorly alongside the pituitary fossa indicates that the The internal carotid artery did not, as in Meryooi- cavernous sinus was a relatively small vessel. So, too, dodon, traverse the tympanic cavity; instead it lay in a was the inferior petrosal sinus, if it was present at all. bony canal formed by the tympanic bulla and the The carotid canal (fig. 27, ICC) is so small that it cc'ild basioccipdtal and basisphenoid (fig. 27, ICC) . not have contained two functional blood vessels. The As Scott (1891) first pointed out, the internal carotid inferior petrosal sinus must, then, have been ex^ra- entered this canal through a foramen anterior to the cranial in its course, or absent. No detailed account of foramen lacerum posterius. The farthest posterior ap­ the venous system of Cannelus is known to the author, pearance of the carotid canal is, therefore, just anterior and it was impossible to determine whether this sinus to this foramen and at the level of the vagina processus is present in living Tylopoda. hyoidei. From this point the canal extends forward 4 mm. to an opening medial to the ostium tympanicum SINUS VENOS0S OSSIS SPHENOIDALIS tubae. After leaving the canal, the artery entered the This venous sinus in Poebrotherium, with its tribu­ cranium through the foramen lacerum medius, which tary subsphenoid canals, differed from that of Mery­ lies dorsal to the anterior end of the bulla (fig. 26, coidodon only in that it extended laterally into the FLM). root of the alisphenoid bone (fig. 28, SVOS, AS). In The carotid canal in Poebrotherium differs from that figure 28 the anterior subsphenoid canals (SSC) are of Camelus only in its less superficial position (because seen, extending transversely in the plane of the section of the more swollen bulla of the former) and in the from their openings at the sides of the basispheroid separation of its posterior foramen from the foramen bone. lacerum posterius. In Poebrotherium^ the size of the The antero-posterior length of the sphenoidal verous carotid canal indicates that the carotid artery was sinus is about 5 mm. already, as it is in Cam-elus, relatively unimportant in Lesbre's illustration of a sagitally sectioned skull of supplying cranial blood, although still functional. Camelus (1903, p. 19) shows that the basisphenoid con­ tains no cavities of any sort. VEINS AND VENOUS SINUSES The above observations suggest that in Poebrothe­ DORSAL SINUS SYSTEM rium the dorsal sinus system was the main patl of Poebrotherium, like Merycoidodon, differed from drainage of blood from the cranial cavity, and the living Artiodactyla in the possession of a venous tribu­ venous blood from the facial region ran posteriorly tary that passed from the orbital plexus through the through the sphenoidal venous sinus and extracranial sinus canal (fig. 15, SC) to the vena collateralis cerebri. paths, rather than through the cranium. The superior cerebral vein in this genus ran ventro- laterally in a space between the ascending plate of the NERVES OF THE SKULL REGION subarcuate fossa (near the anterior end of the pars The arrangement of the cranial nerves of Poebrothe­ petrosa) and the descending plate of the squamosal rium differs from that of Camelus (Lesbre, 1903) only (fig. 27). At the base of the latter plate it entered a in the position of the optic nerves. These nerves lie small (10 mm. in diameter), subspherical sinus venosus very close together throughout their length, and the temporalis (fig. 27, SVT), whose medial and ventral optic foramina (fig. 28, OF) are separated only by walls were formed by the tegmen tympani of the pars the thin medial bony plate formed by the two orfrto- petrosa and the dorsal side of the tympanic bulla. The sphenoids. The great size of the orbits causes tHse size of this sinus is in distinct contrast to that of the bones to be closely pressed together in the midline of homologous sinus in Merycoidodon. the skull—a condition common in small mammals, SMch 148 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 as the Lagomorpha, Tragulidae and Hypertragulidae. The proportion of the occipital surface of th*. skull Because of the larger size of the skull, the optic for­ formed by the supraoccipital is less than in any other amina of the living Tylopoda are separated by the artiodactyl family, because the pars mastoidea is ex­ sphenoidal pneumatic sinus, which lies between the posed on the occipital rather than the lateral surface of orbitosphenoid plates. the skull (fig. 30, M). The basisphenoid bone, like those of Merycoidodon CRANIAL MORPHOLOGY OF LEPTOMERYX and Poebrotherium, contains a cavity for the, sinus Leptomeryx is a member of the family Hypertragu­ venosus ossis sphenoidalis. lidae, which is well represented in the fauna from the The sagittally sectioned skull reveals that th?, post- White River group. The members of this family bear clinoid processes are very high (3.5 mm) in proportion a very close resemblance to the living Tragulidae and to the size of the skull, and that the pituitary fossa, as their relationships have long been a subject of in the Camelidae and Merycoidodon, is shallow. controversy. The alisphenoid bone has a posterior process that Serial sections of the skulls of the Hypertragulidae lies dorsal, not only to the Eustachian tube but also, do not yield as much information as those of larger behind this, dorsal to the tympanic cavity itself. This mammals, because their delicate bones retain fewer process thus reaches farther back than in Merycoidlodon, impressions of the endocranial soft parts. where it lies dorsal only to the ostium tympr.nicum Because well-preserved Leptomeryx skulls are rare, tubae. In Tragulus the alisphenoid seems, insofar as one skull was divided sagittally, the left half serially could be determined from external examination of sectioned at 0.5 mm intervals, and the right half kept skulls in toto, to have the same relationships as in intact for reference. Leptomeryx. The skull on which this study is chiefly based is one The subsphenoid canals, leading to the sinus venosus of Leptomeryx evansi Leidy, M. C. Z. 6566 (figs. 29, ossis sphenoidalis, lie for the most part in the alisphe­ 30). It was collected by S. W. Garman in 1880, in the noid, and their foramina open on its surface (fig. 30, Bad Lands of northeastern Wyoming, and was given FSA). to the Museum of Comparative Zoology by Alexander Considerable portions of the turbinal ossifications of Agassiz. the ethmoid bone were preserved in this specimen. In BONES OF THE SKULL the back part of the nasal cavity are the roots of three The supraoccipital portion of the coossified occipital turbinals thick enough to be tentatively termed endo- bone is, unlike that of the Tragulidae, extended pos­ turbinals. There may well have been more than three teriorly into a nuchal crest, which is filled with diploe of these scrolls in life, but the three preserved ap­ the size of whose cells resembles those of the Camelidae parently occupied more than two-thirds of the posterior more than those of Merycoidodon. 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-. SQ, squamosal bone. FSA, anterior subsphenoid foramen. TO, foramen jugulare spurium. M, pars mastoidea of periotic bone. TT, tympanic bone. 10, occipital bone. VPH, vagina processus hyoidei. PPO, paroccipital process of exoccipital bone. CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 149 wide, roofing the lateral portion of the tympanic cavHy. There is no superficies meatus. The posterior hori­ zontal 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 Artio­ dactyla : 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 .OC projects between the basisphenoid bone and the audi­ tory bulla (fig. 31, VPP) . Lull reports this feature in M Allomerym planiceps (1922). In most mammals the pars cochlearis lies in or near this postion; in Lepto- merym it lies dorsal to it, well within the braincase.

I INCH PAR FIGURE 30.—Leptomeryx evansi, ventral view of basis cranii. M. C. Z. 6566. X 2.

B, auditory bulla. FSM, stylomastoid foramen. CF, condylar foramina. M, pars mastoidea of periotic bone. FLA, foramen lacerum anterius. OC, occiptal bone. FLPt foramen lacerum posterius. PBC, petrobasilar canal. FO, foramen ovale. SQ, squamosal bone. F8A, anterior subsphenoid foramen. 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 maxillo- turbinals (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 re­ flects the shape of the brain. It is cancellous only FIQUKB 31.—Leptomeryx evansi Leidy. X 2. Thick section from sec­ where it fills the gap between cerebrum and cerebellum; tions L 14-36, viewed anteriorly from region of bulla. elsewhere it is little more than 1 mm thick. APP, ascending process of pars petrosa. Like the parietal, the part of the squamosal partici­ AS, alisphenoid. B, bulla. pating in the braincase wall is very thin and non- BS. basisphenoid. FE, facies epitympanica of squamosal. F8, facial sulcus. diploic. ICC, internal carotid canal. The horizontal plate of the squamosal is, relative to PAR, parietal. PET, pars petrosa. the size of the vertical plate, much less well developed PM, pars mastoidea. 8F, subarcuate fossa. SO, squamosal. in Leptomeryss than in most other Artiodactyla. The TO, temporal canal. TT, tegmen tympani. facies epitympanica (fig. 31, FE) is a plate only 2 mm. VPP, ventral process of pars petrosa. 150 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952 The subarcuate fossa of Leptomeryx (fig. 31, SF) that of the Tragulidae, situated between the paroc- resembles that of Merycoidodon and the carnivores in cipital process, the pars mastoidea, the posttympanic being a definite but shallow depression in the dorsal process, and the tympanohyal bone (fig. 30, FSM). wall of the pars petrosa. It is divided into two sub- The tympanohyal separates the foramen from th^ bulla. equal portions by a low antero-posterior ridge, which may indicate that the fossa contained two cerebellar EXTERNAL AUDITORY MEATUS gyri. The subarcuate fossa bears no resemblance to The meatus is short, with a diameter almost ss great that of the Camelidae. as that of the bulla and twice as large as that of the The sulcus facialis of Leptomeryx lies in the extreme Tragulidae. It slants posterodorsally. Its posterior lateral side of the roof of the tympanic cavity (fig. 31, wall is formed by the posttympanic process of tto squa­ FS). It is separated from the squamosal by a ventrally mosal; the tympanic occupies the other three vails of projecting plate of the pars petrosa, the lower end of the tube. In the Tragulidae, however, the tympanic which is inserted in the stylomastoid foramen and ex­ forms the entire meatus. tends to within a few millimeters of the ventral surface of the skull. FARS MASTOIDEA OF THE FERIOTIC BONE As figures 29 and 30 (M) show, the pars mastoidea MIDDLE EAR AND SURROUNDING STRUCTURES is extensively exposed upon the occipital surface of the The epitympanic recess in Leptomeryw is, consider­ skull as in the Carnivora, rather than on its lateral ing the size of the middle ear, a fairly long cavity (fig. surface as in all other mastoid Artiodactyla, includ­ 31). Its anterior part is covered chiefly by the tegmen ing the Tragulidae, and . Thi^1 char­ tympani of the pars petrosa; laterally it is bounded by acteristic of Leptomeryx is probably a primitive one; the squamosal and the tympanic. Farther anteriorly it in this respect Leptomeryx resembles the hypothetical is roofed entirely by the tympanic. ancestor of the amastoid Artiodactyla postulated by The epitympanic recess of Tragulus (van der Helga Pearson (1927, p. 456, 457). Klaauw, 1931) is like that of Leptomeryx. The pars mastoidea is large, relative to that of Mery­ As in all Artiodactyla, no tympanic sinus is present. coidodon and Poebrotheriwn (fig. 31, PM). Here, in The auditory bulla of Leptomeryx (fig. 17, B) is the back part of the bone, is a large cavity which, hav­ hollow, unlike that of most tragulid species. It is ing no direct outlet, must be diploic in nature. In the only moderately inflated, extending ventrally to the front part, the are smaller. level of the basioccipital. It is in contact medially As in the Tragulidae (Milne-Edwards, 186*), the with the petrosal, a ventral process of which separates mastoid process is separated from the bulla by the it from the basioccipital; dorsally with the posterior posttympanic process of the squamosal (fig. 2S). process of the alisphenoid (fig. 31) and laterally with PNEUMATIC SINUSES the squamosal. It is fused to none of these elements. The tympanic gap ("tympanicumdefekt" of Bondy, The maxillary pneumatic sinus is the only one present 1907), or dorsal opening in the inflated auditory bulla, in Leptomeryx. This is also true of the Tragulidae is small in Leptomeryx. The tympanic bone arches (Aubert, 1929), and seems to be typical of small mam­ dorsal to the lateral part of the epitympanic recess, so mals. In Leptomeryx it lies above the cheek tocth row, that the tympanic gap is only half as wide as in Mery­ and extends antero-posteriorly about 5 mm. coidodon and PoebrotJierium. In Leptomeryx^ the gap is closed partly by the pars petrosa, as is usual, and CIRCULATORY SYSTEM OF THE SKUU. partly by the posterior process of the alisphenoid. ARTERIES The tympanic gap does not extend to the anterior No separate posterior carotid foramen is viable on part of the bulla (fig. 31). the basis cranii of Leptomeryx (fig. 30), but the serial The incisura tympanica is absent in Leptomeryx as sections indicate that here, as in Poebrotheriw^ Pro- in the Tragulidae. In the former, the position usually toreodon and a few groups of living mammals, the occupied by the incisura, dorsal to the medial end of internal carotid artery entered the carotid canal the external auditory meatus, is covered within the through an opening anterior to the foramen lacerum tympanic cavity by the ventro-lateral plate of the pars posterius. petrosa which serves as the lateral wall of the facial A cross-section of the posterior portion of the carotid sulcus. Leptomeryx thus could not have possessed a canal in Leptomeryx is seen in figure 31 (ICC), which pars flaccida of the tympanic membrane. shows a canal between the dorsomedial tymparic wall The stylomastoid foramen of Leptomeryx is, like of the tympanic cavity and the ventral side of the CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTTLA Ifl posterior process of the alisphenoid bone. Appar­ these observations add to the existing information con­ ently the artery lies for a short distance in the pos­ cerning evolutionary trends in this order; others, be­ terior third of the petrobasilar canal (fig. 30, PBC), cause of the lack of sufficient data for comparison, can then enters the carotid canal near the dorsal edge of only be recorded in hopes that further studies, bridging the medial wall of the bulla. The artery extends dorsal the gap between the Oligocene and Recent epochs, will to the bulla for only about 2 mm, leaves the canal give them significance. through the anterior carotid foramen, enters the cra­ The characteristics observed may be divided into nium through the foramen lacerum medius, and extends three categories : anteriorly in a groove in the floor of the basis cranii. (a) Primitive characteristics. The internal carotid artery in Leptomeryx was much (b) Characteristics whose significance is doubtful, less superficial in position than that of the Tragulidae, but which are noted because they have never before be^-n in which as a rule, it lies in a simple groove in the observed. medial wall of the bulla. (c) Anatomical peculiarities indicating the habits of VEINS the genus involved, evolutionary tendencies within the

DORSAL SINUS STSTEM artiodactyl group to which it belongs, or its relation­ There is no indication in the Leptomeryx skull of ships to other artiodactyl groups. the details of the dorsal cranial vessels anterior to the The characteristics to which the first two categories transverse sinus, but several of the distributaries of may be applied are listed under the headings usod the sinus are well marked. above for subdividing the morphological discussions, There was no temporal venous sinus in Leptomeryx, then the diagnostic anatomical features are discussed and the temporal canal, bearing the superior cerebral individually. BONES OF THE SKULL vein, was similar to that of the living Artiodactyla. The superior cerebral vein left the brain cavity high PRIMITIVE CHARACTERISTICS up on its lateral wall, extended ventrally in the tem­ 1. In Merycoidodon, the pituitary body was con­ poral canal (fig. 31, TO), between the mastoid and tained between two thin, longitudinally oriented, bony squamosal bones, and left the skull through a foramen plates. These represent the taenia interclinoidalis, t\e just behind the external auditory meatus (fig. 30, TO}. primitive lateral wall of the braincase. As in most Artiodactyla, an anterior branch of the 2. In Merycoidodon and Poebrotherium the super­ temporal canal passed anteriorly in the temporal crest ficies meatus of the squamosal is wide and heavily os^i- to a foramen on its dorsal side in the region of the root fied as compared to that of Leptomeryx and the living of the zygoma. Artiodactyla. The reduction of the superficies in living The transverse sinus of Leptomeryx was also drained genera is concomitant with the shortening and lighten­ by a vein that occupied the post-parietal canal, which ing of the bony meatus. extends posteriorly in the parietal bone and opens upon 3. Merycoidodon is unique among the Artiodactyla the dorsal surface of the braincase through the post- in that the posttympanic process of the squamosal parti­ parietal foramen (fig. 29, FPP). cipates in forming the ventral wall of the exterral

BASILAE SINUS STSTEM auditory meatus. Other than the groove in the cranial floor for the CHARACTERISTICS OF DOUBTFUL SIGNIFICANCE cavernous sinus, and the petrobasilar canal for the 1. The mesethmoid bone of Merycoidodon is inferior petrosal sinus (fig. 30, PBC), there is no evi­ heavily ossified midway between the cribriform plate dence as to the condition of this system in Leptomeryx. and the anterior nares. This suggests that the meseth­ moid and presphenoid bones in this genus ossified sep­ SINUS VENOSUS OSSIS SPHENOIDALIS arately. If this were the case, this genus would be This sinus in Leptomeryx was about 3 mm long. It sharply differentiated from the other Artiodactyla. It differed from those of Merycoidodon and Poebrothe- is probable, however, that the thickness of the meseth­ rium chiefly in that the associated subsphenoid canals moid is simply a part of the heavy ossification of the extended laterally into the alisphenoid bone (fig. 30, entire nasal region in Merycoidodon. FSA). 2. The posterior horizontal process of the alisphenoid MORPHOLOGICAL CONCLUSIONS of Leptomeryx extends far enough posteriorly to close The studies upon which this paper is based have the anterior part of the tympanic gap. This is appar­ yielded a number of new facts concerning the endo- ently most unusual among Artiodactyla, but compara­ cranial anatomy of Oligocene Artiodactyla. Some of tive data are incomplete. 152 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1952

TYMPANIC REGION the membrane to be attached to a process of the squamo­ PRIMITIVE CHARACTERISTICS sal below which lay the incisura tympanica. This evolutionary trend in the Tylopoda conforms 1. The tympanohyal bone of Merycoidodon, Poe- with the already mentioned tendency in all mammals brotherium, and Leptomeryx was inserted loosely into toward closure of the tympanic gap. the vagina processus hyoidei. This is in contrast to the 4. Leptomeryx differs from all other mammals ob­ living Artiodactyla, in which the tympanohyal is fused served by the writer or mentioned in the literature in to the tympanic in the fundus of the vagina. the extreme height of the ascending plate of the pars 2. The tympanic gap of Merycoidodon is larger than petrosa, which serves as the lateral border of the sub- that of living Artiodactyla or of Leptomeryx. This is arcuate fossa. In this genus, this plate composes regarded as primitive because it more closely approxi­ almost the entire medial wall of the temporrl canal. mates the primitive ring-form of the tympanic (van It probably appears unusually high simply because of der Klaauw, 1931) than does the condition in. which the small size of the skull. This supposition is borne the bulla is partly closed dorsally. out by the large size of the pars petrosa as a v^hole in The size of the tympanic gap determines the composi­ proportion to the size of the Leptomeryso skull. It is tion of the roof of the epitympanic recess. If it is a recognized fact that, in small mammals, the organs of small, as in Leptomeryso and most living Artiodactyla, sense occupy, comparatively, a greater space than in the tympanic itself forms most of the roof. If it is larger ones. large, as in Merycoidodon and the Tylopoda, the pars 5. A characteristic which, in the present state of petrosa is the important element involved. knowledge, distinguishes Leptomeryx from all other CHARACTERISTICS OF DOUBTFUL SIGNIFICANCE Artiodactyla is the great ventral extent of that plate of the pars petrosa which forms the lateral wall of the 1. In Merycoidodon, the fissura glaseri is almost en­ facial sulcus. No functional reason for this condition tirely surrounded by the tympanic bone. This is not is evident, nor is it a stage in any known evolutionary the case in any living artiodactyl (van Kampen, 1905), trend in skull development. It is not present in the or in Poebrotheriwn or Leptomeryx. It may, at least Tragulidae, so that it would not seem necessarily asso­ until further information becomes available, be re­ ciated with small skull size. It is therefore her** tenta­ garded as a structure characteristic of Merycoidodon. tively suggested that this peculiarity ma7 be a 2. An entotympanic bone is present in Merycoidodon. hypertragulid (or perhaps only a leptomerycid) Because of the sparseness of knowledge concerning this characteristic. bone, even in Recent Artiodactyla, speculation on this fact is useless. PNEUMATIC SINUSES 3. The meaning of the presence or absence of the PRIMITIVE CHARACTERISTICS incisura tympanica is often difficult of interpretation. It is missing in Leptomeryx and the Tragulidae, prob­ In their small extent and number, and in the lack of ably because the relatively large size of the tympanic any interconnections between them, the pneumatic annulus, together with the great reduction of the hori­ sinuses of the three Oligocene genera here discussed are zontal plate of the squamosal, leaves no room for such primitive. This is most strikingly apparent in Mery­ a structure. The lack of the incisura in these genera coidodon^ whose thick cranial bones were capable of is due to the small size of their skulls. housing a much more complicated sinus system than The condition in the Tylopoda is, however, puzzling. the animal possessed. This bears out Paulli's state­ The incisura tympanica is missing in the living mem­ ment (1900) that the complicated sinus system of bers of the suborder (van Kampen, 1905), but well modern type did not appear until late in the Miocene developed in Poebrotherium. Only one explanation epoch. can at present account for this discrepancy. In the ABTEBIES living Tylopoda a portion of the tympanic bone forms PRIMITIVE CHARACTERISTICS the roof of the external auditory meatus, and thus the 1. In the three Oligocene genera studied, evidence superficies meatus of the squamosal, which performed was found that the internal carotid artery was func­ this function in Poebrotheriivm, is excluded from its tional, a primitive condition in the Artiodactyla. The participation in the attachment of the tympanic mem­ artery was relatively much smaller in Poebrotherium brane. This membrane in modern Tylopoda is, there­ than in Merycoidodon or Leptomeryx. fore, attached throughout its circumference to a ring 2. The absence in Merycoidodon of a rete mirabile of of bone; in Poebrotherium, due to a gap in this ring, it the intracranial part of the internal carotid artery is,, was possible for a dorsal portion (the pars flaccida) of in all probability, also primitive. CRANIAL MORPHOLOGY OF SOME OLIGOCENE ARTIODACTYLA 153

VEINS AND VENOUS SINUSES Course of the internal carotid artery.—In Merycoii- PRIMITIVE CHARACTERISTICS odon, this artery courses through the auditory bulla as 1. The sinus venosus ossis sphenoidalis and the sub- in the Carnivora; in this it differs from all other Artio­ sphenoid canals, possessed by all three of the Oligocene dactyla in which the internal carotid is known. Prob­ genera studied here, were part of an archaic type of ably this is true of all the Merycoidodontidae. venous cranial circulation. This conclusion is based The other two Oligocene genera studied possessed a upon their absence in living Artiodactyla as opposed to carotid canal medial to the bulla, as in the living Tylo­ their apparently widespread occurrence in Oligocene poda and Tragulidae. time, and also upon their presence in primitive living In the study of various phylogenetic lines of the mammals: the Insectivora and Marsupialia, as well as Artiodactyla there was observed a tendency to posterior the Lagomorpha and some Rodentia. The conclusion migration of the posterior carotid foramen: in oth^r that these blood vessels are primitive in character is words, the length of the carotid canal progressively in­ further borne out by their absence in the Merycoidodon- creased. This backward migration culminates when tidae after the late Oligocene. the posterior carotid foramen merges with the foramen 2. The vein of the sinus canal, which was possessed lacerum posterius. by Merycoidodon and Poel>rotherium, was another Migration of the posterior carotid foramen was com­ primitive artiodactyl characteristic. The same evi­ pleted early in the evolution of the Merycoidodontidr^, dence contributes to this conclusion as was cited in the much later in the Camelidae. case of the sinus venosus ossis sphenoidalis. Tendency toward reduction of the internal carotid 3. The presence of a temporal venous sinus in Mery­ artery in the Artiodactyla.—The internal carotid, ab­ coidodon, and to a far lesser extent in Poebrotheri/u/m^ sent or non-functional in almost all living Artiodac­ is due to the fact that the small size of the brain relative tyla, was present and well developed in some Oligocene to skull size leaves space available for the sinus. members of the order, according to the observations 4. The vestigial vena capitis lateralis, observed in recorded here. It was strong in Merycoidodon ard Merycoidodon^ was probably merely a relic from the Leptomeryw; in Poebrotherium, oddly enough, it was embryonic stage, various manifestations of which are relatively no larger than in the living Oamelus. This often encountered in adult modern Mammalia. suggests that, in the Oligocene ancestors of the Tylo­ poda, the reduction in size of this artery may have ANATOMICAL PECULIARITIES INDICATING HABITS, progressed further than in Merycoidodon and Lep- EVOLUTIONARY TRENDS, OR RELATIONSHIPS tomeryx, which apparently represented more archaic Mesethmoid ossification in Merycoidodon.—As has sidelines of artiodactyl phylogeny. been discussed at length above, the extremely strong Internal jugular vein replaced by external jugular as bony nasal septum of this genus resembles closely that dominant efferent cranial vessel.—In Merycoidodon es­ of the peccary, and indicates similar rooting habits. pecially, and to a less degree in Poebrotherwm, most of The thickness and extensive development of the turbi- the blood from the facial region and the brainca^e nal bones of this genus were undoubtedly the result of found its exit from the skull by way of the basilar the unusual degree of ossification of the septum. sinus system and the sphenoidal venous sinus, ard Tendency of Selenodontia to approach the amastoid thence to the internal jugular vein. In the course of condition.—Comparison of Poebrotherium with the liv­ evolution to the modern artiodactyl condition, the sph

Sollas, W. J., 1903, A method for investigation of fossils by serial Tilney, F., 1931, Fossil, brains of some Tertiary Mammals of sections. Philos. Trans. Royal Soc. London, Ser. B, vol. 196, North America. Bull. Neurological Institute, vol. J, pp. pp. 259-265. 430-505. ———1916, The skull of Ichthyosaurus studied in serial sections. Turkewitseh, B. G., 1933, Zur Anatomie des Gehororgane^- der Philos. Trans. Royal Soc. London, Ser. B, vol. 208, pp. 63-126. Saugetiere (Canales semicirculares). Anatomischer An- ———1920, On the structure of Lysorophus, as exposed by serial zeiger, Band 76, pp. 369-408. sections. Philos. Trans. Royal Soc. London, Ser. B, vol. 209, Turner, H. N., 1848, Observations relating to some of tl 3 fo­ pp. 481-527. ramina in the base of the skull in Mammalia. Proc. Zool. Sollas, I. B. J. and Sollas, W. J., 1914, A study of the skull of a Soc. London, vol. 16, p. 63. Dicynodon, by means of serial sections. Philos. Trans. ———1849, On the evidences of affinity afforded by the skull Royal Soc. London, vol. 204, pp. 201-223. in the ungulate mammalia. Proc. Zool. Soc. London, vol. 17, Stehlin, H. G., 1910, Die Saugetiere das Schweizerischen p. 147. Eocaens. Abhand. Schweizerischen Palaeont. Gesell, Band Voit, M., 1909, Das Primordialcranium der Kaninehens. 36, pp. 839-1164. Anatomische Hefte, Abt. I, Heft 116 (Band 38, He*t 3), Stensio, B. A., 1926, On the sensory canals of Pteraspis and pp. 427-616. Palaeaspis. Arkiv fur Zoologie, Band ISA, no. 19. Walton, J., 1928, Use of cellulose peels in paleobotany. Nature, ———1927, The Downtonian and vertebrates of Spltz- 1928, p. 571. bergen. I. Family, Cephalaspidae. Skrif ter om Svalbard og Whitmore, F. C., Jr., 1942, Endocranial anatomy of some Nordishavet, no. 12. Oligocene Artiodactyla (Abstract): Geol. Soc. America Tandler, J., 1899, Zur vergleichenden Anatomie der Kopfarterien bei den Mammalia. Denkschr. Akad. Wien, Band 47, pp. Bull., vol. 53, no. 12, pt. 2, pp. 1842-1843. 677-784. Wilhelm, J., 1924, Zur Bntwicklungsgeschichte der Hinterl Tupt- Thorpe, M. R., 1931, The Osteology of socialis Marsh. schuppe des Rindes. Anatomischer Anzeiger, Band 59 p. 1. Yale Peabody Mus., Bull. 2. Wincza, H., 1896, Uber einige Entwickelungsveranderungen in ———1937, The Merycoidodontidae. Memoirs Peabody Mus. der Gegend des Schadelgrundes bei den Saugetfrieren. Nat. History, vol. 3, pt. 4. Acad. des Sciences de Cracow, Bull. International, p. 326.

INDEX

Page Acknowledgments..______.._____------__------— ------117 Merycoides...... ------122 alisphenoid bone______-._--______-_-___-..._------123 Merycoidodon...... ______—__—_ 118-155 Attomeryx.... ______.__.______..-_____.-._-———....-— 155 culbertsonii.—...... 118,122,125,136 planiceps...... ——______-.__------— --- ——.-.--__.- 149 evolution.-._-_-.-__------______—------154-155 amastoid condition, significance of tendency toward---.__-.-- — ------153 gracilis...... ——.. 122 Amphimeryx...... -———__.______.-._—————-——————--- 135 Merycoidodontidae, evolution______-_____--.____. 154-155 anatomical peculiarities indicating habits, evolutionary trends, or relation­ mesethmoid ossification, significance_.______._____--__-__. 153 ships------—....---— — ..- — —.-_.- — — — ---.-. 153-154 Mesoreodon...... -.. __——_- — --... 122 Anthropopithecui..--..... _------__.-_-_- — __-__-_-_-_-- 145 Microgale...... 138 ones, Ovis—...... -.——— — .————-- 136 midd^ ear and surrounding structures___-----_____. 132-135,145-146,150 arteriesoftheskull-...——..-- —_——.—-.---_-.-- — -- 137-138,147,150-151 Mixtotherium...... 135 morphological conclusions-______-__-___—.-——_ 151-154 basilar sinus system...-----__....__.-_..-.---_____-__..___._--- 139-140,147,151 basiphenoid bone_-_----.-_-.-___-______._-__..----_-_.-----__-.--. 120-123 nerves of skull region.-_.______——__. 141,147-148 Bibliography.______...--______.______.-______-... 155 bonesoftheskull—---—---_ — _--..-_--- — .-- 118-131,141-144,148-149,151 occipital bone...... _----_----_.__.____....._ ...... 118-120 Bos.-... — — ...-—_ — --- — .—-—---. — -.._-.------141 Oldfieldthomasia.-—-—.-...... 130,131,132 Braehyerug...... 135,136 orbitosphenoid bone__.______—_——__——.———-———----- 124 Oreamnos...... -..——._-——— ——————————————— ——— —... 154 Camdut...... 134,139,143,144,146,147 Ovis...... 132,136,141 Omis—----—-——- — -——— — -— — ------122 aries...... __-_...__--.—--.---- 136 carotid artery, significance of course, reduction______-_.__-____-_-----.----- 153 Cebochoerus...... -_._____..--.-...-.-.------.------135 parietal bone...____..__..______.___..__—_------126 characteristics of doubtful significance-_..-_.-__-.-._....-_------.--. 151,152 pars mastoidea of periotic bone..-_.-.______.__. 135,146,150 Choeropotamus...... ______------— ---— 135 pars petrosa of per iotic bone------_....___.....--.__.. 131-132,144,149--150 circulatory system of skull...... __..__.. 137-141,147,150-151 Pearson, quoted____.______.___—_——_.— 135 conclusions, phylogenetic.______..—————— --- 155 preinterparietal bone. -__.._...._——__..__—.———...————— 126 culbertsonii, Merycoidodon—...... 118,122,125,126 periotic bone, pars petrosa of______.__—-.-— 119,144-145 Cfydopidius..—...... 122 Phylogenetic conclusions....-______-_------154-155 planicepa, Attomeryx—...... 149 Dacrytherium...... 135 pneumatic sinuses___. ______. 136-137,146-147,150 Dicoteles...... 124,125,126,137 Poebrotkerium...... 118,131,141-148,150-155 dorsal sinus system__...... ___._...___.__——.——-— 138-139,147,151 evolution______.______.—_——.... 154 eximium...... 141 Entelodon...... 135 labiatum...... ———————.. 141 epitympanic sinus, significance of absence...______.._-. 153 wilioni—...... 141 Eporeodon...... 122,123 presphenoid bone______...... —__... __—__._..—_...... 123-124 Erinaceus...... ______.______.____.--.... 138 primitive characteristics, artery.....-..._...__—...——————————.„ 152 ethmoid bone___.______._.___—. 124-126 bones of skull___. ______- 151 evansi, Leptomeryx...... _____------—- ——.. 148 pneumatic sinuses.. __.__.___.______._._—-————.. 152 eximium, Poebrotherium...... 141 tympanic region..-.______.__—_.-.._.. 162 external auditory meatus.____.______.__.--...____ 135,146,150 veins and venous sinuses..__....„—.———...——-——.————— 168 Promerycockoerus...... 122 frontal bone.______._.-.__.....__ 126-127 Protoreodon—...... 122,137,138, ISO gracilis, Merycoidodon____.______.______... — .—.___-__ 122 scrofa,Stts...... —..... 122 Selected bibliography_...... ___....______.-..---...... 156 Hippopotamus.—...... —.. 135 Simia...... ——..——————————————------——... 146 Homo...... ----- —— ------145 sinus venosus ossis sphenoidalis______-. 140-141,147,151 huanachus, Lama...... 145 Solenodon...... ______.__...__„__...__„.._—__. 138 Hypertragulus...... ———— —.__.___._„..————..— — .-.—-....— 150 squamosal bone____.______.______127-131 Interparietal bone___------____------______._-.-..-----.. 126 subsphenoid veins.______..___.___.__....__—_——_ 140-141 Introduction..___.______-_.._- 117 Sus...... 139 scrofa...... 122 Jugular vein, significance of changes in__....___...... 153 Talpa europaea...... 138 labiatum, Poebrotherium...... -...... 141 Tapirulus...... 135 Lama...... 146 Tragulus...... 133,138,148,150 huanachus...... ___.--___---_._ 145 turbinal bones- ____. ______124-126 Leptomerycinae, evolution...______....____..--__-.__..-. 155 Tylopoda, evolution_-_.—__...... —_——__——__—..——--... 154 Leptomeryx...... 118,148-155 tympanic region...—...... 131-135,144-146,149-150 evansi...... _____.-._..-—.————.. 148 evolution..--.-_-.-__..___---.___-____.______._- 155 veins and venous sinuses..__-______. 138-141,147,151,153 Lepus...... 122,137,138,140 vomer-..__..--._____.._...._——_——__——_...--..___.. 126 cuniculus...... ———— — 140 lobulus petrosus, significance of development..----- —------—-.-. 153 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 , 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 , 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. 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