Cranial Morphology of Some Oligocene / Artiodactyla GEOLOGICAL SURVEY PROFESSIONAL PAPER 243-H Cranial Morphology of Some Oligocene Artiodactyla By FRANK C. WHITMORE, JR. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 1952, PAGES 117-160 GEOLOGICAL SURVEY PROFESSIONAL PAPER 243-H UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1953 UNITED STATES DEPARTMENT OF THE INTERIOR Douglas McKay, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price 35 cents (paper cover) CONTENTS Page Cranial morphology of some Oligocene Artiodactyla—Con. Page Abstract_--_-------___________-__________________ 117 Cranial morphology of Poebrotherium—Continued Introduction _______________________________________ 117 Circulatory system of the skull—Continued Acknowledgments_ ____________________________ 117 Veins and venous sinuses____----___---_- 147 Cranial morphology of some Oligocene Artiodactyla____- 118 Dorsal sinus system_______-___-_-.__ 147 Cranial morphology of Merycoidodon ______________ 118 Basilar sinus system——____________ 147 Bones 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 ear and surrounding structures 150- Vomer__________________________ 126 External auditory meatus— _____________ 150 Preinterparietal and interparietal— _______ 126 Pars mastoidea of the periotic bone.______ 150 Parietal.. ______________________ 126 Pneumatic sinuses__________________________ 150 Frontal—_____________________ 126 Circulatory system of the skull____---____-_-- 150 SquamosaL ____________________________ 127 Arteries ______________-____---___------ 150 Tympanic region. __________________________ 131 Veins____________________---___----__ 151 Pars petrosa of the periotic bone________ 131 Dorsal sinus system_________________ 151 Middle ear 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 skulls 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 nasal cavity 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 mammals. 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 artery 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 paleontology 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 species. 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 Tylopoda 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 Eocene. 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 Pecora. the taxonomic position of groups of animals. On the basis of cranial structure, the family Hypertragulidae 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 Miocene. fossil skulls (Darrah, 1936;