THE SKULL AND DENTITION OF TWO TRITYLODONTID SYNAPSIDS FROM THE LOWER JURASSIC OF WESTERN NORTH AMERICA HANS-DIETER SUES^ Abstract. Based on an extensive collection of specimens from the Kayenta Formation (Lower Ju-rassic) of northeastern Arizona, a detailed anatomical account on the skull of two advanced non-mamma-lian synapsids referable to the family Tritylodonti-dae is presented. The material is referred to Kay-entatherium wellesi D. M. Kermack, 1982 and Dinnehitodon amarali Sues, 1986, respectively; the skull of the latter form is still poorly known. The upper postcanine teeth of Kayentatherium have two buccal, three median, and three lingual cusps whereas those of Dinnehitodon are distin-guished by the presence of only two lingual cusps. Kayentatherium has but a single pair of upper in-cisors, which are enlarged and separated by an eden-tulous gap. The presence of an internarial bar is demonstrat-ed. The quadrate has a prominent stapedial process and articulates only with the crista parotica (anterior paroccipital process). The quadratojugal is a slender bony rod. The dentary has a very tall coronoid pro-cess with a distinctly recurved apex. The postdentary bones form a slender bony rod. The distinct retroar-ticular process consists of a thin central lamina and a posteriorly thickened rim. The postdentary ele-ments were presumably involved in hearing and are extremely similar to the auditory ossicles in the most primitive mammals. The skull bears prominent ectocranial crests and has deep zygomatic arches. The lateral wall of the braincase is completely ossified. An extensive second-ary palate is formed by the maxillae and palatines in Kayentatherium and by the premaxillae and pal-atines in Dinnehitodon. A number of cranial fea-tures are consistent with the hypothesis that trityl-odontid synapsids were herbivorous. Jaw motion was propalinal and included a significant, posteriorly di-rected power stroke. ' Museum of Comparative Zoology, Harvard Uni-versity, Cambridge, Massachusetts 02138. Present Address: Department of Paleobiology, Na-tional Museum of Natural History, NHB E-207 MRC NHB 121, Smithsonian Institution, Washington, D.C. 20560. INTRODUCTION The question of the origin of mammals has occupied the attention of several gen-erations of anatomists and paleontologists. Extensive collections of advanced "mam-mal-like reptiles" and early mammals have been brought together from various re-gions of the world, and, although much detail remains to be clarified, the broader outlines of this important phase in the evolutionary history of the Synapsida are now slowly emerging. There exists a gen-eral consensus at present that mammals are derived from the Cynodontia, a para-phyletic assemblage of advanced synap-sids of primarily Permian and Triassic age. Kemp (1982) and Jenkins (1984) have re-viewed this subject most recently. Certain Late Triassic and Jurassic rep-resentatives of this assemblage already show a host of mammalian characters and indeed have been referred to the Mam-malia on occasion. Among these forms, the Tritylodontidae form a well-defined group, which is characterized principally by the possession of multicuspid molari-form cheek-teeth that meet in precise oc-clusion. They were classified as primitive mammals with affinities to the Multitu-berculata by most authors until the 1940s. The discovery of well-preserved and abundant skeletal remains of Tritylodon-tidae in the Lower Lufeng Series of Yun-nan, China (Young, 1940, 1947), now be-lieved to be Early Jurassic in age, and in Liassic fissure-fillings in Somerset, En-gland (Kiihne, 1943, 1956) led to a revised assessment of the phylogenetic position of this group. Subsequently tritylodontids were regarded as highly derived descen-Bull. Mus. Comp. Zool., 151(4): 217-268, December, 1986 217
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