Anatomy and phylogenetic relationships of Boreogomphodon jeffersoni (Cynodontia: Gomphodontia) from the Upper Triassic of Virginia

ABSTRACT

We present a detailed account on the skeletal structure of the traversodont cynodont Boreogomphodon jeffersoni on the basis of a considerable quantity of excellently preserved craniodental remains and several referred postcranial bones from the Tomahawk Creek Member of the Vinita Formation (Upper Triassic: Carnian) of the Richmond basin (Newark Supergroup) in eastern Virginia. The small size, proportionately short snout and mandible, low number of molariform postcanine teeth, and presence of up to three sectorial postcanines all indicate that most of the specimens recovered to date represent immature individuals. The superbly preserved dental material permits detailed inferences regarding tooth replacement and dental function during ontogeny. Boreogomphodon differs from other known traversodont cynodonts primarily in the possession of lower molariform postcanine teeth with three rather than two anterior cusps in all but the smallest specimens, zygomatic arches that are bowed laterally at about mid-length, and pronounced, irregular sculpturing on the dorsal surface of the snout. Plesiomorphic features of traversodont cynodonts retained by Boreogomphodon include the position of the paracanine fossa anterolingual to the upper canine as well as the presence of a distinct central cusp and a posterior cingulum on the upper molariform postcanines. Phylogenetic analysis suggests the existence of a clade comprising Boreogomphodon plus two other taxa (Arctotraversodon and Nanogomphodon) in the Northern Hemisphere that is the sister group to most other known Middle and Late Triassic traversodonts from Gondwana.

ACKNOWLEDGMENTS

H.-D.S. gratefully acknowledges Peter A. Kroehler (National Museum of Natural History), Paul E. Olsen and Annika K. Johansson (Lamont-Doherty Earth Observatory, Columbia University), Kenneth A. Pitt (Manassas, Virginia), ShayMaria Silvestri (New Jersey State Office, NRCS/USDA), and especially Elizabeth B. Sues for their expert help and cheerful companionship in the field even under inclement conditions. He and Olsen are indebted to the Hon. Vivian Watts, former Secretary of Transportation for the Commonwealth of Virginia, and to Jack Adams for permission to work on their respective properties. William W. Amaral (Museum of Comparative Zoology, Harvard University) and Diane Scott (University of Toronto at Mississauga) prepared several specimens with customary skill. Ann L. M. Davis (formerly Royal Ontario Museum) and Peter A. Kroehler and Fred V. Grady (National Museum of Natural History) assisted with processing for small vertebrate remains. We thank the curators and collection managers of numerous museums for access to and loans of comparative cynodont material. Diane Scott prepared the drawings for Figs. 2, 3, 5, and 11 and Mary A. Parrish (National Museum of Natural History) created the carbon-dust illustrations for Figs. 4 and 6. Scott Whittaker (National Museum of Natural History) and Dee Breger (Lamont-Doherty Earth Observatory, Columbia University) assisted with SEM photography. Christian A. Sidor (University of Washington) and two anonymous reviewers provided helpful comments on the manuscript. We acknowledge financial support for fieldwork and laboratory research through grants from the National Geographic Society (grants 3592-88 and 4232-89 to H.-D.S. and P. E. Olsen), National Science Foundation (NSF EAR-9016677 to H.-D.S. and P. E. Olsen and NSF EAR-9814475 to P. E. Olsen and H.-D.S.), Natural Sciences and Engineering Research Council of Canada (to H.-D.S.), and the Charles Doolittle and Mary Vaux Walcott Fund (to H.-D.S.).