The cranium of<i>Bunomeryx</i>(Artiodactyla: Homacodontidae) from the Upper Eocene Uinta deposits of Utah and its implications for tylopod systematics

Abstract

ABSTRACT Observations of four crania of the homacodont artiodactyl Bunomeryx suggest that this taxon has strong affinities with the Tylopoda. In particular, Bunomeryx possesses a deep mastoid fossa on the endocranial surface of the petrosal, a feature which is confined, within the living artiodactyls, to the Camelidae. Mastoid fossae are also found in members of the Cainotheriidae, Anoplotheridae and Xiphodontidae (collectively Anoplotheroidea), suggesting that the presence of a mastoid fossa may be a feature linking Bunomeryx to more derived artiodactyl taxa in the late Eocene of North America and Europe. Expansion of the ventral margin of the petrosal and its contribution to the roof of the petrobasilar canal, together with a reduction in the size of the internal auditory meatus, are characters which may unite a broader grouping of camelids, anoplotheroids, oreodonts and bunomerycids. While not providing any additional evidence for the ongoing debate on the ruminant affinities of the protoceratids, the information obtained from Bunomeryx may strengthen the case for a monophyletic clade including the taxa listed above, but excluding protoceratids.