Hypertragulus

Abstract: The postcranial skeletal anatomy of the early Eocene dichobunid Diacodexis, the oldest known artiodactyl, is described, together with new material of Bunophorus, Antiacodon, Pentacemylus, and an undetermined homacodontine. All are characterized by relatively slender, elongate limb elements and specialized limb proportions compared to their Eocene contemporaries, indicating that they were uniformly highly adapted for cursorial-saltatorial locomotion. In nearly all postcranial features, known dichobunids are more similar to primitive ruminants such as OligoMiocene Hypertragulus and extant Tragulus and to Cainotherium than to any other artiodactyls. Dichobunids have been considered the stem group of the Artiodactyla. Although the extent of their postcranial modifications suggests that they may be too specialized to occupy this phylogenetic position, the occurrence of a cursorially adapted skeleton in all known dichobunids strengthens the probability that this is the primitive skeletal form for the Artiodactyla. If so, postcranial evolution was minimal in the transition from dichobunids to primitive neoselenodont artiodactyls (camels and ruminants), whereas suoids and other non-neoselenodonts became secondarily generalized.