SYSTEMATICS OF THE LATE OLIGOCENE AND MIOCENE OREODONTS (MERYCOIDODONTIDAE: ARTIODACTYLA): IMPLICATIONS FOR PALEOBIOLOGICAL DATABASES

Oreodonts were a very common and diverse group of sheep-sized ruminant artiodactyls exclusive to North America from the late Eocene through the late Miocene. Their rapid evolution and diversification, and great abundance of specimens have made them key index fossils for most of this interval. Yet their taxonomy has been a chaotic mess, largely due to a series of monographs published between 1940 and 1968 by Schultz and Falkenbach. Their work was plagued by typological oversplitting and inability to recognize differences due to post-mortem deformation, and its failure to consider fossils as members of living populations, or use of any statistical methods. These monographs were long known to be problematic, and there was one unpublished attempt to revise them by Lander in 1977, but no thorough revision of the group was published until Stevens and Stevens (1996) revised the Chadronian-Whitneyan oreodonts. The present work is based on the research of the late Margaret Stevens, and covers all the post-Whitneyan subfamilies of oreodonts, of which only seven are recognized. The invalid subfamilies Desmatochoerinae and Phenacocoelinae of Schultz and Falkenbach are disbanded and their contents dispersed to other groups, because they were based entirely on features due to deformation of specimens. We recognize only 27 valid genera and 74 species of post-Whitneyan oreodonts, down from 39 genera and 169 species published by Schultz and Falkenbach. Oreodonts reached their maximum diversification in the late Arikareean, when 22 different species are recognized, but they also became more disparate and distinctive, with small gracile running forms (Merychyus) and short-legged tapir-like forms with retracted nasals that suggest the presence of a short proboscis (Promerycochoerus, Brachycrus, and Merycochoerus). Most of the oreodont lineages vanished after the early Barstovian, about 14.5 Ma. The very last of the oreodonts was the medium-sized taxon Ustatochoerus from the middle and early late Miocene (late Barstovian to early Hemphillian, or about 14-7 Ma) of North America, surviving about 7 m.y. after all the other oreodonts had vanished. Most paleobiological databases, such as the PBDB, still use the outdated taxonomy with the 39 genera and 169 species (mostly invalid) which we reduced to 27 genera and 74 species, so they cannot be relied upon for any kind of analysis.