THE LAST OF THE OREODONTS: A REVIEW OF THE USTATOCHOERINAE (MERYCOIDODONTIDAE: ARTIODACTYLA)

Oreodonts were a very abundant and diverse group of sheep-sized ruminant artiodactyls in North America from the late Eocene through the late Miocene. The last surviving clade of oreodonts was the subfamily Ustatochoerinae, consisting of the genera Ustatochoerus and Mediochoerus, which was the only group of oreodonts to survive after all other groups had vanished about 14.5 Ma. The taxonomy of all oreodonts has been badly confused for over a century with dozens of invalid taxa, largely due to typological oversplitting and the lack of population thinking and statistical concepts in their systematics. We recognize five valid species of Ustatochoerus: U. leptoscelos new combination from the early Arikareean of Texas; U. tedfordi new species from the early Hemingfordian of Nebraska; U. medius from the late Barstovian, U. major from the Clarendonian, and U. californicus from the early Hemphillian, mostly from Nebraska, but also from South Dakota, Kansas, Texas, New Mexico, Montana, Nevada, and California. This lineage shows a dramatic increase in size through time, along with increased hypsodonty of their molars, molarization of premolars, flaring of the occiput, and other distinctive features. Mediochoerus was an extremely rare genus, known from M. johnsoni (early Hemingfordian) and M. blicki (early Barstovian) of Nebraska, and M. mohavensis (late Barstovian) of California. The last Ustatochoerus vanished about 7 Ma, about the same time as the spread of C4 grasslands, which may have favored obligate grazers with very hypsodont teeth like horses and camels, over oreodonts with their only moderately hypsodont teeth.