OREODONTS FROM THE UPPER OLIGOCENE-LOWER MIOCENE (EARLY ARIKAREEANEARLY HEMINGFORDIAN) SESPE-VAQUEROS FORMATIONS, ORANGE COUNTY, CALIFORNIA, AND THEIR PALEOBIOGEOGRAPHIC IMPLICATIONS

We describe new material of oreodonts from the late Oligocene (early Arikareean, “Geringian” land mammal subage) and early Miocene (early Hemingfordian) of the Sespe-Vaqueros formations in Orange County, California. In addition to the dwarfed leptauchenine Sespia californica, which was previously well known from the early Arikareean exposures of the Sespe Formation, the fossils also include skulls and jaws of the small cursorial oreodont Merychyus elegans from the early Hemingfordian part of the sequence. The distribution of oreodonts in the early Arikareean (“Geringian”) shows some remarkable biogeographic patterns: only Sespia from the Sespe Formation, Sespia and Mesoreodon from the Otay Formation in San Diego, lepatauchenines (Sespia and Leptauchenia) plus Mesoreodon and other groups from the “Geringian” of the High Plains—but only Eporeodon and no leptauchenines from the earliest Arikareean of the John Day Formation in central Oregon. These odd patterns of endemism are surprising because most large herbivorous mammal species of that time interval are widespread and are similar from the High Plains to Oregon to California.