A NEW ANALYSIS OF ANCHITHERINE EQUIDS ACROSS THE EOCENE-OLIGOCENE BOUNDARY IN THE WHITE RIVER GROUP OF THE WESTERN GREAT PLAINS

Anchitherine horses are a subfamily of equids that are abundantly represented in the late Eocene and Oligocene of North America. This group has been heavily studied in the past, but important questions remain. Some studies have focused on the Eocene-Oligocene boundary and have used these equids along with other taxa to study mammalian diet and climate change through this interval. We reexamine two anchitherine genera, Mesohippus and Miohippus, from stratigraphic sequences of the White River Group in western Nebraska and southwestern South Dakota. These sequences span the Chadronian (late Eocene), Orellan (Early Oligocene), and Whitneyan (early Oligocene) North American land-mammal ages. The most recent revision of these genera was done by Prothero and Shubin (1989) and we review the characters they used for taxonomic identification. This includes characters, such as the hypostyle and the articular facet on the third metatarsal, and dental dimensions. To avoid possible biases from combining specimens from different stratigraphic levels, we separated specimens by location and stratigraphic level. We measured the length and width of cheek teeth, and tooth rows of 488 specimens. First molar area serves as a proxy for body mass in horses and other mammals, and may be useful for distinguishing among species. We found that the morphological characters used by Prothero and Shubin were highly variable and were not useful for distinguishing between these genera. We suggest that the development of the articular facet on the third metatarsal is a function of body size and therefore is of no more use than the first molar area. Variability in the first molar area suggests the presence of 3 species in the medial and late Chadronian, 2 species in the Orellan, and probably at least 2 species in the early Whitneyan. Due to a lack of objective criteria separating Mesohippus from Miohippus, we recommend synonymy of these genera, making Mesohippus a junior subjective synonym.