A COMPARISON OF THE TAXONOMIC COMPOSITON OF MEDIAL MIOCENE EQUIDS (MAMMALIA: PERISSODACTYLA) FROM THE MISSION PIT, SOUTH DAKOTA AND ASHFALL FOSSIL BEDS, NEBRASKA

The Mission Pit locality near Mission, South Dakota, has produced a large collection of equid teeth. Ashfall Fossil Beds near Royal, Nebraska, has yielded an extensive collection of equid cranial elements and teeth. The two sites are interpreted to be Clarendonian in age [12.5 to 9.0 Ma], but may represent different periods of time within the Clarendonian North American Land Mammal Age. The two sites also exhibit a similar composition of equid genera, including Pliohippus, Calippus, Protohippus, Cormohipparion, Neohipparion, and Pseudhipparion. Both sites share the same abundance of the equid tribes Hipparionini and Equini. Approximately seventy-five percent of the equids at both sites are members of the Hipparionini tribe, whereas twenty-five percent are of the Equini tribe. The composition of the Equini tribe between the two sites is nearly identical. Only slight differences are observed in the composition of genera within the Hipparionini tribe between the two sites, with the Mission Pit containing a higher abundance of Neohipparion. The striking taxonomic similarity suggests the two sites are potential biochronologic correlatives within the early Clarendonian.