Pleistocene horses from Tarija, Bolivia, and validity of the genus †Onohippidium (Mammalia: Equidae)

ABSTRACT

The rich Pleistocene (Ensenadan) mammalian fauna from Tarija, Bolivia, contains abundant fossil horses. Traditionally, it has been stated that three genera, Equus and the hippidiforms Hippidion and Onohippidium, and as many as five species are represented by the equids in the Tarija fauna. Recently, it has been proposed that Onohippidium is a junior synonym of Hippidion and thus there is only one valid genus of hippidiform at Tarija. The present study reports new data from upper and lower dentitions, metapodials, and the skull (particularly the preorbitai facial fossa) showing that three monospecific equid genera are present in the Pleistocene of Tarija, including the primitive hippidiforms Hippidion principale and Onohippidium devillei, and the advanced equine Equus insulatus. The genus Onohippidium is indeed valid and distinct from Hippidion based on facial characters, dental pattern, and metatarsal proportions. Comparisons with relevant equids from North America confirm that the hippidiform horses share synapomorphies with Pliohippus and that they diversified in North America during the late Miocene prior to dispersal to South America before, or about, 2.5 myr ago, at the beginning of the Uquian (late Pliocene). Equus, which shares synapomorphies with North American Dinohippus, apparently was a later immigrant, first appearing in South America at the beginning of the Ensenadan (early Pleistocene), about 1.5 myr ago.