| Sumario: | Equine material recovered from the Middle Miocene El Camarón and Matatlán Formations (both K-Ar dated ca. 15 Ma, late Early Barstovian) of Oaxaca, southeastern Mexico, is formally described. It consists of isolated upper and lower cheek teeth, whose occlusal patterns and tooth dimensions closely resemble those of Pliohippus, although no positive specific identification is warranted on this basis. Pliohippus sp. from Oaxaca is contemporary with P. mirabilis (late Early-Late Barstovian of Nebraska, Colorado and Florida), the oldest and most plesiomorphic species of the genus. These facts are very significant on several counts: Biogeographically, the Oaxacan find extends the Barstovian/Middle Miocene record of Pliohippus from the Northern Great Plains and the Gulf Coast of the coterminous United States to southeastern Mexico, some 2000 km and at least 10º latitude degrees apart. Phyletically, it gives additional evidence that the differentiation of Pliohippus occurred during the Early Barstovian at least. Paleoecologically, it shows that Pliohippus could thrive both in temperate and tropical environmental settings, either of which could have been the site of such phyletic differentiation. Finally, Pliohippus sp. from Oaxaca is the oldest record of the tribe Equini in México, antedating by ~6–8 Ma the extensive occurrence of equines during the Hemphillian-Blancan of this country.
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