THE LATE PLEISTOCENE EQUIDS FROM NORTHWESTERN OAXACA, SOUTHERN MEXICO

Most of the knowledge about Mexican Pleistocene equids derives from the study of localities in northern and central Mexico. This knowledge is meager in the southern part of the country. Here we report the identified equid species from the Mixteca Alta of Oaxaca. The specimens include cranial and postcranial material recovered from fluvial bars and overbank deposits. The presence of Bison in the fossil localities indicates a Rancholabrean NALMA. Beside these mammals, specimens of glyptodonts, ground sloths, rodents, camels, deer, proboscideans, amphibians, reptiles and mollusks have also been collected; they constitute the Viko vijin local fauna.

About 65 % of the equid specimens from the study area show the diagnostic characters of Equus conversidens, such as: short muzzle that tapers rostrally, masseteric ridge that extend anteriorly to P4, infraorbital foramen set high, teeth series that converge rostrally, teeth rather small, P2 short, U-shaped linguaflexids and moderately deep ectoflexids that do not fully penetrate the molar isthmus. The other identified species is Equus mexicanus, based on the large size of teeth with complexly plicated enamel and long flat protocones, a narial notch that extends over the P2 and P3, thick and deep rami, lower molars with shallow ectoflexids and long isthmuses and limbs bones large and stout.

Body mass estimations of the Oaxacan specimens resulted in a mean of 393 Kg for E. conversidens (similar to the medium-sized zebra E. grevyi), and a body mass of around 440 Kg for the larger E. mexicanus.

Equus conversidens had a wide geographic distribution in North America during the late Pleistocene, from southern Canada to central Mexico; also it has been reported in Chiapas, the Yucatan Peninsula, Guatemala, Honduras and Costa Rica. The record of E. conversidens in the Mixteca Alta is the first for the state of Oaxaca. E. mexicanus has been recorded in Oregon, California and Texas, as well as in northeastern and central Mexico. The record of E. mexicanus in Oaxaca is the southern most in North America.