POST-PLEISTOCENE, PRE-COLUMBIAN HORSES FROM A SITE IN SAN LUIS POTOSI, MEXICO

Both Pleistocene and Holocene vertebrate sites are reasonably common in Western North America, but having a Holocene site superposed on a Pleistocene one, with mixing of human artifacts and extinct horse, is rare. This, however, is the situation at Rancho Carabanchel in San Luis Potosi. Its most unique character lies in producing Equus specimens of Post-Pleistocene to Pre-Columbian age. C-14 dates obtained from this site have yielded ages ranging from 41000 to 930 years before the present (YBP). Six of them occur within a 3310 to 930 YBP time span. Materials on which the dates were obtained include carbon fragments, carbonized wood, snail shells and organic sediments. Most of these were found juxtaposed or else very closely associated with horse bones and/or teeth.

Upper stratigraphic beds at the site contain evidence of ancient humans such as arrow points, lithics and possible carved and polished objects. Some of the lowest artifacts found were in a stratum containing various animals including a jaw with dentition of an extinct horse, Equus cf. mexicanus.

This locality and surrounding area exhibit an abundance of tufa and related deposits. Many springs and standing bodies of water undoubtedly existed here in the past which helped to produce them. This situation led to a site frequented by animals, and in time humans. Tests show that the springs were of cool water in the recent past but might have been warm at some earlier period. Evidence shows that there was exposed land for long periods of time. This evidence comes from burned zones and paleosols within examined beds. Disconformities, while most are not distinct, must be present.

Wooded and open areas surrounded the locality during the Late Pleistocene and Early Holocene. Conditions in general changed from cool and moist to warm and dry. Presence of wooded areas in the past is indicated by rabbit, deer, elk, mountain lion and peccary. Open areas have been determined by presence of horses, bison, camels and mammoth.

Three species of horses were found at Rancho Carabanchel. They are Equus cf. mexicanus, E. cf. conversidens and E. cf. tau. Sufficient numbers of specimens obtained clearly show three distinct species. Size more than dental characters enabled identifications. Equus cf. mexicanus has the greatest representation, followed by E. cf. conversidens and then E. cf. tau. The first two-named species have been sufficiently dated to show that they both lived in Post-Pleistocene time, with E. cf. mexicanus seemingly surviving the longest.