Cranium of Dinohippus mexicanus (Mammalia: Equidae) from the early Pliocene (latest Hemphillian) of central Mexico, and the origin of Equus

Abstract

A newly discovered skull of Dinohippus is described from the latest Hemphillian (early Pliocene) Rancho El Ocote locality of Guanajuato, Mexico, which is dated at 4.8 Ma. This cranium is referred to D. mexicanus, the senior synonym for the latest Hemphillian species otherwise known from several localities in northern Mexico and the southern United States. Although crushed, this is the most complete skull known for this extinct species. With the diagnostic configuration of the dorsal preorbital fossa, distinctive dental pattern, and moderate tooth curvature, this cranium demonstrates a morphology similar to, although slightly more primitive than, that of closely related and slightly more derived Blancan Equus, such as E. simplicidens. As represented by occurrences in central Mexico and southern California, D. mexicanus coexisted with Equus during the middle Blancan from about 4.5 to 3 million years ago. Despite traditional interpretations of anagenetic speciation, the current study demonstrates that primitive species of Equus originated from D. mexicanus by cladogenesis.