AstrohippusandDinohippusfrom the Yepómera local fauna (Hemphillian, Mexico) and implications for the phylogeny of one-toed horses

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Abstract
Astrohippus stocki is characterized by a very complex cheek region with lacrimai and malar fossae that contain smaller concentric pits. Dinohippus mexicanus is characterized by a relatively simple cheek region; the nasomaxillary (“lacrimai”) fossa is moderately developed and the malar fossa is absent. Analysis of characters of the cheek region, when combined with those of the dentition, yield the following phylogenetic hypotheses: 1) A. stocki (and A. ansae) are derivable from Pliohippus s.s. and are only distantly related to Equus; 2) D. mexicanus is a very close relative of some segment of the Equus radiation. A. stocki and D. mexicanus are known from Mexico, Texas, and Florida. These occurrences are biochronologically useful; they indicate a latest Hemphillian age.