The sole extant genus of equids, Equus, is found in the Equidae tribe Equini, which also includes the extinct species Astrohippus, or "Star horse". Fossil remains have been discovered in the Mexican states of Chihuahua, Jalisco, and Guanajuato as well as in Florida and the central United States. The oldest fossils are from the Barstovian period of the Miocene, while the newest fossils are from the early Pliocene Hemphillian faunal stage. Astrohippus's most likely progenitor is Pliohippus, according to an analysis of the tooth morphology, which makes it unlikely that Astrohippus was the ancestor of current horses. R. A. Stirton and W. D. Matthew initially classified the species Astrohippus ansae based on a few cheek teeth that were discovered at the Coffee Ranch quarry in Hemphill County, Texas. The remnants of six more equid genera, including Dinohippus and Nannippus, have been found at this quarry. In 1950, J.F. Lance classified Astrohippus stockii as Pliohippus stockii from the Yepomera location in Chihuahua, Mexico. Quinn relocated the species to Astrohippus five years later. In 1965, O. Mooser identified the species Astrohippus albidens from Mexico; however, in 1988, T. S. Kelly and E. B. Lander moved the taxon to the genus Dinohippus. T. S. Kelly merged A. stockii and A. albidens in 1998; the earlier A. stockii name was kept, while A. albidens was reduced to junior synonym status. O. Mooser noted in 1973 that both species of Astrohippus were found in Guanajuato, Mexico's Ocote Local Fauna.
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