Equidae—Horses // Tapirus—Tapirs
By the Pleistocene, one family of perissodactyls, the rhinoceroses, had become extinct in North America. Extinction of the other two families on the continent was delayed until the end of the Pleistocene, at which time the horses and tapirs disappeared from the continent. All three families survived in the Old World, and the tapirs also survived in South America. The disappearance from the continent was part of the Pleistocene megafaunal extinction and has variously been ascribed to overkill by the entrance of hunting humans into the New World, to the vast climatic/ecological changes occurring at the end of the Pleistocene, or some combination of these (numerous other suggestions also have been made).
Horses are among the most common fossils in both open sites and caves; tapirs are rarer, especially in the Southwest.
The perissodactyls, unlike the artiodactyls, have the main axis of the foot through the third toe rather than between digits III and IV. There is a tendency to lose toes evolutionarily, carried to the extreme by the horses that, by the Pleistocene, retain only one per foot. There also is a tendency for molarization of the premolars where some of the premolars become morphologically and functionally similar to the molars. In the horses, this occurs to the point that three of the four premolars are molariform (the first is vestigial or lost).
Last Update: 5 Dec 2008