PLIOHIPPUS

extinct genus of primitive horses that inhabited North America during the Pliocene Epoch (from 5.3 to 1.6 million years ago). Pliohippus, the first of the one-toed horses, evolved from Merychippus, an ancestor that lived in the preceding Miocene Epoch. The teeth in Pliohippus are more advanced than are those in earlier forms and tend toward the dentition of modern horses: tall, complexly folded teeth. Pliohippus was a grazing and running animal from which modern horses evolved.