FOSSIL HORSES Fossil remains of horses have been found in the Pleistocene formations of all parts of Europe, Asia, Africa, North and South America, and these discoveries show the former wide distribu tion of the genus. Many of the remains found in the later Pleistocene of Europe and Asia have been referred to the domes ticated species (Equus caballus), others to the ass and dziggetai (E. hemionus) ; and some South African fossils to the zebra. Most of the Old World fossils and all of those from the New World are referred to a large number of extinct species, only a few of which are sufficiently known and sufficiently distinct for characterization. The oldest and best known extinct species in the Old World is Equus stenonis, found in the upper Pliocene and older Pleistocene of Europe ; a species of medium size and propor tions near to the lighter breeds of domestic horses, and with a variable tooth-pattern sometimes quite close to that of Plesippus of the American Upper Pliocene. It is regarded by Boule (191o) as including races ancestral to the ass, zebra and the domestic horse, and various intermediate forms have been described from the middle and later Pleistocene of Europe and Asia.
Many fossil species of Equus have been described from the Pleistocene of North and South America. Two of the North American species known from a series of complete skeletons are E. scotti of the lower Pleistocene of Texas and E. occidentalis of California. Both are about as large as an average-sized E. caballus, but E. scotti has more the proportions of a zebra, with relatively large head, deep jaws, short back and relatively small legs and feet. E. occidentalis is similar in these general features but differs in minor characters of teeth and skull. The same is true of E. niobrarensis from the lower Pleistocene of Kansas. In general these species from the lower Pleistocene of North America are intermediate between Plesippus and the modern Equus.
The South American species of Equus are also in a rather primi tive evolutionary stage according to Boule, but of several fairly distinct types. They occur only in the middle and later Pleisto cene, following and finally superseding the aberrant short-legged horses of the Hippidium group. Both are invaders from North America, Hippidium probably derived directly from Pliohippus, Equus through the intermediate Plesippus.
Very little is known of the fossil horses of Africa south of the Sahara, but they seem to be all nearly related to the modern zebras. No fossil Equidae are known from Australasia.
For the Tertiary ancestry of the horse, see EQUIDAE.