The name of their order derives from the fact that their middle toe is larger than the others, and the plane of symmetry of the foot passes through it, a condition called mesaxonic . Most species have three digits on the hindfoot and three or four on the forefoot, but in some only a single digit, the third, remains. Some species have horns , but these are dermal structures without bony cores, and they are located on the nasals or frontals in the midline of the skull. This contrasts with the horns of artiodactyls , which have bone cores, are paired, and are located on the frontals. Molars and premolars are hypsodont in grazing forms such as horses, and brachydont in browsers such as tapirs.
Perissodactyls have a simple stomach , in contrast to the chambered structure of most artiodactyls.
Their cecum is enlarged and sacculate, and in it some bacterial digestion of cellulose takes place. Early in the Tertiary, this was a dominant group that included 14 families and many species. Now, all that remain are 3 living families, with 18 species in all. Their decline accelerated during the Oligocene and coincided with the rise of another group of large herbivorous and cursorial mammals, the artiodactyls . Modern perissodactyls are native to Africa, south and central Asia, southern North America, and northern South America.