Perissodactyls, otherwise known as odd-toed ungulates, compose an order of mammals characterized by an odd number of toes and being hindgut fermenters with somewhat simple stomachs. As large grazers they digest plant cellulose in their intestines rather than in one or more stomach chambers, unlike the even-toed ungulates. The order includes three extant families: Equidae, including horses, donkeys, and zebras, Rhinocerotidae, rhinos and Tapiridae, tapirs, with a total of about 17 species. In spite of their very different appearance they were recognized by the zoologist Richard Owen, who also coined the term, as related families, in the 19th century.

Anatomy

As an adaptation to different habitats and lifestyles, the odd-toed ungulates have developed distinct differences in their build. Common features are in the construction of the limbs and teeth. Rhinos are the largest members of the group, and the extinct Paraceratherium, a hornless rhino from the Oligocene, is considered one of the largest land mammals of all time. At the other extreme, an early member of the order, the prehistoric horse Hyracotherium, had a shoulder height of only 20 cm (7.9 in). Apart from dwarf varieties of the domestic horse and the donkey, perissodactyls reach a body length 180–420 cm (71–165 in) and a weight of 150 to 3,500 kg (330 to 7,720 lb). While rhinos are only sparsely hairy and exhibit a thick epidermis, tapirs and horses have a dense, short coat. Most species are grey or brown, although zebras and young tapirs are striped.