Perissodactyla

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An order of herbivorous, odd-toed, hoofed mammals, including the living horses, zebras, asses, tapirs, rhinoceroses, and their extinct relatives. They are defined by a number of unique specializations, but the most diagnostic feature is their feet. Most perissodactyls have either one or three toes on each foot, and the axis of symmetry of the foot runs through the middle digit.

The perissodactyls are divided into three groups: the Hippomorpha (horses and their extinct relatives); the Titanotheriomorpha (the extinct brontotheres); and the Moropomorpha (tapirs, rhinoceroses, and their extinct relatives). See also Rhinoceros.

Perissodactyls originated in Asia some time before 57 million years ago (Ma). By 55 Ma, the major groups of perissodactyls had differentiated, and migrated to Europe and North America. Before 34 Ma, the brontotheres and the archaic tapirs were the largest and most abundant hoofed mammals in Eurasia and North America. After these groups became extinct, horses and rhinoceroses were the most common perissodactyls, with a great diversity of species and body forms. Both groups were decimated during another mass extinction about 5 Ma, and today only five species of rhinoceros, four species of tapir, and a few species of horses, zebras, and asses cling to survival in the wild. The niches of large hoofed herbivores have been taken over by the ruminant artiodactyls, such as cattle, antelopes, deer, and their relatives.

Most extinct horses were browsers and ate soft, leafy vegetation, but all living horses are grazers, using their sharp incisors and mobile lips to crop low-growing grasses. The only common wild horse, the plains zebra, lives in large herds (up to 100 individuals) and migrates over large areas of grasslands in search of food. However, desert-dwelling asses and Grevy's zebra live in small herds, with a stallion guarding a small harem of mares. Most species of wild horses, including the Grevy's and mountain zebras, all species of onagers and asses, and Przewalski's horse (an ancestor of domesticated horses), are nearly extinct in the wild.

The earliest moropomorphs, such as Homogalax, from strata about 55 million years old, are virtually indistiguishable from the earliest horses. From this unspecialized ancestry, a variety of archaic tapirlike animals diverged. Most retained the simple leaf-cutting teeth characteristic of tapirs and, like brontotheres, died out about Ma when their forest habitats shrank. Only the modern tapirs, with their distinctive long proboscis, survive in the jungles of Central and South America (three species), and southeast Asia (one species). All are stocky, piglike beasts with short stout legs, oval hooves, and a short tail. They have no natural defenses against large predators (such as jaguars or tigers), so they are expert at fleeing through dense brush and swimming to make their escape.

Rhinoceroses have been highly diverse and successful throughout the past 50 million years. They have occupied nearly every niche available to a large herbivore, from dog-sized running animals, to several hippolike forms, to the largest land mammal that ever lived—the 18-ft-tall (6-m), 44,000-lb (20,000-kg) Paraceratherium. Between 20 and 5 Ma, rhinos diversified into several browsing (leaf-eating) lineages, and hippolike grazing lineages, and browser-grazer pairs of rhinos were found all over the grasslands of Eurasia, Africa, and North America. The mass extinction event that occurred about 5 Ma wiped out North American rhinos and decimated most of the archaic rhino lineages in the Old World. During the ice ages, woolly rhinos and their relatives were common all over Eurasia. Their only surviving descendant is the endangered Sumatran rhinoceros. Only a few hundred individuals still live in the mountainous jungles of Sumatra. Four other species of rhino survive in Asia and Africa, but all are on the brink of extinction because of heavy poaching for their horns. See also Mammalia.

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