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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 Rhinoceros, Tapir
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. See Mammalia
(odd-toed ungulates), an order of large or, less frequently, medium-size mammals.
The forelimbs have one, three, or five digits; the hind limbs have one or three digits. The third digit is most highly developed and bears the principal weight of the animal’s body. The terminal digital bones are encased by hooves. The cheek teeth have transverse and longitudinal ridges on the chewing surface and are adapted for grinding coarse vegetable food. The facial portion of the skull is long. Clavicles are absent, and, unlike in Artiodactyla, the femur has a third trochanter. Odd-toed ungulates are herbivorous. The stomach is simple and has a single chamber; the cecum and colon are long and large and have numerous protusions, or pouches, that facilitate digestion of coarse food. The uterus is bicornuate, and the placenta is diffused. There is one pair of mammary glands, located in the region of the groin. The number of young is one.
Odd-toed ungulates are distributed in Africa, Asia, and South America. They have been domesticated on all continents. Wild forms were encountered in southern Europe until the end of the 19th century. Extant representatives of the Perissodactyla make up the three families Equidae, Rhinocerotidae, and Tapiridae.
REFERENCESSokolov, I. I. Kopytnye zveri. Moscow-Leningrad, 1959. (Fauna SSSR: Mlekopitaiushchie, vol. 1, issue 3.)
Mlekopitaiushchie Sovetskogo Soiuza, vol. 1. Moscow, 1961.
I. I. SOKOLOV