Perissodactyls

Order Perissodactyla (odd-toed ungulates) contains herbivorous grazing or browsing ungulates (hoofed mammals) that walk and run entirely on their hooves or digits; the sole of the foot and heel never touch the ground. Domestic horses and donkeys have been part of the human experience for thousands of years. Domestication of horses began about 3100 B.C.E., and of asses approximately 6,000 years ago. Both have accompanied human dispersal over the planet's land surfaces, functioning as sources of transportation, leather, and food. The mule is the sterile offspring of a male donkey (Equus assinus) and female horse (Equus caballus) and has been bred for more than 3,000 years for heavy pulling, as a pack animal, or for riding. Perissodactyls evolved from the Family Phenacodontidae (Late Paleocene to Eocene), member of the extinct Order Condylarthra, a group of ancient herbivorous ungulates from which several mammalian orders originated. The earliest perissodactyls, such as the dog-size Hyra-cotherium, are known from the Early Eocene of North America. The seventeen recent species are arrayed in six genera and three families: Equidae (Equus, eight species of horses, zebras, and ass); Tapiridae (Tapirus, four species); and Rhinocerotidae (Dicerorhi-nus, one species; Rhinoceros, two species; Diceros, one species; Ceratotherium, one species).

In all perissodactyls, body weight is borne by the central terminal digits (mesaxonic). All terminal digits are encased in hooves. Perissodactyls also have elongate skulls because

Cothersium
Two zebras neck-to-neck, Tanzania (Carl and Ann Purcell/Corbis)

the facial region is enlarged to accommodate a complete series of premolars and molars (cheek teeth) with expansive and complex chewing surfaces. Perissodactyls lack a clavicle and horns with bony cores. Unlike most artiodactyls, perissodactyls have a small, simple stomach. Cellulose from their herbivorous diet is broken down in the intestines by microbial action.

Wild populations of horses, zebras, and ass are now found only in Africa, the Middle East, and parts of Asia. This group has an extensive fossil record, providing a superb example of large-scale evolutionary change over a long time. Most of the evolutionary history of Equidae took place in North America, with the greatest diversity of species occurring in the Miocene. Wild equids became extinct at the end of the Pleistocene in North and

South America but persist today in Eurasia and Africa. Domestic horses were brought to the New World during the 1600s.

Tapirs first appear in the Oligocene, and they retain many primitive traits shared with the common ancestors of all perissodactyls. Tapirs have a short proboscis, the front feet have four digits (with a vestigial fifth), and the hind feet have three digits. They inhabit tropical evergreen rain forests in the neotropics and Indomalayan region. Fruit and succulent vegetation constitute their diet.

Rhinoceroses first appear in the Middle Eocene. Recent species occur in Africa and the Indomalayan region, where they inhabit semi-deserts, grasslands, savannas, forests, and marshes. Rhinoceroses have been hunted for about 1,000 years to obtain the horn and other body parts that are valued for their supposed medicinal properties. All species are extremely endangered; the Asian rhinoceroses and African black rhinoceros face extinction.

—Mary Ellen Holden

See also: Black Rhinoceros; Endangered Species; Preservation of Species

Bibliography

Carroll, Robert. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman; Groves, Colin P. 1986. "The Taxonomy, Distribution, and Adaptations of Recent Equids." In Equids in the Ancient World, edited by R. H. Meadow and H.-P. Uerp-mann, pp. 11-65. Wiesbaden: Dr. Ludwig Reichert Verlag; Grubb, Peter. 1993. "Order Perissodactyla." In Mammal Species of the World, 2d ed., edited by Don E. Wilson and DeeAnn M. Reeder, pp. 369-372. Washington, DC: Smithsonian Institution Press; MacFadden, Bruce J. 1992. Fossil Horses: Systematics, Paleobiology, and Evolution of the Family Equidae. New York: Cambridge University Press.

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