Order Artiodactyla is composed of hoofed mammals that have two, or sometimes four, functional toes on each foot. The diversity and abundance of this group is of major economic importance to humans. Through hunting and domestication, most artiodactyl families have been utilized by humans either for transportation or as important sources of meat,
milk, and hides. Some species, such as the giant wild ox (Bos primigenius), are extinct in the wild and are represented today only by their domesticated descendants. Artiodactyls evolved from condylarths, an extinct group of herbivorous placental mammals that originated in the Late Cretaceous. The earliest artiodactyls, such as the rabbit-size Diacodexis, are known from the Early Eocene. Artiodactyls reached their greatest diversity in the Miocene; twenty-one families were represented, including the ten extant families. The more than 220 living species of artiodactyls are arranged in eighty-one genera, ten families, and three suborders: Suidae (pigs), Tayassuidae (peccaries), and Hippopotamidae (hippopotamuses) in
Suborder Suiformes; Camelidae (llamas, vicunas, camels) in Suborder Tylopoda; and Trag-ulidae (mouse deer), Giraffidae (giraffes, okapi), Moschidae (musk deer), Cervidae (deer, munt-jacs), Antilocapridae (pronghorns), and Bovi-dae (antelopes, cattle, goats, sheep, bison, buffalo, anoas) in suborder Ruminantia. Their natural distributions included most worldwide land masses except Antarctica, New Zealand, the Australia-New Guinea region, the West Indies, and the oceanic islands, but a few species have been introduced by humans into some of those places.
There is an impressive range in body size among living artiodactyls. The rabbit-size lesser Malay mouse-deer (Tragulus javanicus) is the smallest, with a head and body up to 75 cm, shoulder height up to 35 cm, and weight up to 8 kg. The hippopotamus (Hippopotamus amphibius) is the most massive, with a body up to 500 cm long, shoulder height up to 165 cm high, and weighing up to 4,500 kg. The giraffe (Giraffa camelopardalis) is the tallest artio-dactyl, reaching a height of up to 5.8 m; it is also the tallest living terrestrial animal.
All artiodactyls are herbivorous, but pigs and peccaries also eat carrion, invertebrates, and small vertebrates. They occur in nearly all terrestrial habitats: tropical and temperate forests, woodlands, savannas, steppes, deserts, and the polar latitudes. Species are terrestrial and active either during the day or night. Hippopotamuses are amphibious, immersed in rivers and lakes during the day, but emerging at night to forage on land. Some species are solitary (anoas, for example), but most live in herds. The wildebeest (Connochaetes taurinus) is a keystone species that significantly affects the structure and stability of its local community and environment.
The diagnostic anatomical feature shared by all artiodactyls is the paraxonic feet: the primary weight-bearing axis of each foot (plane of symmetry) passes between the third and fourth digits. The first digit is absent in all living species, and the lateral digits (two and five) are reduced in size. The Suidae, Hip-popotamidae, Tragulidae, and forelimb of the Tayassuidae have four digits (collared and white-lipped peccaries have three digits on each hind foot); species in the other families are characterized by two complete digits, with the lateral toes either absent or rudimentary. In the Camelidae, Cervidae, Giraffidae, Antilocapridae, and Bovidae, the third and fourth wrist and ankle bones (metapodials) fuse in each foot to form a single cannon bone to which are attached the two toes. The ankle-bone (astragalus) has pulleylike articular surfaces above and below that restrict lateral movement of the hind feet but allow for significant flexion and extension. The digits are encased in hooves in all artiodactyls except camels and their relatives, which bear digits with nails. Most artiodactyls lack a clavicle (shoulder bone), and most species in suborder Ruminantia bear horns or antlers. Upper incisors are reduced in number or absent. Canines are absent or very small in most species but form prominent tusks in pigs and hippopotamuses. Pigs and hippopotamuses have low-crowned molars with chewing surfaces formed by cusps; all the other artio-dactyls have high-crowned molars sculptured by crescent-shaped ridges.
Added to these diagnostic traits are the distinctive structure and function of the artio-dactyl stomach. In most mammals the stomach is a single chamber, but it consists of two chambers in pigs and hippopotamuses, three in camels and mouse deer, and four in all the other artiodactyls. All species in suborder Ruminantia consume a huge amount of vegetation and recycle and reconstitute it in their enlarged and complex stomachs, a process called ruminating (pigs, hippopotamuses, and members of Camelidae are nonruminants). Ruminants graze or browse vegetation (grasses, leaves, the woody parts of trees and shrubs), swallow it after only slightly chewing, then rest in places protected from predators to digest it. The food is high in cellulose but low in nutritional content. The slightly chewed (masticated) vegetation passes into the first chamber, the large rumen, where it is stored and fermented as the animal feeds. During rest, the food is regurgitated from the rumen into the mouth and remasticated (chewing the cud). After a long period of remastication, the vegetation (or cud) is swallowed a second time, bypasses the rumen, and proceeds to the second chamber, the reticulum. From there the food is drawn into a third chamber, the omasum, from where it passes into the fourth chamber, the abomasum, which is lined with glandular tissue and is the counterpart of the single stomach chamber of most other mammals. The stomach chambers contain a diverse bacterial fauna that breaks down cellulose and is crucial to the digestive process. In most other mammals, microbial fermentation of ingested food takes place only in the intestines. Microbial fermentation in the stomach releases proteins, carbohydrates, and lipids, which are more efficiently absorbed by the intestines. Remastication allows more thorough breakdown of the cellulose in plant cell walls, and the bacterial fauna maintains itself by feeding on nitrogen released by digestion of cellulose.
Most artiodactyls were described in the eighteenth and nineteenth centuries, but new species are still being discovered, such as the sao la (Pseudoryx nghetinhensis) and the giant muntjac (Muntiacus vuquangensis), described in the 1990s; both live in tropical forests of Vietnam and Laos.
—Mary Ellen Holden
See also: Bluebuck; Endangered Species; Mammalia;
Paleontology; Preservation of Species
Carroll, Robert. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman; Grubb, Peter. 1993. "Order Artiodactyla." In Mammal Species of the World. 2d ed, edited by Don E. Wilson and DeeAnn M. Reeder, pp. 377-414. Washington, DC: Smithsonian Institution Press; Janis, Christine M., and Kathleen M. Scott. 1987. "The Interrelationships of Higher Ruminant Families with Special Emphasis on the Members of the Cervoidea." American Museum Novitates 2893: 1-85; McKenna, Malcolm C., and Susan K. Bell. 1997. Classification of Mammals above the Species Level. New York: Columbia University Press; Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th ed. Vol. 2. Baltimore: Johns Hopkins University Press; Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy, 4th ed. Orlando, FL: Harcourt.
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