artiodactyla

(′ärd·ē·ō′dak·tə·lə)

(vertebrate zoology) An order of terrestrial, herbivorous mammals characterized by having an even number of toes and by having the main limb axes pass between the third and fourth toes.

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An order comprising the even-toed ungulates (hoofed mammals). There are two main radiations: the predominantly omnivorous Bunodontia, including suoids (such as pigs, peccaries, and hippos); and the more herbivorous Selenodontia, including camels and ruminants (such as deer, giraffe, cattle, sheep, and antelope). Artiodactyla contains about 213 living species, making it the fifth most speciose order of mammals. First known from the early Eocene, artiodactyls have proliferated during the last 55 million years to reach great diversity (especially among the family Bovidae). Their radiation is often contrasted with that of the odd-toed ungulates, or Perissodactyla (horses, rhinos, and tapirs). Artiodactyls are also important for human economy and agriculture, comprising most of the domestic animals, providing milk, wool, and most of the meat supply. See also Perissodactyla.

Artiodactyls are defined by a unique morphology of the ankle joint, possessing a “double-pulley” astragalus (see illustration). This morphological feature is frequently considered to be a key innovation of artiodactyls, but its precise functional significance is poorly understood. Artiodactyls also have a paraxonic foot structure, where the axis of limb support passes between the third and fourth digits. The first toe is usually completely lost. In primitive artiodactyls and in suoids, the foot is four-toed, and the metapodials (hand and foot bones) are short and unfused as in the generalized mammalian condition (illus. a). In more derived artiodactyls, digits 2 and 5 have been reduced or lost entirely, and metapodials 3 and 4 are lengthened and fused to form a “cannon bone” (illus. b). It is this fusion of the metapodials with digits 3 and 4 free that gives ruminants their cloven-hoofed appearance (in contrast with the single and solid hoof of horses).

Representative left hindfeet of artiodactyls, showing the double-pulley astragalus. (a) Primitive condition, as in the Oligocene oreodont Agriochoerus (although the clawed condition in this animal is a secondary one). (b) Derived condition, as in the Miocene camelid Oxydactylus. (After A. S. Romer, Vertebrate Paleontology, 3d ed., University of Chicago Press, 1966)

Despite their host of unique features, in terms of branching patterns artiodactyls represent an early divergence from the main stem of ungulate evolution. The earliest artiodactyls, rabbit-sized animals found in the Eocene of North America and Europe, are commonly known as dichobunids. By the late middle Eocene the earliest members of the two modern radiations, the Bunodontia and the Selenodontia, had arisen from among different groups of dichobunids. The three main types of living artiodactyls—suoids, camelids, and ruminants—can trace their roots back to this Eocene divergence. There has been much debate as to whether whales (order Cetacea) should be included among the Artiodactyla. Whales are clearly the sister taxon to artiodactyls among modern mammals, but some molecular studies imply that they are closely allied with the hippos. Morphologists and paleontologists would generally prefer to keep whales separate from artiodactyls. See also Cetacea.

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IN BRIEF: n. - An order of hooved mammals of the subclass Eutheria (including pigs and peccaries and hippopotami and members of the suborder Ruminantia) having an even number of functional toes.

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The animal order of artiodactylids. Contains the families Antilocapridae (proghorn antelope), Bovidae (antelope, wild cattle, bison, buffalo, wild goats, wild sheep), Camelidae (camels, guanacos, llamas, alpacas, vicuñas), Cervidae (moose, elk), Giraffidae (giraffe, okapi) and Tragulidae (mouse deer).