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By Phil Myers
The artiodactyls are a large and remarkably diverse group of mammals, containing around 220 living species placed in 10 families. The majority live in relatively open habitats, such as plains and savannas, but others dwell in forests, and one group is semiaquatic. Within the order can be found some of the fastest-running mammals, but the Artiodactyla also includes relatively slow and cumbersome species such as pigs and hippos.
Artiodactyls are paraxonic, that is, the plane of symmetry of each foot passes between the third and fourth digits. In all species the number of digits is reduced at least by the loss of the first digit, and the second and fifth digits are small in many. The third and fourth digits, however, remain large and bear weight in all artiodactyls. This pattern has earned them their name, Artiodactyla, which means "even-toed." Artiodactyls stand in contrast to the "odd-toed ungulates," the Perissodactyla, in which the plane of symmetry runs down the third toe. The most extreme toe reduction seen in any artiodactyls (living or extinct) is in forms such as antelope and deer, which have just two functional (weight-bearing) digits on each foot. In such forms the third and fourth metapodials fuse, partially or completely, to form a single bone called a cannon bone. In the hind limb of these species, the bones of the ankle are also reduced in number, and the astragalus becomes the main weight-bearing bone. These traits are probably adaptations for running fast and efficiently.
Artiodactyls tend to share a number of cranial and dental characteristics, but the group is so diverse that there are exceptions to each trait. The anterior part of the skull (in front of the orbits) tends to be long and narrow. Horns or antlers are often present, usually on the 
frontals, which are usually larger than the parietals. All artiodactyls have a postorbital bar or process. The number of teeth is variable, but in many species it is smaller than the number found in perissodactyls. A diastema usually separates anterior and posterior teeth, especially in the lower jaw. Cheek teeth are bunodont in some forms, but more commonly selenodont. The premolars tend to be relatively small and not fully molariform.
Most artiodactyls have modified stomachs, the extreme case being that of groups such as antelope and deer, which have distinctive, four-chambered stomachs. This arrangement appears to be an adaptation that allows members of these groups to make use of microorganisms to decompose cellulose into digestible components. Cellulose is an important constituent of plant tissue that most mammals cannot digest.
Artiodactyls are native to all continents except Australia and Antarctica. The group contains a number of domesticated species. The fossil record of artiodactyls is ancient, going back at least to the earliest Eocene. The group expanded greatly (up to 36 families have been described in the Cenozoic) and its members took on more-or-less modern form in the Miocene. The dominance of this order is correlated with the decline of perissodactyls. It has been suggested that artiodactyls caused most perissodactyls to disappear, or that the disappearance of perissodactyls permitted the radiation of artiodactyls, but we probably can never know whether either of these relationships occurred.
Artiodactyls are usually divided into several suborders. The Suiformes include the suids, tayassuids and hippos, plus a number of extinct families. These animals do not ruminate (chew their cud), and their stomachs may be simple and one-chambered or have up to three chambers. Their feet are usually 4-toed (but at least slightly paraxonic). They have bunodont cheek teeth, and canines are present and tusk-like. The suborder Tylopoda contains a single living family, the Camelidae. Modern tylopods have a 3-chambered, ruminating stomach. Their third and fourth metapodials are fused near the body but separate distally, forming a Y-shaped cannon bone. The navicular and cuboid bones of the ankle are not fused, a primitive condition that separates tylopods from the third suborder the Ruminantia. This last suborder includes the families Tragulidae, Giraffidae, Cervidae, Moschidae, Antilocapridae, and Bovidae, plus a number of extinct groups. In addition to having fused naviculars and cuboids, this suborder is characterized by a series of traits including missing upper incisors, often (but not always) reduced or absent upper canines, selenodont cheek teeth, a 3 or 4-chambered stomach, and third and fourth metapodials usually partially or completely fused.
Literature and references cited
Simpson, C. D. 1984. Artiodactyls. Pp. 563-587 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Click on a name below to learn more about that family:
Family Suidae
Family Tayassuidae
Family Hippopotamidae
Family Camelidae
Family Tragulidae
Family Giraffidae
Family Moschidae
Family Cervidae
Family Antilocapridae
Family Bovidae
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Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
