Artiodactyls are the most diverse, large, terrestrial mammals alive today. They are the fifth largest order of mammals, consisting of 10 families, 80 genera, and approximately 210 species. Although the majority of artiodactyls live in relatively open habitats, they can be found in all habitat types, including some aquatic systems, and are native to every continent, excluding Australia and Antarctica. As would be expected in such a diverse group, artiodactyls exhibit exceptional variation in body size and structure. Body mass ranges from 4000 kg in hippos to 2 kg in lesser Malay mouse deer. Height ranges from 5 m in giraffes to 23 cm in lesser Malay mouse deer.
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 by the loss of the first digit (i.e., pollex), and many species have second and fifth digits that are reduced in size. 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". In contrast, the plane of symmetry in perissodactyls (i.e., odd-toed ungulates) runs down the third toe. The most extreme toe reduction in artiodactyls, living or extinct, can be seen in antelope and deer, which have just two functional (weight-bearing) digits on each foot. In these animals, 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 are divided into 3 suborders. Suiformes includes the suids, tayassuids and hippos, including 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, 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, Ruminantia. This last suborder includes the families Tragulidae, Giraffidae, Cervidae, Moschidae, Antilocapridae, and Bovidae, as well as 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 that are often partially or completely fused.
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