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Even-toed ungulates Fossil range: Early Eocene - Recent
Right-rear foot of a Masai Giraffe (Giraffa camelopardalis tippelskirchi) at the San Diego Zoo
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Infraclass: Eutheria Superorder: Laurasiatheria Order: Artiodactyla* Owen, 1848
Families
Antilocapridae Bovidae Camelidae Cervidae Giraffidae Hippopotamidae Moschidae Suidae Tayassuidae Tragulidae Leptochoeridae † Dichobunidae † Cebochoeridae † Entelodontidae † Anoplotheriidae † Anthracotheriidae † Cainotheriidae † Agriochoeridae † Merycoidodontidae † Leptomerycidae † Protoceratidae † Oromerycidae † Xiphodontidae † Amphimerycidae † Helohyidae † Gelocidae † Dromomerycidae † Raoellidae † Choeropotamidae † Sanitheriidae † Climacoceratidae †

The even-toed ungulates form the mammal order Artiodactyla. They are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in perissodactyls. There are about 220 artiodactyl species, including many that are of great economic importance to humans.

As with many animal groups, even-toed ungulates first appeared during the Early Eocene (about 54 million years ago). In form they were rather like today's chevrotains: small, short-legged creatures that ate leaves and the soft parts of plants. By the Late Eocene (46 million years ago), the three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); and Ruminantia (the goat and cattle group). Nevertheless, artiodactyls were far from dominant at that time: the odd-toed ungulates (ancestors of today's horses and rhinos) were much more successful and far more numerous. Even-toed ungulates survived in niche roles, usually occupying marginal habitats, and it is presumably at that time that they developed their complex digestive systems, which allowed them to survive on lower-grade feed.

The appearance of grasses during the Eocene and their subsequent spread during the Miocene (about 20 million years ago) saw a major change: grasses are very difficult to eat and the even-toed ungulates with their highly-developed stomachs were better able to adapt to this coarse, low-nutrition diet, and soon replaced the odd-toed ungulates as the dominant terrestrial herbivores.

The artiodactyls fall into two groups which, despite underlying similarities, are rather different. The Suina (pigs, hippos, and peccaries) retain four toes of fairly equal size, have simpler molars, short legs, and their canine teeth are often enlarged to form tusks. In general, they are omnivores and have a simple stomach (the two hippopotamus species and the babirusa are herbivorous exceptions). It is possible that Suina is not a natural group. In particular, recent research suggests that the Hippopotamidae (which are likely derived from among the extinct group known as anthracotheres) may be more closely related to the ruminants than to the pigs.

On the other hand, the camelids and the Ruminantia tend to be longer-legged, to walk on only the central two toes (though the outer two may survive as rarely-used dew-claws) and to have more complex cheek teeth well-suited to grinding up tough grasses. They have evolved a highly developed digestive process in which partly-digested food is regurgitated and re-chewed (chewing the cud or cudding). This complex digestion takes place in a multi-chambered stomach, the rumen itself. It allows them to use fermentation by microorganisms to digest cellulose, a plant material which animals cannot digest directly.

Lastly, one group of artiodactyls (which molecular biology suggests were most closely related to Hippopotamidae) returned to the sea to become whales. The conclusion is that Artiodactyla, if it excludes Cetacea, is a paraphyletic group. For this reason, the term Cetartiodactyla was coined to refer to the group containing both artiodactyls and whales.^[1]

• ORDER ARTIODACTYLA
  • Suborder Ruminantia
    • Infraorder Pecora
      • Family Antilocapridae: pronghorn
      • Family Bovidae: cattle, goats, sheep, and antelope
      • Family Cervidae: deer
      • Family Giraffidae: Giraffe and Okapi
      • Family Moschidae: musk deer
    • Infraorder Tragulina
      • Family Tragulidae: chevrotains
  • Suborder Suina
    • Family Hippopotamidae: hippos
    • Family Suidae: pigs
    • Family Tayassuidae: peccaries
  • Suborder Tylopoda
    • Family Camelidae: camels and llamas

• Odd-toed ungulate

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