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Even-toed ungulate

The even-toed ungulates form the mammal order Artiodactyla. They are ungulates whose weight is borne (if they have more than two toes) about equally by the third and fourth toes, rather than mostly or entirely by the third as in perissodactyls. Another key distinguishing feature is the shape of the astragalus (a bone in the hock joint), which has a double-pulley structure in artiodactyls, giving the foot greater flexibility.

There are about 220 artiodactyl species, including many that are of great nutritional, economic and cultural importance to humans.

Evolution

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 food.

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. Now-extinct Artiodactyla which developed during the Miocene include the species Ampelomeryx, Tauromeryx, Triceromeryx, and others.

Suina

Suina (pigs and peccaries) are artiodactyls that retain four toes of fairly equal size, have simpler molars, short legs, and often have enlarged canine teeth that form tusks. In general, they are omnivores and have a simple stomach, except for the two hippopotamus species and the babirusa which are herbivores. Hippopotamidae have been considered a member of Suina, however, recent morphological and genetic research suggests that hippos are more closely related to whales.

Camelids and Ruminantia

Camelids and 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.

Cetaceans

One group of artiodactyls (which molecular biology suggests were most closely related to Hippopotamidae) returned to the sea to become whales. Thus Artiodactyla without Cetacea is a paraphyletic group. For this reason, the term Cetartiodactyla was coined to refer to the group containing both artiodactyls and whales.