The even-toed ungulates (Artiodactyla) are ungulates (hoofed animals) whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates (perissodactyls), e.g. horses.
Artiodactyla comes from the Greek words artios, meaning entire or even numbered, and dactylos for finger or toe. This group includes pigs, peccaries, hippopotamuses, camels, chevrotains (mouse deer), deer, giraffes, pronghorn, antelopes, sheep, goats, and cattle. The group excludes whales even though DNA sequence data indicate that they share a common ancestor, making the group paraphyletic. The more phylogenetically accurate group is Cetartiodactyla.
There are about 220 artiodactyl species, including many that are of great nutritional, economic, and cultural importance to humans.
A further distinguishing feature of the group is the shape of the astragalus, a bone in the ankle joint, which has a double-pulley structure. This gives the foot greater flexibility.
As with many mammal 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.
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