Even-toed ungulates

The **even-toed ungulates** (**Artiodactyla**, Greek ἄρτιος (ártios), meaning 'even', and δάκτυλος (dáktylos), meaning 'finger / toe') are ungulates – hoofed animals – which bear weight equally on two (an even number) of the five toes: their third and fourth toes. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, odd-toed ungulates bear weight on one (an odd number) of the five toes: the third toe. Another difference between the two is that even-toed ungulates digest plant cellulose in one or more stomach chambers rather than in their intestine as the odd-toed ungulates do.

The aquatic cetaceans (whales, dolphins, and porpoises) evolved from even-toed ungulates, so modern taxonomic classification sometimes combines the Artiodactyla and Cetacea into the **Cetartiodactyla**.

The roughly 220 land-based even-toed ungulate species include pigs, peccaries, hippopotamuses, camels, llamas, alpacas, mouse deer, deer, giraffes, antelopes, sheep, goats, and cattle. Many of these are of great dietary, economic, and cultural importance to humans.

## Evolutionary history

See also: Evolution of cetaceans
The oldest fossils of even-toed ungulates date back to the early Eocene (about 53 million years ago). Since these findings almost simultaneously appeared in Europe, Asia, and North America, it is very difficult to accurately determine the origin of artiodactyls. The fossils are classified as belonging to the family Dichobunidae; their best-known and best-preserved member is *Diacodexis*. These were small animals, some as small as a hare, with a slim build, lanky legs, and a long tail. Their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals.
Two formerly widespread, but now extinct, families of even-toed ungulates were Enteledontidae and Anthracotheriidae. Entelodonts existed from the middle Eocene to the early Miocene in Eurasia and North America. They had a stocky body with short legs and a massive head, which was characterized by two humps on the lower jaw bone. Anthracotheres had a large, porcine (pig-like) build, with short legs and an elongated muzzle. This group appeared in the middle Eocene up until the Pliocene, and spread throughout Eurasia, Africa, and North America. Anthracotheres are thought to be the ancestors of hippos, and, likewise, probably led a similar aquatic lifestyle. Hippopotamuses appeared in the late Miocene and occupied Africa and Asia – they never got to the Americas.

The camels (Tylopoda) were, during large parts of the Cenozoic, limited to North America; early forms like Cainotheriidae occupied Europe. Among the North American camels were groups like the stocky, short-legged Merycoidodontidae. They first appeared in the late Eocene and developed a great diversity of species in North America. Only in the late Miocene or early Pliocene did they migrate from North America into Eurasia. The North American varieties became extinct around 10,000 years ago.

Suina (including pigs) have been around since the Eocene. In the late Eocene or the Oligocene, two families stayed in Eurasia and Africa; the peccaries, which became extinct in the Old World, exist today only in the Americas.

South America was settled by even-toed ungulates only in the Pliocene, after the land bridge at the Isthmus of Panama formed some three million years ago. With only the peccaries, lamoids (or llamas), and various species of capreoline deer, South America has comparatively fewer artiodactyl families than other continents, except Australia, which has no native species.