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. Taxonomy and phylogeny The classification of artiodactyls was hotly debated because the ocean-dwelling cetaceans evolved from the land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates (hippopotamuses) are more closely related to the ocean-dwelling cetaceans than to the other even-toed ungulates. This makes the Artiodactyla as traditionally defined a paraphyletic taxon, since it includes animals descended from a common ancestor, but does not include all of its descendants. Phylogenetic classification only recognizes monophyletic taxa; that is, groups that descend from a common ancestor and include all of its descendants. To address this problem, the traditional order Artiodactyla and infraorder Cetacea are sometimes subsumed into the more inclusive Cetartiodactyla taxon. An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon. Classification Order Artiodactyla/Clade Cetartiodactyla Suborder Tylopoda Family Anoplotheriidae? Family Cainotheriidae Family Merycoidodontidae Family Agriochoeridae Family Camelidae: camels and lamoids or llamas (7 extant and 13 extinct species) Family Oromerycidae Family Xiphodontidae Clade Artiofabula Suborder Suina Family Suidae: pigs (19 species) Family Tayassuidae: peccaries (4 species) Family Sanitheriidae Family Doliochoeridae Clade Cetruminantia Clade Cetancodontamorpha Genus Andrewsarchus? Family Entelodontidae Suborder Whippomorpha Family Raoellidae Superfamily Dichobunoidea – paraphyletic to Cetacea and Raoellidae Family Dichobunidae Family Helohyidae Family Choeropotamidae Family Cebochoeridae (Family contains Cebochoerus) Family Mixtotheriidae Infraorder Ancodonta Family Anthracotheriidae – paraphyletic to Hippopotamidae Family Hippopotamidae: hippos (two species) Infraorder Cetacea: whales (about