The even-toed ungulates form the mammal order Artiodactyla, the group that contains the pigs, peccaries, hippopotamuses, camels, chevrotains (mouse deer), deer, giraffes, pronghorn, antelopes, sheep, goats, and cattle. 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 odd-toed ungulates (perissodactyls). Another key distinguishing feature is the shape of the astragalus, a bone in the ankle joint, which has a double-pulley structure. This gives the foot greater flexibility. DNA sequence data indicate that this group is paraphyletic because whales are excluded. 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.