Even-toed ungulates, an order of mammals that includes the living camels, pigs, and ruminants. Descended from the Condylarthra, they underwent a spectacular burst of adaptive radiation in Eocene and early Oligocene times, largely replacing the initially more numerous Perissodactyla. The ankle bone (astragalus) is specialized in artiodactyls to give better spring. The axis of the foot is paraxonic (passing between the third and fourth digits). In primitive, four-toed types, e.g. the pig, the first digit is absent; in advanced forms the second and fifth digits are also reduced or lost. Early forms had an unspecialized dentition, but in the course of evolution the upper incisors were lost in some species, the lower incisors biting against the hardened gum of the upper jaw, an adaptation to a herbivorous diet.