Figure 1. Specializations that define the perissodactyls. (a) Left hind feet of typical perissodactyls, with a line drawn through the axis of symmetry in the third toe. They include (from left) the tapir Tapirus, the primitive three-toed horse Protorohippus, the modern one-toed horse Equus, the brontothere Brontops, and the clawed chalicothere Moropus. Between these last two feet is the ankle bone, or astragalus, of a typical perissodactyl, with its flat, saddle-shaped lower facet. (Modified from Colbert and Morales, 1991.) (b) Crown view of right upper first, second and third molars of typical perissodactyls, showing the characteristic connection of cusps by crests in different groups. The cheek is towards the top, the tongue towards the bottom, and the front of the mouth towards the right in each example. Top row, left: the primitive horse Protorohippus, with four distinct cusps and only weak crests connecting them. Top row, right: the primitive tapir Homogalax, with strong cross-crests connecting the cusps. Second row, left: the rhinoceros Subhyracodon, with crests connected so that they resemble the Greek letter (‘pi’). Second row, right: the brontothere Palaeosyops, with the typical ‘W’-shaped outer crest, and large blunt inner cusps. Bottom row: the chalicothere Moropus, with the ‘W’-shaped outer crest partially connected to the inner cusps with crests. (Modified from Romer, 1966.)

Figure 2. Family tree of the perissodactyls.