PERISSODACTYLA, an order of hoofed, herbivorous mammals, including the tapirs, horses, rhinoceroses and their ex tinct relatives, in all of which the number of digits (toes) on each hindfoot is either three or one. The name Perissodactyla ("odd toes") was given by Sir Richard Owen, to distinguish the group from the Artiodactyla, or even-toed, ungulate (hoofed) mammals, in which the number of functional digits on each foot is usually either four or two.
The earliest known perissodactyls (Lower Eocene) had four toes on each forefoot and three on each hindfoot. These numbers are still retained in the feet of the existing tapirs, but in the ances tral rhinoceroses the outer digit of the forefoot (corresponding to the fifth of the human hand) gradually became reduced and finally disappeared, leaving three toes on each forefoot and three on each hindfoot, as in existing rhinoceroses. In the horse family the earliest known members started with four functional digits in each hand and three in each foot, the only traces of the first and fifth toes of the ancestral mammals being the inner and outer wrist bones (carpals) that formerly supported these digits. In the course of time the whole foot of the early horses grew very long, the side toes were lifted off the ground, became much reduced in size and were finally eliminated, the chief traces of the second and fourth toes of the original five-toed foot being the "splint" bones, or vestigial metacarpals, on the side of the greatly enlarged middle toe. This symmetrical middle toe of the horse therefore represents the logical extreme of the initial perissodactyl tendency for the axis of symmetry to pass through the middle or third digit rather than between the third and fourth digits as in the artiodactyls.
Both the perissodactyls and the artiodactyls were originally small, swift-footed, herbivorous mammals, in which the wrist and heel were raised far above the ground and the animals ran on the enlarged nails, or hooves, of their principal digits. The numerous resemblances between perissodactyls and artiodactyls are chiefly independently acquired adaptations to similar habits. Even in Lower Eocene times (perhaps 6o million years ago) representa tives of the two orders differed so greatly in skeleton and dentition that they must have been derived from widely different families of primitive placental mammals of the Cretaceous epoch. The old
concept of the Perissodactyla and the Artiodactyla as being sub orders of a single group, the Ungulata Vera or Diplarthra, is erro neous and the two "sub-orders" are in fact wholly distinct.
As noted above, the oldest known perissodactyls are found in the Lower Eocene of North America and Europe. It was formerly believed that these were in turn derived from Phenacodus (q.v.), but more recent palaeontological research shows that Phena codas was a specialized side-branch of its own order, the Condy larthra, and is definitely excluded from the ancestry of the Perissodactyla by numerous details of specialization. The direct ancestors of the Eocene Perissodactyla remain unknown. How ever, the comparative morphology of the dentition and of the skeleton indicates that the Perissodactyla, like other orders of placental mammals, were derived eventually from small insectiv orous placental mammals of the Cretaceous period. The origin and evolution of the numerous families of perissodactyls from the Lower Eocene through many ascending horizons of the Tertiary, has been intensively studied in Europe and North America, and their palaeontological history is more fully known than that of any other mammalian order.
The early Eocene perissodactyls were small animals with four toes on each forefoot and three on each hindfoot. The upper molar teeth bore four principal cusps with two oblique cross crests running from the two inner cusps forward and outward, toward the outer side. The two main outer cusps were conical, the second set further in toward the middle than the first, so that the outer half of the crown was oblique. Each lower molar crown was more or less W-shaped in top view. The premolars were not yet molariform, although the last upper premolar was already well on the road toward the molar pattern. The front teeth (in cisors) were all present, small and not peculiarly specialized.