Despite excellent fossil records, the phylogeny of Perissodactyla is not well understood in terms of both the relationship within the order and the position among other orders of mammal. The perissodactyls as well as artiodactyls originated from the Condylarthra, the dominant mammalian herbivores of the early Paleocene (about 65 million years ago [mya]).
Condylarths are considered to be ancestors of many of the other lineages of large mammals. Despite the superficial similarities between horses and cows, rhinos and hippos, tapirs and pigs, the former of each pair belongs to the Perissodactyla, and the later to the Artiodactyla. The similarities between them have largely come about due to convergent evolution. However, mitochondrial genomes studies suggest that the order Perissodactyla is part of one eutherian clade, comprising also Pholidota, Carnivora, and Cetertiodactyla (Artiodactyla and Cetacea). The oldest identifiable perissodactyl fossils are from the early Eocene (about 50 mya). By this time, 14 radiated families were evident. During this epoch, perissodactyls were dominant ungulates, far outnumbering the artiodactyls.
By the end of Oligocene (25 mya), eight families were extinct. By the early Miocene epoch, only the tapirids, rhinocerotids, equids, and Chalicotheriidae remained. This last family included unusual ungulates with large forelimbs and short hind limbs adapted for standing semi-
The current genus Tapirus dates from 20 mya in the Miocene epoch. There are four extant species in the single genus Tapirus. Tapirs belong to among the most primitive large mammals in the world. Fossil evidence of rhinocerotids dates from the late Eocene in Asia and North America. Most of today’s genera date from the Miocene (10–25 mya). They were extinct in North America by the end of the Pliocene (2 mya). Rhinocerotids were abundant and widespread in the Old World until the late Pleistocene epoch (about 60,000 years ago). The largest land mammal that ever lived was a rhinocerotid, Indricotherium transouralicum (Baluchitherium grangeri), which was at least 16.5 ft (5 m) high at the shoulder and to 44,000 lb (20,000 kg) of body mass. Mitochondrial analysis identified a basal divergence between the African and the Asian species about 26 mya. There are four extant genera (Diceros, Rhinoceros, Dicerorhinus, and Ceratotherium), with five living species.
The fossil history of equids is one of the best documented for any mammalian family. This history shows increasing body size or skull proportion and reduction of the number of digits. However, the evolution of equids was not a directed progressive process, but a complex radiation of numerous divergent and overlapping lineages. Equids passed most of their evolution in North America, with migration to Eurasia and Africa during the Miocene and to Central and South America in the Pliocene and Pleistocene epochs. The earliest of the horse-