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Perissodactyla > General of Perissodactyla

Perissodactyla are exclusively large terrestrial herbivores. Also commonly known as perrisodactyls, or odd-toed ungulates, this order is extremely diverse; from the robust, horned white rhinoceros (Ceratotherium simum) to the sleek, slender, and striped mountain zebra (Equus zebra).

Grevy’s zebra

Traditionally, there are three families within the order Perissodactyla: the Tapiridae (tapirs), the Rhinocerotidae (rhinoceroses), and Equidae (asses, horses, and zebras). These three families include six genera and 16 species. 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 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-erect to feed on tall trees. As the perissodactyls declined, there also seems to have been some definite ecological replacement of them by artiodactyls.

Originating in the early Eocene epoch of North America, tapirs migrated into Asia and Central and South America. Tapirs were extirpated throughout most of North America by the late Pleistocene epoch. A combination of migration and extirpation resulted in a discontinuous distribution today. 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-like ancestors, Hyracotherium, appeared in the Eocene, about 54 mya. It was a small dog-size mammal that browsed on low shrubs of forest floor. When grasses extended in the Miocene, equids began to radiate. Overall body size increased, which reduced relative nutritional demands. By the early Pleistocene (2 mya), the one-toed equids had spawned the genus Equus, which rapidly spread. As environment changed, populations became isolated, giving rise to the living species. The first to split off from the equid stream was the Grevy’s zebra (Equus grevyi), which, despite its stripes, is only distantly related to the other two zebra species. However, the ancestor of all equines was probably striped. The horses became extinct only about 10,000 years ago in the New World, but horses were reintroduced by the Spanish conquistador Hernando Cortes in 1519. The number of extant equid species is open to debate: seven to 10 species, all in the genus Equus, are recognized. Many subspecies and regional forms (mainly in zebras) are known.

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