Ceratomorpha
| Odd-toed ungulates Temporal range: 56–0 Ma Paleocene-Holocene |
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| Clockwise from left: plains zebra (Equus quagga), Indian rhinoceros (Rhinoceros unicornis) and South American tapir (Tapirus terrestris) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| (unranked): | Zooamata |
| Order: |
Perissodactyla Owen, 1848 |
Members of the order Perissodactyla, otherwise known as odd-toed ungulates, are mammals characterized by an odd number of toes and by hindgut fermentation with somewhat simple stomachs. Unlike the even-toed ungulates, they digest plant cellulose in their intestines rather than in one or more stomach chambers. The order includes three extant families: Equidae (horses, donkeys, and zebras), Rhinocerotidae (rhinos) and Tapiridae (tapirs), with a total of about 17 species. In spite of their very different appearance, they were recognized as related families in the 19th century by the zoologist Richard Owen, who also coined the order name.
The largest odd-toed ungulates are rhinoceroses, and the extinct Paraceratherium, a hornless rhino from the Oligocene, is considered one of the largest land mammals of all time. At the other extreme, an early member of the order, the prehistoric horse Hyracotherium, had a shoulder height of only 30 to 60 cm (12 to 24 in). Apart from dwarf varieties of the domestic horse and donkey, perissodactyls reach a body length of 180–420 cm (71–165 in) and a weight of 150 to 3,500 kg (330 to 7,720 lb). While rhinos have only sparse hair and exhibit a thick epidermis, tapirs and horses have a dense, short coat. Most species are grey or brown, although zebras and young tapirs are striped.
The main axes of both the front and rear feet pass through the third toe, which is always the largest. The remaining toes have been reduced in size to varying degrees. Tapirs, which are adapted to walking on soft ground, have four toes on their forefeet and three on their hindfeet. Living rhinos have three toes on both the front and hind feet. Modern equines possess only a single toe; however, their feet are equipped with hooves, which almost completely cover the toe. Rhinos and tapirs, by contrast, have hooves covering only the leading edge of the toes, with the bottom being soft.
The ulna and fibula are reduced in horses. A common feature that clearly distinguishes this group from other mammals is the saddle-shaped ankle between the astragalus and the scaphoid, which greatly restricts the mobility of the foot. The thigh is relatively short, and the clavicle is absent.
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