Perissodactyla (, from Ancient Greek περισσός, perissós 'odd' and δάκτυλος, dáktylos 'finger, toe'), or odd-toed ungulates, is an order of ungulates. The order includes about 17 living species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They typically have reduced the weight-bearing toes to three or one of the five original toes, though tapirs retain four toes on their front feet. The nonweight-bearing toes are either present, absent, vestigial, or positioned posteriorly. By contrast, artiodactyls (even-toed ungulates) bear most of their weight equally on four or two (an even number) of the five toes: their third and fourth toes. Another difference between the two is that perissodactyls digest plant cellulose in their intestines, rather than in one or more stomach chambers as artiodactyls, with the exception of Suina, do. The order was considerably more diverse in the past, with notable extinct groups including the brontotheres, palaeotheres, chalicotheres, and the paraceratheres, with the paraceratheres including the largest known land mammals to have ever existed. Despite their very different appearances, they were recognized as related families in the 19th century by the zoologist Richard Owen, who also coined the order's name.
Anatomy The largest odd-toed ungulates are rhinoceroses, and the extinct Paraceratherium, a hornless rhinocerotoid from the Oligocene, is considered one of the largest land mammals of all time. With P. transouralicum being estimated to be 15-20 tonnes at maximum size with estimates as low as 11 tonnes on average. Though an unknown species of the related Dzungariotherium may be slightly larger, being estimated at 20.6 tonnes in weight. At the other extreme, an early member of the order, the prehistoric horse Eohippus, had a withers height of only 30 to 60 cm (12 to 24 in). Apart from dwarf varieties of the domestic horse and donkey, living perissodactyls reach a body length of 180–420 cm (71–165 in) and a weight of 150 to 4,500 kg (330 to 9,920 lb). While rhinos have only sparse hair and exhibit a thick epidermis, tapirs and horses have dense, short coats. Most species are grey or brown, although zebras and young tapirs are striped.
Limbs
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 fore feet and three on their hind feet. 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. Ungulates have stances that require them to stand on the tips of their toes. Equine ungulates with only one digit or hoof have decreased mobility in their limbs, which allows for faster running speeds and agility. Differences in limb structure and physiology between ungulates and other mammals can be seen in the shape of the humerus. For example, often shorter, thicker, bones belong to the largest and heaviest ungulates like the rhinoceros. The ulnae and fibulae are reduced in horses. A common feature that clearly distinguishes this group from other mammals is the articulation between the astragalus, the scaphoid and the cuboid, which greatly restricts the mobility of the foot. The thigh is relatively short, and the clavicle is absent.
Skull and teeth
Odd-toed ungulates have a long upper jaw with an extended diastema between the front and cheek teeth, giving them an elongated head. The various forms of snout between families are due to differences in the form of the premaxilla. The lacrimal bone has projecting cusps in the eye sockets and a wide contact with the nasal bone. The temporomandibular joint is high and the mandible is enlarged. Rhinos have one or two horns made of agglutinated keratin, unlike the horns of even-toed ungulates (Bovidae and pronghorn), which have a bony core. The number and form of the teeth vary according to diet. The incisors and canines can be very small or completely absent, as in the two African species of rhinoceros. In horses, usually only the males possess canines. The surface shape and height of the molars is heavily dependent on whether soft leaves or hard grass make up the main component of their diets. Three or four cheek teeth are present on each jaw half, so the dental formula of odd-toed ungulates is: 0-3 . 0-1 . 2-4 . 31-3 . 1 . 2-4 . 3 × 2 = 30-44 The guttural pouch, a small outpocketing of the auditory tube that drains the middle ear, is a characteristic feature of Perissodactyla. The guttural pouch is of particular concern in equine veterinary practice, due to its frequent involvement in some serious infections. Aspergillosis (infection with Aspergillus mould) of the guttural pouch (also called guttural pouch mycosis) can cause serious damage to the tissues of the pouch, as well as surrounding structures including important cranial nerves (nerves IX-XII: glossopharyngeal, vagus, accessory and hypoglossal nerves) and the internal carotid artery. Strangles (Streptococcus equi equi infection) is a highly transmissible respiratory infection of horses that can cause pus to accumulate in the guttural pouch; horses with S. equi equi colonising their guttural pouch can continue to intermittently shed the bacteria for several months, and should be isolated from other horses during this time to prevent transmission. Due to the intermittent nature of S. equi equi shedding, prematurely reintroducing an infected horse may risk exposing other horses to the infection, even though the shedding horse appears well and may have previously returned negative samples. The function of the guttural pouch has been difficult to determine, but it is now believed to play a role in cooling blood in the internal carotid artery before it enters the brain.
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