This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Perissodactyla" – news · newspapers · books · scholar · JSTOR (October 2021) (Learn how and when to remove this message)

Perissodactyla Temporal range: 56–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Latest Paleocene–present[1]
Clockwise from left: plains zebra (Equus quagga), Indian rhinoceros (Rhinoceros unicornis) and Brazilian tapir (Tapirus terrestris)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Clade:	Perissodactylamorpha
Order:	Perissodactyla Owen, 1848
Families
†Hyopsodontidae[2] †Brontotheriidae (brontotheres) Hippomorpha Equoidea Equidae (horses, asses, and zebras) †Palaeotheriidae Tapiromorpha †Isectolophidae †Ancylopoda †Chalicotherioidea †Chalicotheriidae (chalicotheres) †Eomoropidae †Lophiodontidae Ceratomorpha Tapiroidea †Deperetellidae †Helaletidae Tapiridae (tapirs) Rhinocerotoidea (rhinoceroses sensu lato) †Amynodontidae †Hyracodontidae †Paraceratheriidae †Eggysodontidae Rhinocerotidae (rhinoceroses sensu stricto)

Perissodactyla (/pəˌrɪsoʊˈdæktɪlə/, from Ancient Greek περισσός, perissós 'odd', and δάκτυλος, dáktylos 'finger, toe'^[3]) 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 include 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 rhino from the Oligocene, is considered one of the largest land mammals of all time.^[4] 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).^[5] 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).^[6] 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.^[7]

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.^[8][9] 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, 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.^[10] 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^[10]) and the internal carotid artery.^[10] Strangles (Streptococcus equi equi infection) is a highly transmissible respiratory infection of horses^[11][12] that can cause pus to accumulate in the Guttural Pouch;^[13] horses with S. equi equi colonising their Guttural Pouch can continue to intermittently shed the bacteria for several months,^[14] 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.^[10]

Gut

All perissodactyls are hindgut fermenters. In contrast to ruminants, hindgut fermenters store digested food that has left the stomach in an enlarged cecum, where the food is digested by bacteria. No gallbladder is present. The stomach of perissodactyls is simply built, while the cecum accommodates up to 90 L (24 US gal) in horses. The intestine is very long, reaching up to 26 m (85 ft) in horses. Extraction of nutrients from food is relatively inefficient, which probably explains why no odd-toed ungulates are small; nutritional requirements per unit of body weight are lower for large animals, as their surface-area-to-volume ratio is smaller.

Lack of carotid rete

Unlike artiodactyls, perissodactyls lack a carotid rete,^[15] a heat exchange that reduces the dependence of the temperature of the brain on that of the body. As a result, perissodactyls have limited thermoregulatory flexibility compared to artiodactyls which has restricted them to habitats of low seasonality and rich in food and water, such as tropical forests. In contrast, artiodactyls occupy a wide range of habits ranging from the Arctic Circle to deserts and tropical savannahs.^[15]

Distribution

Most extant perissodactyl species occupy a small fraction of their original range. Members of this group are now found only in Central and South America, eastern and southern Africa, and central, southern, and southeastern Asia.^[16] During the peak of odd-toed ungulate existence, from the Eocene to the Oligocene, perissodactyls were distributed over much of the globe, the major exceptions being Australia and Antarctica. Horses and tapirs arrived in South America after the formation of the Isthmus of Panama around 3 million years ago in the Pliocene. Their North American counterparts died out around 10,000 years ago, leaving only Baird's tapir with a range extending to what is now southern Mexico. The tarpans were pushed to extinction in 19th century Europe. Hunting and habitat destruction have reduced the surviving perissodactyl species to fragmented populations. In contrast, domesticated horses and donkeys have gained a worldwide distribution, and feral animals of both species are now also found in regions outside their original range, such as in Australia.

Lifestyle and diet

Perissodactyls inhabit a number of different habitats, leading to different lifestyles. Tapirs are solitary and inhabit mainly tropical rainforests. Rhinos tend to live alone in rather dry savannas, and in Asia, wet marsh or forest areas. Horses inhabit open areas such as grasslands, steppes, or semi-deserts, and live together in groups. Odd-toed ungulates are exclusively herbivores that feed, to varying degrees, on grass, leaves, and other plant parts. A distinction is often made between primarily grass feeders (white rhinos, equines) and leaf feeders (tapirs, other rhinos).

Reproduction and development

Odd-toed ungulates are characterized by a long gestation period and a small litter size, usually delivering a single young. The gestation period is 330–500 days, being longest in rhinos. Newborn perissodactyls are precocial, meaning offspring are born already quite independent: for example,^[17] young horses can begin to follow the mother after a few hours.^[18] The young are nursed for a relatively long time, often into their second year, reaching sexual maturity around eight or ten years old. Perissodactyls are long-lived, with several species, such as rhinos, reaching an age of almost 50 years in captivity.^[19]

Taxonomy

Outer taxonomy

Traditionally, the odd-toed ungulates were classified with other mammals such as artiodactyls, hyraxes, elephants and other "ungulates". A close family relationship with hyraxes was suspected based on similarities in the construction of the ear and the course of the carotid artery.

Molecular genetic studies, however, have shown the ungulates to be polyphyletic, meaning that in some cases the similarities are the result of convergent evolution rather than common ancestry. Elephants and hyraxes are now considered to belong to Afrotheria, so are not closely related to the perissodactyls. These in turn are in the Laurasiatheria, a superorder that had its origin in the former supercontinent Laurasia. Molecular genetic findings suggest that the cloven Artiodactyla (containing the cetaceans as a deeply nested subclade) are the sister taxon of the Perissodactyla; together, the two groups form the Euungulata.^[20] More distant are the bats (Chiroptera) and Ferae (a common taxon of carnivorans, Carnivora, and pangolins, Pholidota).^[21] In a discredited alternative scenario, a close relationship exists between perissodactyls, carnivores, and bats, this assembly comprising the Pegasoferae.^[22]

According to studies published in March 2015, odd-toed ungulates are in a close family relationship with at least some of the so-called Meridiungulata, a very diverse group of mammals living from the Paleocene to the Pleistocene in South America, whose systematic unity is largely unexplained. Some of these were classified based on their paleogeographic distribution. However, a close relationship can be worked out to perissodactyls by protein sequencing and comparison with fossil collagen from remnants of phylogenetically young members of the Meridiungulata (specifically Macrauchenia from the Litopterna and Toxodon from the Notoungulata). Both kinship groups, the odd-toed ungulates and the Litopterna-Notoungulata, are now in the higher-level taxon of Panperissodactyla. This kinship group is included among the Euungulata, which also contains the even-toed ungulates and whales (Artiodactyla). The separation of the Litopterna-Notoungulata group from the perissodactyls probably took place before the Cretaceous–Paleogene extinction event. "Condylarths" can probably be considered the starting point for the development of the two groups, as they represent a heterogeneous group of primitive ungulates that mainly inhabited the northern hemisphere in the Paleogene.^[23][24]

Modern members

Odd-toed ungulates (Perissodactyla) comprise three living families with around 17 species—in horses, however, the exact count is still controversial. Rhinos and tapirs are more closely related to each other than to horses. The separation of horses from other perissodactyls took place according to molecular genetic analysis in the Paleocene some 56 million years ago, while the rhinos and tapirs split off in the lower-middle Eocene, about 47 million years ago.