Editing Perissodactyla (section)

==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.<ref>{{Cite web |url=http://phenomena.nationalgeographic.com/2015/07/14/the-largest-beasts-to-walk-the-earth/ |archive-url=https://web.archive.org/web/20150715054827/http://phenomena.nationalgeographic.com/2015/07/14/the-largest-beasts-to-walk-the-earth/ |url-status=dead |archive-date=15 July 2015 |title=The Largest Beasts to Walk the Earth |date=14 July 2015 |website=Phenomena |publisher=National Geographic Society }}</ref> At the other extreme, an early member of the order, the prehistoric horse ''[[Eohippus]]'', had a withers height of only {{convert|30|to|60|cm|abbr=on}}.<ref>{{cite encyclopedia |title=Dawn horse |encyclopedia=Encyclopædia Britannica | access-date = 2016-09-07 |url=https://www.britannica.com/animal/dawn-horse}}</ref> Apart from dwarf varieties of the domestic horse and donkey, living perissodactyls reach a body length of {{convert|180|-|420|cm|abbr=on}} and a weight of {{convert|150|to|4,500|kg|abbr=on}}.<ref>{{Cite book |url=https://books.google.com/books?id=2GZm2R9CxocC&q=white+rhino+4500kg&pg=PA63 |title=Namibia |series=Other Places Travel Guide |isbn=9780982261965 |last1=Allen |first1=Jeremiah |date=March 2010}}</ref> 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 [[zebra]]s and young [[tapir]]s are striped.

===Limbs===
[[File:Anatomy of the horses foot.jpg|thumb|Seven figures showing the bones, blood vessels, ligaments and arteries of the hoof and pastern.]]
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.

[[Ungulate]]s have stances that require them to stand on the tips of their toes. [[Equinae|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 [[ungulate]]s 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]].<ref>Hall, Brian K. (2006). Fins into Limbs. University of Chicago Press. ISBN 978-0-226-31337-5.</ref>

The [[ulna]]e and [[fibula]]e are reduced in horses. A [[autapomorphy|common feature]] that clearly distinguishes this group from other mammals is the articulation between the [[talus bone|astragalus]], the [[navicular|scaphoid]] and the [[Cuboid bone|cuboid]], which greatly restricts the mobility of the foot.<ref>{{Cite journal |last=Hussain |first=S. |date=September 1975 |title=Evolutionary and Functional Anatomy of the Pelvic Limb in Fossil and Recent Equidae (Perissodactyla, Mammalia) |url=http://dx.doi.org/10.1111/j.1439-0264.1975.tb00637.x |journal=Anatomia, Histologia, Embryologia: Journal of Veterinary Medicine Series C |volume=4 |issue=3 |pages=193–222 |doi=10.1111/j.1439-0264.1975.tb00637.x |pmid=1199544 |s2cid=34568531 |issn=0340-2096|url-access=subscription }}</ref><ref>{{Cite journal |last=Lorente |first=Malena |date=2019-08-02 |title=What are the most accurate categories for mammal tarsus arrangement? A review with attention to South American Notoungulata and Litopterna |url=http://dx.doi.org/10.1111/joa.13065 |journal=Journal of Anatomy |volume=235 |issue=6 |pages=1024–1035 |doi=10.1111/joa.13065 |pmid=31373392 |pmc=6875937 |issn=0021-8782}}</ref> The thigh is relatively short, and the [[clavicle]] is absent.

===Skull and teeth===
[[File:Hamburg zoo tapir.JPG|thumb|Tapirs are the only extant group of perissodactyls with a trunk.]]

Odd-toed ungulates have a long [[maxilla|upper jaw]] with an extended [[diastema]] between the front and cheek [[Tooth|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 [[wiktionary:agglutinate|agglutinated]] [[keratin]], unlike the horns of [[even-toed ungulate]]s ([[Bovidae]] and [[pronghorn]]), which have a bony core.

The number and form of the teeth vary according to diet. The [[incisor]]s and [[canine tooth|canine]]s 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 [[Molar (tooth)|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 [[Dentition|dental formula]] of odd-toed ungulates is: 
{{DentalFormula |upper= 0-3 . 0-1 . 2-4 . 3|lower= 1-3 . 1 . 2-4 . 3|total= 30-44}}

The [[guttural pouch]], a small outpocketing of the [[Eustachian tube|auditory tube]] that drains the [[middle ear]], is a characteristic feature of Perissodactyla.<ref name="Singh-2018">{{cite book |last1=Singh |first1=Baljit |title=Dyce, Sack and Wensing's Textbook of Veterinary Anatomy |date=2018 |publisher=Elsevier |location=Missouri |isbn=978-0-323442640 |page=511 |edition=5th}}</ref> The guttural pouch is of particular concern in [[Equinae|equine]] [[Veterinarian|veterinary practice]], due to its frequent involvement in some serious infections. [[Aspergillosis]] (infection with ''Aspergillus'' mould) of the guttural pouch (also called [[Guttural pouch#Guttural Pouch Mycosis|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 nerve|glossopharyngeal]], [[Vagus nerve|vagus]], [[Accessory nerve|accessory]] and [[Hypoglossal nerve|hypoglossal nerves]]<ref name="Singh-2018" />) and the [[internal carotid artery]].<ref name="Singh-2018" /> [[Strangles]] (''Streptococcus equi equi'' infection) is a highly transmissible respiratory infection of horses<ref>{{cite web |last1=Rush |first1=Bonnie |title=Strangles (Distemper) in Horses - MSD Veterinary Manual |url=https://www.msdvetmanual.com/horse-owners/lung-and-airway-disorders-of-horses/strangles-distemper-in-horses |website=MSD Veterinary Manual |publisher=MSD |access-date=25 September 2023}}</ref><ref>{{cite web |last1=Lascola |first1=Kara |title=Strangles in Horses - Respiratory Diseases of Horses - MSD Veterinary Manual |url=https://www.msdvetmanual.com/respiratory-system/respiratory-diseases-of-horses/strangles-in-horses |website=MSD Veterinary Manual |publisher=MSD |access-date=25 September 2023}}</ref> that can cause pus to accumulate in the guttural pouch;<ref>{{cite book |last1=Singh |first1=Baljit |title=Dyce, Sack and Wensing's Textbook of Veterinary Anatomy |date=2018 |publisher=Elsevier |location=Missouri |isbn=978-0-323442640 |page=512 |edition=5th}}</ref> horses with ''S. equi equi'' colonising their guttural pouch can continue to intermittently shed the bacteria for several months,<ref>{{cite web |last1=Freeman |first1=Paul |title=Strangles |url=https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0011/158456/Strangles.pdf |website=Strangles |publisher=NSW Government - Department of Primary Industries |access-date=25 September 2023}}</ref> 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.<ref name="Singh-2018" />

===Gut===
All perissodactyls are hindgut fermenters. In contrast to [[ruminant]]s, hindgut fermenters store digested food that has left the [[stomach]] in an enlarged [[cecum]], where the food begins digestion by microbes, with the fermentation continuing in the [[Equine_anatomy#Large_colon|large colon]]. No [[gallbladder]] is present. The stomach of perissodactyls is simply built, while the cecum accommodates up to {{convert|90|L|gal|abbr=on}} in horses. The [[Gastrointestinal tract|small intestine]] is very long, reaching up to {{convert|26|m|abbr=on}} 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]],<ref name="Mitchell Lust 2008 pp. 415–418">{{cite journal |last1=Mitchell |first1=G |last2=Lust |first2=A |date=2008-08-23 |title=The carotid rete and artiodactyl success |journal=Biology Letters |volume=4 |issue=4 |pages=415–418 |doi=10.1098/rsbl.2008.0138 |issn=1744-9561 |pmc=2610139 |pmid=18426746}}</ref> 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.<ref name="Mitchell Lust 2008 pp. 415–418">{{cite journal |last1=Mitchell |first1=G |last2=Lust |first2=A |date=2008-08-23 |title=The carotid rete and artiodactyl success |journal=Biology Letters |volume=4 |issue=4 |pages=415–418 |doi=10.1098/rsbl.2008.0138 |issn=1744-9561 |pmc=2610139 |pmid=18426746}}</ref>