View source for Perissodactyla

{{Short description|Order of hoofed mammals}}
{{more citations needed|date=October 2021}}
{{Use dmy dates|date=October 2020}}
{{Automatic taxobox
| name = Perissodactyla
| fossil_range = {{Fossil range|56|0|earliest=59.2}}[[Paleocene|Latest Paleocene]]–[[Holocene|present]]<ref>{{Cite web|title=PBDB|url=https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=42980&is_real_user=1|access-date=2021-07-18|website=paleobiodb.org}}</ref>
| image = The Perissodactyl.jpg
| image_caption = Clockwise from left: [[plains zebra]] (''Equus quagga''), [[Indian rhinoceros]] (''Rhinoceros unicornis'') and [[South American tapir|Brazilian tapir]] (''Tapirus terrestris'')
| taxon = Perissodactyla
| authority = [[Richard Owen|Owen]], 1848
| subdivision_ranks = Families
| subdivision = *{{extinct}}[[Hyopsodontidae]]<ref>{{cite journal | last1 = Ravel | first1 = Anthony | last2 = Orliac | first2 = Maeva | year = 2014 | title = The inner ear morphology of the 'condylarthran' Hyopsodus lepidus | journal = Historical Biology | volume = 27 | issue = 8| page = 8 | doi = 10.1080/08912963.2014.915823 | s2cid = 84391276 }}</ref>
*{{extinct}}[[Brontotheriidae]] (brontotheres)
*'''Hippomorpha'''
**[[Equoidea]]
***[[Equidae]] (horses, asses, and zebras)
***{{extinct}}[[Palaeotheriidae]]
*'''Tapiromorpha'''
**{{extinct}}[[Isectolophidae]]
**{{extinct}}'''[[Ancylopoda]]'''
***{{extinct}}[[Chalicotherioidea]]
****{{extinct}}[[Chalicotheriidae]] (chalicotheres)
****{{extinct}}[[Eomoropidae]]
***{{extinct}}[[Lophiodontidae]]
**'''Ceratomorpha'''
***[[Tapiroidea]]
****{{extinct}}[[Deperetellidae]]
****{{extinct}}[[Helaletidae]]
****[[Tapiridae]] (tapirs)
***[[Rhinocerotoidea]] (rhinoceroses ''sensu lato'')
****{{extinct}}[[Amynodontidae]]
****{{extinct}}[[Hyracodontidae]]
****{{extinct}}[[Paraceratheriidae]]
****{{extinct}}[[Eggysodontidae]]
****[[Rhinocerotidae]] (rhinoceroses ''sensu stricto'')
}}
[[File:Rhinoceros male 2003.jpg|thumb|The [[white rhinoceros]] is the largest living perissodactyl]]

'''Perissodactyla''' ({{IPAc-en|p|ə|ˌ|r|ɪ|s|oʊ|ˈ|d|æ|k|t|ᵻ|l|ə}}, {{etymology|grc|''περισσός'', perissós|odd||''δάκτυλος'', dáktylos|finger, toe}}<ref>{{cite web|url=https://research.amnh.org/paleontology/perissodactyl/concepts/glossary|archive-url=https://web.archive.org/web/20211120123130/https://research.amnh.org/paleontology/perissodactyl/concepts/glossary|archive-date=20 November 2021|title=Glossary. American Museum of Natural History}}</ref>), or '''odd-toed ungulates''', is an order of [[ungulate]]s. The order includes about 17 living [[species]] divided into three [[Family (taxonomy)|families]]: [[Equidae]] ([[wild horse|horse]]s, [[Asinus|asses]], and [[zebra]]s), Rhinocerotidae ([[rhinoceros]]es), and Tapiridae ([[tapir]]s). 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, [[Vestigiality|vestigial]], or positioned posteriorly. By contrast, [[artiodactyl]]s (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 [[Brontotheriidae|brontotheres]], [[Palaeotheriidae|palaeotheres]], [[Chalicotheriidae|chalicotheres]], and the [[paracerathere]]s, 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 [[Rhinocerotoidea|rhinocerotoid]] 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/ |archive-date=15 July 2015 |title=The Largest Beasts to Walk the Earth |date=14 July 2015 |website=Phenomena |publisher=National Geographic Society }}</ref> With ''P. transouralicum''  being estimated to be 15-20 tonnes at maximum size with estimates as low as 11 tonnes on average.<ref name="Prothero 2013 87 106">Prothero, 2013. pp.&nbsp;87–106</ref> Though an unknown species of the related ''[[Dzungariotherium]]''  may be slightly larger, being estimated at 20.6 tonnes in weight.<ref>{{Cite journal | last1 = Deng | first1 = T. | year = 2022 | title = Body mass of the giant rhinos (Paraceratheriinae, Mammalia) and its tendency in evolution | journal = Historical Biology | pages = 1–12 | doi = 10.1080/08912963.2022.2095908  | s2cid = 250366746 | url = http://www.rhinoresourcecenter.com/pdf_files/165/1657655752.pdf }}</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=978-0-9822619-6-5 |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 of the horse.]]
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) |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}}</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 |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]]s<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 habitats ranging from the Arctic Circle to deserts and tropical savannahs.<ref name="Mitchell Lust 2008 pp. 415–418"/>

==Distribution==
[[File:Indian Rhino 001.jpg|thumb|Restriction of their habitat and poaching threaten the survival of most rhino species, including the [[Indian rhinoceros]] shown here]]
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.<ref>{{Cite web |title=Perissodactyl {{!}} Definition, Characteristics, Species, Examples, Habitat, & Facts {{!}} Britannica |url=https://www.britannica.com/animal/perissodactyl |access-date=2023-05-16 |website=www.britannica.com |language=en}}</ref> 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 [[Great American Interchange|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 [[tarpan]]s 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 [[Grazing (behaviour)|grass feeders]] ([[White rhinoceros|white rhinos]], [[Equus (genus)|equines]]) and [[Browsing (herbivory)|leaf feeders]] (tapirs, other rhinos).

==Reproduction and development==
[[File:Tapirbaby.jpg|thumb|A young [[South American tapir|Brazilian tapir]]]]
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 [[Precociality|precocial]], meaning offspring are born already quite independent: for example,<ref>{{Cite book|title=Zookeeping: An Introduction to the Science and Technology|publisher=University of Chicago Press|year=2013|location=Chicago|page=273}}</ref> young horses can begin to follow the mother after a few hours.<ref>{{Cite web|url=https://equusmagazine.com/management/mare_foal_behavior_121508-8331|title=Understand Normal Mare and Foal Behavior|last=Williams|first=Jennifer|date=15 December 2008|website=equusmagazine.com|archive-url=https://web.archive.org/web/20170513183546/https://equusmagazine.com/management/mare_foal_behavior_121508-8331|archive-date=13 May 2017|access-date=4 February 2019}}</ref> The young are nursed for a relatively long time, often into their second year, with rhinos reaching sexual maturity around eight or ten years old, but horses and tapirs maturing around two to four years old.<ref>{{cite book |last1=Angus |first1=McKinnon |last2=Noah |first2=Heninger |title=Equine Reproduction |date=2011 |publisher=Wiley-Blackwell |isbn=978-0-8138-1971-6 |page=1015 |edition=2nd}}</ref><ref>{{cite web |title=Tapirs (extant/living species; Tapirus spp.) Fact Sheet: Reproduction & Development |url=https://ielc.libguides.com/sdzg/factsheets/tapirs/reproduction |website=International Environment Library Consortium |publisher=San Diego Zoo Wildlife Alliance Library |access-date=12 June 2024}}</ref> Perissodactyls are long-lived, with several species, such as rhinos, reaching an age of almost 50 years in captivity.<ref>Ballenger, Liz; Myers, Phil. "Rhinocerotidae (rhinoceroses)". Animal Diversity Web. Retrieved 2021-03-27."Ballenger, Liz; Myers, Phil. "Rhinocerotidae (rhinoceroses)". Animal Diversity Web. Retrieved 2021-03-27.</ref>

==Taxonomy==
{{See also|List of perissodactyls}}

===Outer taxonomy===
Traditionally, the odd-toed ungulates were classified with other mammals such as [[Even-toed ungulate|artiodactyls]], [[hyrax]]es, [[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 [[Polyphyly|polyphyletic]], meaning that in some cases the similarities are the result of [[convergent evolution]] rather than [[common ancestor|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 [[cetacea]]ns as a deeply nested subclade) are the sister taxon of the Perissodactyla; together, the two groups form the [[Ungulate|Euungulata]].<ref name="Graur et al. 1997">{{cite journal|author1=Dan Graur|author2=Manolo Gouy|author3=Laurent Duret|title=Evolutionary Affinities of the Order Perissodactyla and the Phylogenetic Status of the Superordinal taxa Ungulata and Altungulata Molecular Phylogenetics and Evolution|year= 1997|pages= 195–200|doi= 10.1006/mpev.1996.0391|volume= 7|issue=2|journal=Molecular Phylogenetics and Evolution|pmid=9126561}}</ref> More distant are the [[bat]]s (Chiroptera) and [[Ferae]] (a common taxon of carnivorans, [[Carnivora]], and pangolins, [[Pholidota]]).<ref name="Hu et al. 2012">{{cite journal|author1=Jingyang Hu|author2=Zhang Yaping|author3=Li Yu|title=Summary of Laurasiatheria (Mammalia) Phylogeny|journal=Zoological Research|year=2012|pages=65–74|doi=10.3724/sp.j.1141.2012.e05-06e65|pmid=23266984|volume=33|issue=6|s2cid=30067369 |doi-access=free|hdl=1807/64815|hdl-access=free}}</ref> In a discredited alternative scenario, a close relationship exists between perissodactyls, carnivorans, and bats, this assembly comprising the [[Pegasoferae]].<ref name="Nishihara et al. 2006">{{cite journal|author1=Hidenori Nishihara|author2=Masami Hasegawa|author3=Norihiro Okada|title=Pegasoferae, an unexpected mammalian clade revealed by tracking ancient retroposon insertions|journal=Proceedings of the National Academy of Sciences USA |volume=103 |issue=26|pages=9929–9934 |doi=10.1073/pnas.0603797103 |pmid=16785431|pmc=1479866|year=2006|bibcode=2006PNAS..103.9929N|doi-access=free}}</ref>

{| class="wikitable" style="float:right;"
|-
|+ Internal taxonomy of the Euungulata after Welker ''et al.'' 2015<ref name="Welker et al. 2015"/>
|-
|{{Clade|style=font-size:75%; line-height:100%
|label1=[[Euungulata]]
|1={{Clade
     |1=[[Artiodactyla]] (Even-toed ungulates and cetaceans)
     |label2=[[Panperissodactyla]]
     |2={{Clade
          |1='''Perissodactyla'''(Odd-toed ungulates)
          |2=[[extinct|†]] [[Meridiungulata]] (South American ungulates,<br />especially [[Notoungulata]] and [[Litopterna]])
          }}
     }}
}}
|}

According to studies published in March 2015, odd-toed ungulates are in a close family relationship with at least some of the so-called [[South American native ungulates|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 (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]].<ref name="Welker et al. 2015">{{cite journal|author1=Frido Welker|author2=Matthew J. Collins|author3=Jessica A. Thomas|author4=Marc Wadsley|author5=Selina Brace|author6=Enrico Cappellini|author7=Samuel T. Turvey|author8=Marcelo Reguero|author9=Javier N. Gelfo|author10=Alejandro Kramarz|author11=Joachim Burger|author12=Thomas Jane Oates|author13=David A. Ashford|author14=Peter D. Ashton|author15=Keri Rowsell|author16=Duncan M. Porter|author17=Benedikt Kessler|author18=Roman Fischer|author19=Carsten Baessmann|author20=Stephanie Kaspar|author21=Jesper V. Olsen|author22=Patrick Kiley|author23=James A. Elliott|author24=Christian D. Kelstrup|author25=Victoria Mullin|author26=Michael Hofreiter|author27=Eske Willerslev|author28=Jean-Jacques Hublin|author29=Ludovic Orlando|author30=Ian Barnes|author31=Ross DE MacPhee|title=Ancient protein resolve the evolutionary history of Darwin's South American ungulates|journal=Nature|doi=10.1038/nature14249|volume=522|issue=7554|pages=81–84|pmid=25799987|year=2015|bibcode=2015Natur.522...81W|s2cid=4467386|url=http://eprints.whiterose.ac.uk/91438/1/Welker_postprint.docx|hdl=11336/14769|hdl-access=free}}</ref><ref>{{cite journal|author1=Ross MacPhee|author2=Frido Welker|author3=Jessica Thomas|author4=Selina Brace|author5=Enrico Cappellini|author6=Samuel Turvey|author7=Ian Barnes|author8=Marcelo Reguero|author9=Javier Gelfo|author10=Alejandro Kramarz|title=Ancient protein sequencing Resolves litoptern and notoungulate superordinal affinities|journal=The History of Life: A View from the Southern Hemisphere|year=2014|page=186}}</ref>

===Modern members===
Odd-toed ungulates 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. According to molecular genetic analysis, the separation of horses from other perissodactyls took place 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. 
{| class="wikitable" style="float:right;"
|-
|+ Internal relationships of extant Perissodactyla<ref>{{cite journal|author1= Christelle Tougard|author2=Thomas Delefosse|author3=Catherine Hänni|author4=Claudine Montgelard|title=Phylogenetic Relationships of the Five Extant Rhinoceros species (Rhinocerotidae, Perissodactyla) Based on Mitochondrial Cytochrome b and 12S rRNA gene|journal=Molecular Phylogenetics and Evolution|year=2001|pages=34–44|doi=10.1006/mpev.2000.0903|volume=19|issue=1|pmid=11286489|bibcode=2001MolPE..19...34T |url=http://doc.rero.ch/record/16553/files/PAL_E3210.pdf }}</ref><ref>{{cite journal|author1=Cynthia C. Steiner|author2=Oliver A. Ryder|title=Molecular phylogeny and evolution of the Perissodactyla|journal=Zoological Journal of the Linnean Society|year=2011|pages=1289–1303|doi=10.1111/j.1096-3642.2011.00752.x|volume=163|issue=4|doi-access=free}}</ref><ref>{{cite journal|author1=Mario A. Cozzuol|author2=Camila L. Clozato|author3=Elizete C. Holanda|author4=Flávio HG Rodrigues|author5=Samuel Nienow|author6=Benoit de Thoisy|author7=Rodrigo Redondo|author8=AF Fabrício R. Santos|title= A new species of tapir from the Amazon|journal=Journal of Mammalogy|volume=94|issue=6|page=1331 to 1345|doi=10.1644/12-MAMM-A-169.1
|year=2013|doi-access=free |bibcode=2013JMamm..94.1331C }}</ref>
|-
|{{Clade|style=font-size:75%; line-height:100%
|label1='''Perissodactyla'''
|1={{Clade
     |1={{Clade
             |label1=[[Equidae]]
             |1={{Clade
                    |1=''[[Equus ferus]]''
                    |2={{Clade
                          |1=''[[Equus asinus]]''
                          |2={{Clade
                                |1={{Clade
                                      |1=''[[Equus hemionus]]''
                                      |2=''[[Equus kiang]]''
                                      }}
                                |2={{Clade
                                      |1=''[[Equus zebra]]''
                                      |2={{Clade
                                            |1=''[[Equus grevyi]]''
                                            |2=''[[Equus quagga]]''
                                            }}
                                      }}
                                }}
                          }}
                    }}
             |2={{Clade
                    |label1=[[Tapiridae]]
                    |1={{Clade
                          |1=''[[Tapirus indicus]]''
                          |2={{Clade
                                |1=''[[Tapirus bairdii]]''
                                |2={{Clade
                                      |1=''[[Tapirus kabomani]]''
                                      |2={{Clade
                                            |1=''[[Tapirus pinchaque]]''
                                            |2=''[[Tapirus terrestris]]''
                                            }}
                                      }}
                                }}
                          }}
                    |label2=[[Rhinocerotidae]]
                    |2={{Clade
                          |1={{Clade
                                |1=''[[Ceratotherium simum]]''
                                |2=''[[Diceros bicornis]]''
                                }}
                          |2={{Clade
                                |1=''[[Dicerorhinus sumatrensis]]''
                                |2={{Clade
                                      |1=''[[Rhinoceros sondaicus]]''
                                      |2=''[[Rhinoceros unicornis]]''
                                      }}

                                }}
                          }}
                    }}
              }}
      }}
}}
|}

* '''Order Perissodactyla'''
**'''Suborder Hippomorpha'''
***'''Family [[Equidae]]''': horses and allies, seven species in one genus
**** [[Wild horse]], ''Equus ferus''
*****[[Tarpan]], [[Extinction|†]]''Equus ferus ferus''
***** [[Przewalski's horse]], ''Equus ferus przewalskii''
***** Domestic [[horse]], ''Equus ferus caballus''
**** [[African wild ass]], ''Equus africanus''
*****[[Nubian wild ass]], ''Equus africanus africanus''
***** [[Somali wild ass]], ''Equus africanus somaliensis''
***** Domesticated ass ([[donkey]]), ''Equus africanus asinus''
***** [[Atlas wild ass]], [[Extinction|†]]''Equus africanus atlanticus''
**** [[Onager]] or Asiatic wild ass, ''Equus hemionus''
***** [[Mongolian wild ass]], ''Equus hemionus hemionus''
***** [[Turkmenian kulan]], ''Equus hemionus kulan''
***** [[Persian onager]], ''Equus hemionus onager''
***** [[Indian wild ass]], ''Equus hemionus khur''
***** [[Syrian wild ass]], [[Extinction|†]]''Equus hemionus hemippus''
**** [[Kiang]] or Tibetan wild ass, ''Equus kiang''
***** Western kiang, ''Equus kiang kiang''
***** Eastern kiang, ''Equus kiang holdereri''
***** Southern kiang, ''Equus kiang polyodon''
**** [[Plains zebra]], ''Equus quagga''
***** [[Quagga]], [[Extinction|†]]''Equus quagga quagga''
***** [[Burchell's zebra]], ''Equus quagga burchellii''
***** [[Grant's zebra]], ''Equus quagga boehmi''
***** [[Maneless zebra]], ''Equus quagga borensis''
***** [[Chapman's zebra]], ''Equus quagga chapmani''
***** [[Crawshay's zebra]], ''Equus quagga crawshayi''
***** [[Selous' zebra]], ''Equus quagga selousi''
**** [[Mountain zebra]], ''Equus zebra''
*****[[Cape mountain zebra]], ''Equus zebra zebra''
*****[[Hartmann's mountain zebra]], ''Equus zebra hartmannae''
****[[Grévy's zebra]], ''Equus grevyi''
** '''Suborder Ceratomorpha'''
***'''Family [[Tapir]]idae''': tapirs, five species in one genus
****[[South American tapir|Brazilian tapir]], ''Tapirus terrestris''
**** [[Mountain tapir]], ''Tapirus pinchaque''
**** [[Baird's tapir]], ''Tapirus bairdii''
**** [[Malayan tapir]], ''Tapirus indicus''
****[[Tapirus kabomani|Kabomani tapir]], ''Tapirus kabomani''
*** '''Family [[Rhinocerotidae]]''': rhinoceroses, five species in four genera
****[[Black rhinoceros]], ''Diceros bicornis''
***** [[Southern black rhinoceros]], [[Extinction|†]]''Diceros bicornis bicornis''
***** [[North-eastern black rhinoceros]], [[Extinction|†]]''Diceros bicornis brucii''
***** [[Chobe black rhinoceros]], ''Diceros bicornis chobiensis''
***** [[Uganda black rhinoceros]], ''Diceros bicornis ladoensis''
***** [[Western black rhinoceros]], [[Extinction|†]]''Diceros bicornis longipes''
***** [[Eastern black rhinoceros]], ''Diceros bicornis michaeli''
***** [[South-central black rhinoceros]], ''Diceros bicornis minor''
***** [[South-western black rhinoceros]], ''Diceros bicornis occidentalis''
**** [[White rhinoceros]], ''Ceratotherium simum''
***** [[Southern white rhinoceros]], ''Ceratotherium simum simum''
***** [[Northern white rhinoceros]], ''Ceratotherium simum cottoni''
**** [[Indian rhinoceros]], ''Rhinoceros unicornis''
**** [[Javan rhinoceros]], ''Rhinoceros sondaicus''
***** Indonesian Javan rhinoceros, ''Rhinoceros sondaicus sondaicus''
***** Vietnamese Javan rhinoceros, ''[[Rhinoceros sondaicus annamiticus]]''
***** [[Lesser Indian rhinoceros|Indian Javan rhinoceros]], [[Extinction|†]]''Rhinoceros sondaicus inermis''
**** [[Sumatran rhinoceros]], ''Dicerorhinus sumatrensis''
***** Western Sumatran rhinoceros, ''Dicerorhinus sumatrensis sumatrensis''
*****[[Bornean rhinoceros|Eastern Sumatran rhinoceros]], ''Dicerorhinus sumatrensis harrissoni''
***** [[Northern Sumatran rhinoceros]], [[Extinction|†]]''Dicerorhinus sumatrensis lasiotis''

===Prehistoric members===
[[File:Paleotherium magnum.jpg|thumb|Restoration of ''[[Palaeotherium]]'', a [[palaeothere]] genus first formally described in 1804]]
[[File:ChalicotheriumDB1.jpg|thumb|Live reconstruction of [[chalicothere]] ''[[Anisodon]] grande'' (formerly ''Chalicotherium grande'')]]
There are many perissodactyl fossils of multivariant form. The major lines of development include the following groups:

*[[File:Paraceratherium-Scale-Diagram-SVG-Steveoc86.svg|thumb|Scale diagram of the [[paraceratheriid]] ''[[Paraceratherium]],'' one of the largest land mammals to have ever existed]]Brontotherioidea were among the earliest known large mammals, consisting of the families of [[Brontotheriidae]] (synonym Titanotheriidae), the most well-known representative being ''[[Megacerops]]'' and the more basal family [[Lambdotheriidae]]. They were generally characterized in their late phase by a bony horn at the transition from the nose to the frontal bone and flat molars suitable for chewing soft plant food. The Brontotheroidea, which were almost exclusively confined to North America and Asia, died out at the beginning of the Upper [[Eocene]].
*[[Equoidea]] also developed in the Eocene. [[Palaeotheriidae]] are known mainly from Europe. In contrast, the horse family ([[Equidae]]) flourished and spread. Over time this group saw a reduction in toe number, extension of the limbs, and the progressive adjustment of the teeth for eating hard grasses.
*[[Chalicotherioidea]] represented another characteristic group, consisting of the families [[Chalicothere|Chalicotheriidae]] and [[Lophiodontidae]]. The Chalicotheriidae developed claws instead of hooves and considerable extension of the forelegs. The best-known genera include ''[[Chalicotherium]]'' and ''[[Moropus]]''. Chalicotherioidea died out in the [[Pleistocene]].
*Rhinocerotoidea (rhino relatives) included a large variety of forms from the Eocene up to the Oligocene, including dog-size leaf feeders, semiaquatic animals, and also huge long-necked animals. Only a few had horns on the nose. The [[Amynodontidae]] were hippo-like, aquatic animals. [[Hyracodontidae]] developed long limbs and long necks that were most pronounced in the ''[[Paraceratherium]]'' (formerly known as ''Baluchitherium'' or ''Indricotherium''), the second largest known land mammal ever to have lived (after ''[[Palaeoloxodon namadicus]]''<ref>{{cite journal | author = Larramendi A | year = 2016 | title = Withers height, body mass and shape of proboscideans | url = http://www.app.pan.pl/article/item/article/item/app001362014.html?pdf=39| journal = Acta Palaeontologica Polonica | volume = 61 | doi = 10.4202/app.00136.2014 | doi-access = free }}</ref>). The rhinos (Rhinocerotidae) emerged in the Middle Eocene; five species survive to the present day.
*[[Tapiroidea]] reached their greatest diversity in the Eocene, when several families lived in Eurasia and North America. They retained a primitive physique and were noted for developing a trunk. The extinct families within this group include the [[Helaletidae]].{{citation needed|date=August 2020}}
*Several mammal groups traditionally classified as [[condylarth]]s, long-understood to be a [[wastebasket taxon]], such as [[Hyopsodontidae|hyopsodontids]] and [[Phenacodontidae|phenacodontids]], are now understood to be part of the odd-toed ungulate assemblage. Phenacodontids seem to be stem-perissodactyls, while hyopsodontids are closely related to horses and brontotheres, despite their more primitive overall appearance.
*[[Desmostylia]]<ref>{{cite journal |author1=Cooper L. N. |author2=Seiffert E. R. |author3=Clementz M. |author4=Madar S. I. |author5=Bajpai S. |author6=Hussain S. T. |author7=Thewissen J. G. M. |display-authors= 3| year = 2014 | title = Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls | journal = PLOS ONE| volume = 9 | issue = 10| article-number = e109232 | doi = 10.1371/journal.pone.0109232 | pmid = 25295875 | pmc=4189980|bibcode=2014PLoSO...9j9232C |doi-access=free }}</ref> and [[Anthracobunidae]] have traditionally been placed among the [[Afrotheria|afrotheres]], but they may actually represent stem-perissodactyls. They are an early lineage of mammals that took to the water, spreading across semi-aquatic to fully marine niches in the [[Tethys Ocean]] and the northern [[Pacific Ocean|Pacific]]. However, later studies have shown that, while anthracobunids are definite perissodactyls, desmostylians have enough mixed characters to suggest that a position among the Afrotheria is not out of the question.<ref name= "Gheerbrant 2016 e0157556">{{cite journal |last1= Gheerbrant| first1= Emmanuel| last2= Filippo| first2= Andrea | last3= Schmitt|first3= Arnaud | year = 2016 | title = Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco | journal = PLOS ONE| volume = 11 | issue = 7| article-number = e0157556 | doi = 10.1371/journal.pone.0157556 | pmid=27384169 | pmc=4934866|bibcode=2016PLoSO..1157556G | doi-access= free}}</ref>
* '''Order Perissodactyla'''<ref name="Prothero 2009"/>
**'''Superfamily Brontotherioidea'''
*** †[[Brontotheriidae]]
**'''Suborder Hippomorpha'''
*** †[[Hyopsodontidae]]<ref>{{cite journal |last1= Ravel| first1= Anthony | last2= Orliac| first2= Maeva | year = 2014 | title = The inner ear morphology of the 'condylarthran' Hyopsodus lepidus | journal = Historical Biology | volume = 27 | issue = 8| page = 8 | doi = 10.1080/08912963.2014.915823 |s2cid=84391276 }}</ref>
*** †[[Pachynolophidae]]<ref name="Badiola2003">{{cite journal |last1= Badiola |first1= A. |last2= Pareda-Suberbiola |first2= X. |last3= Cuesta |first3= M. |date= 2003 |title= A new species of ''Pachynolophus'' (Mammalia, Perissodactyla) from Zambrana (Álava, Basque-Cantabrian Region). Phylogenetic analysis of ''Pachynolophus'' and first mention in the Late Eocene of the Iberian Peninsula |journal= Geobios |volume= 38 |issue= 1 |pages= 1–16 |doi= 10.1016/j.geobios.2003.07.005}}</ref>
*** '''Superfamily Equoidea'''
**** †[[Indolophidae]]<ref name="Tsubamoto2003">{{cite journal |last1= Tsubamoto |first1= T. |last2= Tun |first2= S.T. |display-authors = etal |date= 2003 |title= Reevaluation of some ungulate mammals from the Eocene Pondaung Formation, Myanmar |journal= Paleontological Research |volume= 7 |issue= 3 |pages= 219–43 |doi= 10.2517/prpsj.7.219|s2cid= 84061253 |doi-access= free |bibcode= 2003PalRe...7..219T }}</ref>
**** †[[Palaeotheriidae]] (might be a basal perissodactyl grade instead)
** '''Clade Tapiromorpha'''
*** †[[Isectolophidae]]<ref name="Holbrook 2001"/> (a basal family of Tapiromorpha; from the Eocene epoch)
*** †'''Suborder [[Ancylopoda]]'''
**** †[[Lophiodontidae]]
**** '''Superfamily [[Chalicotherioidea]]'''
***** †[[Eomoropidae]] (basal grade of chalicotheroids)
***** †[[Chalicothere|Chalicotheriidae]]
*** '''Suborder Ceratomorpha'''
**** '''Superfamily [[Rhinocerotoidea]]'''
***** †[[Amynodontidae]]
***** †[[Hyracodontidae]]
**** '''Superfamily [[Tapiroidea]]'''
***** †[[Deperetellidae]]
***** †[[Rhodopagidae]] (sometimes recognized as a subfamily of deperetellids)
***** †[[Lophialetidae]]<ref name=Gong2019>{{cite journal |last1= Gong |first1= Y. |last2= Wang |first2= Y. |display-authors = etal |date= December 2019 |title= Dietary adaptations and palaeoecology of Lophialetidae (Mammalia, Tapiroidea) from the Eocene of the Erlian Basin, China: combined evidence from mesowear and stable isotope analyses |journal= Palaeontology |volume= 63 |issue= 4 |pages= 547–564 |doi= 10.1111/pala.12471|s2cid= 214053761 }}</ref>
***** †[[Eoletidae]] (sometimes recognized as a subfamily of lophialetids)
** †[[Anthracobunidae]]<ref name="Cooper2014">{{Cite journal | doi = 10.1371/journal.pone.0109232| pmid = 25295875| title = Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls| journal = PLOS ONE| volume = 9| issue = 10| article-number = e109232| date = 2014-10-08| last1 = Cooper | first1 = L. N. | last2 = Seiffert | first2 = E. R. | last3 = Clementz | first3 = M. | last4 = Madar | first4 = S. I. | last5 = Bajpai | first5 = S. | last6 = Hussain | first6 = S. T. | last7 = Thewissen | first7 = J. G. M.| pmc=4189980| bibcode = 2014PLoSO...9j9232C| doi-access = free}}</ref> (a family of stem-perissodactyls; from the Early to Middle Eocene epoch)
** †[[Phenacodontidae]]<ref name="Cooper2014"/> (a clade of stem-perissodactyls; from the Early Palaeocene to the Middle Eocene epoch)

===Higher classification of perissodactyls===
{| class="wikitable" style="float:right;"
|-
|+ Internal classification of Perissodactyla<ref name="Holbrook et al. 2011"/>
|-
|{{Clade|style=white-space:nowrap;font-size:75%;line-height:100%
|label1='''Perissodactyla'''
|1={{Clade
  |label1=[[Hippomorpha]]
  |1={{Clade
     |label1=[[Equoidea]]
     |1={{Clade
        |1=[[Palaeotheriidae]] (†)
        |2=[[Equidae]]
        }}
     |label2=[[Brontotherioidea]]
     |2={{Clade
        |1=[[Lambdotheriidae]] (†)
        |2=[[Brontotheriidae]] (†)
        }}
     }}
   |label2=[[Tapiromorpha]]
   |2={{Clade
      |1=[[Isectolophidae]] (†)
      |2={{Clade
         |label1=[[Ancylopoda]]
         |1={{Clade
            |1=[[Lophiodontidae]] (†)
            |2=[[Chalicotheriidae]] (†)
            }}
         |label2=[[Ceratomorpha]]
         |2={{Clade
            |label1=[[Tapiroidea]]
            |1={{Clade
                |1=[[Helaletidae]] (†)
                |2=[[Tapiridae]]
                }}
            |label2=[[Rhinocerotoidea]]
            |2={{Clade
               |1=[[Amynodontidae]] (†)
               |2={{Clade
                  |1=[[Hyracodontidae]] (†)
                  |2=[[Rhinocerotidae]]
                  }}
               }}
            }}
        }}
     }}

  }}
}}
|}
Relationships within the large group of odd-toed ungulates are not fully understood. Initially, after the establishment of "Perissodactyla" by [[Richard Owen]] in 1848, the present-day representatives were considered equal in rank. In the first half of the 20th century, a more systematic differentiation of odd-toed ungulates began, based on a consideration of fossil forms, and they were placed in two major suborders: Hippomorpha and Ceratomorpha. The Hippomorpha comprises today's horses and their extinct members ([[Equus (genus)|Equoidea]]); the Ceratomorpha consist of tapirs and rhinos plus their extinct members ([[Tapiroidea]] and [[Rhinocerotoidea]]).<ref name="Simpson 1945">{{cite journal|author=George Gaylord|title=The Principles of Classification and a Classification of Mammals|journal=Bulletin of the American Museum of Natural History |year=1945|pages=252–258}}</ref> The names Hippomorpha and Ceratomorpha were introduced in 1937 by Horace Elmer Wood, in response to criticism of the name "Solidungula" that he proposed three years previously. It had been based on the grouping of horses and Tridactyla and on the rhinoceros/tapir complex.<ref>{{cite journal|author=Horace Elmer Wood|title=Revision of the Hyrachyidae|journal=Bulletin of the American Museum of Natural History|year=1934|pages=181–295}}</ref><ref>{{cite journal|author=Horace Elmer Wood|title=Perissodactyl suborders|journal=Journal of Mammalogy|year=1937|page=106|doi=10.1093/jmammal/18.1.106|volume=18|issue=1 }}</ref> The extinct brontotheriidae were also classified under Hippomorpha and therefore possess a close relationship to horses. Some researchers accept this assignment because of similar dental features, but there is also the view that a very basal position within the odd-toed ungulates places them rather in the group of ''Titanotheriomorpha''.<ref name="Holbrook et al. 2011">{{cite journal|author1=Luke T. Holbrook|author2=Joshua Lapergola|title= A new genus of perissodactyl (Mammalia) from the Bridgerian of Wyoming, with comments on basal perissodactyl phylogeny|journal=Journal of Vertebrate Paleontology|year=2011|volume=31|issue=4|pages=895–901|doi=10.1080/02724634.2011.579669|bibcode=2011JVPal..31..895H |s2cid=84351811}}</ref><ref name="Schoch 1989">{{cite book|author=Robert M. Schoch|chapter=A brief historical review of perissodactyl classification |title=The Evolution of Perissodactyls |publisher=Oxford University Press|year=1989|pages=13–23}}</ref>

Originally, the [[Chalicothere|Chalicotheriidae]] were seen as members of Hippomorpha, and presented as such in 1941. William Berryman Scott thought that, as claw-bearing perissodactyls, they belong in the new suborder Ancylopoda (where Ceratomorpha and Hippomorpha as odd-toed ungulates were combined in the group of Chelopoda).<ref>{{cite journal|author=William Berryman Scott|title=Part V: Perissodactyla|journal=The Mammalian Fauna of the White River Oligocene Transactions of the American Philosophical Society |series=New Series|volume=28|issue=5|year=1941|pages=747–964|doi=10.2307/1005518|jstor=1005518}}</ref> The term Ancylopoda, coined by [[Edward Drinker Cope]] in 1889, had been established for chalicotheres. However, further morphological studies from the 1960s showed a middle position of Ancylopoda between Hippomorpha and Ceratomorpha. [[Leonard Radinsky|Leonard Burton Radinsky]] saw all three major groups of odd-toed ungulates as peers, based on the extremely long and independent phylogenetic development of the three lines.<ref>{{cite journal|author=Leonard B. Radinsky|title=''Paleomoropus'', a new early Eocene chalicothere (Mammalia, Perissodactyla), and a revision of Eocene chalicotheres|journal=American Museum Novitates|year=1964|pages=1–28}}</ref> In the 1980s, Jeremy J. Hooker saw a general similarity between Ancylopoda and Ceratomorpha based on dentition, especially in the earliest members, leading to the unification in 1984 of the two submissions in the interim order, ''Tapiromorpha''. At the same time, he expanded the Ancylopoda to include the ''Lophiodontidae''. The name "Tapiromorpha" goes back to Ernst Haeckel, who coined it in 1873, but it was long considered synonymous to Ceratomorpha because Wood had not considered it in 1937 when Ceratomorpha were named, since the term had been used quite differently in the past.<ref>{{cite journal|author=JJ Hooker|title=A primitive ceratomorph (Perissodactyla, Mammalia) from the Early Tertiary of Europe|journal=Zoological Journal of the Linnean Society|year=1984|pages=229–244|doi=10.1111/j.1096-3642.1984.tb00545.x|volume=82|issue=1–2}}</ref> Also in 1984, Robert M. Schoch used the conceptually similar term Moropomorpha, which today applies synonymously to Tapiromorpha.<ref>{{cite journal|author=Robert Milton Schoch|title=Two unusual specimens of the Yale Peabody Museum Helaletes in collections, and some comments on the ancestry of the Tapiridae (Perissodactyla, Mammalia)|publisher=Peabody Museum, Yale University|year=1984|pages=1–20}}</ref> Included within the Tapiromorpha are the now extinct Isectolophidae, a sister group of the Ancylopoda-Ceratomorpha group and thus the most primitive members of this relationship complex.<ref name="Schoch 1989"/><ref name="Holbrook 2001">{{cite journal|author=Luke T. Holbrook|title=Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla)|journal=Zoological Journal of the Linnean Society|year=2001|pages=1–54|doi=10.1111/j.1096-3642.2001.tb02270.x|volume=132|issue=1 |doi-access=free}}</ref>

==Evolutionary history==

===Origins===
The evolutionary development of Perissodactyla is well documented in the fossil record. Numerous finds are evidence of the [[adaptive radiation]] of this group, which was once much more varied and widely dispersed. ''[[Radinskya]]'' from the late [[Paleocene]] of East Asia is often considered to be one of the oldest close relatives of the ungulates.<ref>{{cite book|author1=Malcolm C. McKenna|author2=Chow Minna|author3=Suyin Ting|author4=Luo Zhexi|chapter=''Radinskya yupingae'', a perissodactyl-like mammal from the Late Palaeocene of China |title=The Evolution of Perissodactyls |publisher=Oxford University Press |year=1989|pages=24–36}}</ref> Its 8&nbsp;cm skull must have belonged to a very small and primitive animal with a π-shaped crown pattern on the enamel of its rear molars similar to that of perissodactyls and their relatives, especially the rhinos.<ref>{{cite book|author=Kenneth D. Rose|title=The Beginning of the Age of Mammals |publisher=Johns Hopkins University Press|year=2006|pages=242–267}}</ref> Finds of ''[[Cambaytherium]]'' and ''Kalitherium'' in the Cambay shale of western India indicate an origin in Asia dating to the Lower [[Eocene]] roughly 54.5 million years ago.<ref>{{cite journal|author1=Sunil Bajpai|author2=Vivesh Kapur|author3=Debasis P. Das|author4=BN Tiwari|author5=N. Saravanan|author6=Ritu Sharma|title=Early Eocene Land Mammals from Vastan Lignite Mine, District Surat (Gujarat), western India|journal=Journal of the Palaeontological Society of India|year=2005|volume=50 |issue=1 |pages=101–113|doi=10.1177/0971102320050106 |bibcode=2005JPalS..50..101B |url=http://repository.ias.ac.in/94013/1/Early_Eocene_land.pdf}}</ref><ref>{{cite journal|author1=Sunil Bajpai|author2=Vivesh Kapur|author3=JGM Thewissen|author4=Debasis P. Das|author5=BN Tiwari|title=New Early Eocene cambaythere (Perissodactyla, Mammalia) from the Vastan Lignite Mine (Gujarat, India) and on evaluation of cambaythere relationships|journal=Journal of the Palaeontological Society of India|year=2006|pages=101–110|url=https://www.researchgate.net/publication/268436397}}</ref> Their teeth also show similarities to ''Radinskya'' as well as to the [[Tethytheria]] clade.<ref>{{cite journal|author1=Kenneth D. Rose|author2=Thierry Smith|author3=Rajendra S. Rana|author4=Ashok Sahni|author5=H. Singh|author6=A. Pieter Missiaen|title=Early Eocene (Ypresian) Continental vertebrate assemblage from India, with description of a new anthracobunid (Mammalia, tethytheria )|journal=Journal of Vertebrate Paleontology|year=2006|pages=219–225|doi=10.1671/0272-4634(2006)26[219:eeycva]2.0.co;2|volume=26|issue=1 |s2cid=86206151 }}</ref><ref>{{cite journal|author=Kishor Kumar|title=Comments on 'Early Eocene Land Mammals from Vastan Lignite Mine, District Surat (Gujarat), western India' by Bajpai|journal=Journal of the Palaeontological Society of India|year=2005|pages=101–113, 2005|url=https://www.researchgate.net/publication/256077442}}</ref> The saddle-shaped configuration of the navicular joints and the mesaxonic construction of the front and hind feet also indicates a close relationship to Tethytheria.{{citation needed|reason=technical and specific|date=June 2016}} However, this construction deviates from that of ''Cambaytherium'', indicating that it is actually a member of a sister group. Ancestors of Perissodactyla may have arrived via an island bridge from the Afro-Arab landmass onto the Indian subcontinent as it drifted north towards Asia.<ref>{{cite journal|author1=Kenneth D. Rose|author2=Luke T. Holbrook|author3=Rajendra S. Rana|author4=Kishor Kumar|author5=Katrina E. Jones|author6=Heather E. Ahrens|author7=Pieter Missiaen|author8=Ashok Sahni|author9=Thierry Smith|title=Early Eocene fossils suggest that the mammalian order Perissodactyla originated in India|journal=Nature Communications|year=2014|doi=10.1038/ncomms6570|volume=5|issue=1 |page=5570|pmid=25410701|bibcode=2014NatCo...5.5570R|doi-access=free|hdl=1854/LU-5972511|hdl-access=free}}</ref> A study on ''Cambaytherium'' suggests an origin in India [[Insular India|prior]] or near its collision with Asia.<ref>{{Cite journal|url=https://www.tandfonline.com/doi/full/10.1080/02724634.2020.1761370|doi=10.1080/02724634.2020.1761370|title=Anatomy, Relationships, and Paleobiology of Cambaytherium (Mammalia, Perissodactylamorpha, Anthracobunia) from the lower Eocene of western India|year=2019|last1=Rose|first1=Kenneth D.|last2=Holbrook|first2=Luke T.|last3=Kumar|first3=Kishor|last4=Rana|first4=Rajendra S.|last5=Ahrens|first5=Heather E.|last6=Dunn|first6=Rachel H.|last7=Folie|first7=Annelise|last8=Jones|first8=Katrina E.|last9=Smith|first9=Thierry|journal=Journal of Vertebrate Paleontology|volume=39|issue=sup1 |pages=1–147|bibcode=2019JVPal..39S...1R |s2cid=226263139|url-access=subscription}}</ref>

The alignment of hyopsodontids and phenacodontids to Perissodactyla in general suggests an older Laurasian origin and distribution for the clade, dispersed across the northern continents already in the early Paleocene. These forms already show a fairly well-developed molar morphology, with no intermediary forms as evidence of the course of its development.<ref name="Gheerbrant Emmanuel, Filippo Andrea, Schmitt Arnaud 2016 e0157556">{{cite journal |author1=Gheerbrant Emmanuel |author2=Filippo Andrea |author3=Schmitt Arnaud | year = 2016 | title = Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco | journal = PLOS ONE| volume = 11 | issue = 7| article-number = e0157556 | doi = 10.1371/journal.pone.0157556 | pmid=27384169 | pmc=4934866|bibcode=2016PLoSO..1157556G |doi-access=free }}</ref> The close relationship between [[Meridiungulata|meridiungulate]] mammals and perissoodactyls in particular is of interest since the latter appeared in [[South America]] soon after the [[Cretaceous–Paleogene extinction event|K–T event]], implying rapid ecological radiation and dispersal after mass extinction.<ref>{{cite journal |author1=Halliday Thomas John Dixon |author2=Upchurch Paul |author3=Goswami Anjali | year = 1833 | title = Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 283 | issue = 1833| article-number = 20153026 | doi = 10.1098/rspb.2015.3026 | pmid=27358361 | pmc=4936024}}</ref>

===Phylogeny===
[[File:Hyracotherium Eohippus hharder.jpg|thumb|''[[Eohippus]]'', an early relative of the horse, is one of the oldest-known perissodactyls]]
The Perissodactyla appeared relatively abruptly at the beginning of the Lower Paleocene about 63&nbsp;million years ago, both in North America and Asia, in the form of [[Phenacodontidae|phenacodontids]] and [[Hyopsodontidae|hyopsodontids]]. The oldest finds from an extant group originate among other sources, from ''[[Sifrhippus]]'', an ancestor of the horses from the Willswood lineup in northwestern [[Wyoming]].<ref>{{cite journal |author1=Ross Secord |author2=Jonathan I. Bloch |author3=Stephen GB Chester |author4=Doug M. Boyer |author5=Aaron R. Wood |author6=Scott L. Wing |author7=Mary J. Kraus |author8=Francesca A. McInerney |author9=John Krigbaum |title=Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum |journal=Science |volume=335 |issue=6071 |year=2012 |pages=959–962 |doi=10.1126/science.1213859 |pmid=22363006|bibcode=2012Sci...335..959S |s2cid=4603597 |url=https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1314&context=geosciencefacpub |url-access=subscription }}</ref><ref>{{cite journal |author=David J. Froehlich |title=The systematics and taxonomy of the early Eocene equids (Perissodactyla) |journal=Zoological Journal of the Linnean Society |year=2002 |pages=141–256 |doi=10.1046/j.1096-3642.2002.00005.x |volume=134 |issue=2|doi-access=free }}</ref> The distant ancestors of tapirs appeared not too long after that in the Ghazij lineup in [[Balochistan]], such as ''Ganderalophus'', as well as ''Litolophus'' from the Chalicotheriidae line, or ''Eotitanops'' from the group of brontotheriidae.<ref>{{cite journal |author1=Pieter Missiaen |author2=Philip D. Gingerich |title=New Early Eocene tapiromorph perissodactyls from the Ghazij formation of Pakistan, with implications for mammalian biochronology in Asia |journal=Acta Palaeontologica Polonica |year=2012 |pages=21–34 |doi=10.4202/app.2010.0093 |volume=57 |issue=1 |hdl=1854/LU-3178691|doi-access=free |bibcode=2012AcPaP..57...21M |hdl-access=free }}</ref><ref>{{cite journal |author1=Pieter Missiaen |author2=Gregg F. Gunnell|author3=Philip D. Gingerich |title=New brontotheriidae (Mammalia, Perissodactyla) from the Early and Middle Eocene of Pakistan with implications for mammalian paleobiogeography |journal=Journal of Paleontology|year=2011 |volume=85 |issue=4 |pages=665–677 |doi=10.1666/10-087.1|bibcode=2011JPal...85..665M |s2cid=129473193}}</ref> Initially, the members of the different lineages looked quite similar, with an arched back and generally four toes on the front and three on the hind feet. ''[[Eohippus]]'', which is considered a member of the horse family, outwardly resembled ''[[Hyrachyus]]'', the first representative of the rhino and tapir line.<ref>{{cite journal |author1=Hlawatsch, Kerstin |author2=Erfurt, Jörg |title=Zahnmorphologie und stratigraphische Verbreitung von ''Hyrachyus minimus'' (Perissodactyla, Mammalia) in den eozänen Geiseltalschichten |trans-title=Tooth morphology and stratigraphic distribution of ''Hyrachyus minimus'' (Perissodactyla, Mammalia) in the Eocene Geiseltal layers |journal=Hallesches Jahrbuch für Geowissenschaften |year=2007 |volume=Suppl. 23 |pages=161–173}}</ref> All were small compared to later forms and lived as fruit and foliage eaters in forests. The first of the [[megafauna]] to emerge were the [[brontothere]]s, in the Middle and Upper Eocene. ''Megacerops'', known from North America, reached a withers height of {{convert|2.5|m|abbr=on}} and could have weighed just over {{convert|3|MT|ST}}. The decline of brontotheres at the end of the Eocene is associated with competition arising from the advent of more successful herbivores.<ref name="Prothero 2009"/><ref>{{cite book |author=Christine Janis |chapter=An evolutionary history of browsing and grazing ungulates |title=The Ecology of Browsing and Grazing |volume=195 |editor1=Iain J. Gordon |editor2=Herbert H. T. Prins |publisher=Springer |year=2008 |pages=21–45 |doi=10.1007/978-3-540-72422-3_2 |series=Ecological Studies |isbn=978-3-540-72421-6}}</ref>

More successful lines of odd-toed ungulates emerged at the end of the Eocene when dense jungles gave way to steppe, such as the chalicotheriid rhinos, and their immediate relatives; their development also began with very small forms. ''[[Paraceratherium]]'', one of the largest mammals ever to walk the earth, evolved during this era.<ref name=Benton>{{cite book |author=Benton, Michael J. |year=1997 |title=Vertebrate Palaeontology |publisher=Chapman & Hall |location=London |page=343 |isbn=0-412-73810-4}}</ref><ref>{{cite journal |date=1993 |title=The Largest land mammal ever imagined |url=http://www.rhinoresourcecenter.com/pdf_files/129/1293772499.pdf |journal=Zoological Journal of the Linnean Society |volume=108 |pages=85–101 |doi=10.1111/j.1096-3642.1993.tb02560.x |author1=Mikael Fortelius |author2=John Kappelmann|issue=1 }}</ref> They weighed up to {{convert|20|MT|ST}} and lived throughout the Oligocene in Eurasia. About 20&nbsp;million years ago, at the onset of the Miocene, the perissodactyls first reached Africa when it became connected to Eurasia because of the closing of the [[Tethys Ocean]]. For the same reason, however, new animals such as the [[mammoth]]s also entered the ancient settlement areas of odd-toed ungulates, creating competition that led to the extinction of some of their lines. The rise of [[ruminant]]s, which occupied similar ecological niches and had a much more efficient digestive system, is also associated with the decline in diversity of odd-toed ungulates. A significant cause for the decline of perissodactyls was climate change during the Miocene, leading to a cooler and drier climate accompanied by the spread of open landscapes. However, some lines flourished, such as the horses and rhinos; anatomical adaptations made it possible for them to consume tougher grass food. This led to open land forms that dominated newly created landscapes. With the emergence of the [[Isthmus of Panama]] in the [[Pliocene]], perissodactyls and other megafauna were given access to one of their last habitable continents: South America.<ref>{{cite journal |author=Matthew Colbert |title=New Fossil Discoveries and the History of ''Tapirus'' |journal=Tapir Conservation |year=2007 |pages=12–14}}</ref><ref>{{cite journal |display-authors=6 |author1=Ludovic Orlando |author2=Jessica L. Metcalf |author3=Maria T. Alberdi |author4=Miguel Telles Antunes-Dominique Bonjean |author5=Marcel Otte |author6=Fabiana Martin |author7=Véra Eisenmann |author8=Marjan Mashkour|author9=Flavia Morello |author10=Jose L. Prado |author11=Rodolfo Salas Gismondi-Bruce J Shockey |author12=Patrick J. Wrinn |author13=Sergei K. Vasil'ev |author14=Nikolai D. Ovodov |author15=Michael I. Cherry Blair Hopwood |author16=Dean Male |author17=Jeremy J. Austin |author18=Catherine Hänni |author19=Alan Cooper |title=Revising the recent evolutionary history of equids using ancient DNA |journal=Proceedings of the National Academy of Sciences USA |year=2009 |pages=21754–21759 |doi=10.1073/pnas.0903672106 |volume=106 |issue=51 |pmid=20007379 |pmc=2799835|bibcode=2009PNAS..10621754O |doi-access=free }}</ref> However, many perissodactyls became extinct at the end of the ice ages, including American horses and the ''[[Elasmotherium]]''. Whether over-hunting by humans (overkill hypothesis), climatic change, or a combination of both factors was responsible for the extinction of ice age mega-fauna, remains controversial.<ref name="Prothero 2009">{{cite journal |author=Donald, R. |title=Evolutionary Transitions in the Fossil Record of Terrestrial Hoofed Mammals |journal=Evolution: Education and Outreach |year=2009 |pages=289–302 |doi=10.1007/s12052-009-0136-1 |volume=2 |issue=2|doi-access=free }}</ref>

==Research history==
[[File:Richard Owen 1856.jpg|thumb|right|Richard Owen, 1856]]
In 1758, in his seminal work ''[[Systema Naturae]]'', [[Carl Linnaeus|Linnaeus]] (1707–1778) classified horses (''Equus'') together with [[hippo]]s (''Hippopotamus''). At that time, this category also included the tapirs (''Tapirus''), more precisely the lowland or South American tapir (''Tapirus terrestus''), the only tapir then known in Europe. Linnaeus classified this tapir as ''Hippopotamus terrestris'' and put both genera in the group of the ''Belluae'' ("beasts"). He combined the rhinos with the [[Glires]], a group now consisting of the [[Lagomorpha|lagomorphs]] and [[rodent]]s. [[Mathurin Jacques Brisson]] (1723–1806) first separated the tapirs and hippos in 1762 with the introduction of the concept ''le tapir''. He also separated the rhinos from the rodents, but did not combine the three families now known as the odd-toed ungulates. In the transition to the 19th century, the individual perissodactyl genera were associated with various other groups, such as the [[proboscidea]]n and [[even-toed ungulate]]s. In 1795, [[Étienne Geoffroy Saint-Hilaire]] (1772–1844) and [[Georges Cuvier]] (1769–1832) introduced the term "pachyderm" ([[Pachydermata]]), including in it not only the rhinos and elephants, but also the hippos, pigs, [[peccary|peccari]]es, tapirs and hyrax.<ref name="Simpson 1945"/><ref name="Schoch 1989"/><ref>{{cite journal|author1=Étienne Geoffroy Saint-Hilaire|author2=Georges Cuvier|title=Memoire sur une nouvelle division of Mammifères, et sur les principes qui doivent servir de base dans cette sorte de travail|journal=Magasin Encyclopédique|year=1795|pages=164–190}}</ref><ref>{{cite journal|author=Georges Cuvier|title=Le Règne Animal distribue d'après son organisation pour servir de base à l'histoire naturelle des animaux|journal=Introduction à l'Anatomie Comparée|volume=1|year=1817|pages=1–540}}</ref> The horses were still generally regarded as a group separate from other mammals and were often classified under the name ''Solidungula'' or ''Solipèdes'', meaning "one-hoof animal".<ref>{{cite book|author=Johann Friedrich Blumenbach|title=Handbook of Natural History|year=1779|pages=168–448}}</ref><ref>{{cite journal|author=Georges Cuvier|title=Tableau Elementaire de l'histoire naturelle des animaux|year=1798|url=https://archive.org/stream/tableaulment00cuvi#page/142/mode/2up|pages=1–710}}</ref>

In 1861, [[Henri Marie Ducrotay de Blainville]] (1777–1850) classified ungulates by the structure of their feet, differentiating those with an even number of toes from those with an odd number. He moved the horses as ''solidungulate'' over to the tapirs and rhinos as ''multungulate'' animals and referred to all of them together as ''onguligrades à doigts impairs'', coming close to the concept of the odd-toed ungulate as a systematic unit. [[Richard Owen]] (1804–1892) quoted Blainville in his study on fossil mammals of the [[Isle of Wight]] and introduced the name ''Perissodactyla''.<ref name="Simpson 1945"/><ref name="Schoch 1989"/>

In 1884, [[Othniel Charles Marsh]] (1831–1899) came up with the concept ''[[Mesaxonia]]'', which he used for what are today called the odd-toed ungulates, including their extinct relatives, but explicitly excluding the hyrax. ''Mesaxonia'' is now considered a synonym of ''Perissodactyla'', but it was sometimes also used for the true odd-toed ungulates as a subcategory (rhinos, horses, tapirs), while ''Perissodactyla'' stood for the entire order, including the hyrax. The assumption that hyraxes were ''Perissodactyla'' was held well into the 20th century.<ref>{{cite book|author1=Donald R. Prothero|author2=Robert M. Schoch|chapter=Classification of the Perissodactyla|title=The Evolution of Perissodactyls |publisher=Oxford University Press |year=1989|pages=530–537}}</ref> Only with the advent of molecular genetic research methods had it been recognized that the hyrax was not closely related to perissodactyls but rather to elephants and manatees.<ref name="Graur et al. 1997"/><ref>{{cite journal|author1=Rodolphe Tabuce|author2=Laurent Marivaux|author3=Mohammed Adaci|author4=Mustapha Bensalah|author5=Jean-Louis Hartenberger|author6=Mohammed Mahboubi|author7=Fateh Mebrouk|author8=Paul Tafforeau|author9=Jean-Jacques Jaeger|title=Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade|journal=Proceedings of the Royal Society B|year=2007|pages= 1159–1166|doi=10.1098/rspb.2006.0229|volume=274|issue=1614|pmid=17329227|pmc=2189562 |bibcode=2007PBioS.274.1159T }}</ref>

==Interactions with humans==
The domestic horse and the donkey play an important role in human history, particularly as transport, work and pack animals. The [[domestication]] of both species began several millennia BCE. Due to the motorisation of agriculture and the spread of automobile traffic, such use has declined sharply in Western industrial countries; riding is usually undertaken more as a hobby or sport. In less developed regions of the world, traditional uses for these animals are, however, still widespread. To a lesser extent, horses and donkeys are also kept for their meat and their milk.

In contrast, the existence in the wild of almost all other odd-toed ungulates species has declined dramatically because of hunting and habitat destruction. The [[quagga]] is extinct and [[Przewalski's horse]] was once eradicated in the wild.

Threat levels, according to the [[International Union for Conservation of Nature]] as of 2026:<ref name=IUCN>{{cite web |title=The IUCN Red List of Threatened Species |url=https://www.iucnredlist.org/search?taxonomies=100194&searchType=species |website=IUCN Red List of Threatened Species |access-date=25 February 2026}}</ref>

*Four species are considered [[critically endangered]]: the [[Javan rhinoceros]], the [[Sumatran rhinoceros]], the [[black rhinoceros]] and the [[African wild ass]].
*Five species are [[Endangered species|endangered]]: the [[mountain tapir]], the [[Baird's tapir|Central American tapir]], the [[Malayan tapir]], the Przewalski's horse and [[Grévy's zebra]].
*Three species are considered [[vulnerable species|vulnerable]]: the [[Indian rhinoceros]], the [[South American tapir]] and the [[mountain zebra]].
*The [[onager]], the [[plains zebra]] and the [[white rhinoceros]] are [[Near-threatened species|near-threatened]]; however, the northern subspecies, ''Ceratotherium simum cottoni'' ([[northern white rhinoceros]]) is close to extinction.
*The [[kiang]] is not considered at risk ([[Least-concern species|least concern]]).

=== Conservation ===
Hunting and habitat loss due to land conversion and human encroachment are the most significant threats to the three endangered species of tapir. The Malayan tapir's inland forest habitat is of particular concern, as this land is being deforested rapidly and converted into palm oil plantations. Climate change is shifting the suitable range of mountain tapirs further up the Andes Mountains, reducing their available habitat. Hunting of mountain and Baird's tapirs in Central and South America for their meat is common and is made easier by climate change, as population densities are forcibly increased. Although hunting is illegal in protected areas throughout this region, regulations are often ignored or unenforced. Conservation efforts for tapirs primarily consist of legal protections from hunting and international trade, though proposals of habitat protection and restoration at the local level are underway in all affected countries.<ref name=IUCN/>

==References==
{{Reflist}}

==Further reading==
* Martin S. Fischer: Mesaxonia (Perissodactyla) Perissodactyla. In: Wilfried Westheide, Reinhard Rieger (eds.): ''Systematic Zoology. Part 2: Vortex or craniotes''. Spektrum Akademischer Verlag, Heidelberg and Berlin 2004, pp 646–655, {{ISBN|3-8274-0307-3}}.
*Ronald M. Nowak: ''Walker's Mammals of the World''. 6th edition. Johns Hopkins University Press, Baltimore 1999, {{ISBN|0-8018-5789-9}}.
*Thomas S. Kemp: ''The Origin & Evolution of Mammals'' Oxford University Press, Oxford, 2005. {{ISBN|0-19-850761-5}}.
*AH Müller: ''Textbook of Paleozoology, Volume III: vertebrates'', Part 3: Mammalia. 2nd edition. Gustav Fischer Verlag, Jena and Stuttgart 1989. {{ISBN|3-334-00223-3}}.
*Don E. Wilson, DeeAnn M. Reeder (eds.): ''[[Mammal Species of the World]]'', 3rd edition. The Johns Hopkins University Press, Baltimore 2005 {{ISBN|0-8018-8221-4}}.
* {{citation | author = Ronald M. Nowak | title = Walker's Mammals of the World | isbn = 978-0-8018-5789-8 | year = 1999 | publisher = Johns Hopkins University Press | location = Baltimore | edition = 6th | lccn = 98023686 | language = en <!--| GBS-id = T37sFCl43E8C | Ltwork = 257350 -->}}

{{Perissodactyla Genera|P.}}
{{Mammals}}
{{Perissodactyla}}
{{Taxonbar|from=Q25374}}
{{Authority control}}

[[Category:Extant Ypresian first appearances]]
[[Category:Perissodactyla| ]] <!-- First in its category-->
[[Category:Mammal orders]]
[[Category:Taxa named by Richard Owen]]
[[Category:Panperissodactyla]]