Editing Perissodactyla (section)

==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 |pages=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}}</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| page = 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| page = 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 }}</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 |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>