Editing Perissodactyla (section)

===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>