Editing Perissodactyla (section)

===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>