Editing Litopterna (section)

== Diversity ==
[[File:Reports of the Princeton University Expeditions to Patagonia, 1896-1899. J. B. Hatcher in charge (1901) (14749571352).jpg|left|thumb|Skeleton of ''[[Theosodon]]'' (Macraucheniidae)]]
[[File:Thoatheriumknight.jpg|left|thumb|Historic life restoration of ''[[Thoatherium]]'' (Proterotheriidae)]]
The body forms of many litopterns, notably in the limb and skull structure, are broadly similar to those of living [[ungulate]]s, unlike other [[South American native ungulate]] groups, which are often strongly divergent from living ungulates.<ref name="Croft-2021" /> Paleocene and Eocene litopterns generally had small body masses, with ''[[Protolipterna]]'' ([[Protolipternidae]]) estimated to have had a body mass of {{Convert|0.5-1.5|kg|lb|abbr=}}, though the Eocene [[Sparnotheriodontidae|sparnotheriodontids]] were considerably larger, with estimated body masses of around {{Convert|400|kg|lb|abbr=}}. Most proterotheriids had body masses of around {{Convert|15 to 80|kg|lb}} while many macraucheniids had body masses of around {{Convert|80–120|kg|lb}}. Some of the last macraucheniids like ''[[Macrauchenia]]'' were considerably larger, with body masses around a ton.<ref name="Croft-2020" /> [[Adianthidae]] generally had small body masses, with members of the genus ''[[Adianthus]]'' estimated to weigh {{Convert|7.4-20|kg|lb}}. Members of the proterotheriid subfamily Megadolodinae are noted for having [[bunodont]] (rounded cusp) molar teeth, which is largely unique to litopterns among South American native ungulates.<ref name="Carrillo-2023" /><ref name="Croft-2020" /> Litopterns of the mid-late Cenozoic had hinge-like limb joints and [[Hoof|hooves]] similar to those of modern ungulates, with the weight being supported on three toes in macraucheniids and one in proterotheriids, with the protherotheriid ''[[Thoatherium]]'' developing greater toe reduction than that present in living horses.<ref name="Croft-2020" /> Macraucheniids had long necks and limbs.<ref name="Schmidt-2014">{{Cite journal |last1=Schmidt |first1=Gabriela I. |last2=Ferrero |first2=Brenda S. |date=2014-07-29 |title=Taxonomic reinterpretation of Theosodon hystatus Cabrera and Kraglievich, 1931 (Litopterna, Macraucheniidae ) and phylogenetic relationships of the family |url=http://www.tandfonline.com/doi/abs/10.1080/02724634.2014.837393 |journal=Journal of Vertebrate Paleontology |language=en |volume=34 |issue=5 |pages=1231–1238 |doi=10.1080/02724634.2014.837393 |bibcode=2014JVPal..34.1231S |hdl=11336/18953 |s2cid=86091386 |issn=0272-4634|hdl-access=free }}</ref> 

Members of the macraucheniid subfamily Macraucheniinae saw the progressive migration of the nasal opening to the top of the skull,<ref>{{Cite journal |last1=Forasiepi |first1=Analía M. |last2=MacPhee |first2=Ross D. E. |last3=Del Pino |first3=Santiago Hernández |last4=Schmidt |first4=Gabriela I. |last5=Amson |first5=Eli |last6=Grohé |first6=Camille |date=2016-06-22 |title=Exceptional Skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) From the Late Miocene of Argentina: Anatomy, Systematics, and Paleobiological Implications |url=http://www.bioone.org/doi/10.1206/0003-0090-404.1.1 |journal=Bulletin of the American Museum of Natural History |language=en |volume=404 |pages=1–76 |doi=10.1206/0003-0090-404.1.1 |s2cid=89219979 |issn=0003-0090}}</ref> which was often historically suggested to indicate the presence of a trunk, though other authors have suggested that a [[moose]]-like prehensile lip,<ref>{{Cite journal |last1=Moyano |first1=Silvana Rocio |last2=Giannini |first2=Norberto Pedro |date=November 2018 |title=Cranial characters associated with the proboscis postnatal-development in Tapirus (Perissodactyla: Tapiridae) and comparisons with other extant and fossil hoofed mammals |url=https://linkinghub.elsevier.com/retrieve/pii/S0044523118301001 |journal=Zoologischer Anzeiger |language=en |volume=277 |pages=143–147 |doi=10.1016/j.jcz.2018.08.005|s2cid=92143497 |doi-access=free |hdl=11336/86349 |hdl-access=free }}</ref> or a [[Saiga antelope|saiga]]-like nose to filter dust<ref>{{Cite journal |last1=Blanco |first1=R. Ernesto |last2=Jones |first2=Washington W. |last3=Yorio |first3=Lara |last4=Rinderknecht |first4=Andrés |date=October 2021 |title=Macrauchenia patachonica Owen, 1838: Limb bones morphology, locomotory biomechanics, and paleobiological inferences |url=https://linkinghub.elsevier.com/retrieve/pii/S0016699521000425 |journal=Geobios |language=en |volume=68 |pages=61–70 |bibcode=2021Geobi..68...61B |doi=10.1016/j.geobios.2021.04.006|url-access=subscription }}</ref> are more likely.

{{Multiple image
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| image1            = Macraucheniidae skulls.jpg
| image2            = Macraucheniidae skulls 2.jpg
| footer            = Skulls of the macraucheniids (A) ''[[Theosodon]]'', (B) ''[[Scalabrinitherium]]'', (C) ''[[Macrauchenia]]'', portraying how the nasal bones shifted backwards on the skull, with the nasal opening following suit.
| align             = center
}}