Litopterna

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Litopterna (from Template:Langx "smooth heel") is an extinct order of South American native ungulates that lived from the Paleocene to the Pleistocene-Holocene around 62.5 million to 12,000 years ago (or possibly as late as 3,500 years ago), and were also present in Antarctica during the Eocene. They represent the second most diverse group of South American ungulates after Notoungulata.<ref name="Croft-2020">Template:Cite journal</ref> It is divided into nine families, with Proterotheriidae and Macraucheniidae being the most diverse and last surviving families.<ref name="Carrillo-2023">Template:Cite journal</ref>

DiversityEdit

File:Reports of the Princeton University Expeditions to Patagonia, 1896-1899. J. B. Hatcher in charge (1901) (14749571352).jpg

Skeleton of Theosodon (Macraucheniidae)

File:Thoatheriumknight.jpg

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 ungulates, 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 Template:Convert, though the Eocene sparnotheriodontids were considerably larger, with estimated body masses of around Template:Convert. Most proterotheriids had body masses of around Template:Convert while many macraucheniids had body masses of around Template:Convert. 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 Template:Convert. 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 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">Template:Cite journal</ref>

Members of the macraucheniid subfamily Macraucheniinae saw the progressive migration of the nasal opening to the top of the skull,<ref>Template:Cite journal</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>Template:Cite journal</ref> or a saiga-like nose to filter dust<ref>Template:Cite journal</ref> are more likely.

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EcologyEdit

Litopterns were likely hindgut fermenters.<ref name="Croft-2021">Template:Cite journal</ref> At least some macraucheniids like Macrauchenia are suggested to have been mixed feeders feeding on both browse and grass.<ref>Template:Cite journal</ref> Sparnotheriodontids are suggested to have been browsers.<ref>Template:Cite journal</ref> Some proterotheriids are suggested to have been browsers,<ref>Template:Cite journal</ref> while some members proterotheriid subfamily Megadolodinae like Megadolodus have been suggested to have been omnivorous with at least part of their diet consisting of hard fruit.<ref name="Carrillo-2023" />

Evolutionary historyEdit

Litopterna, like other "South American native ungulates" is thought to have originated from groups of archaic "condylarths" that migrated from North America.<ref name="Croft-2020" /> Sequencing of the collagen proteome and mitochondrial genome of Macrauchenia has revealed that litopterns are true ungulates, sharing a common ancestor with Notoungulata, and with their closest living relatives being Perissodactyla (the group containing living equines, rhinoceros and tapirs) as part of the clade Panperissodactyla, with the split from Perissodactyla being estimated at around 66 million years ago.<ref name="WelkerCollins2015">Template:Cite journal</ref><ref>Template:Cite journal</ref> The relationship of Litopterna to other South American native ungulate groups is uncertain, though it may be closely related to the "condylarth" group Didolodontidae.<ref name="Croft-2020" /> The earliest litopterns appeared during the early Paleocene, around 62.5 million years ago.<ref>Template:Cite journal</ref>

Aside from South America, sparnotheriodontids are also known from the Eocene aged La Meseta Formation in the Antarctic Peninsula, representing the only record of litopterns on the Antarctic continent. Litopterns declined during the Pliocene and Pleistocene, likely as a result of climatic change and competition with recently immigrated North American ungulates who arrived as part of the Great American interchange, following the connection of the previously isolated North and South America via the Isthmus of Panama.<ref name="Croft-2020" /> Macrauchenia, Xenorhinotherium (Macraucheniidae) and Neolicaphrium (Proterotheriidae) were the last surviving genera of litopterns. All became extinct at the end of the Late Pleistocene around 12,000 years ago as part of the end-Pleistocene extinction event, along with most other large mammals in the Americas, co-inciding with the arrival of the first humans to the continent.<ref name="Schmidt-2014" /> A study in 2025 suggested that Xenorhinotherium may have survived until the late Holocene based on a specimen radiocarbon dated to 3,493–4,217 years cal. Before Present (BP).<ref>Template:Cite journal</ref> It is possible that hunting had a causal role in their extinction.<ref name="SDWebb">Template:Cite journal</ref><ref name="Marshal1990">Template:Cite journal</ref><ref name="SDWebb2">Template:Cite journal</ref><ref name="Croft-2020" />