Skeleton ofTheosodon (Macraucheniidae)Historic life restoration ofThoatherium (Proterotheriidae)
The body forms of many litopterns, notably in the limb and skull structure, are broadly similar to those of livingungulates, unlike otherSouth American native ungulate groups, which are often strongly divergent from living ungulates.[3] Paleocene and Eocene litopterns generally had small body masses, withProtolipterna (Protolipternidae) estimated to have had a body mass of 0.5–1.5 kilograms (1.1–3.3 lb), though the Eocenesparnotheriodontids were considerably larger, with estimated body masses of around 400 kilograms (880 lb). Most proterotheriids had body masses of around 15 to 80 kilograms (33 to 176 lb) while many macraucheniids had body masses of around 80–120 kilograms (180–260 lb). Some of the last macraucheniids likeMacrauchenia were considerably larger, with body masses around a ton.[1]Adianthidae generally had small body masses, with members of the genusAdianthus estimated to weigh 7.4–20 kilograms (16–44 lb). Members of the proterotheriid subfamily Megadolodinae are noted for havingbunodont (rounded cusp) molar teeth, which is largely unique to litopterns among South American native ungulates.[2][1] Litopterns of the mid-late Cenozoic had hinge-like limb joints andhooves similar to those of modern ungulates, with the weight being supported on three toes in macraucheniids and one in proterotheriids, with the protherotheriidThoatherium developing greater toe reduction than that present in living horses.[1] Macraucheniids had long necks and limbs.[4]
Members of the macraucheniid subfamily Macraucheniinae saw the progressive migration of the nasal opening to the top of the skull,[5] which was often historically suggested to indicate the presence of a trunk, though other authors have suggested that amoose-like prehensile lip,[6] or asaiga-like nose to filter dust[7] are more likely.
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.
Litopterns were likelyhindgut fermenters.[3] At least some macraucheniids likeMacrauchenia are suggested to have been mixed feeders feeding on both browse and grass.[8] Sparnotheriodontids are suggested to have beenbrowsers.[9] Some proterotheriids are suggested to have been browsers,[10] while some members proterotheriid subfamily Megadolodinae likeMegadolodus have been suggested to have beenomnivorous with at least part of their diet consisting of hard fruit.[2]
Litopterna, like other "South American native ungulates" is thought to have originated from groups of archaic "condylarths" that migrated from North America.[1] Sequencing of thecollagenproteome andmitochondrial genome ofMacrauchenia has revealed that litopterns are true ungulates, sharing a common ancestor withNotoungulata, and with their closest living relatives beingPerissodactyla (the group containing livingequines,rhinoceros andtapirs) as part of the cladePanperissodactyla, with the split from Perissodactyla being estimated at around 66 million years ago.[11][12] The relationship of Litopterna to other South American native ungulate groups is uncertain, though it may be closely related to the "condylarth" groupDidolodontidae.[1] The earliest litopterns appeared during the earlyPaleocene, around 62.5 million years ago.[13]
Aside from South America, sparnotheriodontids are also known from theEocene agedLa Meseta Formation in the Antarctic Peninsula, representing the only record of litopterns on the Antarctic continent. Litopterns declined during thePliocene andPleistocene, likely as a result of climatic change and competition with recently immigrated North American ungulates who arrived as part of theGreat American interchange, following the connection of the previously isolated North and South America via theIsthmus of Panama.[1]Macrauchenia,Xenorhinotherium (Macraucheniidae) andNeolicaphrium (Proterotheriidae) were the last surviving genera of litopterns. All became extinct at the end of theLate Pleistocene around 12,000 years ago as part of theend-Pleistocene extinction event, along with most other large mammals in the Americas, co-inciding with the arrival of the first humans to the continent.[4] A study in 2025 suggested thatXenorhinotherium may have survived until the late Holocene based on a specimenradiocarbon dated to 3,493–4,217 yearscal.Before Present (BP).[14] It is possible that hunting had a causal role in their extinction.[15][16][17][1]
McKenna, Malcolm C; Bell, Susane K (1997).Classification of Mammals Above the Species Level. New York: Columbia University Press.ISBN978-0-231-11013-6.
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