Litopterna

Ancient Greek: λῑτή πτέρνα "smooth heel") is an extinct order of South American native ungulates that lived from the Paleocene to the end of the Pleistocene-early Holocene around 63 million-12,000 years ago, and were also present in Antarctica during the Eocene. They represent the second most diverse group of South American ungulates after Notoungulata.^[1] It is divided into nine families, with Proterotheriidae and Macraucheniidae being the most diverse and last surviving families.^[2]

bunodont (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 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.^[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 a moose-like prehensile lip is more likely.^[6]

hindgut fermenters.^[3] At least some macraucheniids like Macrauchenia are suggested to have been mixed feeders feeding on both browse and grass.^[7] Sparnotheriodontids are suggested to have been browsers.^[8] Some proterotheriids are suggested to have been browsers,^[9] 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.^[2]

tapirs) as part of the clade Panperissodactyla, with the split from Perissodactyla being estimated at around 66 million years ago.^[10][11] The relationship of Litopterna to other South American native ungulate groups is uncertain, though it may be closely related to the "condylarth" group Didolodontidae. The earliest litopterns appeared during the early Paleocene, around 63 million years ago.^[1]

Quaternary extinction event, along with most other large mammals in the Americas, co-inciding with the arrival of the first humans to the continent. It is possible that hunting had a causal role in their extinction.^{[12][13][14][1]}