Litopterna

Litopterna (from Ancient Greek: λῑτή πτέρνα "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 West Antarctica during the Eocene. They represent the second most diverse group of South American ungulates after Notoungulata. It is divided into nine families, with Proterotheriidae and Macraucheniidae being the most diverse and last surviving families.

Diversity

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. Paleocene and Eocene litopterns generally had small body masses, with Protolipterna (Protolipternidae) estimated to have had a body mass of 0.5–1.5 kilograms (1.1–3.3 lb), though the Eocene sparnotheriodontids 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 like Macrauchenia were considerably larger, with body masses around a ton. Adianthidae generally had small body masses, with members of the genus Adianthus estimated to weigh 7.4–20 kilograms (16–44 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. 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.