Litopterna (from "smooth heel") is an extinction 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.
Diversity
The body forms of many litopterns, notably in the limb and skull structure, are broadly similar to those of living
, 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 , though the Eocene sparnotheriodontids were considerably larger, with estimated body masses of around . Most proterotheriids had body masses of around while many macraucheniids had body masses of around . Some of the last macraucheniids like
Macrauchenia were considerably larger, with body masses around a ton.
generally had small body masses, with members of the genus
Adianthus estimated to weigh . 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
Hoof 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.
Macraucheniids had long necks and limbs.
Members of the macraucheniid subfamily Macraucheniinae saw the progressive migration of the nasal opening to the top of the skull,
Ecology
Litopterns were likely hindgut fermenters.
At least some macraucheniids like
Macrauchenia are suggested to have been mixed feeders feeding on both browse and grass.
Sparnotheriodontids are suggested to have been browsers.
Some proterotheriids are suggested to have been browsers,
while some members proterotheriid subfamily Megadolodinae like
Megadolodus have been suggested to have been
Omnivore with at least part of their diet consisting of hard fruit.
Evolutionary history
Litopterna, like other "South American native ungulates" is thought to have originated from groups of archaic "
" that migrated from North America.
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
Equidae,
rhinoceros and
tapirs) as part of the clade Panperissodactyla, with the split from Perissodactyla being estimated at around 66 million years ago.
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 62.5 million years ago.
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.
Classification
Further reading
External links
