Giant Ground Sloths Used Bipedal Locomotion

Artists often depict giant ground sloths standing on their hind legs while reaching into trees to eat leaves, but rare preserved tracks in Argentina show they occasionally walked for considerable distances using just their hind legs.  (Not unlike the bear in this youtube video  Other tracks show they also walked on all 4 limbs, so they had diverse methods of locomotion.  Ground sloths were able to observe their environment for greater distances when they walked upright, giving them an evolutionary advantage they shared with man.  It would have been hard for a predator to ambush them, and when prepared, this powerful mammal, protected with armor under its thick fur and armed with long claws, was more than a match for any other beast till men with spears came along.  Ground sloths were one of the few South American mammals that were able to successfully colonize North America in the Great American Biotic Interchange that took place  when a landbridge formed between the 2 continents.  Their ability to dig deep underground burrows enabled some species to colonize Alaska and Canada (See: )  I think ground sloths were so unique, that if I was able to bring just 1 species of extinct Pleistocene mammal back to life, I would choose a species of ground sloth.


Location of fossil sites where ground sloth tracks have been found.


A megatherium once walked here.  The Atlantic Ocean was probably about 50 miles to the east of this site when this ground sloth treaded this ground.

Shasta ground sloths in woodland

Like tree sloths, ground sloths browsed on leaves.  Some species ate forbs growing in grasslands.

The fossil sites where preserved tracks of ground sloths have been found are located near the coast of Argentina.  Collectively, they are known as Monte Hermoso cliffs, first studied by the Charles Darwin.  His discovery here of fossils of extinct species similar to existing species probably sparked his conception of the Theory of Evolution.  Most of the sites in this region include both fossil remains and fossil tracks, also known as ichnofossils.  Like on the coastal plain of southeastern North America, geologically older fossil sites are located farther inland than younger sites because high sea levels destroy the latter.  The oldest fossil site at Monte Hermoso dates to the late Miocene/early Pliocene before the Great American Biotic Interchange.  All the fossils at this more ancient site are from species that evolved in isolation when South America was an island continent.  A South American land mammal age was named after this site–Monthermosan.  Fossils of boryhianid, a 300 lb marsupial carnivore, were found here along with many species of archaic ungulates and rodents that evolved in South America such as cavies and capybaras.

Macrauchenia–an archaic South American ungulate.  One genus did survive to the late Pleistocene.




Toxodon–another archaic South American ungulate.  One genus of toxodon also survived till the late Pleistocene.

The Playa del Blanco site, also in this region, dates to about 16,000 BP and includes fossils of endemic species that originally evolved in South America along with the more recent North American invaders.  Scientists excavated 5 species of ground sloths, 2 species of glyptodonts, armadillos, mastodon, horses, llamas, deer, and sabertooths.  Trackways of many of these species have also been preserved.  Only 2 genuses of endemic ungulates still occurred here by then–toxodons and macrauchenia.  At nearby Pehuen Co, fossils and ichnofossils are estimated to date to about 12,000 BP.  Scientists have discovered the fossil tracks of flamingos and 30 other species of birds, cougar, bear, deer, llama, stegomastodon, horse, macrauchenia, megatherium, and a human that evidentally wore shoes or sandals.

The geological process that preserved these ichnofossils is interesting.  During the Ice Age, much of earth’s atmosphere became locked in glacial ice.  As a result, the Atlantic Ocean receded many miles to the east and rivers dried up.  Precipitation, that today drains through rivers, instead flowed into low areas of the terrain, forming temporary pools and ponds.  Animals attracted to the water left tracks in the mud.  Eolian sandstorms, common in this arid environment, covered the tracks.  Eventually, these sand dunes transformed into sandstone.  Today, ocean waves are eroding into this sandstone, revealing the ancient trackways.  Give the Argentinean government credit for protecting these rare and fascinating fossil sites.


Buyan, Cristana; Teresa Manera, Gustavo Politis, Silvia Annayo

“Following the Tracks of the First South Americans”


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