Some Giant Ground Sloths Dug Long Burrows

Many interesting habits of the extinct species of Pleistocene megafauna will remain unknown to science because dead fossil specimens inadequately represent the complete behavior repertoire of once living animals.   For example no scientist would have ever guessed that some species of giant ground sloths dug long underground chambers.  Sure, they had big claws and were anatomically built for digging, but scientists assumed they merely dug for roots and tubers.  The surprising discovery of paleoburrows dug by 2 different species of extinct ground sloth reveals a habit no zoologist would have predicted.

Glossotherium, also known as Harlan’s ground sloth.  It lived in North and South America.  It’s 1 of 2 species that we know for sure dug burrows.  Other species probably did as well.

Scelerodotherium, also known as Darwin’s ground sloth.  It lived in South America and also dug burrows.

Since 1928, scientists have discovered 42 paleoburrows dug by giant ground sloths in the Mar del Plata region of Argentina.  These sites are near the Atlantic coast not far from Buenos Aires and are located on floodplains next to or directly in sea cliffs.  They range in age from Pliocene to late Pleistocene.  The tunnels are from 2-4 feet wide and as much as 70 feet long, and they are multichambered.  Some are filled with collapsed sediment while others are still intact.

Photos of giant ground sloth tunnels dug into sea cliffs located in the Mar del Plata region of Argentina.  Click to enlarge.  The photos are from the below referenced paper.

A geologist discovered the first ground sloth burrow known to science in 1928, but it was a minor footnote ignored by paleontologists for 70 years.  This burrow contained the skeleton of a Scelerodotherium and was filled with volcanic ash.  Scelerodotherium was a 1600 pound ground sloth with a skull resembling that of an anteater.  Vertebrate paleontologists at first rejected the idea that ground sloths dug burrows because they thought the animals were too big to be fossorial creatures.  The largest extant burrowing mammal is the African aardvark which grows to 200 pounds.  However, the authors of the below referenced paper determined that these burrows were dug by ground sloths.  The claw marks on the sides and roofs of the tunnels match those of 2 different species of ground sloths–Scelerodotherium (Darwin’s ground sloth) and Glossotherium (Harlans’s ground sloth).  Scelerodotherium was restricted to South America, but Harlan’s ground sloth lived in South and North America, including what’s now Georgia where its fossil remains were unearthed in Yarbrough cave, Bartow County and a few coastal sites.  Harlan’s ground sloth grew to 2400 pounds and was the larger of the 2 species.  Both species had long claws, well developed triceps muscles shaped for digging, and the ability to balance on 2 limbs–all characteristics that enabled them to dig tunnels.  Paleoburrows of armadilloes and pampatheres (extinct giant armadilloes) have also been discovered in the Mar del Plata region.

Ground sloths probably dug their long underground chambers for 2 reasons.  When not feeding, they could retreat into their burrows to avoid predation.  With their backs protected on 3 sides by dirt walls, they could easily defend themselves from a frontal attack by using their long claws.  More importantly, the tunnels provided the sloths with some protection from the elements.  The edentates–the order including sloths, anteaters, and armadilloes–are primitive mammals with poorly developed thermoregulatory systems.  During extremly cold or hot weather, ground sloths could stay in their underground chambers and remain well insulated.  This adaptation explains how some species of ground sloths survived in cold climates.  Fossils of Jefferson’s ground sloth have been found as far north as Alaska and the Canadian Northwest Territories.  Although there’s no direct evidence Jefferson’s ground sloth dug burrows, I think we can safely assume it did based on its fossil distribution.

If we could travel back in time to visit the Pleistocene, ground sloths might be a rare site, even when they were common.  They likely stayed in their burrows through most of the winter, emerging only during warm days to feed.  During hot months, they probably were nocturnal, feeding in the darkness and returning to their burrows shortly after the sun rose.  Their preference for fossorial living explains why their fossils are so often found in caves.  Caves are ready made burrows that provided protection from the elements.

Many extinct and extant organisms used or even depended upon ground sloth burrows.  The fossil remains of a glyptodont were found in 1 ground sloth burrow.  Glyptodonts were physically incapable of digging their own.  Giant tortoises too probably made use of sloth burrows, possibly explaining how this frost sensitive species survived as far north as Bartow County, Georgia during the Ice Age. (See https://markgelbart.wordpress.com/2011/04/15/the-extinct-pleistocene-giant-tortoise-hesperotestudo-crassicutata-must-have-been-able-to-survive-light-frosts/).

Extant gopher tortoises dig burrows that provide habitat and refuge for dozens of other vertebrate and invertebrate species.  There’s no telling how many animals made use of ground sloth burrows.

Reference:

Vizcaino, Sergis; et. al.

“Pleistocene Burrows in the Mar del Plata area (Argentina) and their Probable Builders”

Acta Palaeontologica Polonica 46 (2) 2001 pp. 280-301

See also https://markgelbart.wordpress.com/2011/03/30/does-a-species-of-giant-ground-sloth-still-exist-in-the-amazon-rain-forest/

And https://markgelbart.wordpress.com/2012/08/19/is-the-9-banded-armadillo-dasypus-novemcinctus-a-dwarf-mutation-of-the-pleistocene-species-dasypus-bellus/

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Tags: Darwin's ground sloth, Glossotherium, Harlan's ground sloth, Mar del Plata region, paleoburrows, Scelerodotherium