Posted by: Rob Viens | August 28, 2013

When Sloths Ruled the Earth

Back on the Beagle, Darwin’s “whole day [was] consumed in telling [his] travellers tales” (Aug 27). That leaves us time to continue to delve into the fascinating world of prehistoric sloths and their relatives as we continue:

Sloth Week logo

Back in the Pleistocene (about 2.6 million to 10,000 years ago) sloths were not just restricted to the trees (like they are today), and they grew a lot bigger. Modern sloth species grow to be about 10-30 pounds (~4-8 kg), with a length of about 18-28 inches (~45-70 cm). Their ancestors were a wee bit bigger.  One species found in North America, for example, was Eremotherium eomigrans.  This friendly fellow is estimated to have been about 20 feet (6 m) long and have weighed more than 5 tons (~4,550 kg).  That is 500x the size of their modern descendants! (And to think, like their modern relatives, they just ate plants.)

Ground sloths show up in the South American fossil record in the later part of the Eocene Epoch (about 35 million years ago). Between that time and the present the bones of around 80 to 100 different genera  have been found (sources vary a bit on this, presumably because many of these genera are represented by a small number of bones). Darwin was not the first to describe ground sloths.  George Cuvier wrote about them in 1796 (one species, Catonyx cuvieri, is named after him)  and the first written reports from North America came from Thomas Jefferson in 1797 (and there is a species named Megalonyx jeffersonii, too). But Darwin’s discoveries of several different sloth genera in a small region of South America contributed significantly to the world’s collection of sloth fossils (and knowledge).  A significant portion of the first volume of the Zoology of the Voyage of the Beagle (Fossil Mammalia) is devoted to these fossil discoveries.

Illustration of some sloth bones (Scelidotherium) from the Zoology of the Voyage of the Beagle

Scelidotherium bones

As it turns out, all prehistoric ground sloths are currently classified into three large families – the Megatheridae, Northrotheridae and the Mylodonitidae. Today a few words on the last family – more on the other two in the next post.

I actually wrote a little bit about one member of the Mylodonitidae family last year when Darwin extracted one of its jaw bones near Punta Alta.  That particular species was ultimately named after Darwin by Richard Owen (for more on  Mylodon darwinii see Darwin’s Sloth).

In 1833, Darwin also found a nearly complete fossil of another member of Mylodonitidae – a member of the genus Scelidotherium. As ground sloth weights go, these guys were on the lower end – “only” weighing about 600 pounds to 1 ton.

Scelidotherium leptocephalum (from the blog Bienvenue chez EMILIE)

Scelidotherium leptocephalum

Reconstruction of Scelidotherium leptocephalum (by Xstor)

Scelidotherium leptocephalum

Yet another Mylodonid collected by Darwin was Glossotherium.  The genera name, which means “tongue beast”, came from the presence of a large tongue cavity described by Richard Owen.  It is likely that these shaggy sloths used their large tongues like a modern giraffe, to strip the leaves off branches.

Glossotherium robustum (from A History of Land Mammals in the Western Hemisphere, 1913)

Glossotherium robustum

Interestingly, the modern three-toed sloths (Bradypodidae family), are probably most closely related to the Mylodontids sloths (though recent research my call that into question, so stay tuned).

Tomorrow – a few words on the other two sloth families, and a more recent discovery of an aquatic ground sloth.  How cool is that! (RJV)

PS – I forgot to mention that the images in the title banner include a Glyptodon (from Arthur’s Dinosaur Clipart) and a Mylodon (from Skeleton of the Mylodon, a fossil American Sloth of Pliocene age  by Taylor, 1904).

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