Posted by: Rob Viens | August 29, 2013

Lumbering Giants and Swimming Sloths

On the 28th, Darwin was done telling stories and back to preparing for the next leg of his overland journey:

“Actively employed in arranging things, in order to start to Buenos Ayres by land. — the feeling of excitement quite delightful after the indolence of the week spent at the fort of Bahia Blanca.” (Aug 28)

Meanwhile, here in 2013, I still have sloth on the brain, so it is time for the next installment of:

Sloth Week logo

As I mentioned yesterday, there are basically three extinct families of prehistoric ground sloths – the Megatheridae, Northrotheridae and the Mylodonitidae. (Technically, there are also some extinct ground sloths that come from the Megalonychidae family which includes the living two-toed sloths.) Today, lets take a look at the Megatheridae and Northrotheridae (which incidentally are lumped into the same “infraorder” – the Megatheria).

First –the oldest and largest of the ground sloths families – the Megatheridae. One of the largest members of the family (reaching a length of about 20 feet (6 m)) was Eremotherium.

Eremotherium scale (from the Museu Geológico da Bahia)

eremotherium and human

Darwin found the head of another giant Megatheridae at Punta Alta (which Owen called Megatherium cuvieri – now more correctly called Megatherium americanum)

Megatherium reconstruction (from Wikipedia Commons):

Megatherium reconstruction

Here is a cool thought – some evidence (tracks and bone structure) suggests that Megatherium walked on its hind legs (e.e., it was bipedal).  Analysis of their tracks imply that they could move about the same speed as a walking human. Imaging that 20-foot-tall critter lumbering up behind you! (Good news is, like all sloths, Megatherium was still a vegetarian.)

Of the three extinct sloth families, Northrotheridae appears to be the youngest – with representatives appearing about 12 million years ago in the fossil record.

Several Northrotheridae fossils have been found in caves, and it is believed that they (as well as some of their relatives) may have used caves (or burrows that they dug) to help stay warm.  In some more recent cave deposits (like the one shown below in Arizona from the National Park Service) paleontologists have found  actually sloth dung.

sloth dung

One member of the Northrotheridae family was Thalassocnus – which what might have been a fully aquatic ground sloth. Yup, that’s right, a giant ground sloth that returned to the seas, spent much of its life in the ocean and feed out on the continental shelf – a lifestyle sort of like a walrus. It is believed that they used their (distinctive) sloth claws to anchor themselves into the ocean floor while feeding. Darwin did not discover any fossils from this genus, and in fact, to this day they have only been found in Peru and Chile.  But since the return to the sea by mammals over the past ~50 million years is one of my favorite evolutionary stories, I had to include them here. (Interestingly, the 5 reported species of Thalassocnus show more and more aquatic adaptations from the older species to the newer.)

Thalassocnus skeleton from the Natural History Museum in Paris (from Wikipedia Commons):


A reconstruction of the aquatic sloth (draw by Bill Parsons and posted on

aquatic sloth

Although Darwin did not bring home any Northrotheridae fossils, he did find remains that Richard Own classified as being a member of the family that includes modern two-toed sloths – the Megalonychidae.  Specifically he found the lower jaw bone of a Megalonyx jeffersonii – the sloth made famous by Thomas Jefferson a few decades before. That being said, it appears that although it has been found as far north as Alaska,  Megalonyx is not found as far south as Argentina. In fact, all of the ground sloths belonging to the Megalonychidae appear to evolved as sloths migrated northward into the Caribbean and North America and islands of the Caribbean Sea and Gulf of Mexico (yes – sloths even made it to Cuba and the Caribbean islands – more on that later.) So it is likely that this fossil came from a different genus of ground sloth.

Megalonyx jeffersonii (from Wikipedia Commons):

Megalonyx jeffersonii

Next a look at another giant prehistoric Xenarthra that Darwin dug up – the Glyptodon. (RJV)


  1. Sloths that swim! As if I needed more to be excited about the giant ground sloths.

    • Tell me about it! I couldn’t believe it when I first read about giant aquatic sloths. Imagine if they had continued to evolve and had fully adapted to the water. We’d have a sort-of sloth equivalent of the manatee!

  2. […] you recall – Megatherium is a giant round sloth (read more on megatherium at Lumbering Giants and Swimming Sloths).  However, Darwin later updates his assessment of the skull (and shares a little more about its […]

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