Posted by: Rob Viens | September 27, 2012

Darwin’s Sloth

A couple of quiet days for Darwin as he enjoyed the “sterility” of his surroundings and moved the boat:

“The weather is most beautiful.— Passing from the splendor of Brazil to the tame sterility of Patagonia has shown to me how very much the pleasure of exercise depends on the surrounding scenery.”  (Sept 26)

“That no time may be lost during the altering of the Schooner, we have changed our anchorage & stood further out, so as to survey some of the outer banks.” (Sept 27)

The “rhinoceros” that Darwin pulled out of the cliffs of Punta Alta a couple of days ago was actually the jaw of a Mylodon – a giant ground sloth. In fact, Richard Owen called it Mylodon darwinii – possibly the first species that Darwin discovered to bear his name.

Mylodon jaw fossil collected at Punta Alta, Published by Smith, Elder & Co, 65 Cornhill, London (from Zoology of the Voyage of the Beagle Vol.1, Richard Owen):


Owen and Darwin also comapred the fossils to the genus Megatherium – another type of giant ground sloth.  Mylodon and Megatherium were once thought to be closely related, but are now considered to be different families.  Darwin’s bones were the type bones for Mylodon.

Mylodon was found throughout Patagonia – a 400+ lb (~200 kg), 10 foot (~3 m) sloth that either grazed in the grasslands and/or browsed on the trees and shrubs.  Compare this to modern sloths which are typically not more than about 20 lbs (~10 kg) and other prehistoric giant sloths that weighed several tons. Mylodon was essentially the smallest of the “giant” sloths (there were at least 40 genera in the New World during the Pleistocene.) Within their skin they had bony plates (called osteoderms) that served as a sort of body armor. That and their long sharp claws (probably used for digging roots) keep predators away (at least until humans came along).

Mylodon darwinii by Rudolph Ludwig (1861)


Based on the geologic context, Darwin also estimated the age of the fossil and its surrounding deposits, suggesting that it was relatively young:

“I think, we are justified (although some of the shells are at present unknown to conchologists) in considering the shingle strata at Punta Alta, as belonging to an extremely modern epoch.” (Zoology of the Voyage of the Beagle Vol.1, Richard Owen (quote from the Introduction by Darwin))

Turns out he was right – the fossils were probably deposited in the last 40,000 years (a drop in the bucket in the geologic record which consists of billions of years).

Mylodon went extinct in Patagonia at the end of the last glacial period, about 10,000 years ago.

Mylodon reconstruction (from Encyclopedia Britanica)




  1. There is a video series on Darwin that I have often used in majors bio that actually depicts Darwin finding the sloth fossil…

  2. […] time, unlike the giant ground sloths fossils that Darwin had uncovered a few weeks earlier (see Darwin’s Sloth), it was some little critters whose bones he dug out of the cliff face.  Zoology of the Voyage of […]

  3. […] that his is most proud of, I’d say that finding fossils of recently extinct megafauna (See Darwin’s Sloth) and riding with the gauchos (see Shootin’ and Ridin’ with the Gauchos and Learning […]

  4. […] It was the second time Darwin sent crates of specimens back to John Henslow and would be the last time until he returned from his long journey to points south.  Along with numerous examples of plants and animals, this set of crates contained Darwin’s important fossil finds from Bahia Blanco (see Darwin’s Sloth) […]

  5. Reblogged this on GNN.

  6. […] 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). […]


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