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)