On 23 September 1832, Charles Darwin made an important discovery at Punta Alta, near Bahia Blanca, South America. He recorded the find in his Beagle diary:
Sunday Sept: 23rd 
A large party was sent to fish in a creek about 8 miles distant; great numbers of fish were caught. — I walked on to Punta alta to look after fossils; & to my great joy I found the head of some large animal, imbedded in a soft rock. — It took me nearly 3 hours to get it out: As far as I am able to judge, it is allied to the Rhinoceros. — I did not get it on board till some hours after it was dark. —
The bones were eventually shipped back to Britain, where the great anatomist Richard Owen identified them as belonging to an extinct giant ground sloth, Megatherium.
Megatherium's close relatives, the tree sloths, still live in South America. In later years, Darwin realised the fact that surviving species were often found in the same locations as closely related extinct ones suggested a geological succession of organic beings. This realisation helped convince him of the concept of descent with modification: a key element of his theory of evolution.