"I had no idea at the time, to what kind of animal these remains belonged".
C. Darwin 1839
During the first two years of his voyage aboard HMS Beagle, Charles Darwin collected a considerable number of fossil mammals from various localities in Argentina and Uruguay. He recovered his first fossils at Punta Alta on September 23, 1832, and the last two years later at Puerto San Julián.
The fossils were packaged and sent to his former mentor the botanist/geologist John Stevens Henslow, deposited in the Royal College of Surgeons in London, and finally studied and named by Richard Owen between 1837 and 1845. Based on the fossil material Owen described a variety of Pleistocene mammals, including Equus curvidens, Glossotherium sp., Macrauchenia patachonica, Mylodon darwini, Scelidotherium leptocephalum and Toxodon platensis.
Unfortunately during April 10 and 11.1941 the paleontological collection of the Royal College was heavenly damaged by bombardment, almost 95% of the collection got lost. Beginning in 1946 the remaining material was transferred to the Natural History Museum in London, where it is still housed.
Fossils were known in South America since before the Spanish conquistadores, but interpreted as the remains of mythical creatures or giants annihilated by the gods. In 1774 the English Jesuit Thomas Falkner wrote:
"On the banks of the River Carcarania, or Tercero, about three or four leagues before it enters into the Parana, are found great numbers of bones, of an extraordinary bigness, which seem human. There are some greater and some less, as if they were of persons of different ages. I have seen thigh-bones, ribs, breast-bones, and pieces of skulls. I have also seen teeth, and particularly some grinders which were three inches in diameter at the base. These bones (as I have been informed) are likewise found on the banks of the Rivers Parana and Paraguay, as likewise in Peru. The Indian Historian, Garcilasso de la Vega Inga, makes mention of these bones in Peru, and tells us that the Indians have a tradition, that giants formerly inhabited those countries, and were destroyed by God for the crime of sodomy. I myself found the shell of an animal, composed of little hexagonal bones, each bone an inch in diameter at least; and the shell was near three yards over. It seemed in all respects, except it's size, to be the upper part of the shell of the armadillo; which, in these times, is not above a span in breadth."
22 years later the French naturalist George Cuvier published the first scientific work on a fossil South American mammal, and named it the giant sloth Megatherium americanum. In 1806 Cuvier described preliminary three proboscidean types, attributing them to the genus Mastodon. After these first investigations, there was almost no further research, in 1838 Owen wrote in his opening paragraph on his work on the fossil mammals collected by Darwin:
"It may be expected that the description of the osseous remains of extinct Mammalia, which rank amongst the most interesting results of Mr. Darwin's researches in South America, should be preceded by some account of the fossil mammiferous animals which have been previously discovered in that Continent. The results of such a retrospect are, however, necessarily comprised in a very brief statement; for the South American relics of extinct Mammalia, hitherto described, are limited, so far as I know, to three species of Mastodon, and the gigantic Megatherium."
The young Darwin got some of the first fossil determination wrong, so he attributed found osteoderms (regarded by Owen to belong to the giant "armadillo" Glyptodon) to Megatherium, following a reconstruction by Cuvier of an armoured ground sloth, and molars of Toxodon as remains of a giant rodent (but even Owen admitted that these teeth's bear a certain resemblance to those of rodents).
Owen by his part got the general relationship of this mammals incorrect, attributing some genera closer to existing animal-groups then they were in fact.
Influenced by the proposal of Owen, Darwin got convicted that "The most important result of this discovery, is the confirmation of the law that existing animals have a close relation in form with extinct species." (1839), surely a further clue for Darwin that species are not isolated and immutable in time.
Ironically in the error of Darwin there is a ray of truth, toxodonts are today considered highly derived native South American ungulates, distantly related phylogenetically to rodents and guanacos, whereas the large glyptodonts are not the ancestors of armadillos, but to the contrary, the latter are antecedent to the former.
FERNICOLA; VIZCAINO & DE IULIIS (2009): The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle. Revista de la Asociacon Geologica Argentina. 64(1): 147-159