South America 1550 to 1650?

After looking at a few websites, I found out that one of the last few Megatheriums, or giant ground sloth, were spotted in South America during 1550 when an Argentinean explorer, Ramon Lista, was hunting with a group of men when an extremely large animal walked slowly across their path. He describes it as looking like a monstrous Armadillo, and claims that firing their rifles at it had no affect, as if it had some kind of protective armour under it's hair. Later, a farmer invited Ramon into a cave near his property where he had seen a very large animal's hide. When Ramon studied the hide, he found hard bone like plates underneath the hair. Ramon then claimed that the hide had to of belonged to the same species of animal he had previously sighted. Right now, I'm trying to find out what the conditions of South America were like between then and a couple thousand years back so i can found out what mainly caused the extinction of the Giant Ground Sloths.