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The first discovery of the fossil of Megatherium was on the bank of the Lujan River in Argentina, uncovered in 1788 by Manuel Torres. In the following year, it was shipped to the Museo Nacional de Ciencias Naturales in Madrid, where it now resides. Reassembled by a museum employee by the name Juan Bautista Bru, assigned with the task of drawing the majority of the skeleton, as well as some specific bones in particular.

Based on Bru’s illustrations of the creature, comparative anatomist Georges Cuvier had determined the relationships and appearance of Megatherium. He pushed to publish his first paper on the topic in the year of 1796, a documented transcript of an older lecture at the French Academy of Sciences. He published a paper on the subject once more in 1804; this paper in particular was republished in his own book Recherches sur les ossemens fossiles de quadrupèdes. In the year 1796, Cuvier released another paper, assigning the fossil the scientific name Megatherium americanum. Cuvier had ultimately determined that Megatherium was a sloth, and at first believed that it used its large claws for climbing trees, like modern sloths, although he later changed his hypothesis to support a subterranean lifestyle, with the claws used to dig tunnels.

Megatherium is one of the largest land mammals known to have existed, weighing up to 4 metric tons and measuring up to 6 m (20 ft) in length from head to tail. It is the largest-known ground sloth, as big as modern elephants, and would have only been exceeded in its time by a few species of mammoth. The group is known primarily from its largest species, M. americanumMegatherium species were members of the abundant Pleistocene megafauna large mammals that lived during the Pleistocene epoch.

It had a robust skeleton with a large pelvic girdle and a broad muscular tail. Its large size enabled it to feed at heights unreachable by other contemporary herbivores. Rising on its powerful hind legs and using its tail to form a tripod, Megatherium could support its massive body weight while using the curved claws on its long forelegs to pull down branches with the choicest leaves. This sloth, like a modern anteater, walked on the sides of its feet because its claws prevented it from putting them flat on the ground. Although it was primarily a quadraped, its trackways show that it was capable of bipedal locomotion. Biomechanical analysis also suggests it had adaptations to bipedalism.

In the south, the giant ground sloth flourished until about 10,500 radiocarbon years BP (8,500 BCE). Most cite the appearance of an expanding population of human hunters as the cause of its extinction. There are a few late dates of around 8,000 BP and one of 7,000 BP for Megatherium remains, but the most recent date viewed as credible is about 10,000 BP. The use of bioclimatic envelope modeling indicates that the area of suitable habitat for Megatherium had shrunk and become fragmented by the mid-Holocene. While this alone would not likely have caused its extinction, it has been cited as a possible contributing factor. Legends surrounding this creature imply that it may still persist into modern times, in the mythological name Mapinguari, a creature said to stand at 7 feet tall and covered in thick matted fur. The creature has one eye, long sharp powerful claws capable of tearing down small trees and animals. The most mysterious yet frightening feature of this beast is that is has a gigantic mouth on it’s torso which uses to devour other beasts of the field.