Megatherium (Megatherium americanum)

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Megatherium americanum, or more commonly the "giant ground sloth," was a very large mammalian herbivore that lived in the arid steppes of South America during the last ice age. Along with most of the large Pleistocene fauna, the Megatherium became extinct around 11,000 years ago. At 20 to 25 feet in length and weighing close to 5 tons, the Megatherium was the largest of the ground-dwelling sloths and was proportional to a modern elephant in size.

Fossil forms of Megatherium are numerous. Dozens of complete skeletons have been discovered, making it the best known of the large ground sloths. The first Megatherium specimen to be examined by scientists was sent to Spain in 1789. Immediately the scientist could see that the bones belonged to a very large plant-eating animal with a distinct shape and characteristics.

The Megatherium had a robust skeleton with a large pelvic girdle and a broad muscular tail. It's large size and specially adapted body made it possible to feed at heights otherwise unobtainable to other contemporary herbivores. Rising on its powerful hind legs and using its tail to form a tripod, the Megatherium was able to support its massive body weight while using its long forelegs with curved claws to pull down branches with the choicest leaves. Its large deep jaw is believed to have housed a long tongue, as in modern tree sloths, which it would then use to pull leaves into its mouth.

The Megatherium would have ambled from place to place on all fours at a relatively slow pace. The bones in its feet were so oddly arranged that they were unable to place them flat on the ground. It walked on the sides of its feet with the toes and claws pointing inward. The long claws on the forelegs were primarily used for tree browsing but would also have been a formidable defense against predators such as saber-toothed cats. The Megatherium also had built-in body armor. A small section of remarkably well-preserved hide was found in a cave in 1895 by a rancher. The hide was studded with small, hard, calcium nodules that would have aided against penetrating bites and slashes from Pleistocene predators.

At least five separate species of ground sloth have been identified from the upper Pliocene and Pleistocene (7 to 2 million years ago). Originating in South America, they eventually migrated into North America about 9 million years ago. The North American species such as the bear-sized Nothrotheriops shastensis and the ox-sized Megalonyx jeffersonii, though large by any standards, were dwarfed by their South American cousin Megatherium americanum. They also became extinct around 11,000 years ago at the end of the Ice Age. Their closest living relatives are armadillos, anteaters, and the small tree-dwelling sloths of South America.