Name: Megatherium (Greek for "giant beast"); pronounced meg-ah-THEE-ree-um

Megatherium (Greek for "giant beast"); pronounced meg-ah-THEE-ree-um Habitat: Woodlands of South America

Woodlands of South America Historical Epoch: Pliocene-Modern (five million-10,000 years ago)

Pliocene-Modern (five million-10,000 years ago) Size and Weight: About 20 feet long and 2-3 tons

About 20 feet long and 2-3 tons Diet: Plants

Plants Distinguishing Characteristics: Large size; giant front claws; possible bipedal posture

About Megatherium (the Giant Sloth)

Megatherium is the poster genus for the giant megafauna mammals of the Pliocene and Pleistocene epochs: this prehistoric sloth was as big as an elephant, about 20 feet long from head to tail and weighing in the neighborhood of two to three tons. Fortunately, for its fellow mammals, the Giant Sloth was restricted to South America, which was cut off from the earth's other continents during most of the Cenozoic Era and thus bred its own particular assortment of plus-sized fauna (a bit like the bizarre marsupials of modern-day Australia). When the central American isthmus formed, about three million years ago, populations of Megatherium migrated to North America, eventually spawning giant-sized relatives like Megalonyx, the fossils of which were described in the late 18th century by the future U.S. President Thomas Jefferson.

Giant sloths like Megatherium led much different lifestyles than their modern relatives. Judging by its huge, sharp claws, which measured almost a foot long, paleontologists believe Megatherium spent most of its time rearing up on its hind legs and ripping the leaves off trees, but it may also have been an opportunistic carnivore, slashing, killing and eating its fellow, slow-moving South American herbivores. In this regard, Megatherium is an interesting case study in convergent evolution: if you ignore its thick coat of fur, this mammal was anatomically very similar to the tall, pot-bellied, razor-clawed breed of dinosaurs known as therizinosaurs (the most imposing genus of which was the huge, feathered Therizinosaurus), which went extinct about 60 million years earlier. Megatherium itself went extinct shortly after the last Ice Age, about 10,000 years ago, most likely from a combination of habitat loss and hunting by early Homo sapiens.

As you might expect, Megatherium captured the imagination of the public just beginning to come to terms with the concept of giant extinct animals (much less the theory of evolution, which wasn't formally proposed, by Charles Darwin, until the mid-19th century). The first identified specimen of the Giant Sloth was discovered in Argentina in 1788, and definitively pegged as a sloth a few years later by the French naturalist Georges Cuvier (who at first thought Megatherium used its claws to climb trees, and then decided it burrowed underground instead!) Subsequent specimens were discovered over the next few decades in various other South American countries, including Chile, Bolivia, and Brazil, and were some of the world's best-known and best-loved prehistoric animals until the start of the golden age of dinosaurs.