Giant Ground Sloth

Megatherium

Paleontologists have identified an amazing variety of fossils of at least 23 different kinds and sizes of prehistoric sloths. The largest and grandest of these was the Megatherium, the extinct genus of the family Bradypodidae. This giant stood a staggering seven meters tall, and weighed a whopping seven tons! Megatherium, which means giant beast, lived from some 35 million years ago until around 11,000 years ago, which coincides with the last Ice Age. Theories abound in the scientific world about the disappearance of these incredible creatures. Among them is the suggestion that the fast changes of climate killed them, or that human hunters that had crossed into Alaska killed them for food as they moved throughout North America. Another hypothesizes that these southern sloths had lived on an island that held few carnivores for thousands of years. During the Great American Interchange, northern animals learned to watch out for carnivores that traveled south at the same time they did. The southern herbivores had not learned to be cautious as they traveled north. Many of them may have been eaten by their new neighbors. Another theory suggests that disease finished off these giants of the Americas. Unfortunately there were no journalists thousands of years ago, reporting back to the news room live, so there are still some very intriguing mysteries and gaps in our knowledge about these incredible animals.

“Salvador”, the giant pictured in the photo on the right, is a life-size replica of the Megatherium, thought to be the ancestor of our modern three-fingered sloth, Bradypus He guards over the Sloth Sanctuary.

Megalonyx

Megalonyx is the Greek name for another of the giant ground sloths, and means “great claw”. The name Megalonyx was proposed by future U.S. President Thomas Jefferson in 1797, based on fossil specimens found in a cave located in West Virginia, and given to Jefferson by a friend*. Megalonyx jeffersonii, of the family Megalonychidae, was a large, heavily built animal about 8 to 10 feet (2.5–3 m) long. Its maximum weight may have been as much as 800 pounds. This is medium-sized among the giant ground sloths.

Like other ground sloths it had a blunt snout, massive jaw, and large, peg-like teeth. The hind limbs were plantigrade (flat-footed) and this, along with its stout tail, allowed it to rear up into a semi-erect position to feed on tree leaves. The forelimbs had three highly developed claws that were probably used to strip leaves and tear off branches. M. jeffersonii was apparently the most wide-ranging giant ground sloth. Fossils are known from many Pleistocene sites in the United States, including most of the states east of the Rocky Mountains as well as along the west coast. It was the only ground sloth to range as far north as the present-day Yukon  and Alaska.
In the fall of 2010, the first specimen ever found in Colorado was discovered at the Ziegler Reservoir site near Snowmass Village (in the Rocky Mountains at an elevation of 8,874 feet).
Why the Giant Ground Sloth, as with other megafauna of the Miocene epoch, grew to such enormous size is a mystery (though theories abound).

Besides their bulk, these sloths were distinguished by their significantly longer front than hind legs, a clue that they used their long front claws to rope in copious amounts of vegetation. As big as it was, though, Megalonyx was a mere pup compared to the truly giant Megatherium. As to whether the Giant Ground Sloth was as slow and deliberate as a modern sloth, that’s something we may never know!

Megatherium and Megalonyx are distant relatives of today’s modern two- and three-fingered sloths that live in Central and South America.