Megatherium

Megatherium; Temporal range: Pleistocene
	Megatherium fossil, from James Parkinson, "Organic Remains of a Former World", c.1830.
Conservation status
	Fossil
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Superorder:	Xenarthra
Order:	Pilosa
Family:	Megatheriidae
Genus:	Megatherium; Cuvier, 1796

Megatheriinae were a subfamily of elephant-sized ground sloths that lived from 2 million to 8,000 years ago. Their smaller ground sloth cousins were the Mylodon.

Characteristics

File:Giant Sloth.jpg

Giant sloth

Unlike its living relative, the tree sloth, Megatherium was one of the largest mammals to walk the Earth. Weighing as much as an African bull elephant, it had huge claws on its feet. These claws meant that it could not put its feet flat on the ground and so, like a modern anteater, it had to walk on the sides of its feet. Its footprints show that it walked mainly on its hind legs. When it stood on its hind legs, it was about twice the height of an elephant, or about twenty feet tall. They were one of the abundant Pleistocene megafauna - a wide variety of very large mammals that lived during the Pleistocene epoch.

Megatherium had a robust skeleton with a large pelvic girdle and a broad muscular tail. Its large size and specially adapted body made it possible to feed at heights otherwise unobtainable by other contemporary herbivores. Rising on its powerful hind legs and using its tail to form a tripod, 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 the modern tree sloth, which it would then use to pull leaves into its mouth.

Some recent morpho-functional analysis (Bargo, 2001) indicate that M. americanum was well adapted for strong and mainly vertical biting. The teeth are extremely hypsodont and bilophodont, and the sagittal section of each loph is triangular with a sharp edge. This suggests that the teeth were used mainly for cutting, rather than grinding, and that hard and fibrous food was not the main dietary component.

There is a common misbelief that the sabre-toothed cat Smilodon hunted Megatherium, but the sloths were far too big for even this large cat to attack. Richard Fariña and Ernesto Blanco of the Universidad de la República in Montevideo have analysed a fossil skeleton of M. americanum and discovered that its olecranon—the part of the elbow to which the triceps muscle attaches—was very short. This adaptation is found in carnivores and optimises speed rather than strength. The researchers say this would have enabled M. americanum to use its claws aggressively, like daggers (Fariña and Blanco, 1996). The conclusion is that due to its nutrient-poor habitats, Megatherium may have actually taken over the kills of Smilodon. A number of adult Glyptodon fossils have also been found where the shell was turned upside down. This hints at Megatherium scavenging or hunting this animal, as no other known animal existed in South America during that period that could flip an adult Glyptodon.

Distribution

It was formerly thought that Megatherium lived only in South America. However, a University of Florida research team recently found a skeleton in North America. This was a new species of giant sloth, which weighed as much as an African bull elephant, more than 5 tons. Unlike previously discovered species, it had 5 digits and 4 claws instead of 4 digits and three claws.

A giant sloth skeleton was found during construction of approach lighting for Seattle-Tacoma International Airport in Seattle, Washington, USA. The skeleton now resides at the Burke Museum, University of Washington.

Habits

Little is known about the giant ground sloth, but it is known that it was big and powerful. When it stood on its hind legs it was about twice the height of an elephant. Its very thick skin, which was covered with dense, heavy fur, protected it from predators. The giant ground sloth was an herbivorous animal that fed mainly on plants that grew on the ground. It is also believed that the giant sloth lived in groups, but it may have lived singly in caves.

Food & Feeding

The giant ground sloth lived in the lightly wooded areas of South America, feeding on the leaves such as yuccas, agaves, and grasses. The closely related genus Eremotherium lived in more tropical environments further north. Pulling itself upright to sit on its haunches or to stand, the giant ground sloth balanced its weight with its tail. It then tugged at plants with is feet, digging them up with the five sharp claws on each foot. The sloth used its simple teeth to grind down food before swallowing it, and its highly developed cheek muscles helped in this process. The sloth's stomach was able to digest coarse and fibrous food. For millions of years, the sloth had no enemies to bother it, so it was probably a diurnal feeder. It is likely that it spent a lot of time resting to aid digestion.

Evolution

When the ancestors of the giant ground sloth became established in South America, the land bridge joining North and South America was submerged. The sloth and other edentates (simple toothed creatures) developed undisturbed and were unique to this part of the world. The edentate families looked very different but had certain things in common: Simple teeth and different numbers of vertebrae to provide varying degrees of spinal movement. Nearly six million years passed before the land dividing the continents appeared again. Some sloths and other edentates then moved further north and became widespread before they disappeared completely. In the south, the giant ground sloth flourished until about 10,000 years ago. Some cite the appearance of human hunters as the cause of its extinction, others climatic changes; however, the actual cause is unknown.

Some Brazillian scientists believe that reports of a seven foot tall creature called a mapinguary (pronounced ma-ping-wahr-EE) may be a remnant population of the species. It is said to emit a disagreeble odor and have thick, matted fur that makes it nearly impervious to bullets and arrows. Expeditions to find specimens have proved fruitless. Other scientists believe that the reports are legends based on human contact with the last megatheriums.

References

Bargo, M. S. (2001). "The ground sloth Megatherium americanum: Skull shape, bite forces, and diet". Acta Palaeontologica Polonica. 46 (2): 173–192.

Fariña, R. A. (1996). "Megatherium, the stabber". Proceedings of the Royal Society of London. 263: 1725–1729. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)