Megatherium

Cladus: Eukaryota
Supergroup: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Superclassis: Tetrapoda
Classis: Mammalia
Subclassis: Theria
Infraclassis: Placentalia
Ordo: Pilosa
Subordo: Folivora
Familia: †Megatheriidae
Subfamilia: Megatheriinae
Tribus: Megatheriini
Subtribus: Megatheriina
Genus: Megatherium
Species: M. americanum - M. altiplanicum - M. gallardoi - M. istilarti - M. medinae - M. parodii - M. sundti - M. tarijense

Name

Megatherium Cuvier, 1796

Megatherium ("Great Beast") was a genus of elephant-sized ground sloths endemic to Central America and South America that lived from the Pliocene through Pleistocene existing approximately 5.3 million years.[1] Its size was exceeded by only a few other land mammals, including mammoths and the even larger Paraceratherium.

The rhinoceros-sized Promegatherium is suggested to be the ancestor of Megatherium.

Characteristics

Megatherium was one of the largest mammals known, weighing up to eight tons, about as much as an African bull elephant. Although it was primarily a quadruped, its footprints show that it was capable of assuming a bipedal stance. When it stood on its hind legs it was about twenty feet (6 m) tall, twice the height of an elephant. 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. Megatherium species were members of the abundant Pleistocene megafauna, 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 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. Its jaw is believed to have housed a long tongue, which it would then use to pull leaves into its mouth, similar to the modern tree sloth.

Some recent morpho-functional analysis[2] indicates that M. americanum was adapted for strong vertical biting. The teeth are hypsodont and bilophodont, and the sagittal section of each loph is triangular with a sharp edge. This suggests the teeth were used for cutting, rather than grinding, and that hard fibrous food was not the primary dietary component.
Megatherium americanum skeleton, Muséum national d'Histoire naturelle, Paris

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 like daggers.[3] They suggest that to add nutrients to its diet, Megatherium may have taken over the kills of Smilodon. Based on the estimated strength and mechanical advantage of its biceps, it has been proposed that Megatherium could have overturned adult glyptodonts (large, armored xenarthrans, related to armadillos) as a means of scavenging or hunting these animals.[3] However, other zoologists have described this proposal as "fanciful".[4]

Habitat

Megatherium inhabited woodland and grassland environments of the lightly wooded areas of South America.[5]

Distribution

Megatherium lived in parts of South America, where it was an endemic species, as recently as 10,000 years ago. An example of these most recent finds is at Cueva del Milodon in Patagonian Chile.[6] The closely related genus Eremotherium lived in more tropical environments further north, and in North America

Habits

The giant ground sloth lived mostly in groups, but it may have lived singly in caves. While mostly vegeterian, it has been suggested that it may have used its size and strength to take over the kills of sabre-toothed cat Smilodon and to scavenge or hunt large, armored glyptodonts. For millions of years, the sloth had no enemies to bother it, so it was probably a diurnal animal.

Diet and feeding habits

The giant ground sloth was an herbivore, feeding on the leaves such as yuccas, agaves, and grasses. While it fed chiefly on terrestrial plants it could also stand on its hind legs, using its tail as a balancing tripod and reach for upper growth vegetation. It would pull itself upright to sit on its haunches or to stand and then tugged at plants with its 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. It is likely that it spent a lot of time resting to aid digestion.

While it has been suggested that the giant sloth may have been a carnivore, this is still a controversial claim.[7] It is depicted as both a herbivore and a scavenger in the BBC documentary Walking with Beasts. It is depicted as an omnivore in the Animal Planet special Giant Monsters. In another BBC documentary, Prehistoric America, Megatherium is depicted as an herbivore.

Evolution

The ground sloths, as with all other xenarthrans, evolved in isolation in South America, while it was an island during the Paleogene. During the Pliocene, the Central American Isthmus formed, causing the Great American Interchange, and a mass extinction of much of the indigenous South American megafauna. Ground sloths were largely unaffected and continued to thrive in spite of competition from the northern immigrants. In fact, ground sloths were among the various South American animal groups to migrate northwards, into North America, where they remained and flourished until the late Pleistocene.[8] In the south, the giant ground sloth flourished until about 10,000 years ago. Most cite the appearance of an expanding population of human hunters as the cause of its extinction.

The oldest (and smallest) species of Megatherium is M. altiplanicum of Pliocene Bolivia.[9] It was very similar to the Miocene ground sloth, Promegatherium, and was about the size of a rhinoceros. Species of Megatherium became larger and larger, with the largest species, M. americanum of the late Pleistocene, reaching the size of an African Elephant.

Notes

^ A. E. Zurita, A. A. Carlini, G. J. Scillato-Yané and E. P. Tonni. 2004. Mamíferos extintos del Cuaternario de la Provincia del Chaco (Argentina) y su relación con aquéllos del este de la región pampeana y de Chile. Revista geológica de Chile 31(1):65-87
^ M.S. Bargo (2001)
^ a b Fariña and Blanco (1996)
^ P. S. Martin (2005), p. 35
^ M.C. McKenna (1997)
^ C. Michael Hogan (2008) Cueva del Milodon, Megalithic Portal [1]
^ [2][dead link]
^ P. S. Martin (2005)
^ Saint-André & De Iuliis (2001)

References

Bargo, M. S. (2001). "The ground sloth Megatherium americanum: Skull shape, bite forces, and diet". Acta Palaeontologica Polonica 46 (2): 173–192. Retrieved 2010-07-05.
Fariña, R. A.; R. E. Blanco (1996). "Megatherium, the stabber". Proceedings of the Royal Society of London 263 (1377): 1725–1729. doi:10.1098/rspb.1996.0252. JSTOR 50663. PMID 9025315.
Martin, P. S. (2005). Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America. University of California Press. ISBN 0520231414.
McKenna, M. C, and S. K. Bell. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp.
Saint-André, P-A.; De Iuliis, G. (2001). "The smallest and most ancient representative of the genus Megatherium Cuvier, 1796 (Xenarthra, Tardigrada, Megatheriidae), from the Pliocene of the Bolivian Altiplano". Geodiversitas 23 (4): 625–645. Retrieved 2010-07-05.