Megatherium

This specimen of M. americanum in Madrid, the first discovered (in 1788), was the first prehistoric animal skeleton mounted (in 1795).

The first fossil specimen of Megatherium was discovered in 1788 by Manuel Torres, on the bank of the Luján River in Argentina. The fossil was shipped to Museo Nacional de Ciencias Naturales in Madrid the following year, where it remains. It was reassembled by museum employee Juan Bautista Bru, who also drew the skeleton and some individual bones.[4]

Based on Bru's illustrations, comparative anatomist Georges Cuvier determined the relationships and appearance of Megatherium. He published his first paper on the subject in 1796, a transcript of a previous lecture at the French Academy of Sciences. He published on the subject again in 1804; this paper was republished in his book Recherches sur les ossemens fossiles de quadrupèdes.[3] In his 1796 paper, Cuvier assigned the fossil the scientific name Megatherium americanum. Cuvier determined that Megatherium was a sloth, and at first believed that it used its large claws for climbing trees, like modern sloths, although he later changed his hypothesis to support a subterranean lifestyle, with the claws used to dig tunnels.[3]

Fossils of Megatherium and other western megafauna proved popular with the Georgian era public until the discovery of the dinosaurs some decades later.

Since the original discovery, numerous other fossil Megatherium skeletons have been discovered across South America, in Argentina, Bolivia, Brazil, Chile, Colombia (Quipile, Cundinamarca),[5][6] Guyana, Paraguay, Peru and Uruguay.[2] New species in the genus Megatherium, M. urbinai and M. celendinense, have been described in 2004 and 2006 respectively.[7] M. celedinense is named after Celendin, Cajamarca Province in the Peruvian Andes.[8]

Evolution

M. americanum

Ground sloths are a diverse group belonging to superorder Xenarthra, a group of South American origin which also includes extinct pampatheres and glyptodonts, as well as living tree sloths, anteaters, and armadillos. The family to which Megatherium belongs, Megatheriidae, is related within superfamily Megatheroidea to the extinct families Nothrotheriidae and Megalonychidae, and to living three-toed sloths of family Bradypodidae, as deduced recently from collagen[9] and mitochondrial DNA[10] sequences obtained from fossils.

The subgenus and species Megatherium (Pseudomegatherium) tarijense appears to be a junior synonym of M. americanum, and merely a small individual.[11] The ground sloths, as with all other xenarthrans, evolved in isolation in South America, while it was an island continent 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. 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.[12]

The rhinoceros-sized Promegatherium of the Miocene is suggested to be the ancestor of Megatherium. The oldest (and smallest) species of Megatherium is M. altiplanicum of Pliocene Bolivia.[1] It was very similar to Promegatherium, and was also about the size of a rhinoceros. M. tarijense has been regarded as a medium-sized Megatherium species, larger than M. altiplanicum, but smaller than M. americanum. It roamed from the Tarija Basin in Bolivia to Yantac in Peru.[11] Species of Megatherium became larger over time, with the largest species, M. americanum of the Late Pleistocene, reaching the size of an African elephant.

Restoration

Megatherium is one of the largest land mammals known to have existed, weighing up to 4 tonnes[13] and measuring up to 6 m (20 ft) in length from head to tail.[14][15] It is the largest-known ground sloth, as big as modern elephants, and would have only been exceeded in its time by a few species of mammoth. The group is known primarily from its largest species, M. americanum. Megatherium species were members of the abundant Pleistocene megafauna, large mammals that lived during the Pleistocene epoch.

M. americanum pelvis

M. americanum foot

It 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. 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. Although it was primarily a quadruped, its trackways show that it was capable of bipedal locomotion. Biomechanical analysis also suggests it had adaptations to bipedalism.[16]

Megatherium had a narrow, cone-shaped mouth and prehensile lips that were probably used to select particular plants and fruits.[17] While some evidence suggests the animal could use its tongue to differentiate and select its foliage, the lips probably had a more important role in this. Like other sloths, Megatherium lacked the enamel, deciduous dentition, and dental cusp patterns of other mammals. Instead of enamel, the tooth displays a layer of cementum, orthodentine, and modified orthodentine, creating a soft, easily abraded surface.[18] The teeth of M. americanum exhibit extreme hypsodonty, indicative of its gritty, fibrous diet. Their teeth laterally show interlocking V-shaped occlusion surfaces, though they are nearly square in cross-section and exhibit bilophodonty. The teeth are spaced equidistantly in a series, located posteriorly in the mouth, which leaves space at the predentary; there is no diastema, though the length of this tooth row and of the predentary spout can vary by species. Analysis of wear and the biomechanics of the masticatory muscles suggests that they chewed vertically. Megatheres displayed deeper jaws than other sloth clades.[13] Some of the elements in the oral cavity of Megatherium were fused together: a rigidly articulated stylohyal and epihyal, and the apparatus lies farther anteriorly, which together with the elongated, steeply inclined mandibular symphysis, indicates a relatively shorter geniohyoid muscle and thus more limited capacity for tongue protrusion.[19] It was well adapted for strong, predominantly orthal (up-down) movement for eating rough vegetation. Megatherium possessed the narrowest muzzle of all ground sloths from the Pleistocene. This leads paleobiologists to believe that it was a very selective eater. It had the ability to pick and choose which leaves and twigs it would consume. Megatherium had a large narrow prehensile lip that was capable of grabbing and tearing off particular leaves and twigs and other sorts of vegetation.[17]

According to one study, Megatherium was probably mostly hairless (like modern elephants) due to its large mass making it susceptible to overheating.[20]

Megatherium inhabited woodland and grassland environments of the lightly wooded areas of South America[21] where it was an endemic species, as recently as 10,000 years ago. Megatherium was adapted to temperate, arid or semiarid open habitats. An example of these most recent finds is at Cueva del Milodón in Patagonian Chile.[22] The closely related genus Eremotherium (that has been classified occasionally as part of Megatherium)[23] lived in more tropical environments further north, and invaded temperate North America as part of the Great American Interchange.

Restoration of M. americanum by Robert Bruce Horsfall

The giant ground sloth lived mostly in groups, but it may have lived singly in caves. It probably had mainly a browsing diet in open habitats, but also it probably fed on other moderate to soft tough food. For millions of years, the sloth did not have many enemies to bother it, so it was probably a diurnal animal.

The giant ground sloth was a herbivore, feeding on 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.

M. americanum skull

The claw

A recent morpho-functional analysis[13] 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 that the teeth were used for cutting, rather than grinding, and that hard fibrous food was not the primary dietary component.

While it has been suggested that the giant sloth may have been partly carnivorous, this is a controversial claim. 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. 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.[24] However, noting that sloths lack the carnassials typical of predators and that traces of bone are absent from the many preserved deposits of sloth dung, Paul Martin has described this proposal as "fanciful".[upper-alpha 1] Carbon isotope analysis has found that Megatherium has isotope values similar to other megafaunal herbivores such as mammoths, glyptodonts, and Macrauchenia, and significantly unlike omnivorous and carnivorous mammals, suggesting that Megatherium was an obligate herbivore.[25]

In the south, the giant ground sloth flourished until about 10,500 radiocarbon years BP (8,500 BCE). Most cite the appearance of an expanding population of human hunters as the cause of its extinction.[26] There are a few late dates of around 8,000 BP and one of 7,000 BP[27] for Megatherium remains, but the most recent date viewed as credible is about 10,000 BP.[28] The use of bioclimatic envelope modeling indicates that the area of suitable habitat for Megatherium had shrunk and become fragmented by the mid-Holocene. While this alone would not likely have caused its extinction, it has been cited as a possible contributing factor.[29]

• Paleotoca

Wikimedia Commons has media related to Megatherium.

• BBC – Walking With Beasts – Megatherium
• London Review of Books – Bare Bones – Steven Shapin