Giant Ground Sloth

Megatherium (/mɛɡəˈθɪəriəm/ meg-ə-THEER-ee-əm; from Greek méga (μέγα) 'great' + theríon (θηρίον) 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene[1] through the end of the Pleistocene.[3] It is best known for the elephant-sized type species M. americanum, sometimes called the giant ground sloth, or the megathere, native to the Pampas through southern Bolivia during the Pleistocene. Various other smaller species belonging to the subgenus Pseudomegatherium are known from the Andes. Megatherium is part of the sloth family Megatheriidae, which also includes the similarly elephantine Eremotherium, which was native to tropical South America and southern North America. Only a few other land mammals equaled or exceeded M. americanum in size, such as large proboscideans (e.g., elephants) and the giant rhinoceros Paraceratherium. Megatherium was first discovered in 1788 on the bank of the Luján River in Argentina. The holotype specimen was then shipped to Spain the following year wherein it caught the attention of the paleontologist Georges Cuvier, who was the first to determine, by means of comparative anatomy, that Megatherium was a sloth.[4] Megatherium became extinct around 12,000 years ago during the Quaternary extinction event, which also claimed most other large mammals in the New World. The extinction coincides with the settlement of the Americas, and a kill site where a M. americanum was slaughtered and butchered is known, suggesting that hunting could have caused its extinction.
Megatherium americanum is one of the largest land mammals known to have existed, weighing up to 4 t (4.4 short tons)[6] and measuring up to 6 m (20 ft) in length from head to tail.[7][8] 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. Megatherium species were members of the abundant Pleistocene megafauna, large mammals that lived during the Pleistocene epoch.

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.[9]

According to one study, Megatherium was probably mostly hairless (like modern elephants) due to its large size giving it a small surface-area-to-volume ratio, making it susceptible to overheating.[10]

Mouth
Megatherium had a narrow, cone-shaped mouth and prehensile lips that were probably used to select particular plants and fruits.[11] Megatherium also possessed the narrowest muzzle of all ground sloths from the Pleistocene, possibly meaning it was a very selective eater, able to carefully pick and choose which leaves and twigs to consume.[11] 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.[12] In Megatherium, the stylohyal and epihyal bones (parts of the hyoid bone which supports the tongue and is located in the throat) were fused together, and the apparatus lies farther upwards the throat, which, together with the elongated, steeply inclined mandibular symphysis, indicates a relatively shorter geniohyoid muscle and thus more limited capacity for tongue protrusion.[13] Analysis of wear and the biomechanics of the chewing muscles suggests that they chewed vertically. Megatheres displayed deeper jaws than other sloths.[6]

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.[12] The teeth of M. americanum exhibit extreme hypsodonty, indicative of its gritty, fibrous diet. Their teeth in side view show interlocking V-shaped biting surfaces, though they are nearly square in cross-section and exhibit bilophodonty. The teeth are spaced equidistantly in a series, located in the back of 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.