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Megatherium ( ; from Greek méga () 'great' + theríon () 'beast') is an extinct genus of endemic to South America that lived from the Early Pliocene through the end of the Late Pleistocene. It is best known for the elephant-sized type species Megatherium americanum, primarily known from the Pampas, but ranging southwards to northernmost Patagonia and northwards to southern Bolivia during the late Middle Pleistocene and Late Pleistocene. Various other species belonging to the subgenus Pseudomegatherium and ranging from sizes comparable to M. americanum down to considerably smaller, are known from the Andean region.
The first (holotype) specimen of Megatherium was discovered in 1787 on the bank of the Luján River in what is now northern Argentina. The specimen was then shipped to Spain the following year wherein it caught the attention of the French paleontologist Georges Cuvier, who named the animal in 1796 and was the first to determine, by means of comparative anatomy, that Megatherium was a giant sloth.
Megatherium is part of the sloth family Megatheriidae, which also includes the closely related and similarly giant Eremotherium, comparable in size to M. americanum, which was native to tropical South America, Central America and North America as far north as the southern United States.
Megatherium americanum is thought to have been a browser that fed on the foliage and twigs of trees and shrubs using a black rhinoceros–like prehensile upper lip. Despite its large body size, Megatherium americanum is widely thought to have been able to adopt a bipedal posture at least while standing, which allowed it to feed on high-growing leaves, as well as possibly to use its claws for defense.
Megatherium became extinct around 12,000 years ago as part of the end-Pleistocene extinction event, simultaneously with the majority of other large mammals in the Americas. The extinctions followed the first arrival of humans in the Americas, and one and potentially multiple where M. americanum was slaughtered and butchered is known, suggesting that hunting could have been a factor in its extinction.
At the direction of the cabinets main taxidermist Juan Bautista Bru, the specimen was then mounted for public exhibition (which remains unaltered in the modern museum display). In 1796 a scientific description of the skeleton was published authored by Bru along with engineer Joseph Garriga, with engravings by Manuel Navarro. As the work was going through the process of publication in 1795, preliminary prints of the paper were obtained by French diplomat Philippe-Rose Roume who was in Madrid at the time, who sent them to the National Museum of Natural History ( Muséum national d'histoire naturelle) in Paris, France, where they were seen by French anatomist and paleontologist Georges Cuvier.
Cuvier, working solely from the prints from Madrid and not visiting the specimen personally, and using comparative anatomy with "edentate" mammals (now recognised as members of the order Xenarthra) in the collection of the Paris museum, correctly recognised that the remains represented those of a giant sloth, and an animal that was entirely extinct and not living. In early 1796, somewhat before the full publication of the work by Bru, Garriga and Navarro, Cuvier published a paper naming the species Megatherium americanum (literally "Great American beast"), becoming the first fossil mammal to be identified with both a genus and species name. Which description had priority has been controversial in the past. Cuvier later wrote a fuller description in 1804, which was republished in his famous 1812 book Recherches sur les ossemens fossiles de quadrupèdes. Cuvier identified Megatherium as a sloth primarily on the basis of its skull morphology, the dental formula and the shoulder, while regarding the anatomy of its limbs as more similar to and . Cuvier suggested that based on the proportions of its limbs (which are approximately equal to each other), that Megatherium did not jump or run, nor crawl like living sloths, with the presence of a clavicle and well developed crests on the humerus, suggesting to Cuvier that the animal probably used its forelimbs to grasp. A later publication in 1823 by Cuvier suggested that giant carapaces found in the Pampas also belonged to Megatherium, but British paleontologist Richard Owen in 1839 demonstrated that these actually belonged to another extinct group of xenarthrans called glyptodonts that were related to armadillos.Fernicola, J. C., Vizcaino, S. F., & De Iuliis, G. (2009). The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle. Revista De La Asociación Geológica Argentina, 64(1), 147–159. Retrieved from https://revista.geologica.org.ar/raga/article/view/1339
Additional remains of Megatherium were collected by Charles Darwin during the Voyage of the Beagle in the 1830s, these remains were assigned by Richard Owen in 1840 to the species Megatherium cuvieri, which had been named by Anselme Gaëtan Desmarest in 1822. These remains are now assigned to M. americanum., London, England|200x200px]]Owen later wrote a monograph series from 1851 to 1860 thoroughly describing the anatomy of M. americanum.Owen R (1851) On the Megatherium ( Megatherium americanum, Blumenbach). Part I. Preliminary observations on the exogenous processes of vertebrae. Phil Trans R Soc Lond 141:719–764Owen R (1855) On the Megatherium ( Megatherium americanum, Cuvier and Blumenbach). Part II. Vertebrae of the trunk. Phil Trans R Soc Lond 145:359–388Owen R (1856) On the Megatherium ( Megatherium americanum, Cuvier and Blumenbach). Part III. The skull. Phil Trans R Soc Lond 146:571–589Owen R (1858) On the Megatherium ( Megatherium americanum, Cuvier and Blumenbach). Part IV. Bones of the anterior extremities. Phil Trans R Soc Lond 148:261–278Owen R (1859) On the Megatherium ( Megatherium americanum, Cuvier and Blumenbach). Part V. Bones of the posterior extremities. Phil Trans R Soc Lon149:809–829Owen R (1861) Memoir on the Megatherium, or Giant Ground-sloth of America ( Megatherium americanum, Cuvier). Williams and Norgate, London
From the late 19th century onward additional species of Megatherium were described. In 1888 Argentine explorer Francisco Moreno erected the species Megatherium filholi for remains found in the Late Pleistocene of Argentina.Moreno, F.P. (1888): Informe preliminar de los progresos del Museo La Plata durante el primer semestre de 1888. – Boletín del Museo La Plata, 1: 1-35. In 1880 Paul Gervais and Florentino Ameghino described the species M. tarijense from remains of Pleistocene age found in Bolivia. In 1893 Rodolfo Amando Philippi erected the species M. sundti and M. medinae from remains found in the Pleistocene of Bolivia and Chile, respectively.Philippi, R.A. 1893a. Vorläufige Nachricht über fossile Säugethierknochen von Ulloma, Bolivia. Zeitschrift der deutschen geologischen Gesellschaft 45: 87–96.Philippi, R.A. 1893b. Noticias preliminares sobre huesos fósiles de Ulloma. Anales de la Universidad de Chile 82: 499–506. In 1921, Florentino's brother Carlos Ameghino and Lucas Kraglievich described the species Megatherium gallardoi based on remains found in the Pampas of Northern Argentina, of Early-Middle Pleistocene age.C. Ameghino, L. Kraglievich Descripción del " Megatherium gallardoi" C. Amegh. descubierto en el Pampeano inferior de la ciudad de Buenos Aires Anales del Museo Nacional de Historia Natural de Buenos Aires, 31 (1921), pp. 134–156Brandoni D., Soibelzon E. & Scarano A. 2008. — On Megatherium gallardoi (Mammalia, Xenarthra, Megatheriidae) and the Megatheriinae from the Ensenadan (lower to middle Pleistocene) of the Pampean region, Argentina. Geodiversitas 2008 (4): 793-804. In 2001, the species M. altiplanicum was described based on remains found in the Pliocene of Bolivia. In 2004, the species Megatherium urbinai was erected based on remains found in Pleistocene aged deposits in Peru. In 2006, the species Megatherium celendinense was erected for remains of Pleistocene age found in the Peruvian Andes.
Mitochondrial DNA sequences obtained from M. americanum indicates that ( Bradypus) are their closest living relatives. Phylogeny of sloths after Delsuc et al. 2019.
Megatheriidae is suggested to have diverged from other sloth families during the Oligocene, around 30 million years ago. The subfamily to which Megatherium belongs, Megatheriinae, first appeared in the Middle Miocene in Patagonia, at least 12 million years ago, represented by the genus Megathericulus. The earliest known remains of the genus Megatherium are known from the Pliocene, found in Bolivia ( M. altiplanicum) and the Pampas (indeterminate species), dating to at least 3.6 million years ago. M. altiplanicum is suggested to be more closely related to M. americanum than to species of Pseudomegatherium. Phylogeny of Megatheriinae after Pujos, 2006: Megatherium americanum first appears in the fossil record during the second half of the Middle Pleistocene, from around 400,000 years ago.
Although some authors have suggested that Megatherium was an omnivore, isotopic analysis has supported an entirely herbivorous diet for Megatherium. Megatherium americanum is suggested to have been a browser that was a selective feeder on the foliage, twigs and fruits of trees and shrubs.Bargo, M. S., & Vizcaíno, S. F. (2008). Paleobiology of Pleistocene Ground Sloths (Xenarthra, Tardigrada): Biomechanics, Morphogeometry and Ecomorphology Applied to the Masticatory Apparatus. Ameghiniana, 45(1), 175–196. The sharp cusps of the teeth served to shear plant material. Megatherium is widely thought to have been able to adopt a bipedal posture to use its forelimbs to grasp vegetation, though whether it was capable of moving in this posture is uncertain. Analysis of injuries on the clavicles of M. americanum individuals suggests that the species probably habitually moved in a quadrupedal posture and assumed a bipedal posture next to trees to feed on high-growing leaves, likely using its forelimbs to brace itself against the tree trunk, as well as to pull down higher branches within reach of its prehensile lip. Isotopic analysis suggests that some individuals of M. americanum at certain times and places also consumed grass. The smaller Megatherium tarijense has been suggested to have had a mixed feeding-browsing diet. Preserved attributed to Megatherium suggests that its diet included plants like Fabiana, Ephedra (Ephedra breana), beebrush, Junellia, and Chuquiraga.H. Gregory McDonald and Gerardo de Iuliis: Fossil history of sloths. In: Sergio F. Vizcaíno and WJ Loughry (eds.): The Biology of the Xenarthra. University Press of Florida, 2008, pp. 51–52.
Whether or not Megatherium had a slow metabolism like living tree sloths is uncertain. Analysis of the nutrient foramina in the diaphysis (shaft) of the femur of Megatherium americanum shows that they are more similar to those of other large living mammals like elephants than living tree sloths, which may suggest that it had a metabolism more similar to non-xenarthran mammals and was capable of vigorous activity similar to living elephants. However, isotopic analysis of teeth suggests that Megatherium had a somewhat lower body temperature than non-xenarthran mammals, around , comparable to that of living tree sloths, implying a lower metabolic rate. Megatherium americanum has been traditionally reconstructed as being covered with a thick coat of fur. Due to its very large body size, some authors have alternatively argued that Megatherium americanum was probably relatively hairless like modern elephants in order to Thermoregulation.Fariña, R. A. (2002). Megatherium, the hairless: appearance of the great Quaternary sloths (Mammalia; Xenarthra). Ameghiniana, 39(2), 241–244. However this has been disputed, with other authors suggesting based on thermodynamic modelling assuming a living xenarthran-like metabolism that Megatherium species probably had a dense coat of fur around thick to be able to tolerate the relatively cool environments they inhabited.
Based on fossil trackways and the anatomy of its inner ear, which is considerably different from living sloths and more similar to those of armadillos, species of Megatherium, while probably not capable of swift locomotion due to limitations of their skeletal anatomy, were likely significantly more agile and mobile than living sloths, which are only capable of moving . Species of Megatherium likely relied on their large adult body size to protect themselves against predators. Like many other large mammals, Megatherium is suggested to have had a slow life cycle in accordance with a K-selection strategy. Megatherium americanum is suggested to have given birth to a single large offspring at a time.
The anatomy of its forelimb bones suggests that M. americanum had the ability to rapidly and powerfully extend its arms, which likely made its claws effective stabbing weapons. It may have used its claws like this to defend itself, as living tree sloths do. Although some authors in the 19th century suggested that Megatherium engaged in digging behaviour, this has been disputed by other scholars, and the morphology of its limb bones do not appear to display significant adaptations to digging unlike some other ground sloths like Mylodontidae.
In the Pampas, Megatherium americanum lived alongside other megafauna species, including the large ground sloth Lestodon, along with the smaller (but still large) ground sloths Mylodon, Glossotherium, and Scelidotherium, the (very large armadillos with fused round carapaces covering the body) Glyptodon, Doedicurus, and Panochthus, the large camel-like ungulate Macrauchenia and rhinoceros-like Toxodon, the gomphothere (elephant-relative) Notiomastodon, the equines Hippidion and Equus neogeus, the large short-faced bear Arctotherium, and the large sabertooth cat Smilodon.S.F. Vizcaíno, R.A. Fariña, J.C. Fernicola Young Darwin and the ecology and extinction of Pleistocene South American fossil mammals Rev. Asoc. Geol. Argent., 64 (1) (2009), pp. 160-169 The range of Megatherium americanum overlaps little with its similarly sized tropical relative Eremotherium, with their co-occurrence only confidently reported from a few localities in Southern Brazil, and it is unclear whether they were contemporary at these localities.
The youngest unambiguous dates for Megatherium are from the end of the Late Pleistocene. Supposed early Holocene dates obtained for Megatherium americanum and other Pampas megafauna have been questioned, with suggestions that they are likely due to Humic substance contamination of the collagen used to radiocarbon date the bones. Megatherium disappeared simultaneously along with the vast majority (>80%) of other large () South American mammals, as part of the end-Pleistocene extinction event. 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.
Towards the end of the Late Pleistocene, humans first arrived in the Americas, with some of the earliest evidence of humans in South America being the Monte Verde site in Chile, dating to around 14,500 years Before Present (~12,500 BC). The extinction interval of Megatherium and other megafauna coincides with the appearance and abundance of Fishtail points, which are suggested to have been used to hunt megafauna, across the Pampas region and South America more broadly. At the Paso Otero 5 site in the Pampas of northeast Argentina, Fishtail points are associated with burned bones of Megatherium americanum and other extinct megafauna. The bones appear to have been deliberately burned as a source of fuel. Due to the poor preservation of the bones there is no clear evidence of human modification.G. Martínez, M. A. Gutiérrez, Paso Otero 5: A summary of the interdisciplinary lines of evidence for reconstructing early human occupation and paleoenvironment in the Pampean region, Argentina, in Peuplements et Préhistoire de l’Amérique, D. Vialou, Ed. (Muséum National d’ Histoire Naturelle. Departement de Prehistoire, U.M.R, Paris, 2011), pp. 271–284.
There is evidence for the butchery of Megatherium by humans. Two M. americanum bones, an Ulnar nerve and an atlas vertebra, from separate collections, bear cut marks suggestive of butchery, with the latter suggested to represent an attempt to exploit the contents of the head. A kill site dating to around 12,600 years Before Present (BP), is known from Campo Laborde in the Pampas in Argentina, where a single individual of M. americanum was slaughtered and butchered at the edge of a swamp, which is the only confirmed giant ground-sloth kill site in the Americas. At the site several stone tools were present, including the fragment of a projectile point. Another possible kill site is Arroyo Seco 2 near Tres Arroyos in the Pampas in Argentina, where M. americanum bones amongst those of other megafauna were found associated with human artifacts dating to approximately 14,782–11,142 cal yr BP. This hunting may have been a factor in its extinction.
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