Megatherium

The earliest specimen of Megatherium americanum was discovered in 1787 by Manuel de Torres, a Dominican friar and naturalist, from a ravine on the banks of the Lujan River in what is now northern Argentina, which at the time was part of the Viceroyalty of the Río de la Plata in the Spanish Empire. Torres described the bones as a 'wonder and providence of the Lord'. On the orders of the then viceroy of la Plata, Nicolás Cristóbal del Campo, Marqués de Loreto, the specimen was moved to the capital Buenos Aires. There the skeleton was drawn for the first time by José Custodio Sáa y Faria in a horse-like posture. Campo summoned a number of local indigenous leaders to ask if they had heard of the animal. The skeleton was then transferred by Campo to the Royal Cabinet of Natural History of Madrid (now the National Museum of Natural Sciences MNCN) in 7 crates, which had arrived and been unpacked by late 1788. At the direction of the cabinets main taxidermist Juan Bautista Bru, the specimen was then mounted for public exhibition (The specimen is still on display in the National Museum of Natural Sciences today, with the mount still unaltered from Bru's original effort). 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. 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"), 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. 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. 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. In 2001, the species M. altiplanicum was described based on remains found in the Pliocene of Bolivia. In 2006, the species Megatherium celendinense was erected for remains of Pleistocene age found in the Peruvian Andes. == Taxonomy and evolution ==

Megatherium is divided into 2 subgenera, Megatherium and Pseudomegatherium. Taxonomy according to Pujos (2006) and De Iuliis et al (2009): • Subgenus Megatherium • †M. altiplanicum • †M. americanum • †M. gallardoi Ameghino & Kraglievich, 1921 • Subgenus Pseudomegatherium • †M. celendinense • †M. medinae • †M. sundti • †M. tarijense • †M. urbinai Megatherium gallardoi Ameghino & Kraglievich, 1921 from the Pampas dating to the Early to Middle Pleistocene Megatherium gaudryi Moreno (1888) from Argentina, of uncertain temporal provenance but possibly Pliocene in age, may also be valid. Mitochondrial DNA sequences obtained from M. americanum indicates that three-toed sloths (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 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. == Description ==

Size M. americanum is one of the largest known ground sloths, with a total body length of around . Volumetric analysis suggests that a full grown M. americanum weighed around , comparable to an Asian elephant. The Late Pleistocene Andean-Altiplano Pseudomegatherium species Megatherium celendinense was likely comparable in size. These species were only rivalled in size amongst ground sloths by the closely related Eremotherium and the distantly related Lestodon. The Chilean Pseudomegatherium species M. sundti was much smaller, with an estimated body mass of only , with the Peruvian Megatherium urbinai, Bolivian Megatherium tarijense and the Chilean Megatherium medinae (all also belonging to Pseudomegatherium) also having a considerably smaller body size than M. americanum. The Pliocene Megatherium (Megatherium) species M. altiplanicum has been estimated to weigh . Skull and jaws The head of Megatherium is relatively small compared to body size. The skull of M. americanum has a relatively small cranial cavity (and thus brain) relative to skull size, with the skull having extensive sinus spaces. In many species of Megatherium, the lower jaw is relatively deep, which served to accommodate the very long hypselodont (evergrowing) teeth, The skull of M. americanum has a relatively narrow snout/muzzle with a ossified nasal septum, and is suggested to have had a thick prehensile upper lip, similar to that of the living black rhinoceros, which compensated for the lack of teeth at the front of the jaws. The morphology of the hyoid bones in Megatherium suggests that they were relatively rigid, this along with the short distance between the hyoid and the mandibular symphysis (the joint connecting the two halves of the lower jaw) suggests that the tongue had limited ability to protrude, and thus Megatherium did not have a long prehensile tongue, contrary to what was often historically suggested. The skull and jaws of M. americanum show adaptation to powerful vertical biting. The foot was heavily modified from those of other mammals and earlier ground sloths, with a reduction in the number of digits on the inner part of the foot (digits I and II being lost), the increase in the size and robustness (thickness) of the metapodial elements of the outer digits, with the loss or reduction of the phalangeal bones. The calcaneum is wide and elongate posteriorly. The foot is suggested to have been inwardly rotated, historically the foot was suggested to be near vertical, though a recent study suggests that the angle was much shallower. The weight was primarily borne on the outer digits and the calcaneum. M. urbinai differs from M. americanum and other Megatherium species in the shape and position of the feet and hand bones, including the metacarpals, metatarsals, ectocuneiform, hamate/unciform, navicular and astragalus/talus. == Ecology ==

Glyptodon. Remains of Megatherium americanum have been found in low elevation areas to the east of the Andes mountains in northern Patagonia, the Pampas and adjacent areas in what is now northern Argentina, Uruguay, Paraguay, southern Bolivia and Rio Grande do Sul in southern Brazil. During the Last Glacial Period, the Pampas was generally drier than it is at present with many areas exhibiting a steppe-like environment dominated by grass, with some areas of woodland. 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. The sharp cusps of the teeth served to shear plant material. 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 coprolites attributed to Megatherium suggests that its diet included plants like Fabiana, Ephedra (Ephedra breana), beebrush, Junellia, and Chuquiraga. 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 has been argued to suggest 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. 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. 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. 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 glyptodonts (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. 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. == Relationship with humans and extinction ==

During the Late Pleistocene, six species of Megatherium were present in South America, including M. americanum in the Pampas and adjacent regions, and the 5 species of Pseudomegatherium in the vicinity of the Andes. 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 II 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. There is evidence for the butchery of Megatherium by humans. Two M. americanum bones, an ulna 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. This hunting may have been a factor in its extinction. == Cultural references ==

The Megatherium Club, named for the extinct animal and founded by William Stimpson, was a group of Washington, D.C.–based scientists who were attracted to that city by the Smithsonian Institution's rapidly growing collection, from 1857 to 1866. == References ==