User:Hemiauchenia/sandboxMegatherium

Research history
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. 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, though what kind of animal it actually was unknown. 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), which had been founded a decade prior 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 in 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 "edenate" 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. Cuver identified Megatherium as a sloth primarily on the basis of its skull morphology and the dental formula and the shoulder, while regarding the anatomy of its limbs as more similar to armadillos and anteaters. 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 cavicle 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 xenathrans called glyptodonts that were related to armadillos.

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 Desmarest in 1822. These remains are now assigned to M. americanum.

Owen later wrote a monograph series from 1851 to 1860 thoroughly describing the anatomy of M. americanum.

From the late 19th century onward additional species of Megatherium were described. In 1888 and 1921 the species Megatherium filholi (Late Pleistocene) and Megatherium gallardoi (Early-Middle Pleistocene) a were named based on remains found in the Pampas. The validity of these species is disputed, with some authors considering them to be synonyms of ''M. americanum.  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. 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. 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 Peru. 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, with at least 7 species typically being considered valid. Taxonomy according to Pujos (2006) and De Iuliis et al (2009):


 * Subgenus Megatherium
 * †M. altiplanicum Saint-André & de Iuliis 2001
 * †M. americanum Cuvier 1796

Megatherium gallardoi Ameghino & Kraglievich, 1921 from the Pampas dating to the Pleistocene has been either regarded as a valid species of Megatherium (Megatherium) or a junior synomym of M. americanum. The species Megatherium filholi Moreno, 1888 also from the Pleistocene of the Pampas region, historically regarded to be a junior synonym of M. americanum representing juvenile individuals has been suggested to be valid by some recent authors. Megatherium gaudryi Moreno (1888) from Argentina, of uncertain temporal provenance but possibly Pliocene in age, may also be valid.
 * Subgenus Pseudomegatherium Kraglievich 1931
 * †M. celendinense Pujos 2006
 * †M. medinae Philippi 1893
 * †M. sundti Philippi 1893
 * †M. tarijense Gervais & Ameghino, 1880
 * †M. urbinai Pujos & Salas 2004

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 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 millon 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.

Size
M. americanum is one of the largest known ground sloths. Volumetric analysis suggests that a full grown M. americanum weighed around 3700-3800 kg, comparable to an 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. The Chilean Pseudomegatherium species M. sundti was much smaller, with an estimated body mass of only 1253 kg, 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 1465 kg.

Skull and jaws
The head of Megatherium is relatively small compared to body size. The skull of M. americanum has a relatively narrow snout/muzzle with a ossified nasal septum, and is suggested to have had a muscular prehensile upper lip. The skull is roughly cylindrical in shape, with the cranial region of the skull being narrrow. The jugal bone of M. americanum has a strongly developed ascending and descending processes. In many species of Megatherium, the lower jaw is relatively deep, which served to accomodate the very long hypselodont (evergrowing) teeth, which are considerably proportionally longer than those of other ground sloths. Like other ground sloths, the number of teeth in the jaw is reducted to 5 and 4 teeth in each half of the upper and lower jaws, respectively, and the teeth lack enamel. The teeth of Megatherium americanum have sharp crests separated by v-shaped valleys, which interlock with the teeth on the opposing jaw. The skull and jaws of M. americanum show adaptation to powerful vertical biting. M. americanum and M. altiplanicum are distinguished from species of the subgenus Pseudomegatherium by the fusion of the maxilla and premaxilla, while members of Pseudomegatherium are distinguished from those species by their flat occipital condyles.

Axial skeleton
Like other xenathrans, the posterior trunk vertebrae of Megatherium americanum have additional xenarthrous processes that articulate with the other vertebrae. The ischium was connected to the caudal vertebrae, forming a synsacrum. The sacrum was composed of 5 vertebrae. The pubic symphysis is reduced. The tail is large in size.

Limbs
The bones of the forelimbs of M. americanum are relatively elongate and thin. The three fingers in the middle of the hand bore claws, while the cuneiform bones did not touch the ulna. The femur was massive and roughly rectangular in shape. As in most megatheriines, the tibia and fibula of Megatherium species are fused together at their proximal (closest to hip) end, while in M. americanum and M. tarijense, they are also fused together at their distal (closest to foot) ends. The foot was headily 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 reduction/loss of digit V and 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 based on various characters of the feet and hands.

Ecology
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 of trees and shrubs. 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. Due to its very large body size, some authors have argued that Megatherium americanum was probably hairless like modern elephants for thermodynamic reasons. Megatherium americanum inhabited temperate, arid-to semi arid open habitats.

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 moving at considerable speed due to limitiations of their skeletal anatomy were likely significantly more agile and mobile than living sloths (likely capable of moving a few kilometers an hour, as opposed to around 0.5-0.6 km/h in living tree sloths). Species of Megatherium likely relied on their large adult body size to protect themselves against predators, due to their inability to outrun them. Like other ground sloths, Megatherium likely used the claws on its hands for defense.