genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

ground sloth Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. They varied widely in size with the largest, belonging to genera '' Lestodon'', ''Eremotherium'' and ''Megatherium'', being around the size of elephants. ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 through the end of the

Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

. It is best known for the

elephant Elephants are the largest living land animals. Three living species are currently recognised: the African bush elephant ('' Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian elephant ('' Elephas maximus ...

-sized

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''Megatherium americanum'', primarily known from the

Pampas The Pampas (; from Quechua 'plain'), also known as the Pampas Plain, are fertile South American low grasslands that cover more than and include the Argentine provinces of Buenos Aires, La Pampa, Santa Fe, Entre Ríos, and Córdoba; all o ...

, but ranging southwards to northernmost

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

and northwards to southern

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

during the late

Middle Pleistocene The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...

and Late Pleistocene. Various other species belonging to the subgenus ''Pseudomegatherium'' ranging in size comparable to considerably smaller than ''M. americanum'' are known from the Andean region. The first (

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

) 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 Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...

the following year wherein it caught the attention of the French paleontologist

Georges Cuvier Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...

, who named the animal in 1796 and was the first to determine, by means of

comparative anatomy Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species). The science began in the classical era, continuing in t ...

, that ''Megatherium'' was a giant sloth. ''Megatherium'' is part of the sloth family

Megatheriidae Megatheriidae is a family of Extinction, extinct ground sloths that lived from approximately 23 Annum, mya—11,000 years ago. Megatheriids appeared during the Oligocene, Late Oligocene (Deseadan in the South American land mammal age, SALMA cl ...

, which also includes the closely related and similarly giant ''

Eremotherium ''Eremotherium'' (from Greek for "steppe" or "desert" "beast": ἔρημος "steppe or desert" and θηρίον "beast") is an extinct genus of giant ground sloth in the family Megatheriidae. ''Eremotherium'' lived in southern North America, Cen ...

'', 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 The black rhinoceros (''Diceros bicornis''), also called the black rhino or the hooked-lip rhinoceros, is a species of rhinoceros native to East Africa, East and Southern Africa, including Angola, Botswana, Eswatini, Kenya, Lesotho, Malawi, Moza ...

–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 kill sites where ''M. americanum'' was slaughtered and butchered is known, suggesting that hunting could have been a factor in its extinction.

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 The Viceroyalty of the Río de la Plata or Viceroyalty of Buenos Aires ( or Virreinato de Buenos Aires or ) meaning "River of the Silver", also called the "Viceroyalty of River Plate" in some scholarly writings, in southern South America, was ...

in the

Spanish Empire The Spanish Empire, sometimes referred to as the Hispanic Monarchy (political entity), Hispanic Monarchy or the Catholic Monarchy, was a colonial empire that existed between 1492 and 1976. In conjunction with the Portuguese Empire, it ushered ...

. 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 Buenos Aires, controlled by the government of the Autonomous City of Buenos Aires, is the Capital city, capital and largest city of Argentina. It is located on the southwest of the Río de la Plata. Buenos Aires is classified as an Alpha− glob ...

. 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 (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 The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...

(''Muséum national d'histoire naturelle'') in Paris, France, where they were seen by French anatomist and paleontologist

Georges Cuvier Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...

. Cuvier, working solely from the prints from Madrid and not visiting the specimen personally, and using

comparative anatomy Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species). The science began in the classical era, continuing in t ...

with "edentate" mammals (now recognised as members of the order

Xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a superorder and major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and ...

) 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 Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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 Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...

and the shoulder, while regarding the anatomy of its limbs as more similar to

armadillo Armadillos () are New World placental mammals in the order (biology), order Cingulata. They form part of the superorder Xenarthra, along with the anteaters and sloths. 21 extant species of armadillo have been described, some of which are dis ...

s and

anteater Anteaters are the four extant mammal species in the suborder Vermilingua (meaning "worm tongue"), commonly known for eating ants and termites. The individual species have other names in English and other languages. Together with sloths, they ar ...

s. 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 The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...

and well developed crests on the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

, 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 Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomy, comparative anatomist and paleontology, palaeontologist. Owen is generally considered to have been an outstanding naturalist with a remarkabl ...

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 Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

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 Anselme Gaëtan Desmarest (6 March 1784 – 4 June 1838) was a French Zoology, zoologist and author. He was the son of Nicolas Desmarest and the father of Eugène Anselme Sébastien Léon Desmarest. Career Desmarest was a disciple of Georges Cu ...

in 1822. These remains are now assigned to ''M. americanum.'' Megatherium Crystal Palace

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 Argentine explorer Francisco Moreno erected the species ''Megatherium filholi'' for remains found in the Late Pleistocene of Argentina.'''' In 1880

Paul Gervais Paul Gervais (full name: François Louis Paul Gervais) (26 September 1816 – 10 February 1879) was a French palaeontologist and entomologist. Biography Gervais was born in Paris, where he obtained the diplomas of doctor of science and of medic ...

and

Florentino Ameghino Florentino Ameghino (born Giovanni Battista Fiorino Giuseppe Ameghino; September 19, 1853 – August 6, 1911) was an Argentine naturalist, paleontologist, anthropologist and zoologist, whose fossil discoveries on the Argentine Pampas, especial ...

described the species ''M. tarijense'' from remains of Pleistocene age found in Bolivia. In 1893

Rodolfo Amando Philippi Rodolfo Amando (or Rudolph Amandus) Philippi (14 September 1808 – 23 July 1904) was a German–Chilean paleontologist and zoologist. Philippi contributed primarily to malacology and paleontology, but also published a major work on Diptera of C ...

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, of Early-Middle Pleistocene age.Brandoni 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.

Taxonomy and evolution

''Megatherium'' is divided into 2

subgenera In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...

, ''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 The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...

has sometimes been regarded as a synonym 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.

Mitochondrial DNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...

sequences obtained from ''M. americanum'' indicates that

three-toed sloth The three-toed or three-fingered sloths are arboreal neotropical mammals. They are the only members of the genus ''Bradypus'' (meaning "slow-footed") and the family Bradypodidae. The five living species of three-toed sloths are the brown-throa ...

s (''Bradypus'') are their closest living relatives. Phylogeny of sloths after Delsuc et al. 2019.

Megatheriidae Megatheriidae is a family of Extinction, extinct ground sloths that lived from approximately 23 Annum, mya—11,000 years ago. Megatheriids appeared during the Oligocene, Late Oligocene (Deseadan in the South American land mammal age, SALMA cl ...

is suggested to have diverged from other sloth families during the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

, around 30 million years ago. The subfamily to which ''Megatherium'' belongs,

Megatheriinae Megatheriinae is a subfamily of the Megatheriidae, an extinct family of ground sloths that lived from the Middle Miocene to the Early Holocene. Classification Within the Megatheriidae there are two (possibly three) subfamilies; the Megatheriinae ...

, first appeared in the

Middle Miocene The Middle Miocene is a sub-epoch of the Miocene epoch (geology), epoch made up of two Stage (stratigraphy), stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0. ...

in

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

, at least 12 million years ago, represented by the genus '' Megathericulus''. The earliest known remains of the genus ''Megatherium'' are known from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 ''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 The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...

, from around 400,000 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 ''Eremotherium'' (from Greek for "steppe" or "desert" "beast": ἔρημος "steppe or desert" and θηρίον "beast") is an extinct genus of giant ground sloth in the family Megatheriidae. ''Eremotherium'' lived in southern North America, Cen ...

'' 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 .Saint-André P.-A. & de Iuliis G. 2001. �
The smallest and most ancient representative of the genus ''Megatherium'' Cuvier, 1796 (Xenarthra, Tardigrada, Megatheriidae), from the Pliocene of the Bolivian Altiplano
''Geodiversitas'' 2001 (4): 625–645.

Skull and jaws

The head of ''Megatherium'' is relatively small compared to body size. The skull is roughly cylindrical in shape, with the cranial region of the skull being narrow. The jugal bone of ''M. americanum'' has strongly developed ascending and descending processes.Bargo, M.S. 2001
The ground sloth ''Megatherium americanum'': Skull shape, bite forces, and diet
– ''Acta Palaeontologica Polonica'' 46,2, 173–192. The skull of ''M. americanum'' has a relatively small

cranial cavity The cranial cavity, also known as intracranial space, is the space within the skull that accommodates the brain. The skull is also known as the cranium. The cranial cavity is formed by eight cranial bones known as the neurocranium that in human ...

(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, which are considerably proportionally longer than those of other ground sloths. Like other ground sloths, the number of teeth in the jaw is reduced 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. These teeth were self-sharpening, akin to rodent incisors. The skull of ''M. americanum'' has a relatively narrow snout/muzzle with a

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

nasal septum The nasal septum () separates the left and right airways of the Human nose, nasal cavity, dividing the two nostrils. It is Depression (kinesiology), depressed by the depressor septi nasi muscle. Structure The fleshy external end of the nasal s ...

, and is suggested to have had a thick prehensile upper lip, similar to that of the living

black rhinoceros The black rhinoceros (''Diceros bicornis''), also called the black rhino or the hooked-lip rhinoceros, is a species of rhinoceros native to East Africa, East and Southern Africa, including Angola, Botswana, Eswatini, Kenya, Lesotho, Malawi, Moza ...

, 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 In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral ha ...

(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. ''M. americanum'' and ''M. altiplanicum'' are distinguished from species of the subgenus ''Pseudomegatherium'' by the fusion of the

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

and

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

, while members of ''Pseudomegatherium'' are distinguished from those species by their flat occipital condyles.

Axial skeleton

Like other xenarthrans, the posterior trunk vertebrae of ''Megatherium americanum'' have additional xenarthrous processes that articulate with the other vertebrae. The

ischium The ischium (; : is ...

was connected to the

caudal vertebrae Caudal vertebrae are the vertebrae of the tail in many vertebrates. In birds, the last few caudal vertebrae fuse into the pygostyle, and in apes, including humans, the caudal vertebrae are fused into the coccyx. In many reptiles, some of the caud ...

. The caudal vertebrae were combined into a synsacrum. The

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...

was composed of 5 vertebrae. The

pubic symphysis The pubic symphysis (: symphyses) is a secondary cartilaginous joint between the left and right superior rami of the pubis of the hip bones. It is in front of and below the urinary bladder. In males, the suspensory ligament of the penis attache ...

is reduced. The tail is large in size.

Limbs

The bones of the forelimbs of ''M. americanum'' are relatively slender. The three fingers in the middle of the hand bore claws, while the cuneiform hand bones did not touch the

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

. The

olecranon The olecranon (, ), is a large, thick, curved bony process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...

process of the ulna was relatively short. Like other xenarthrans, but unlike most other mammals, ''Megatherium'' possesses

clavicle The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...

s (collarbones), which serves to support the forelimb. Like other sloths, the clavicle is merged with the

acromion In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", : acromia) or summit of the shoulder is a bony process on the scapula (shoulder blade). Together with the coracoid process, it extends laterally over the sh ...

of the

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

. The femur was massive and roughly rectangular in shape. As in most megatheriines, the

tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

and

fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...

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 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 In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel; : calcanei or calcanea) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is t ...

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 In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular skeleton, appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones (wrist, wris ...

, metatarsals, ectocuneiform, hamate/unciform, navicular and astragalus/talus.

Ecology

Remains of ''Megatherium americanum'' have been found in low elevation areas to the east of the Andes mountains in northern

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

, the

Pampas The Pampas (; from Quechua 'plain'), also known as the Pampas Plain, are fertile South American low grasslands that cover more than and include the Argentine provinces of Buenos Aires, La Pampa, Santa Fe, Entre Ríos, and Córdoba; all o ...

and adjacent areas in what is now northern Argentina, Uruguay, Paraguay, southern Bolivia and

Rio Grande do Sul Rio Grande do Sul (, ; ; "Great River of the South") is a Federative units of Brazil, state in the South Region, Brazil, southern region of Brazil. It is the Federative units of Brazil#List, fifth-most populous state and the List of Brazilian s ...

in southern Brazil. ''Megatherium americanum'' inhabited

temperate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...

,

arid Aridity is the condition of geographical regions which make up approximately 43% of total global available land area, characterized by low annual precipitation, increased temperatures, and limited water availability.Perez-Aguilar, L. Y., Plata ...

-to

semi arid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of sem ...

open habitats. During the Last Glacial Period, the Pampas was generally drier than it is at present with many areas exhibiting a

steppe In physical geography, a steppe () is an ecoregion characterized by grassland plains without closed forests except near rivers and lakes. Steppe biomes may include: * the montane grasslands and shrublands biome * the tropical and subtropica ...

-like environment dominated by grass, with some areas of woodland. 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. 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

coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...

s 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 Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

like living tree sloths is uncertain. Analysis of the nutrient foramina in the

diaphysis The diaphysis (: diaphyses) is the main or midsection (shaft) of a long bone. It is made up of cortical bone and usually contains bone marrow and adipose tissue (fat). It is a middle tubular part composed of compact bone which surrounds a centr ...

(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 Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding 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 hairless like modern elephants for thermodynamic reasons. 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 moving at considerable speed due to limitations of their skeletal anatomy (with one study estimating a max speed of approximately or , a fraction of the or top speed observed for living elephants) 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 ground sloths such as mylodontids. 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

glyptodont Glyptodonts are an extinct clade of large, heavily armoured armadillos, reaching up to in height, and maximum body masses of around 2 tonnes. They had short, deep skulls, a fused vertebral column, and a large bony carapace made up of hundreds o ...

s (very large armadillos with fused round carapaces covering the body) '' Glyptodon'', '' Doedicurus'', and '' Panochthus'', the large camel-like ungulate ''

Macrauchenia ''Macrauchenia'' ("long llama", based on the now-invalid llama genus, ''Auchenia'', from Greek "big neck") is an extinct genus of large ungulate native to South America from the Pliocene or Middle Pleistocene to the end of the Late Pleistocene. I ...

'' and rhinoceros-like '' Toxodon'', the gomphothere (elephant-relative) '' Notiomastodon'', the equines '' Hippidion'' and '' Equus neogeus'', the large short-faced bear ''

Arctotherium ''Arctotherium'' ("bear beast") is an extinct genus of the Pleistocene Tremarctinae, short-faced bears endemic to Central America, Central and South America. ''Arctotherium'' migrated from North America to South America during the Great American In ...

'', and the large sabertooth cat ''

Smilodon ''Smilodon'' is an extinct genus of Felidae, felids. It is one of the best known saber-toothed predators and prehistoric mammals. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats ...

.'' The range of ''Megatherium americanum'' overlaps little with its similarly sized tropical relative ''

Eremotherium ''Eremotherium'' (from Greek for "steppe" or "desert" "beast": ἔρημος "steppe or desert" and θηρίον "beast") is an extinct genus of giant ground sloth in the family Megatheriidae. ''Eremotherium'' lived in southern North America, Cen ...

,'' 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 The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

, 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 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 acid Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown lead ...

contamination of the

collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

used to radiocarbon date the bones. ''Megatherium'' disappeared simultaneously along with the vast majority (>80%) of other large (

megafauna In zoology, megafauna (from Ancient Greek, Greek μέγας ''megas'' "large" and Neo-Latin ''fauna'' "animal life") are large animals. The precise definition of the term varies widely, though a common threshold is approximately , this lower en ...

l) 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 II site in Chile, dating to around 14,500 years

Before Present Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...

(~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 The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

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.

Cultural references

The ''

Megatherium Club The Megatherium Club was a group of scientists based in Washington, D.C. as a result of the Smithsonian Institution's rapidly growing collection, meeting from 1857 to 1866. It was founded by William Stimpson. Many of the members had no formal ed ...

'', named for the extinct animal and founded by

William Stimpson William Stimpson (February 14, 1832 – May 26, 1872) was an American scientist. He was interested particularly in marine biology. Stimpson became an important early contributor to the work of the Smithsonian Institution and later, director o ...

, was a group of

Washington, D.C. Washington, D.C., formally the District of Columbia and commonly known as Washington or D.C., is the capital city and federal district of the United States. The city is on the Potomac River, across from Virginia, and shares land borders with ...

–based scientists who were attracted to that city by the

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

's rapidly growing collection, from 1857 to 1866.

References

External links

BBC – ''Walking With Beasts'' – Megatherium

''London Review of Books'' – ''Bare Bones'' – Steven Shapin
* {{Taxonbar, from=Q310387 Prehistoric sloths Prehistoric placental genera Pliocene xenarthrans Pleistocene xenarthrans Piacenzian first appearances Holocene extinctions Pliocene mammals of South America Pleistocene mammals of South America Fossils of Argentina Fossils of Bolivia Fossils of Brazil Fossils of Chile Fossils of Colombia Fossils of Paraguay Fossils of Peru Fossils of Uruguay Fossil taxa described in 1796 Taxa named by Georges Cuvier Neogene Argentina