Presidential paleontology
One species of ground sloth is named after Thomas Jefferson. The future third president had a well-known interest in fossils, and a friend had sent him some bones that had been found in a cave in West Virginia. Jefferson first thought the bones belonged to a large lion and called it the "Great Claw," or Megalonyx, according to the Thomas Jefferson Encyclopedia. In 1797, as he was preparing a paper on the find for the American Philosophical Society, he saw an engraving of a sloth skeleton and realized that his fossil was similar and that his classification was wrong. Later, Jefferson was credited with discovering the extinct sloth, which was named Megalonyx jeffersonii.
The six modern species of sloths are all arboreal, so they are called tree sloths. These sloths are small-bodied and weigh less than 20 pounds. Many of their extinct relatives were much larger and lived on the ground. Because of this, they are referred to as ground sloths.
Megalonyx jeffersonii was the largest of the ground sloths in family Megalonychidae, reaching the size of an ox when fully grown, said Ken Wilkins, an associate dean for sciences and professor of biology at Baylor University. Megalonyx sloths grew to around 9.8 feet (3 meters) long and weighed up to 2,205 lbs. (1,000 kilograms), according to the San Diego Zoo.
Giant ground sloths evolved in South America around 35 million years ago. Around 8 million years ago, they migrated into North America, according to the San Diego Natural History Museum.
Giant ground sloths preferred forests along rivers or lakes, but they also lived during the Pleistocene period, also known as the Great Ice Age. At its peak, as much as 30 percent of the Earth's surface was covered by glaciers and parts of the northern oceans were frozen, according to the San Diego Natural History Museum. This made for a very cold environment that few animals could endure.
By the end of the Great Ice Age, around 11,700 years ago, many believe that the giant ground sloths had become extinct. Some argue that they were around for many more thousands of years, though, surviving on islands in the Caribbean.
Ground sloths were herbivores, meaning they ate vegetation. Their peg-like teeth were ideal for this diet, but they also had other body parts that played a large part in their meals. "They had long curved claws, likely an adaptation for foraging for grabbing branches and stripping foliage from tree limbs, as well as for protection from predators," Wilkins told Live Science.
Their hind foot structure and posture of the ground sloths also helped it with meal time. They likely relied on their robust hind feet, in combination with a stout tail, to support their massive bodies when rearing on their hindquarters to reach high into trees for forage, Wilkins explained.
Megalonyx fossils have been recovered from about 150 sites across North America, according to the Illinois State Museum. Some have been found as far north as Alaska and the Northwest Territories of Canada. They have also been found in California, Arizona and New Mexico, as well as northern Mexico. In the Midwest, most of them have been found in caves, including sites in Missouri, Minnesota, Illinois, Ohio, Kentucky and Tennessee.
Notes and references[]
Giant ground sloths were large, lumbering beasts that lived in the Americas during the Ice Age. They were directly related to today's modern sloths. They were also distantly related to anteaters and armadillos.
Small ground sloths[]
Illustration of the Puerto Rican sloth (Acratocnus odontrigonus), depicted without the long tail characteristic of ground sloths, by the American Museum of Natural History.
The smaller semi-arboreal ground sloths of the West Indies are believed to have gone extinct much later than the mainland sloths, perhaps persisting into the colonial era: according to Walker's Mammals of the World, their bones have been discovered in European middens alongside those of introduced domestic pigs.[35] Yehos, small, ape-like creatures with claws (which are not a feature of primates) reported from Cuba, The Bahamas, Hispaniola, Trinidad, and Puerto Rico, may represent living or late-surviving forms of these smaller, arboreal West Indian ground sloths.[35][36]
Missionary Thomas Bridges originally believed the saapaim, a shaggy, sheep-sized animal with large claws reported from the deepest forests of Tierra del Fuego in Chile, could have been "a sloth". Austin Whittall disagrees because of the saapaim's reportedly large and powerful incisor teeth.[37]
Giant ground sloths[]
Although the first alleged evidence of modern-day ground sloth survival came from Patagonia,[5] there have been few recorded sightings of ground sloth-like animals from the region. A collection of somewhat nebulous clawed monsters in Patagonian tradition have been connected with the Patagonian ground sloth,[7] including the succarath, lobo-toro, and ellengassen.[5] Florentino Ameghino also claimed that he had heard may stories of a:
...mysterious quadruped [...] in the interior of the territory of Santa Cruz, living in burrows hollowed out in the soil, and usually only coming out at night. According to the reports of the Indians, it is a strange creature, with long claws and a terrifying appearance, impossible to kill because it has a body impenetrable alike to firearms and missiles.
A statue of Mylodon at Puerto Natales, Chile, photographed by Wikipedia user Haplochromis.
Ameghino also wrote that Ramón Lista, governor of Santa Cruz, told himself, his brother Carlos Ameghino, and several other people of a sighting he'd had of a large, hairy, pangolin-like creature:
He came across it one day during one of his journeys in the interior of the territory of Santa Cruz , but in spite of all his efforts he was unable to cap- ture it. Several shots failed to stop the animal, which soon disappeared in the brushwood; all search for its recovery being useless. Lista retained a perfect recollection of the impression this encounter made upon him. According to him the animal was a a pangolin (
), almost the same as the Indian one, both in size and in general aspect, except that in place of scales, it showed the body to be covered with a reddish grey hair. He was sure that if it were not a pangolin, it was certainly an edentate nearly allied to it.
Hesketh Prichard, who was sent to Patagonia to search for the Mylodon, could not find a record of the encounter in any of the books written by Lista, who was dead by the time of the skin's discovery.[10] Prichard did not believe a large animal could live in Patagonia's dense Valdivian forests, which he did not explore. However, he admitted that "in addition to the regions visited by our Expedition, there are, as I have said, hundreds and hundreds of square miles about, and on both sides of the Andes, still unpenetrated by man. A large portion of this country is forested, and it would be presumptuous to say that in some hidden valley far beyond the present ken of man some prehistoric animal may not still exist. Patagonia is, however, not only vast, but so full of natural difficulties".[10]
During the second voyage of the HMS Beagle, Charles Darwin noted the South American belief that "there exists in Paraguay an animal larger than a bullock, & which goes by the name of "gran bestia", and wrote that, "if no credit is given to the actual existence of the "gran bestia", "then it must have been based on sightings of perfect Megatherium skeletons, as, according to him, the resemblance to a giant sloth is too striking to be accidental.[12]
Both Darwin and Captain Fitz-Roy were told by a Bahia Blanca garrison commandant named Rodriguez that he had seen a gran bestia chained in Paraguay. It resembled a hog with talons (or "great claws"), and, although only a few months old, it already stood about four feet high. Fitz-Roy seemingly thought the animal could have been a yaquaru, a sabre-toothed cat-like animal,[13] whilst Darwin believed it was simply a tapir.[12]
Illustration of a giant ground sloth by Hermann Trappman.
Cryptids resembling cow-sized ground sloths have frequently been reported from the Amazon regions of Brazil, Peru, Ecuador, and Colombia. The most famous of these is the mapinguari,[3] which is, however, also described as an ape-like animal. David Oren and the cryptozoologists of the Centre for Fortean Zoology, including Karl Shuker, believe the mapinguari may be a living ground sloth;[14] older cryptozoologists such as Bernard Heuvelmans and Ivan T. Sanderson, as well as a number of modern cryptozoologists including Loren Coleman and Dale A. Drinnon, believe it is a giant hominid.[15] Much of the confusion arises from mixed-up accounts of different animals,[15] and the application of the name "mapinguari" to a number of different cryptids. Heuvelmans did associate a 21' long "human" footprint, supposedly made by the mapinguari, with either Megatherium or the Triassic Prestosuchus.[5]
Traditional depiction of the mapinguari as a cyclopean humanoid compared to a giant ground sloth, drawn by Ray Troll.
According to Dale A. Drinnon, there have been consistent generic reports of creatures resembling bear-sized ground sloths from the Amazon since colonial times. Native accounts generally stress their large claws, which in more dramatic stories are said to be stained red with the blood of their victims. Drinnon refers to them as "wolfskins," and says that the old term su is still used to refer to them in parts of South America (see below).[15] In 1953, explorer Leonard Clark claimed that a piece of Megatherium skin had recently been found in an "upland cave".[16]
In 1997, anthropologist Glenn Shepard Jr. learned from some Peruvian Machiguenga people of the segamai, a cow-sized animal which can walk both quarupedally and bipedally, with dark matted fur and a snout "similar to" a giant anteater's. It is said to live in caves in the remote cloud and foothill forests, where it feeds on Cyclanthaceae plants and palm piths. The Machiguenga are terrified of it due to its reputedly aggressive behaviour, and it has a number of alleged characteristics in common with the mapinguari: it is said to be impervious to bullets, has a terrible roar, and supposedly generates an odour which stupefies or knocks out anyone who comes close to it.[17] Shepard Jr. that it might be a bear: the Machiguenga, who knew spectacled bears (Tremarctos ornatus) well, "expressed great surprise and affirmed that the two animals are completely different".[18] One of the tribe matter-of-factly told him that he had seen a segamai at Lima's Natural History Museum when he was a student,[17] and when Shepard checked, he discovered that the museum had a diorama featuring a model of a giant ground sloth.[19] However, the student had never seen the segamai himself, and had previously assumed it to be mythical.[17]
The well-known theory that the mapinguari could be an extant ground sloth was first posited by David Oren, who has gathered more than a hundred claimed sightings during his time in Brazilian Amazonia. He initially regarded the mapinguari as a myth, but after speaking with a number of people in the Tapajós River basin who claimed to have seen it, Oren came to believe that the animal being described could only be a ground sloth.[20][18]
In a 1993 paper for Goeldiana Zoologia, Oren demonstrated how each of the mapinguari's characteristics, as they were then known, were consistent with a human-sized mylodontid ground sloth. Almost all known hair samples from mummified ground sloths are reddish in colour. They are believed to have walked with their claws turned inwards, which would give rise to stories of backwards feet, as the unusual curvature would lead people to interpret the tracks the wrong way around, and fossil ground sloth tracks have been misinterpreted as giant human footprints, which they closely resemble in the past (Heuvelmans attributed backwards footprints in Asia to bears for similar reasons). Oren suggests that the more common, round track attributed to the mapinguari could be the imprint of the tip of its powerful tail. The fossilised faeces of ground sloths are almost identical to those of horses, just like the mapinguari. The mapinguari's reputed invulnerability could be explained twofold, by a mylodontids triple-layered bone ossicles covering the shoulders, back, and thighs, as well as the powerful, almost-fused ribcage present in some ground sloths — the combination of both characteristics would explain why only a shot to the navel or face can kill a mapinguari. Although ground sloths are famous for their great size, not all were so large, and since forest animals are frequently smaller than their open-environment cousins, it would make sense for a jungle ground sloth to be smaller. Oren's study of ground sloth hyoid bones suggested to him that they would have been capable of loud vocalisations, as indeed modern sloths are. Only the monkey-like face is inconsistent with what is known of ground sloths, but Oren suggested that some species could have had such faces, as the tree sloths do.[20][21]
When Oren proposed a mylodontid identity, he had not spoken to anyone who claimed to have killed a specimen, and the hunters he spoke to afterwards did not mention the more fantastic traits, and gave details which both reinforced the ground sloth theory - including a head like a horse as opposed to a monkey, a slightly larger size and build, and hooked claws like those of an anteater - and suggested a slightly different familial identity, including four peg-like "canine teeth". Following the hunters' accounts, Oren modified his theory, suggesting that the mapinguari would be a megalonychid, not mylodontid, ground sloth: megalonychids, almost alone amongst the ground sloths, had frontal caniniform teeth (as did some specimens of mylodontids such as Glossotherium and Lestodon), and walked on the soles of their hind feet, so a sloth with "fangs" and a flat-footed locomotion could only be a megalonychid.[18] Reconstructions of Megalonyx have twice been identified as the mapinguari by claimed eyewitnesses (Geovaldo and Salinas). Although megalonychids did not have ossicles like mylodontids, they still had very powerful ribcages. Additionally, some accounts ascribe the mapinguari's invulnerability to its hair, not any sort of armour—although the Karitiana kida harara, which is supposed to have fangs, is said to have "pebbles" under its skin. Oren also admitted that the hunters did not describe a ground sloth-like tail.[18]
Theories related to ground sloths have also been put forward to explain some of the mapinguari's more fantastic characteristics. It was suggested that the abdomen mouth (which is not always described) could be a specialised scent gland which discharges a foul smell (which is almost always described) as a defense mechanism.[21] Josh Gates, on the other hand, suggested that the description of a stomach mouth could be derived from people getting a bad look at a bipedal ground sloth holding its claws, which curve upwards, in front of its abdomen.[22]
The kida harara of southwestern Brazil is often equated with the mapinguari. During Pat Spain's "animal identity parade" interview with kida harara eyewitness Geovaldo for Beast Man, the animals which he is shown (onscreen) to have no reaction to are an African elephant, a Bengal tiger, a spectacled bear, a white rhinoceros, and a gorilla. He showed no recognition of the spectacled bear, and thought the gorilla could be some sort of monkey. Geovaldo recognised the giant anteater and stated that the animal he had seen was "much, much different," with the only slight similarity being in the arms. When an image of the ground sloth Megalonyx was shown, Geovaldo unhesitatingly nodded and identified it as what he had seen, stating that "it was kind of like that. I think that was the animal. I really think that looks like it. Its arms were just like that." One difference he noted was that the claws on what he saw were similar, but even larger - other than that, it had the same body, the same arms, and the same face.[23]
Illustration of an encounter with a living Megatherium by Zdeněk Burian, from the 1956 edition of the novel Plutonia.
Other, lesser-known Amazonian cryptids which may be ground sloths include the jucucu, which is also frequently lumped together with the mapinguari;[24] the Orinoco giant sloth, which allegedly grows up to 5 metres (16 feet) long, and uses its large claws to browse from the tops of trees and dig up roots;[25] and the ujea, the description of which reminded researcher Angel Morant Forés of a giant sloth.[26] There are also dubious stories of a smaller, supposedly predatory ground sloth, called the xolchixe or "tiger sloth", from the Brazilian Amazon.[15]
At least eight genera of ground sloths are known from the Pleistocene western Amazon, including two megatheriids, two megalonychids, and four mylodontids. There is no general consensus regarding the reason for their apparent extinction. Brad Rancy theorised that during the Pleistocene the region was cool, dry, wooded savannah, and that the expansion of the warm, wet rainforest wiped them out, leading Oren to suggest that a smallish species living in the gallery forests which existed at the time could have adapted to forest life.[20] However, according to current beliefs, much of what is now the western Amazon was already tropical rainforest during the Late Pleistocene, with savannah to the south and east. Although no alleged sightings of ground sloths in the Amazon had been reported by the publication of Bernard Heuvelmans' On the Track of Unknown Animals (1955) - in which the examination of supposed living ground sloths focuses on the Patagonian ground sloth - Heuvelmans did predict that they might survive there. He noted that, since giant ground sloths were almost certainly wiped out by human hunting and not climate change, it is difficult to see why they would go extinct in regions uninhabited or sparsely inhabited by humans, particularly the tropical regions of northern South America. He concluded his examination of Patagonian ground sloths by writing:
Slaughtered by the nomadic hunting Indians, both in the pampas of the south and the green prairies of the north, the largest sloths would have retreated, as the jaguar did, to the tropical forests, where they could find a safer refuge. [...] it is not difficult to see how the medium-sized ground-sloths might have survived in wooded savannah or sparse forest, or even on the fringes of or in clearings in the densest of jungles. For the great ground-sloths were not destroyed by any revolutionary geological or climate change. From the number of their remains in kitchen middens it is clear that these large and peacable beasts, like so many other species, were victims of man's gluttony. If such is the case, what has happened to them in their impenetrable retreat in the vast Amazonian
of the Andes, through which they passed in the course of ages? It is hard to see what, in the peace of these forests rarely inhabited by man, could have led to their extinction. Only human traps were able to put an end to these armoured brutes against which beasts of prey were powerless. Might they not still live in this 'green hell' and find it a heaven of peace?
Ivan T. Sanderson suspected that reports of cave cows from Belize, describing hairy lizard-shaped animals, referred to medium-sized living ground sloths.[27]
Illustration of a living Megalonyx in the United States by artist NocturnalSea on DeviantArt.
A handful of cryptids alleged to be living giant sloths have been sporadically reported from the United States, most notably the Appalachian ground sloth; the sheepsquatch, which is reported from a similar range, is also sometimes speculated to be a giant sloth.[28] The Beasts of Sherman, allegedly sighted in New York in the 1960s, were explicitly identified as white giant sloths by the eyewitness,[29][30] and the Beast of Boonville was said to have been a giant sloth captured in Mexico which had escaped captivity.[31] Certain researchers also theorise that Bigfoot may be a living giant ground sloth.[primary source needed]
The saytoechin, a large, hairy animal with claws reported from the Yukon, was identified with a picture of a giant ground sloth by Native American informants.[32] These informants also reported that a saytoechin had been shot in a lake by a white man; this man's story, independently told by himself, described a sloth-like animal with a 3-foot-long tail.[33] Ben S. Roesch suggests that, if it is a giant sloth, the saytoechin's purpoted predation of beavers is a misinterpretation, and that the animal really rips open beaver hives to eat the branches and vegetation they are made of.[34]
Roesch also suggests that the giant squirrels of Nova Scotian Micmac lore were late-surviving ground sloths. Although they never harmed people, these animals were a nuisance to the Micmacs because they ate their houses, which were made of bark. According to the Micmacs, the giant squirrels eventually disappeared, but as the Micmac's legends are believed to occur no further back in history than around 500 B.C., the animals must have survived into medieval times in this area.[34] Roesch also briefly speculated that the camel-like urchow of the Nass Valley could be another living ground sloth.[34]
Hunting of ground sloths
A number of kill sites are known for ground sloths in the Americas, these include Campo Laborde in the Pampas of Argentina, where an individual of Megatherium americanum was butchered at the edge of a swamp, dating to approximately 12,600 years Before Present (BP),[63] with another potential Megatherium kill site being Arroyo Seco 2 in the same region, dating to approximately 14,782–11,142 cal yr BP.[64] In northern Ohio, a Megalonyx jeffersoni skeleton dubbed the "Firelands Ground Sloth" has cut marks indicative of butchery, dating to 13,738 to 13,435 years BP.[65] At the Santa Elina rockshelter in Mato Grosso Brazil, a specimen of Glossotherium is associated with hearths and stone tools, dating to 11,833–11,804 years BP. At Fell's Cave in southern Chilean Patagonia, a specimen of Mylodon with fractured and burned bones associated with human activity has been dated to approximately 12,766–12,354 years BP.[64]
Humans are believed to have entered the New World via Beringia, a land bridge which connected Asia and North America during the last glacial maximum. Mosimann and Martin (1975) suggested the first of these nomads descended from hunting families who had acquired the skills to track down and kill large mammals.[66] By this time, humans had developed proficient hunting weapons, including the Clovis points, which were narrow, carved stone projectiles used specifically for big game. A couple of hundred years later, the atlatl became widely used, which allowed them to throw spears with greater velocity.[67] These inventions would have allowed hunters to put distance between them and their prey, potentially making it less dangerous to approach ground sloths.
Certain characteristics and behavioral traits of the ground sloths made them easy targets for human hunting and provided hunter-gatherers with strong incentives to hunt these large mammals.
Ground sloths often fed in open fields.[68] Recent studies have attempted to discover the diet of ground sloths through fossils of their dung. Analysis of these coproliths have found that ground sloths often ate the foliage of trees, hard grasses, shrubs, and yucca; these plants were located in areas that would have exposed them,[69] making them susceptible to human predation. Ground sloths were not only easy to spot, but had never interacted with humans before, so would not have known how to react to them. Additionally, these large mammals waddled on their hind legs and front knuckles, keeping their claws turned in. Their movement and massive build (some weighed up to 3,000 kilograms (6,600 lb)) imply they were relatively slow mammals.[7]
These reasonable after-the-fact inferences from the evidence might explain why ground sloths would have been easy prey for hunters, but are not certain.[70]
While ground sloths would have been relatively easy to spot and approach, big game hunters' weapons would have been useless from farther than 9.1 metres (30 ft) away. It would have been difficult to take down a ground sloth with a spear-thrower and would have required extensive knowledge of the species. Additionally, the ground sloths' already thick hide was fortified by osteoderms, making it difficult to penetrate.[62][71]
Since ground sloths thrived in an environment filled with large predators, they evidently would have been able to also defend themselves against human predation, so there is no reason to expect that they would have been "easy pickings". When feeding, they had enough strength to use their long, sharp claws to tear apart tree branches; presumably their strength and formidable claws would be dangerous for hunters that attempted to attack them at close quarters.[72] But fossilized evidence of humans hunting on ground sloth in White Sands National Park suggests that the slow-moving giant sloths were likely easy prey for early humans possibly hurling spears.[58][59]
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Megatherium, largest of the ground sloths, an extinct group of mammals belonging to a group containing sloths, anteaters, glyptodonts, and armadillos that underwent a highly successful evolutionary radiation in South America in the Cenozoic Era (beginning 65.5 million years ago). The size of these animals approximated that of a modern elephant, and they were equipped with large claws and teeth; the latter were confined to the sides of the jaw, because the animal fed largely on the leaves of trees and bushes. Ground sloths appeared briefly in North America during the Pleistocene Epoch (2.6 million to 11,700 years ago), when a land connection was established between the American continents.
: any of various often huge extinct American edentates related to the recent sloths
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Illustration of living giant sloths in the Amazon by William Rebsamen.
Ancient rock art at Guaviare, Colombia, sometimes interpreted as a giant ground sloth.
Ground sloths, giant sloths, or giant ground sloths were a group of sloths belonging to four families (Megalonychidae, which also includes the extant two-toed sloths, Megatheriidae, Nothrotheriidae, and Mylodontidae; some authorities also consider Scelidotheriidae, Orophodontidae, and Rathymotheriidae distinct families) which lived in the Americas from the Eocene until at least 8,000 years ago, with an accepted later date of around 1550 years ago for the Caribbean species. They are generally considered to have been wiped out by human hunting.
Ever since the first ground sloth fossils were discovered in the late 18th Century, it has been theorised that they may still persist in remote parts of the Americas, and a fairly large number of cryptids have been identified as possibly being ground sloths, if they are real.[1][2][3] Cryptids which may be living ground sloths have been reported from Argentina, The Bahamas, Belize, Bolivia, Brazil, Canada, Chile, Colombia, Cuba, the Dominican Republic, Ecuador, Haiti, Mexico, Paraguay, Peru, Trinidad and Tobago, the United States, and Venezuela: as noted by Karl Shuker, "there is no doubt that a medium-sized species of surviving ground sloth would solve a number of currently unresolved cryptozoological conundra,"[4] and much larger, smaller, and even aquatic ground sloths have also been put forward as identities for certain cryptids.
Early European beliefs[]
When the first remains of Megatherium were discovered in 1788, they were believed to have belonged to a living animal, an enormous mole which had burrowed to the surface by accident, and been scorched to death by the sun.[5] According to Bernard Heuvelmans, after the fossils had been studied and revealed as a giant sloth, the King of Spain instructed explorers to capture a live Megatherium for him.[5]
When Thomas Jefferson described the giant sloth Megalonyx (which he believed to be a lion) in 1797, he assumed the animal was still extant, and asked Lewis and Clark, as they planned their famous expedition in 1804 to 1806, to keep an eye out for living specimens of Megalonyx, as this would support his case.[5][6] Following the discovery that it was a sloth and not a lion, French naturalist Jean Baptiste Bory de Saint-Vincent agreed with Jefferson regarding its survival.[6]
In 1848, British naturalist and antiquarian Colonel Charles Hamilton Smith theorised based on American Indian legends that the first people in the Americas had coexisted with giant ground sloths and mastodons - however, he did not believe that either animal was still alive.[6]
Aquatic ground sloths[]
Reconstruction of Thalassocnus by Velizar Simeonovski.
At least two genera of giant sloth, the small freshwater Ahytherium of Brazil and the larger, marine Thalassocnus of Peru, were aquatic animals. Although they are believed to have gone extinct before the other giant sloths, around 3 million years ago, Austin Whittall speculates that a late-surviving (post-Ice Age) freshwater Patagonian species of the bulky Thalassocnus could explain several Patagonian lake monsters.[38]
The ningen of the Antarctic Ocean, described as a 20' to 30' human-like being, has been connected with large marine sloths like Thalassocnus by some.[primary source needed]
Diverse group of extinct sloth species
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. Ground sloths represent a paraphyletic group, as living tree sloths are thought to have evolved from ground sloth ancestors.
The early evolution of ground sloths took place during the late Paleogene and Neogene of South America, while the continent was isolated. At their earliest appearance in the fossil record, they were already distinct at the family level. Sloths dispersed into the Greater Antilles during the Oligocene, and the presence of intervening islands between the American continents in the Miocene allowed a dispersal of some species into North America. They were hardy as evidenced by their high species diversity and their presence in a wide variety of environments, extending from the far south of Patagonia (Cueva del Milodón Natural Monument) to Alaska.[1][2][3] Sloths, and xenarthrans as a whole, represent one of the more successful South American groups during the Great American Interchange after the connection of North and South America during the late Pliocene with a number of ground sloth genera migrating northwards. One genus, Thalassocnus, even adapted for marine life along the Pacific coast of South America during the late Miocene and Pliocene epochs.
Ground sloths, which were represented by over 30 living species during the Late Pleistocene, abruptly became extinct on the American mainland as part of the end-Pleistocene extinction event around 12,000 years ago, simultaneously along with the majority of other large animals in the Americas. Their extinction has been posited to be the result of hunting by recently arrived humans and/or climate change.[4][5] A number of kill sites are known where humans butchered ground sloths dating just prior to their extinction.
The Caribbean ground sloths, the most recent survivors, lived on Cuba and Hispaniola, possibly until 1550 BCE. However, radiocarbon dating suggests an age of between 2819 and 2660 BCE for the last occurrence of Megalocnus in Cuba.[6] They survived 5,000–6,000 years longer in the Caribbean than on the American mainland, which correlates with the later colonization of this area by humans.[7]
Ground sloths varied widely in size from under 100 kilograms (220 lb) in the Caribbean ground sloths, to 3,700–4,100 kilograms (8,200–9,000 lb) in the largest ground sloth genera Megatherium, Lestodon and Eremotherium.[8][9] The bodies of ground sloths were generally barrel-shaped, with a broad pelvis.[10] Like other xenarthrans, the adult teeth of ground sloths lacked enamel, with the tooth surface being composed of relatively soft orthodentine.[11] The number of teeth in the jaws is considerably reduced in comparison to other mammals, with most ground sloths only having 5 and 4 teeth in each half of the upper and lower jaws respectively, with some ground sloths exhibiting further tooth number reduction.[12] These teeth were rootless[12] and were continuously growing (hypselodont), and typically have a relatively simple morphology.[13] Some ground sloths have canine-like teeth at the front of the jaws separated from the other teeth by a gap (diastema).[10] The skull shapes of ground sloths are highly variable. Those with narrow muzzles are likely to have had prehensile lips, while those with wider muzzles are likely to have had mobile tongues.[13] The hands of ground sloths have ungual phalanges that indicate that they had well developed claws.[14] In many ground sloth families (Megatheriidae, Mylodontidae, Scelidotheriidae and Nothrotheriidae), the hindfoot is inwardly rotated, meaning sole faces inwards and that the body weight was primarily borne on the fifth metatarsus and the calcaneum.[15]
Ground sloths are generally regarded as herbivores, with some being browsers,[16] others grazers,[17] and some intermediate between the two as mixed feeders (both browsing and grazing),[18] though a number of authors have argued that some ground sloths may have been omnivores.[19] Sloths that had longer snouts are presumed to have had greater olfactory acuity, but appear to have also had less binocular vision and poorer ability to localize sounds. A number of extinct sloth species are thought to have had hearing abilities optimized for low frequencies, perhaps related to use of infrasound for communication.[20][21] Some ground sloths are suggested to have dug burrows.[22][23] Their skeletal anatomy suggests that they were incapable of running, and relied on other strategies to defend against predators,[24] though they were likely significantly more active and agile than living tree sloths.[25] Ground sloths were likely able to adopt a bipedal stance while stationary, allowing the forelimbs to be used to grasp vegetation as well as to use their claws for defence, though whether they were capable of moving in this posture is uncertain.[26][24] Some ground sloths have been suggested to be able to climb.[27] Some authors have suggested ground sloths were largely solitary animals, like living sloths,[28] though other authors have argued that at least some ground sloths are likely to have engaged in gregarious behaviour.[29] Whether or not ground sloths had a slow metabolism like living xenarthrans (including living sloths) is debated.[19]
Like living sloths, ground sloths likely only gave birth to a single offspring at a time,[30][31] with likely several years between the birth of offspring. At least some ground sloths engaged in long-term parental care, with one adult (presumably female) Megalonyx found with two juveniles of different ages, with the oldest juvenile suggested to be 3–4 years old.[31] Juvenile ground sloths may have clung to the body of their mother for some time following birth, as occurs in living tree sloths.[32]
The earliest unambiguous fossil evidence of ground sloths comes from the early Oligocene.[33] Ground sloths had dispersed into the Caribbean already by 31 million years ago, as evidenced by a femur found in Puerto Rico.[34] During the Miocene, sloths diversified, with the major families of sloths appearing during this period,[34] with diversity waxing and waning over the course of the Miocene. Megalonychid and mylodontid sloths had migrated into North America by the Late Miocene, around 10 million years ago. At the end of the Miocene, ground sloth diversity declined, though their diversity would remain largely stable throughout the Pliocene and Pleistocene periods, up until their extinction. During the Pliocene and Pleistocene, as part of the Great American Interchange, additional lineages of sloths migrated into Central and North America.[35] Prior to their extinction, there were over 30 living species of ground sloths across the Americas during the Late Pleistocene.[8]
Paleontologists assign more than 80 genera of ground sloths to multiple families.[36]
The megalonychid ground sloths first appeared in the Late Eocene, about 35 million years ago, in Patagonia. Megalonychids first reached North America by island-hopping, prior to the formation of the Isthmus of Panama. Some lineages of megalonychids increased in size as time progressed. The first species of these were small and may have been partly tree-dwelling, whereas the Pliocene (about 5 to 2 million years ago) species were already approximately half the size of the huge Late Pleistocene Megalonyx jeffersonii from the last ice age. Some West Indian island species were as small as a large cat; their dwarf condition typified both tropical adaptation and their restricted island environment. This small size also enabled them a degree of arboreality.[37]
Megalonyx, which means "giant claw", was a widespread North American genus that lived past the close of the last (Wisconsin) glaciation, when so many large mammals died out. Remains have been found as far north as Alaska[38] and the Yukon.[39][40] Ongoing excavations at Tarkio Valley in southwestern Iowa may reveal something of the familial life of Megalonyx. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations.[41][42]
The earliest known North American megalonychid, Pliometanastes protistus, lived in the southern U.S. about 9 million years ago and is believed to have been the predecessor of Megalonyx. Several species of Megalonyx have been named; in fact it has been stated that "nearly every good specimen has been described as a different species".[39] A broader perspective on the group, accounting for age, sex, individual and geographic differences, indicates that only three species are valid (M. leptostomus, M. wheatleyi, and M. jeffersonii) in the late Pliocene and Pleistocene of North America,[43] although work by McDonald lists five species. Jefferson's ground sloth has a special place in modern paleontology, for Thomas Jefferson's letter on Megalonyx, read before the American Philosophical Society of Philadelphia in August 1796, marked the beginning of vertebrate paleontology in North America.[39] When Lewis and Clark set out, Jefferson instructed Meriwether Lewis to keep an eye out for ground sloths. He was hoping they would find some living in the Western range. Megalonyx jeffersonii was appropriately named after Thomas Jefferson.[39]
The megatheriid ground sloths are relatives of the megalonychids; these two families, along with the family Nothrotheriidae, form the infraorder Megatheria. Megatheriids appeared later in the Oligocene, some 30 million years ago, also in South America. The group includes the heavily built Megatherium (given its name 'great beast' by Georges Cuvier[44]) and Eremotherium, which are the largest known ground sloths, thought to have had body masses of 3.5-4 tons.[8] The skeletal structure of these ground sloths indicates that the animals were massive. Their thick bones and even thicker joints (especially those on the hind legs) gave their appendages tremendous power that, combined with their size and fearsome claws, provided a formidable defense against predators.
The earliest megatheriid in North America was Eremotherium eomigrans which arrived 2.2 million years ago, after crossing the recently formed Panamanian land bridge. With more than five tons in weight, 6 meters in length, and able to reach as high as 17 feet (5.2 m), it was larger than an African bush elephant bull. Unlike relatives, this species retained a plesiomorphic extra claw. While other species of Eremotherium had four fingers with only two or three claws, E. eomigrans had five fingers, four of them with claws up to nearly a foot long.[45]
Recently recognized, ground sloths of Nothrotheriidae are often associated with those of the Megatheriidae, and together the two form the superfamily Megatherioidea. The most prominent members of the group are the South American genus Thalassocnus, known for being aquatic, and Nothrotheriops from North America.
The last ground sloths in North America belonging to Nothrotheriops died so recently that their subfossil dung has remained undisturbed in some caves. One of the skeletons, found in a lava tube (cave) at Aden Crater, adjacent to Kilbourne Hole, New Mexico, still had skin and hair preserved, and is now at the Yale Peabody Museum. The largest samples of Nothrotheriops dung can be found in the collections of the Smithsonian Museum. Another Nothrotheriops was excavated at Shelter Cave, also in Doña Ana County, New Mexico.[citation needed]
The mylodontid ground sloths together with their relatives the scelidotheriids form the Mylodontoidea, the second radiation of ground sloths. The discovery of their fossils in caverns associated with human occupation led some early researchers to theorize that the early humans built corrals when they could procure a young ground sloth, to raise the animal to butchering size.[46] However, radiocarbon dates do not support simultaneous occupation of the site by humans and sloths.[47] Subfossil remains like coproliths, fur and skin have been discovered in some quantities. The American Museum of Natural History has exhibited a sample of Mylodon dung from Argentina with a note that reads "deposited by Theodore Roosevelt".[48][49][50][51] Mylodontids are the only ground sloths confirmed to have had osteoderms embedded within their skin, though osteoderms were only present in a handful of genera and absent in many others.[52]
The largest mylodontid is Lestodon, with an estimated mass of 3,400–4,100 kilograms (7,500–9,000 lb).[53]
The ground sloth family Scelidotheriidae was demoted in 1995 to the subfamily Scelidotheriinae within Mylodontidae.[54][55] Based on collagen sequence data showing that its members are more distant from other mylodontids than Choloepodidae, it was elevated back to full family status in 2019.[56] Together with Mylodontidae, the enigmatic Pseudoprepotherium and two-toed sloths, the scelidotheriids form the superfamily Mylodontoidea. Chubutherium is an ancestral and very plesiomorphic member of this subfamily and does not belong to the main group of closely related genera, which include Scelidotherium and Catonyx.
The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee et al., 2019).[56]
Radiocarbon dating places the disappearance of ground sloths in what is now the United States at around 11,000 years ago. The Shasta ground sloth (Nothrotheriops shastensis) visited Rampart Cave (located on the Arizona side of the Lake Mead National Recreation Area) seasonally, leaving behind a massive stratified subfossilized dung deposit, and seemed to be flourishing from 13,000 until 11,000 BP, when the deposition suddenly stopped.[57] Steadman et al. argue that it is no coincidence that studies have shown that ground sloths disappeared from an area a few years after the arrival of humans.[7] Trackways preserved in New Mexico (probably dating from 10 to 15.6 thousand years ago) that appear to show a group of humans chasing or harassing three Nothrotheriops or Paramylodon ground sloths may record the scene of a hunt. The tracks are interpreted as showing seven instances of a sloth turning and rearing up on its hind legs to confront its pursuers, while the humans approach from multiple directions, possibly in an attempt to distract it.[58][59][60]
Those who argue in favor of humans being the direct cause of the ground sloths' extinction point out that the few sloths that remain are small sloths that spend most of their time in trees, making it difficult for them to be spotted. Although these sloths were well hidden, they still would have been affected by the climate changes that others claim wiped out the ground sloths. Additionally, after the continental ground sloths disappeared, insular sloths of the Caribbean survived for approximately 6,000 years longer, which correlates with the fact that these islands were not colonized by humans until about 5500 yr BP.[7]
It is difficult to find evidence that supports either claim on whether humans hunted the ground sloths to extinction.[61] Removing large amounts of meat from large mammals such as the ground sloth requires no contact with the bones; tool-inflicted damage to bones is a key sign of human interaction with the animal.[62]
