Ground sloths are a diverse group of

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

sloth Sloths are a Neotropical realm, Neotropical group of xenarthran mammals constituting the suborder Folivora, including the extant Arboreal locomotion, arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of move ...

s in the

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

ian superorder

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 ...

. They varied widely in size with the largest, belonging to genera '' Lestodon'', ''

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 ''

Megatherium ''Megatherium'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Late Pleistocene. It is best known for the elephant-sized type spe ...

'', being around the size of

elephants Elephants are the Largest and heaviest animals, 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 ele ...

. 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 The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...

and

Neogene The Neogene ( ,) is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period million years ago. It is the second period of th ...

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 The Greater Antilles is a grouping of the larger islands in the Caribbean Sea, including Cuba, Hispaniola, Puerto Rico, and Jamaica, together with Navassa Island and the Cayman Islands. Seven island states share the region of the Greater Antille ...

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 ...

, and the presence of intervening islands between the American continents in the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

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 Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

. Sloths, and xenarthrans as a whole, represent one of the more successful South American groups during the

Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which land ...

after the connection of North and South America during the late

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Thalassocnus,'' 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 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 ...

, abruptly became extinct on the American mainland as part of the end-Pleistocene extinction event around 12,000 years ago, simultaneously 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. 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 Cuba, officially the Republic of Cuba, is an island country, comprising the island of Cuba (largest island), Isla de la Juventud, and List of islands of Cuba, 4,195 islands, islets and cays surrounding the main island. It is located where the ...

and

Hispaniola Hispaniola (, also ) is an island between Geography of Cuba, Cuba and Geography of Puerto Rico, Puerto Rico in the Greater Antilles of the Caribbean. Hispaniola is the most populous island in the West Indies, and the second-largest by List of C ...

, possibly until 1550 BCE. However,

radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for Chronological dating, determining the age of an object containing organic material by using the properties of carbon-14, radiocarbon, a radioactive Isotop ...

suggests an age of between 2819 and 2660 BCE for the last occurrence of '' Megalocnus'' in

. They survived 5,000–6,000 years longer in the

Caribbean The Caribbean ( , ; ; ; ) is a region in the middle of the Americas centered around the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean, mostly overlapping with the West Indies. Bordered by North America to the north, Central America ...

than on the American mainland, which correlates with the later colonization of this area by humans.

Description

Ground sloths varied widely in size from under in the Caribbean ground sloths, to in the largest ground sloth genera ''

'', '' Lestodon'' and ''

.'' The bodies of ground sloths were generally barrel-shaped, with a broad pelvis. The skull shapes of ground sloths are highly variable. Like other xenarthrans, the adult teeth of ground sloths lacked enamel, with the tooth surface being composed of relatively soft orthodentine. 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. These teeth were rootless and were continuously growing (hypselodont), and typically have a relatively simple morphology. There are generally no teeth at the front of the jaws. In order to be able to grasp food, those whose skulls exhibit narrow muzzles are likely to have had prehensile,

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 upper lips, while those with wider muzzles are likely to have had a square,

white rhinoceros The white rhinoceros, also known as the white rhino or square-lipped rhinoceros (''Ceratotherium simum''), is the largest extant species of rhinoceros and the most Sociality, social of all rhino species, characterized by its wide mouth adapted f ...

like upper-lip, used in combination with mobile tongues. Some ground sloths have canine-like teeth at the front of the jaws separated from the other teeth by a gap (diastema). The hands of ground sloths have ungual phalanges that indicate that they had well developed claws. 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.

Ecology

Ground sloths are generally regarded as herbivores, with some being browsers, others grazers, and some intermediate between the two as mixed feeders (both browsing and grazing), though a number of authors have argued that some ground sloths may have been omnivores. 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 Infrasound, sometimes referred to as low frequency sound or incorrectly subsonic (subsonic being a descriptor for "less than the speed of sound"), describes sound waves with a Audio frequency, frequency below the lower limit of human audibility ...

for communication. Some ground sloths are suggested to have dug

burrow file:Chipmunk-burrow (exits).jpg, An eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...

s. Their skeletal anatomy suggests that they were incapable of running, and relied on other strategies to defend against predators, though they were likely significantly more active and agile than living tree sloths. 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. Some ground sloths have been suggested to be able to climb. Some authors have suggested ground sloths were largely solitary animals, like living sloths, though other authors have argued that at least some ground sloths are likely to have engaged in gregarious behaviour. Whether or not ground sloths 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 xenarthrans (including living sloths) is debated. Like living sloths, ground sloths likely only gave birth to a single offspring at a time, 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. Juvenile ground sloths may have clung to the body of their mother for some time following birth, as occurs in living tree sloths.

Evolution

The earliest unambiguous fossil evidence of ground sloths comes from the early

. Ground sloths had dispersed into the Caribbean already by 31 million years ago, as evidenced by a femur found in Puerto Rico. During the

, sloths diversified, with the major families of sloths appearing during this period, 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 The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

periods, up until their extinction. During the Pliocene and Pleistocene, as part of the

, additional lineages of sloths migrated into Central and North America. Prior to their extinction, there were over 30 living species of ground sloths across the Americas during the

Families

Paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

s assign more than 80

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

of ground sloths to multiple families.

Megalonychidae

The megalonychid ground sloths first appeared in the

Late Eocene The Priabonian is, in the International Commission on Stratigraphy, ICS's geologic timescale, the latest age (geology), age or the upper stage (stratigraphy), stage of the Eocene epoch (geology), Epoch or series (stratigraphy), Series. It spans ...

, about 35 million years ago, in Patagonia. Megalonychids first reached North America by island-hopping, prior to the formation of the

Isthmus of Panama The Isthmus of Panama, historically known as the Isthmus of Darien, is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North America, North and South America. The country of Panama is located on the i ...

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

'' 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.J.L. White (1993) '' 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 and the

Yukon Yukon () is a Provinces and territories of Canada, territory of Canada, bordering British Columbia to the south, the Northwest Territories to the east, the Beaufort Sea to the north, and the U.S. state of Alaska to the west. It is Canada’s we ...

. Ongoing excavations at Tarkio Valley in southwestern

Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...

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. 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".Harrington (1993) 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, although work by McDonald lists five species. Jefferson's ground sloth has a special place in modern

paleontology Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure ge ...

, for

Thomas Jefferson Thomas Jefferson (, 1743July 4, 1826) was an American Founding Fathers of the United States, Founding Father and the third president of the United States from 1801 to 1809. He was the primary author of the United States Declaration of Indepe ...

's letter on ''Megalonyx'', read before the

American Philosophical Society The American Philosophical Society (APS) is an American scholarly organization and learned society founded in 1743 in Philadelphia that promotes knowledge in the humanities and natural sciences through research, professional meetings, publicat ...

Philadelphia Philadelphia ( ), colloquially referred to as Philly, is the List of municipalities in Pennsylvania, most populous city in the U.S. state of Pennsylvania and the List of United States cities by population, sixth-most populous city in the Unit ...

in August 1796, marked the beginning of

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

paleontology in North America. When

Lewis and Clark Lewis may refer to: Names * Lewis (given name), including a list of people with the given name * Lewis (surname), including a list of people with the surname Music * Lewis (musician), Canadian singer * " Lewis (Mistreated)", a song by Radiohe ...

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.

Megatheriidae

The megatheriid ground sloths are relatives of the megalonychids; these two families, along with the family Nothrotheriidae, form the

infraorder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between Family_(biology), family and Class_(biology), class. In biological classification, the order is a taxonomic rank used in the classific ...

Megatheria. Megatheriids appeared later in the Oligocene, some 30 million years ago, also in South America. The group includes the heavily built ''

'' (given its name 'great beast' by

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 ...

) and ''

,'' which are the largest known ground sloths, thought to have had body masses of 3.5-4 tons. 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

Panama Panama, officially the Republic of Panama, is a country in Latin America at the southern end of Central America, bordering South America. It is bordered by Costa Rica to the west, Colombia to the southeast, the Caribbean Sea to the north, and ...

nian land bridge. With more than five tons in weight, 6 meters in length, and able to reach as high as , it was larger than an

African bush elephant The African bush elephant (''Loxodonta africana''), also known as the African savanna elephant, is a species of elephant native to sub-Saharan Africa. It is one of three extant elephant species and, along with the African forest elephant, one ...

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.

Nothrotheriidae

Recently recognized, ground sloths of Nothrotheriidae are often associated with those of the

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 ...

, 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 New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, 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,

Mylodontidae

Paramylodon fossil at Texas Memorial Museum

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. However, radiocarbon dates do not support simultaneous occupation of the site by humans and sloths. Subfossil remains like coproliths, fur and skin have been discovered in some quantities. The

American Museum of Natural History The American Museum of Natural History (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...

has exhibited a sample of '' Mylodon'' dung from Argentina with a note that reads "deposited by

Theodore Roosevelt Theodore Roosevelt Jr. (October 27, 1858 – January 6, 1919), also known as Teddy or T.R., was the 26th president of the United States, serving from 1901 to 1909. Roosevelt previously was involved in New York (state), New York politics, incl ...

". Mylodontids are the only ground sloths confirmed to have had

osteoderms Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of Extant taxon, extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, Temnospondyli, ...

embedded within their skin, though osteoderms were only present in a handful of genera and absent in many others. The largest mylodontid is '' Lestodon'', with an estimated mass of .

Scelidotheriidae

The ground sloth family

Scelidotheriidae Scelidotheriidae is a family of extinct ground sloths within the order Pilosa, suborder Folivora and superfamily Mylodontoidea, related to the other extinct mylodontoid family, Mylodontidae, as well as to the living two-toed sloth family Choloepo ...

was demoted in 1995 to the subfamily Scelidotheriinae within Mylodontidae. 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. Together with Mylodontidae, the enigmatic '' Pseudoprepotherium'' and two-toed sloths, the scelidotheriids form the superfamily Mylodontoidea. ''

Chubutherium ''Chubutherium'' is an extinct genus of ground sloth from the Late Oligocene and Early Miocene of Chubut Province, Argentina. Description This species of ground sloth was assigned to the family Scelidotheriidae, which was subsequently demoted ...

'' 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''.

Phylogeny

The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee ''et al.''., 2019).

Extinction

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 Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

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. 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. 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. 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. It is difficult to find evidence that supports either claim on whether humans hunted the ground sloths to extinction. 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.

Hunting of ground sloths

Kill sites

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 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 ...

(BP), with another potential ''Megatherium'' kill site being Arroyo Seco 2 in the same region, dating to approximately 14,782–11,142 cal yr BP. 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. 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.

Hunting weapons

Humans are believed to have entered the New World via

Beringia Beringia is defined today as the land and maritime area bounded on the west by the Lena River in Russia; on the east by the Mackenzie River in Canada; on the north by 70th parallel north, 72° north latitude in the Chukchi Sea; and on the south ...

, a

land bridge In biogeography, a land bridge is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonize new lands. A land bridge can be created by marine regression, in which sea le ...

which connected Asia and North America during the

last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Last Glacial Coldest Period, was the most recent time during the Last Glacial Period where ice sheets were at their greatest extent between 26,000 and 20,000 years ago. Ice sheets covered m ...

. 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. 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. These inventions would have allowed hunters to put distance between them and their prey, potentially making it less dangerous to approach ground sloths.

Advantages

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. 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, 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 ) imply they were relatively slow mammals. 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.

Difficulties

While ground sloths would have been relatively easy to spot and approach, big game hunters' weapons would have been useless from farther than 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

osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amph ...

s, making it difficult to penetrate. 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. 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.

References

Sources

*; (2): 227–228. * * Harrington, C.R. (1993)
Yukon Beringia Interpretive Center – Jefferson's Ground Sloth
Retrieved 2008-JAN-24. * Hogan, C.M. (2008)
''Cueva del Milodon'', Megalithic Portal
Retrieved 2008-APR-13 * Kurtén, Björn and Anderson, Elaine (1980): ''Pleistocene Mammals of North America.'' Columbia University Press, New York. * McKenna, Malcolm C. & Bell, Susan K. (1997): ''Classification of Mammals Above the Species Level.'' Columbia University Press, New York. * Nowak, R.M. (1999): ''Walker's Mammals of the World'' (Vol. 2). Johns Hopkins University Press, London. * * *

External links

Picture and information about a ground sloth skeleton on display at the University of Georgia's Science Library.''Eremotherium'' in Florida.Western Center for Archaeology and Paleontology
Hemet, CA {{Taxonbar, from=Q933583 Clawed herbivores Eocene first appearances Mammal common names Miocene xenarthrans Oligocene xenarthrans Pleistocene xenarthrans Pliocene xenarthrans Prehistoric mammals of South America Prehistoric mammals of North America Prehistoric sloths