Ground sloth

(A, top left) Eremotherium laurillardi (B, top right), Lestodon armatus (C, middle left) Mylodon darwinii (D, middle right) Glossotherium robustum (E, bottom left) and Catonyx cf. C. cuvieri'' (F, bottom right) Ground sloths varied widely in size from under in the Caribbean ground sloths, to in the largest ground sloth genera Megatherium, Lestodon and Eremotherium. The bodies of ground sloths were generally barrel-shaped, with a broad pelvis. The skull shapes of ground sloths are highly variable. 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 Some ground sloths have canine-like teeth at the front of the jaws separated from the other teeth by a gap (diastema). 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 for communication. Some ground sloths are suggested to have dug burrows. 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 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 like living xenarthrans (including living sloths) is debated. 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. == Evolution ==

The earliest unambiguous fossil evidence of ground sloths comes from the early Oligocene. Ground sloths had dispersed into the Caribbean already by 31 million years ago, as evidenced by a femur found in Puerto Rico. During the Miocene, sloths diversified, with the major families of sloths appearing during this period, Prior to their extinction, there were over 30 living species of ground sloths across the Americas during the Late Pleistocene. == Families ==

Paleontologists assign more than 80 genera of ground sloths to multiple families. Megalonychidae 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. 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. 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. 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". 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, 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.) and Eremotherium, 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 Panamanian 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 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, 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. Mylodontidae '', Texas Memorial Museum, University of Texas at Austin 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 has exhibited a sample of Mylodon dung from Argentina with a note that reads "deposited by Theodore Roosevelt". 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. The largest mylodontid is Lestodon, with an estimated mass of . Scelidotheriidae The ground sloth family Scelidotheriidae 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. }} == Extinction ==

'' anteater in an upright defensive stance similar to those presumed to have been adopted by ground sloths, per trackways preserved in New Mexico ized Nothrotheriops shastensis dung in Rampart Cave, Arizona (NPS, 1938) 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. 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. 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. 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 (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. 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, 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. 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. 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 osteoderms, 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==