Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. They varied widely in size; the largest belonged to the genera Lestodon, Eremotherium and Megatherium, and were roughly the size of modern-day 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. 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 northward. One genus, Thalassocnus, was adapted to 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 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 known kill sites, where ground sloths were butchered by humans, date to just prior to their extinction. The Caribbean ground sloths, the most recent survivors, lived on Cuba, Hispaniola, and Puerto Rico, possibly until 1550 BCE. However, radiocarbon dating suggests an age of between 2819 and 2660 BCE for the last occurrence of Megalocnus in Cuba. 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.
Description
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. 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-like upper lips, while those with wider muzzles are likely to have had a square, white rhinoceros 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 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 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 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.
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