Ground sloths are a group of extinct sloths that belong to the mammalian superorder Xenarthra. They ranged in size, with the largest species found in the genera Lestodon, Eremotherium, and Megatherium. These sloths were about the same size as modern-day elephants. Ground sloths form a paraphyletic group, meaning that living tree sloths are believed to have evolved from ground sloth ancestors.
The early development of ground sloths occurred during the late Paleogene and Neogene periods in South America, when the continent was isolated from others. When they first appeared in the fossil record, they were already distinct at the family level. Sloths spread to the Greater Antilles during the Oligocene period. During the Miocene period, islands between the American continents allowed some sloth species to move into North America. They were resilient, as shown by their many species and wide range of habitats, from southern Patagonia (Cueva del Milodón Natural Monument) to Alaska. Sloths, along with other xenarthrans, were among the most successful South American groups during the Great American Interchange, which happened after North and South America became connected in the late Pliocene. Some ground sloth genera moved northward during this time. One genus, Thalassocnus, lived in the ocean along the Pacific coast of South America during the late Miocene and Pliocene periods.
Ground sloths had more than 30 living species during the Late Pleistocene. They suddenly disappeared from the American mainland around 12,000 years ago, during the end-Pleistocene extinction event, which also caused the extinction of many other large animals in the Americas. Scientists think their extinction was caused by hunting by humans who had recently arrived and/or changes in climate. Evidence of human activity, such as sites where ground sloths were killed, has been found just before their extinction.
Caribbean ground sloths were the last to survive, living on Cuba and Hispaniola until about 1550 BCE. However, radiocarbon dating shows that the last known Megalocnus in Cuba lived between 2819 and 2660 BCE. These sloths survived 5,000 to 6,000 years longer in the Caribbean than on the American mainland, which matches the later arrival of humans in the region.
Description
Ground sloths ranged in size from less than 100 kilograms (220 pounds) in Caribbean ground sloths to 3,700–4,100 kilograms (8,200–9,000 pounds) in the largest species, such as Megatherium, Lestodon, and Eremotherium. Their bodies were generally barrel-shaped, with a wide pelvis. The shapes of their skulls varied greatly. Like other xenarthrans, the adult teeth of ground sloths did not have enamel, and instead had a softer material called orthodentine. Ground sloths had far fewer teeth than most mammals, with most species having 5 teeth in each half of the upper jaw and 4 teeth in each half of the lower jaw. Some species had even fewer teeth. These teeth lacked roots and continuously grew throughout the sloth’s life, usually having a simple shape. Most ground sloths did not have teeth at the front of their jaws. Sloths with narrow muzzles likely had prehensile, upper lips similar to those of black rhinoceroses, while those with wider muzzles likely had square-shaped upper lips like those of white rhinoceroses, used with mobile tongues to grasp food. Some ground sloths had canine-like teeth at the front of their jaws, separated from other teeth by a gap. Their hands had claw-like structures, indicating well-developed claws. In several ground sloth families, the hindfoot was rotated inward, so the soles of the feet faced inward, and the body weight was mainly supported by the fifth toe and the heel bone.
Ecology
Ground sloths are typically considered herbivores, meaning they mainly eat plants. Some ate plants from trees, others ate grasses, and some did both. Some scientists believe certain ground sloths may have also eaten other foods, like insects or small animals. Sloths with longer snouts likely had a better sense of smell but had less ability to see with both eyes and had trouble locating sounds. Some extinct sloth species are thought to have heard very low sounds, possibly using them to communicate. Some ground sloths may have dug burrows for shelter. Their bones suggest they could not run and used other ways to avoid predators. They were probably more active and agile than today’s tree sloths. Some ground sloths could stand on two legs while still, using their front legs to grab plants and their claws to defend themselves. It is unclear if they could move while standing this way. Some ground sloths may have been able to climb. Scientists disagree about whether ground sloths lived alone like modern sloths or if some lived in groups. It is also debated whether ground sloths had a slow metabolism, like modern sloths.
Like modern sloths, ground sloths likely had one baby at a time, with many years between births. Some ground sloths cared for their young for a long time. One adult Megalonyx was found with two young sloths of different ages, with the older one possibly 3–4 years old. Baby ground sloths may have clung to their mothers for a time after birth, similar to modern tree sloths.
Evolution
The earliest clear fossil evidence of ground sloths comes from the early Oligocene period. By 31 million years ago, ground sloths had spread to the Caribbean, as shown by a femur discovered in Puerto Rico. During the Miocene period, sloths became more varied, with major families of sloths appearing during this time. Some groups of sloths became more common, while others decreased in number throughout the Miocene. By the Late Miocene, around 10 million years ago, megalonychid and mylodontid sloths had moved into North America. At the end of the Miocene, ground sloth diversity decreased, but it remained mostly stable during the Pliocene and Pleistocene periods until their extinction. During the Pliocene and Pleistocene, as part of the Great American Interchange, new sloth groups moved into Central and North America. Before their extinction, more than 30 species of ground sloths lived across the Americas during the Late Pleistocene.
Families
Paleontologists classify more than 80 types of ground sloths into several families.
Megalonychid ground sloths first appeared in the Late Eocene, about 35 million years ago, in Patagonia. These sloths reached North America by moving between islands before the Isthmus of Panama connected the continents. Some groups of megalonychids grew larger over time. Early species were small and may have lived partly in trees, while Pliocene (about 5 to 2 million years ago) species were roughly half the size of the large Late Pleistocene Megalonyx jeffersonii from the last ice age. Some West Indian island species were about the size of a large cat; their small size was common for tropical animals and those living on islands. This small size also helped them climb trees.
Megalonyx, meaning "giant claw," was a widespread North American group that lived after the last (Wisconsin) ice age, when many large mammals died out. Fossils have been found as far north as Alaska and the Yukon. Excavations at Tarkio Valley in southwestern Iowa may reveal more about how Megalonyx lived. An adult was found with two young sloths of different ages, suggesting 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 ancestor of Megalonyx. Many species of Megalonyx have been named, with some sources stating that "nearly every good specimen has been described as a different species." A broader view, considering age, sex, individual differences, and geography, suggests only three species (M. leptostomus, M. wheatleyi, and M. jeffersonii) were valid in the late Pliocene and Pleistocene of North America, though some studies list five species. Jefferson's ground sloth holds a special place in paleontology because Thomas Jefferson’s letter about Megalonyx, read before the American Philosophical Society in August 1796, marked the start of vertebrate paleontology in North America. Jefferson told Meriwether Lewis to look for ground sloths during the Lewis and Clark expedition, hoping to find living sloths in the West. Megalonyx jeffersonii was named after Thomas Jefferson.
Megatheriid ground sloths are relatives of megalonychids. These two families, along with Nothrotheriidae, form the infraorder Megatheria. Megatheriids appeared in the Oligocene, about 30 million years ago, in South America. This group includes the large Megatherium (named "great beast" by Georges Cuvier) and Eremotherium, the largest known ground sloths, which may have weighed 3.5 to 4 tons. Their strong bones and thick joints, especially on their hind legs, gave them powerful limbs. Combined with their size and large claws, these features helped them defend against predators.
The earliest megatheriid in North America was Eremotherium eomigrans, which arrived 2.2 million years ago after the Panamanian land bridge formed. Weighing over five tons, measuring 6 meters long, and standing up to 17 feet (5.2 meters) tall, it was larger than an African bush elephant bull. Unlike other Eremotherium species, which had four fingers with two or three claws, E. eomigrans had five fingers, four of which had claws nearly a foot long.
Ground sloths of Nothrotheriidae are often linked to those of Megatheriidae, and together they form the superfamily Megatherioidea. Prominent members include 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 recently enough that their old dung remains in some caves. A skeleton found in a lava tube (cave) at Aden Crater, near Kilbourne Hole in New Mexico, still had skin and hair preserved and is now at the Yale Peabody Museum. The largest samples of Nothrotheriops dung are in the Smithsonian Museum. Another Nothrotheriops was found at Shelter Cave in Doña Ana County, New Mexico.
Mylodontid ground sloths and their relatives, the scelidotheriids, form the Mylodontoidea, the second major group of ground sloths. Fossils found in caves with human remains led some researchers to think early humans built corrals to raise young sloths for meat. However, radiocarbon dates show humans and sloths did not live at the same sites. Sub
Extinction
Radiocarbon dating shows that ground sloths in what is now the United States disappeared about 11,000 years ago. The Shasta ground sloth (Nothrotheriops shastensis) visited Rampart Cave, located in Arizona near Lake Mead National Recreation Area, seasonally. It left behind thick layers of old, hardened dung, and seemed to be thriving from 13,000 to 11,000 years before present (BP), when the dung deposits suddenly stopped. Some scientists, like Steadman and others, say it is not a coincidence that ground sloths disappeared in an area shortly after humans arrived. In New Mexico, trackways (footprints) from 10,000 to 15,600 years ago show humans possibly chasing or harassing three ground sloths. The tracks suggest the sloths turned and stood on their hind legs to face humans, who approached from different directions.
Some scientists believe humans caused the extinction of ground sloths. Today’s remaining sloths are small and live in trees, making them hard to see. However, even these sloths would have been affected by climate changes that some say caused the extinction of larger ground sloths. After ground sloths on the mainland disappeared, sloths in the Caribbean survived for about 6,000 years longer, which matches the timing of human arrival in the region around 5,500 years ago.
Finding clear evidence of whether humans hunted ground sloths to extinction is difficult. Removing meat from large animals like sloths does not require touching their bones. Instead, signs of human interaction, such as damage to bones caused by tools, are key clues.
Several kill sites for ground sloths have been found in the Americas. In Argentina’s Pampas, a Megatherium americanum was butchered near a swamp about 12,600 years ago. Another possible kill site, Arroyo Seco 2, dates to 14,782–11,142 years ago. In northern Ohio, a Megalonyx jeffersoni skeleton, called the "Firelands Ground Sloth," shows cut marks from butchery, dating to 13,738–13,435 years ago. In Brazil’s Santa Elina rockshelter, a Glossotherium was found with hearths and stone tools, dating to 11,833–11,804 years ago. In Chile’s Fell’s Cave, a Mylodon with broken and burned bones linked to human activity dates to 12,766–12,354 years ago.
Humans are thought to have entered the Americas through Beringia, a land bridge connecting Asia and North America during the last ice age. Scientists like Mosimann and Martin (1975) suggested early humans were skilled hunters who tracked and killed large animals. By this time, humans used tools like Clovis points—narrow, carved stone projectiles for hunting big game. Later, the atlatl (a spear-throwing tool) allowed hunters to throw spears faster, giving them more distance from their prey. These tools may have made it safer to approach ground sloths.
Ground sloths had traits that made them easy targets for humans. They often fed in open areas, and their diet, found in fossilized dung, included tree leaves, grasses, shrubs, and yucca. These plants grew in open spaces, making sloths visible to humans. Sloths had never met humans before, so they did not know how to react to them. They moved slowly, waddling on their hind legs and front knuckles, with claws turned inward. Their large size (some weighed up to 3,000 kilograms) and slow speed made them vulnerable.
However, hunting ground sloths would have been challenging. Hunters could not effectively use weapons from more than 9.1 meters (30 feet) away. Their thick skin, reinforced by bony plates, made them hard to pierce. Ground sloths also lived in areas with large predators, suggesting they could defend themselves. When feeding, they used their strong claws to tear branches, which could have been dangerous to humans trying to attack them up close. Fossil evidence from White Sands National Park, however, suggests that early humans may have hunted slow-moving giant sloths by throwing spears at them.