Ground sloth

Ground sloths were a diverse group of large, extinct sloths that lived in the Americas. They belonged to the mammalian superorder Xenarthra and came in many sizes, with some of the biggest, like Lestodon, Eremotherium, and Megatherium, being about as big as modern-day elephants. Unlike the tree sloths we see today, which climbed in trees, ground sloths lived on the ground and roamed across many landscapes.

These fascinating animals first evolved in South America when it was isolated from other continents. Over time, they spread to places like the Greater Antilles, North America, and even as far north as Alaska. They were very successful, living in many different environments from the cold south of Patagonia to the northern reaches of Alaska. After North and South America joined, many ground sloth species moved north as part of the Great American Interchange.

Ground sloths were very numerous during the Late Pleistocene, with over 30 species living across the Americas. However, they suddenly went extinct around 12,000 years ago, along with many other large animals. Scientists think this might have been because humans hunted them or because of changes in the climate. Some ground sloths lived much longer on islands in the Caribbean, like Cuba and Hispaniola, surviving until around 1550 BCE.

Description

Ground sloths came in many different sizes. Some were small, weighing less than 100 kilograms, while others, like Megatherium, Lestodon, and Eremotherium, were as big as modern elephants, weighing between 3,700 and 4,100 kilograms. They had strong, barrel-shaped bodies and broad pelvises. Their skulls varied a lot in shape.

These sloths had unique teeth. Unlike most animals, their teeth did not have enamel and were made of a softer material called orthodentine. They had fewer teeth than other mammals, usually just five in the upper jaw and four in the lower jaw. These teeth kept growing throughout their lives and were simple in shape. Many ground sloths did not have teeth at the front of their mouths. Those with narrow faces might have had upper lips like a black rhinoceros to help grab food, while those with wider faces might have had square lips like a white rhinoceros, working together with their tongues. Some had special teeth at the front separated from the others by a gap.

Their hands had strong claws, and in many ground sloth families, their back feet were turned inward, with their weight resting on certain bones.

Ecology

Ground sloths were plants eaters, with some eating leaves and others eating grass. Some might have eaten a mix of both. Sloths with longer noses may have had a better sense of smell but worse eyesight and hearing. Some ground sloths might have dug holes to live in. They could not run and had to use other ways to stay safe from danger. They might have been able to stand on two legs to reach plants and use their claws for protection. Some may have been able to climb.

Like living sloths, ground sloths probably had one baby at a time, with several years between each baby. Some ground sloths took care of their young for a long time. One adult sloth was found with two young sloths of different ages, with the oldest being about 3 to 4 years old. Young sloths may have stayed close to their mother after birth, just like living tree sloths.

Evolution

The earliest clear fossils of ground sloths date back to the early Oligocene. By 31 million years ago, ground sloths had already reached the Caribbean, as shown by a bone found in Puerto Rico. During the Miocene, sloths diversified into many different families. By the Late Miocene, around 10 million years ago, some types of sloths had moved into North America. At the end of the Miocene, the number of ground sloth species decreased, but their variety stayed about the same during the Pliocene and Pleistocene periods. During these times, more types of sloths moved into Central and North America as part of the Great American Interchange. Before they went extinct, there were over 30 different species of ground sloths living in the Americas during the Late Pleistocene.

Families

Paleontologists classify more than 80 genera of ground sloths into several families.

Megalonychidae

Main article: Megalonychidae

The megalonychid ground sloths appeared first in the Late Eocene, around 35 million years ago, in Patagonia. They reached North America before the Isthmus of Panama formed. Some grew larger over time. Early species were small and might have lived partly in trees, but Pliocene species were already about half the size of the huge Late Pleistocene Megalonyx jeffersonii, which lived during the last ice age. Some island species were as small as large cats, adapting to their island homes.

Megalonyx, meaning "giant claw", lived in North America until after the last (Wisconsin) glaciation. Fossils have been found as far north as Alaska and the Yukon. Excavations in Tarkio Valley, southwestern Iowa, suggest that adult Megalonyx cared for young of different ages.

Megatheriidae

Main article: Megatheriidae

The megatheriid ground sloths are related to megalonychids. They appeared later in the Oligocene, about 30 million years ago, also in South America. This group includes the very large Megatherium and Eremotherium, some of the biggest known ground sloths, weighing as much as 3.5–4 tons. Their strong bones and joints, along with their size and big claws, helped protect them from attackers.

Eremotherium eomigrans arrived in North America 2.2 million years ago, after the Panamanian land bridge formed. It was larger than an African bush elephant, measuring over 6 meters long and standing up to 17 feet tall, and had extra claws on one hand.

Nothrotheriidae

Main article: Nothrotheriidae

Ground sloths in Nothrotheriidae are often grouped with those in Megatheriidae. The most well-known are the South American Thalassocnus, which lived in water, and Nothrotheriops from North America.

The last Nothrotheriops in North America died out recently enough that their subfossil dung can still be found in some caves. One skeleton found in a lava tube at Aden Crater, near Kilbourne Hole, New Mexico, still had skin and hair. This is now at the Yale Peabody Museum.

Mylodontidae

Main article: Mylodontidae

The mylodontid ground sloths, along with their close relatives the scelidotheriids, are the second major group of ground sloths. Early ideas that humans raised young ground sloths in corrals were proven wrong by dating, which showed humans and sloths did not live together at those sites. Scientists have found subfossil remains like fur and skin. The American Museum of Natural History displays a sample of Mylodon dung from Argentina, labeled as being deposited by Theodore Roosevelt. Mylodontids are the only ground sloths known to have had osteoderms in their skin, though not all species had them.

The largest mylodontid was Lestodon, estimated to weigh between 3,400–4,100 kilograms.

Scelidotheriidae

Main article: Scelidotheriidae

The ground sloth family Scelidotheriidae was moved to a subfamily in 1995, but later restored to full family status in 2019 based on genetic data. Along with Mylodontidae and the puzzling Pseudoprepotherium and two-toed sloths, scelidotheriids make up the superfamily Mylodontoidea. Chubutherium is an early member that is not closely related to the main group, which includes Scelidotherium and Catonyx.

Extinction

Ground sloths disappeared from what is now the United States around 11,000 years ago. Evidence from Rampart Cave in Arizona shows that the Shasta ground sloth lived there until about that time. Some scientists think humans may have played a role in their disappearance, as these large animals vanished from areas shortly after humans arrived. Tracks in New Mexico suggest humans may have chased ground sloths.

There are several places in the Americas where ground sloths appear to have been hunted by humans. For example, in Argentina, a Megatherium was found with signs of butchering near a swamp about 12,600 years ago. In Ohio, a Megalonyx skeleton shows cuts from tools, dating to around 13,700 years ago. These findings show that early humans may have hunted these large animals.

Ground sloths had some traits that might have made them easier targets. They often fed in open areas where they could be seen, and they had never met humans before, so they didn’t know how to react. They moved slowly on their hind legs and front knuckles, and some weighed as much as 3,000 kilograms (6,600 lb). However, they also had thick hides and strong claws that could defend themselves. Despite this, evidence suggests early humans did hunt them, possibly using spears from a distance.