The ground sloths were an incredibly diverse group that ranged dramatically in size, from the 100kg Caribbean ground sloth to the four-tonne, 3.5m-tall Megatherium, says Will Newton.
The descendants of these giant ground sloths are still alive today, but they live exclusively in trees, aren’t nearly as large, and are only represented by six species - all of which are endemic to the tropical rainforests of Central and South America.
- Why are sloths so slow?
- It's slow, covered in algae, likes hanging out upside down and is always smiling - meet the sloth – a rainforest cutie that is a surprisingly good swimmer
Where did ground sloths live?
The ground sloths are a large group of extinct sloths that lived from the Late Eocene (around 30 million years ago) right up until the Middle Holocene (around 5,000 years ago). They first appeared in South America, but after the Ismuth of Panama rose up from the seafloor and formed a land bridge between the Americas roughly 2.7 million years ago, they spread to North America and established themselves as one of the most successful pan-American mammals.
As a group, ground sloths lived alongside some of the most iconic megafauna ever to set foot in the Americas, including phorusrhacids, or terror birds, mammoths, woolly rhinos, sabre-tooth tigers, and even prehistoric humans.
It’s widely thought that prehistoric humans, who migrated to the Americas from Eurasia during the Late Pleistocene roughly 20,000 years ago, drove the ground sloths to extinction - but more on that later.
How diverse were ground sloths?
The Late Pleistocene (129,000-11,700 years ago) was a particularly prosperous time for ground sloths - as many as 30 species lived during this short period, distributed across the Americas. The majority of these species lived on the ground, hence the name, and are generally regarded as herbivores, with some interpreted as browsers, others grazers, and several a mixture of both.
The browsers fed on a variety of leaves, soft shoots, and fruits from high-growing trees and shrubs, while the grazers had a diet that largely consisted of low-lying plants, like grasses.
However, there were a couple of ground sloths that bucked this generic herbivore trend and evolved to occupy different ecological niches.
What was Thalassocnus?
Thalassocnus was one of these trendsetting ground sloths; it was roughly the size of a cow and based on the density of its bones and the high placement of its nostrils, it’s thought that it spent most of its time in water, diving for seaweed and seagrasses. The seas weren’t a safe place during this period, though, and Thalassocnus was likely prey for sharks and an ancestor of today’s sperm whales known as Acrophyseter.
What was Mylodon?
Another peculiar ground sloth was Mylodon - a 4m-long, 2-tonne behemoth that had a taste for meat. From isotopic studies of amino acids extracted from some well-preserved fossils, researchers have discovered that Mylodon occasionally ate meat, likely from carcasses it may have stumbled across during its search for the grasses it typically dined on.
This suggests that not all ground sloths were strict vegetarians and that some may have occasionally engaged in opportunistic carnivory, just like many herbivorous mammals do today, such as cows, goats, deers, and hippos.
What specialist adaptations did ground sloths have?
The Americas were a dangerous place during the Late Pleistocene, even before the arrival of prehistoric humans roughly 20,000 years ago. It’s clear from ground sloths’ skeletal anatomy that they were incapable of running, so when it came to a fight-or-flight encounter with one of the Americas’ many predators (e.g. the infamous sabre-tooth tiger Smilodon), they probably always chose the former.
To defend themselves, ground sloths had long, sharp claws on the ends of several of their fingers - the 6m-long Eremotherium had four such claws, each nearly a foot in length. They also had size on their side; with thick bones and even thicker joints they could strike with a surprising amount of power. It’s unlikely that many predators went after a fully grown ground sloth - to do so would have meant certain death, especially if the predator found themselves on the receiving end of a well-timed slap.
Their immense bulk didn’t just help them ward off predators, ground sloths also used their size - and their ability to stand on two legs - to reach vegetation that would have been out of reach to most other herbivores of the time.
The largest ground sloths, Megatherium, Lestodon, and Eremotherium, were probably capable of reaching heights of up to 5.5m, which is about the height of an average residential street light.
It’s thought some ground sloths were also capable of burrowing and may have even dug the 1.2m-high, 1.8m-wide, and 20m-long branched tunnels found in the Pampean region of South America. There are scratches on the walls and roofs of these tunnels that match those of a ground sloth known as Scelidotherium - a smaller than average ground sloth, standing 1.1m tall at the shoulder and weighing in at ~850kg. There are some signs that the slightly larger ground sloth Glossotherium may have also dug tunnels, which would make it the largest burrowing animal of all time.
Why did ground sloths become extinct?
The majority of ground sloths disappeared at the end of the Pleistocene (around 11,700 years ago), facing extinction at the same time as many other iconic American megafauna, such as woolly rhinos, sabre-tooth tigers, and mastodons. However, a small population managed to survive for a few more thousand years in the Caribbean, specifically on the islands of Cuba and Hispaniola. These islands were connected to the mainland during glacial periods, allowing the ancestors of the last-surviving ground sloths to move over and establish themselves before they were cut off by the Caribbean Sea.
Megalocnus, a pig-sized, possibly cave-dwelling ground sloth, is generally considered to be the last of its long lineage. The last occurrence of Megalocnus in the Caribbean has been radiocarbon dated to between 2819 and 2669 BCE, which is around the same time that the pyramids were built in Ancient Egypt. It’s thought a small population of woolly mammoths also survived until this time, although on an island off the coast of Siberia, known as Wrangel Island.
The general consensus is that ground sloths, and many other megafauna from the Late Pleistocene and Early Holocene, faced extinction at the hands of our ancestors. There’s lots of evidence to suggest our ancestors hunted ground sloths, particularly in South America where a number of kill sites containing remains of butchered ground sloths have been found. Some of these remains belong to one of the biggest ground sloths, Megatherium, suggesting their sheer size wasn’t enough to deter hungry humans.
There’s also an incredible site in White Sands National Park, New Mexico, where the footprint of a ground sloth - likely made by a Paramylodon - has been found alongside sets of footprints made by humans.
This site paints an interesting picture of a group of hunters either hot on the tail of a ground sloth, or engaged in a fight to the death. There’s no evidence to suggest whether or not the hunt was successful, but given the stone-tipped hurling spears humans are known to have used at the time, it seems unlikely that the ground sloth escaped without significant injury.
While humans certainly contributed to the demise and subsequent extinction of ground sloths, it’s thought they were also victims of large, continent-spanning climatic changes that occurred towards the end of the Pleistocene. As temperatures warmed and ice sheets retreated, the environments that ground sloths and other megafauna once thrived in changed and, as a result, many populations started to collapse. This, combined with hunting pressures from humans, was enough to spark an America-wide extinction of many groups of megafauna, including ground sloths.
- What was the first ever apex predator? Meet the prehistoric Anomalocaris who ruled before the dinosaurs
- Prehistoric cats: meet 10 ruthless feline hunters that roamed the planet thousands of years ago
- At a staggering 3 metres tall the extinct prehistoric Gigantopithecus is the biggest ape ever, but it wouldn't be the heaviest...
- These prehistoric terror birds – with 'walking bolt cutters' for beaks – would have petrified early humans