The Fascinating Ancestry of the Sloth

While sloths today are lethargic tree-dwellers, their ancestors were different beasts entirely. But what happened to these wonderful animals? And what does Thomas Jefferson have to do with them?

The Sloths that we are familiar with can best be described as lethargic mop buckets. Present-day sloths are among the slowest and most conservative animals on the planet, typically only moving a whopping 40 meters per day (1) within their treetop homes of South America. Sloths are covered in algae and, once a week, undertake slow, laborious trips to the forest floor to defecate; to put it bluntly, they are not the prettiest animals around. While modern sloths may be lazy and odd, their extinct counterparts weren’t so lame. These extinct sloths, which I will formally refer to as ‘ground sloths’ for the duration of this article, were the opposite of their modern counterparts in many ways and are some of the most peculiar animals of all time.

Ground sloth size

Not Your Average Sloth

The most obvious difference between the ground sloths and their modern counterparts revolves around their size. The largest Sloth species alive today is Hoffman’s Two-Toed sloth, which weighs in at a meager 7.7 kilograms (17 pounds); about the size of a bowling ball. On the other hand, the largest genus of ground sloth, Megatherium, weighed an astonishing 4 tonnes and stood at heights of 4 meters (when it reared back on its hind legs). The size of Megatherium is comparable to that of an African elephant, with only a few extinct elephant and rhinoceros’ species being larger amongst land mammals. Even smaller genera of ground sloth, such as the American genus Megalonyx, were relatively large with a weight of over 1 tonne. While the colossal size difference between extinct and extant sloths may seem odd, it is a common trend amongst paleontology that includes a surprising amount of animal families.

‘Ground Sloths’ for a Reason

Besides size, the overall lifestyle of ground sloths was far different from that of their modern cousins. As opposed to hanging from trees, ground sloths spent their time, well, on the ground; most species walked on all fours and had the ability to occasionally rear up on their hind legs. Their forearms were equipped with massive claws, which were used primarily as tools to grab and pull vegetation, for defense against predators and lastly, for digging. There is substantial evidence for this last function, as numerous burrows attributed to ground sloths have been found in Brazil, with some spanning hundreds of meters and being large enough to walk through.

On top of massive size and claws, ground sloths also had a hidden defense against predators. Underneath their skin lay ‘dermal ossicles’, which formed a layer of armor that protected them from attack. Even though ground sloths weren’t invincible (more on that later), their combined mechanisms of defense made them dangerous opponents for any predators daring enough to challenge them head on.


Thomas Jefferson, Presidential Paleontologist?

Amongst all the accomplishments of American president Thomas Jefferson, having a ground sloth named after him may just be the strangest. As it turns out, Jefferson took great interest in the giant bones of mysterious and exotic creatures from across America, bones which we recognize today as fossils that belonged mostly to mammoths and mastodons. His pièce de résistance came four years ahead of his presidency in 1797, when Jefferson received a set of strange claw bones that had originated from West Virginia. Jefferson theorized that the bones belonged to a giant cat that still roamed the American east, since the idea of extinction was a relatively new and unproven concept during Jefferson’s life. He named his mystery species Megalonyx, which translates to “giant claw” in Latin. Years later, Jefferson’s specimen would be reclassified as a ground sloth, as paleontologists noticed the similarity between Jefferson’s claws and those of the South American ground sloth Megatherium. Despite the change in identity, the genus name Megalonyx survived the transition, with its holotype species name-Megalonyx Jeffersonii being made out to honour its discoverer. In fact, the name Megalonyx is often negated for a name that is far easier to remember: Jefferson’s ground sloth.



If I had to choose, I would say that the most peculiar genus of extinct sloth was Thalassocnus, the sloth that became aquatic. The fossils of Thalassocnus are often found on the west coast of South America in marine deposits alongside extinct whale species, clear evidence of aquatic behaviours. Since multiple species of Thalassocnus are known, each of which representing a different point of time spanning millions of years, paleontologists have been able to identify a clear transition in the behaviour of Thalassocnus. Whereas older species seem to have been adapted to eating vegetation close to shore, younger species show clear adaptations to venturing into deeper water, including denser bones to accommodate their natural buoyancy and teeth enamel that reflects a diet of aquatic vegetation, namely seagrass and algae. Due to the absence of clear swimming adaptations, Thalassocnus likely spent their time in water walking along the seabed, using their signature sloth claws to ground them. While their life in the shallow seas of the Miocene may sound like peaches and cream, Thalassocnus would have experienced constant danger in the form of carnivorous toothed whales, including the giant sperm whale Livyatan and dozens of shark species, including the infamous Carcharocles (Otodus?) Megalodon. Despite the danger, Thalassocnus grew increasingly well adapted to life in the sea as time went on, their evolution ultimately being cut short when cooling global temperatures forced them into extinction 3 million years ago.


A Trip To America

For over 100 million years, South America was isolated from the rest of the world. This isolation helped to foster the evolution  a myriad of strange and unique animals, including giant tank-like armadillos, 10 foot tall killer birds (the aptly named Terror Birds) and the ground sloths. This changed for segmented periods of time starting 3 million years ago when North and South America finally collided. This event is known as the great American interchange and saw hundreds of species travel between the continents, with animals such as bears, saber-tooth cats, elephants and horses travelling south, and animals like opossums, armadillos and the ground sloths travelling north. While animals moving into South America mostly adapted well to their new habitats, the South American fauna moving north had a more difficult time. Species moving north rarely exceeded modern-day Texas, and even when they managed to, only did so for a short period of time. The ground sloths were an exception to this however, as they are substantially represented in American fossil beds in locations such as Florida, California, Alaska and even the Northwest Territories. Despite heavy competition from mammoths, mastodons and other large megaherbivores, ground sloths were extremely successful in North America despite the failures of their South American counterparts. The reasoning behind their success is unclear, but based on their success in North America, it is entirely possible that they may have travelled across the Bering strait and into Asia if it was possible at the time.

Made Extinct By People?

Ground sloths are currently extinct. Along with the vast majority of large mammals across the planet, they died out at the end of the last ice age, some 10,000 years ago. The cause of this mass extinction has long been a mystery, but two prevailing theories have long been maintained. The first theory pertains to human hunting. This theory states that it was humans that forced the extinction of animals such as mammoths, woolly rhinos and the ground sloths through practises of overkill. This fact does have substantial evidence to support it, including fossil sites that show evidence of human predation on ground sloths and mammoths, amongst other species. While this theory does have its merits, it seems unlikely that humans alone could be responsible for a simultaneous global extinction, especially when considering the fact that humans had been around for some tens of thousands of years prior. The other prevailing theory has to do with drastic climate changes, which brought upon extreme changes to the habitats of these megaherbivores. Like the human hunting theory, it doesn’t seem plausible that all megafauna species across the world would be affected. In all likelihood, a combination of the two and some other currently unknown factor contributed to the extinction of the megafauna, thus killing of the ground sloths for good.

While it may be unfortunate that we can’t witness 4 tonne sloths in person, watching their seventeen-pound cousins will have to suffice. I must admit, they aren’t the worst things to look at; have you ever seen a baby sloth?

mamma sloth

For Avedis.


I do not take credit for any illustrations found in this article. 

Thalassocnus illustrated by Dr. Mark Witton, which can be found at his blog here or his twitter here

Megatherium header image illustrated by Brian Engh, who can be found at his twitter here

Jefferson and Megalonyx illustrated by Tom Dunne, found here

Sloth family found here

Megatherium skeleton found here

Brewer, Pip. “What Was Megatherium?” Natural History Museum,,metres%20(12%20feet)%20tall.

Collins, R. Lee. “Mylodont (Ground Sloth) Dermal Ossicles from Colombia, South America.” Journal of the Washington Academy of Sciences, vol. 23, no. 9, 1933, pp. 426–429. JSTOR, Accessed 15 July 2020

“Jefferson’s Ground Sloth.” Jefferson’s Ground Sloth | Explore the Ice Age Midwest, Illinois State Museum,

Jenner, Andrew. “Get Lost in Mega-Tunnels Dug by South American Megafauna.” Discover Magazine, Discover Magazine, 9 May 2020,

Jewett, Thomas O. “Thomas Jefferson: Paleontologist.” Online Tutoring, Classes, and Test Prep – Varsity Tutors,

Lax, Aaron. “10 Facts about Sloths, Nature’s Slowest Animals.” World Animal Protection, 18 May 2020,

Life through the Ages II: Twenty-First Century Visions of Prehistory, by Mark P. Witton and Charles Robert Knight, Indiana University Press, 2020, pp. 122–123.

McDonald, H. G., et al. “The Ground Sloth Megalonyx from Pleistocene Deposits of the Old Crow Basin, Yukon, Canada.” Arctic, vol. 53, no. 3, 2000, pp. 213–220. JSTOR, Accessed 16 July 2020.

Pappas, Stephanie. “What Is Gondwana?” LiveScience, Purch, 7 June 2013,

Presslee, Samantha, et al. “Palaeoproteomics Resolves Sloth Relationships.” Nature News, Nature Publishing Group, 6 June 2019,

Woodward, Aylin. “Humans Once Hunted and Butchered Giant Ground Sloths in South America, 12,600-Year-Old Bones Reveal.” Business Insider, Business Insider, 7 Mar. 2019,

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