Today, the six living species of sloths are usually found dangling from tree branches, or going viral on YouTube. But sloths used to be a lot more diverse—and a lot bigger. The extinct ground sloths pursued all sorts of different lifestyles and came in just about every imaginable shape and size. Some were cow-like grazers; others might have been accomplished burrowers; and, believe it or not, a few even dined beneath the ocean waves.
1. THE LARGEST WERE ELEPHANT-SIZED.
Megatherium (above) means “giant beast”—a fitting name for a creature that weighed several tons, reached 20 feet in length, and—when reared up on its hind legs—stood over 12 feet tall. The biggest sloth of all time, Megatherium americanum, occupied South America between five million and eleven thousand years ago. Above the equator, its slightly-smaller cousin, the 6000-pound Eremotherium, managed to spread as far north as New Jersey.
2. MOST WALKED ON THE SIDES OF THEIR HIND FEET.
All ground sloths were predominantly quadrupedal. While they were more than capable of standing up on two legs (more about this later), the animals preferred to get around on four—but individual species differed widely from each other in terms of limb posture.
Scientists have divided ground sloths up into four recognized families, and only one—the megalonychids—stood flat on their rear feet like humans do. Because of the shapes of their ankle and/or hind claws, sloths from the megatheriid, mylodontid, and nothrotheriid families had to trudge along by putting weight on the outer sides of their feet.
3. AT LEAST SOME HAD ARMOR PLATES.
Buried in the skin of the mylodontid ground sloths—including the Harlan’s ground sloth, whose range extended from Florida to Washington state—were a series of small bony discs. Known as “osteoderms,” these little knobs (nickel-sized in Harlan’s ground sloth) were mostly clustered around the back, shoulders, and neck and would have acted like protective chainmail.
This trait isn’t all that unusual. A few modern animals, including armadillos and crocodilians, also have osteoderms of some kind—as did many dinosaurs.
4. MANY USED THEIR TAILS TO FORM “TRIPODS.”
For these animals, standing up on two limbs required some extra stability. Whenever a ground sloth did this, its muscular tail would act like another leg, helping to support its considerable body weight.
5. ONE SPECIES IS NAMED AFTER THOMAS JEFFERSON.
The Sage of Monticello’s importance to American paleontology cannot be understated. In 1796, Jefferson—a respected armchair naturalist—received some curious bones from western Virginia (modern West Virginia). This find wasn’t all that unusual—similar-looking fossils had also emerged in Kentucky and other parts of Virginia. Still, Jefferson spoke at length about the big-clawed mystery animal at a 1797 meeting of the American Philosophical Society in Philadelphia. The future president dubbed this creature Megalonyx, or “great claw.” Though we now know that it was a large, flat-footed sloth, Jefferson originally mistook the animal for an enormous lion or tiger-like carnivore.
Currently, four different species of Megalonyx are recognized; the most famous, Megalonyx jeffersonii, was named in Jefferson's honor. On March 8, 2008, West Virginia recognized the animal as its official state fossil.
6. HUMANS PROBABLY ATE THEM.
What killed off the woolly mammoth, the scimitar cat, and North America’s other ice age mega-mammals? Homo sapiens usually gets a good chunk of the blame. Scientists have long speculated that humans killed and devoured ground sloths—but, for many years, there was no physical evidence to support this idea. Then, in 2008, incriminating scars were found on the femur of an Ohio Megalonyx. The 13,000-year old fossil is riddled with 41 unusual cuts that appear to have been left by manmade tools.
As archaeologist Haskel Greenfield points out, we’ll likely never know if early Americans killed this animal or merely scavenged its remains. “The only thing that is clear,” he said in 2012, “is that there are disarticulation marks: they were separating the limbs from each other; they were cutting the joints. And some marks show that they were filleting the meat off the bone.”
7. THERE WERE SEAGOING “GROUND” SLOTHS.
Imagine a sloth that’s trying hard to be a marine iguana. You’ve just pictured a member of the Thalassocnus genus. These Peruvian herbivores, which lived 8 to 4 million years ago, dove into the ocean for their supper. Hooked claws helped them latch onto submerged, seaweed-covered rocks; once anchored, a Thalassocnus could consume marine algae. Over time, evolution fitted the amphibious sloths with increasingly dense ribs and limb bones. Therefore, younger species were less buoyant—and probably more aquatic—than their ancestors had been.
8. WE’VE FOUND A MUMMIFIED GROUND SLOTH.
Rivaling a black bear in size, Nothrotheriops would have been dwarfed by behemoths like Megatherium. Still, we know more about it than any other ground sloth thanks to one amazing find. Eleven thousand years ago, a New Mexican Nothrotheriops stumbled into a volcanic gas vent and died. Then, in either 1927 or 1928 (sources differ), a group of explorers happened upon the incredibly well preserved body. Not only were almost all of its ligaments and bones intact, but this Nothrotheriops also came with a few muscle fibers. Even more interestingly, the specimen retained some original skin—covered by rough, yellowish hair. The cherry on top was an accompanying dung ball, which helped confirm that Nothrotheriops ate a diverse array of plants—including cacti fruit, yuccas, and saltbushes.
In 1928, Yale’s Peabody Museum of Natural History acquired the mummy, and today, visitors can find the creature on display in Mammal Hall.
9. CERTAIN SLOTHS MIGHT HAVE BEEN REALLY GOOD DIGGERS.
Your average ground sloth was—in all likelihood—a browsing herbivore, pulling down tree branches with its strong forelimbs. However, the mylodontids may have also gathered food by digging for it. Experts argue that their wide, flattened claws look like ideal tools for excavating roots and tubers [PDF].
Believe it or not, mylodontids might have even been burrowers. Several huge, prehistoric tunnels have been found in Argentina [PDF]. Made sometime during the Pleistocene epoch (between 2.6 million and 11,700 years ago), these were natural marvels, with the longest stretching 130 feet from end to end. What could have possibly dug them? Two top suspects are Scelidotherium and Glossotherium: a pair of mylodonts with claws that match scratch marks found inside the burrows.
10. ONE HOLDOUT DIDN’T GO EXTINCT UNTIL RATHER RECENTLY.
The Caribbean islands seem like an unlikely place for ground sloths to have made their last stand—but that’s exactly where it happened [PDF]. Mainland North America lost all of its indigenous species around 11,000 years ago, and half a millennium later, South America, too, became a ground sloth-free continent.
But despite these extinctions, some ground sloths didn't die out until much later. Hispaniola and Cuba were home to assorted dwarf species. Descended from their full-sized counterparts on the mainlands, these mammals were resilient. Ultimately, Megaloncus rodens was the last ground sloth standing: Radiocarbon dating indicates that this 200-pound vegetarian waddled across Cuba as recently as 4200 years ago.