About 8 million years ago, off the coast of Peru, a large mammal used its powerful claws to pull itself along the ocean floor. About 5 million years ago, a similar creature was burrowing underground in Argentina. About 20,000 years ago, another member of this same group of animals climbed the cliffs as high as skyscrapers. But today, the only living relatives of these animals hang upside down from trees. They poop only once a week and move so slowly that algae grow on their fur.
Over time, sloths have seen many territories, and their ancestors evolved to fill a surprising and diverse array of niches. The story of sloths is one of astounding ecological variability, with some foraging in the seas and others still hiding from predators in towering cliffs. So how did an entire family of burrow-digging, sea-faring, cliff-hopping sloths go extinct, and their only living relatives end up in the trees?
How sloths went from the seas to the trees?
Sloths are members of the order Xenarthra, including armadillos and anteaters. It’s not clear what the last common ancestor of all Xenarthrans was. But genetic studies of living species suggest they probably lived more than 65 million years ago. This means this early xenarthran lived among the feet of the last non-avian dinosaurs. But it probably resembled an anteater or an armadillo more than a modern sloth in that it was built for digging. All Xenarthrans share features that are perfect for burrowing.
For example, they all have long, curved claws and extra contact points between their vertebrae and pelvis. It gives them a stiffer lower back to free up their front limbs for digging. Since the days of that common ancestor back in the Cretaceous Period, Xenarthrans have crawled beyond the forest floor and into many other ecosystems.
Sloths Fossil records
Sloths have been preserved in the fossil record since the early Oligocene. Their fossil record is huge in South America, representing about 80 to 90 different genera over 30 million years. Many even lived in the same area. One 17-million-year-old site in Patagonia was home to 11 different genera of sloths from 3 different subfamilies!
Usually, herbivores of the same size compete with each other for food. So how did so many sloths live together without competing for each other into extinction? The answers to that question can help us understand why sloths have been able to occupy so many niches over time.
Ancient sloths likely had a very low basal metabolism, much like modern ones. That means that when they weren’t doing much, they weren’t using as much energy as other mammals of the same size. It is typical of all xenarthrans, and there’s some evidence that it was the case for ancient sloths.
For example, some research has revealed a relationship between an animal’s body temperature and the amino acids found in its bone collagen. A study of the Shasta Ground Sloth fossils, a giant sloth from the Pleistocene, found that it probably had a body temperature of about 35 degrees celsius. That’s colder than it sounds.
Another thing that helped ancient sloths thrive in weird places. They could eat all kinds of stuff that no other animals were interested in, such large numbers. We don’t have any fossils of their stomachs. But they probably had digestive systems that were as complex and efficient as modern sloths, with four chambers. It allowed them to digest tough, fibrous leaves that other herbivores didn’t bother. The last thing that gave these ancient animals an unexpected advantage was their mouths.
Their teeth don’t have any enamel in both extinct and living sloths, so they aren’t as hard. But they’re always growing, so sloths can eat almost anything without running out of tooth surfaces. So, sloths could make a good living by having a deficient metabolism, efficient gut, and teeth that don’t get worn away. So they didn’t have to compete with other herbivores or each other.
In the Late Miocene Epoch, the sea sloth Thalassocnus was hard to say 8 million years ago. The Peruvian coast was a hot desert without much food for a large herbivore. So to make ends meet, an early species of Thalassocnus turned to the sea and the grasses that grew in the shallow water.
Over about 4 million years, it became more specialized for aquatic life. It developed denser ribs and limb bones to help weigh it down as it began foraging deeper waters. Also, it grew in size, eventually becoming as big as a modern sea lion. Still, Thalassocnus probably wasn’t a great swimmer. Instead of having, it used its big claws to pull itself along the ocean floor against the force of the crashing waves. It was an important adaptation because those waves were dangerous:
- Several fossils of Thalassocnus have been found with broken and re-healed leg bones, signs of trauma from being swept away and bashed against the rocks.
But sloths’ massive claws allowed them to thrive in other habitats too. In Brazil and Argentina, massive burrows reaching almost 80 meters long have been found in Pliocene and Pleistocene rocks. Some of these burrows are so big that they’re practically caves. Also, giant ground sloths were the only things around that could dig a big hole. Even though these sloths were big and slow, they probably didn’t have to work too hard to dig.
These caves are found in rocks with lots of fissures and faults. So Lestodon and other sloths likely pulled loose rocks out of the wall instead of furiously digging. Once they were done, they had managed to make a useful refuge for themselves. In addition to providing a safe place to live, the stable climate of the caves probably helped keep the sloths’ body temperatures steady.
Other sloths lived in caves that didn’t have to dig for themselves. The Peruvian sloth: Diabolotherium, or “devil beast,” from the Late Pleistocene. Unlike other ground sloths, Diabolotherium had a much greater range of motion in its elbows and forearms, making it an excellent rock climber. Its fossils are found almost exclusively in caves, including 300 meters up a cliff face.
Sloths in trees
While Diabolotherium was climbing cliffs, other sloths were doing what we know them best for: Living in trees. As early as the Miocene Epoch, semi-arboreal sloths can be found in the fossil record like Hapalops or Nematherium. Both of them had flexible elbow joints suggesting they could climb trees, and they’re probably ancestral cousins of the modern 2-toed sloth.
They weigh between 40 and 90 kilograms, from a deer to a kangaroo. The very biggest branches could have only supported them. They didn’t hang upside-down. So sloths have inhabited caves, dived in the seas, lived on the ground, and climbed up in the trees. With their super low metabolism, they could find nutrients others couldn’t. But this brings us to perhaps the weirdest part of the whole story of sloth evolution: the sloths that survived to the modern day.
Both modern genera of living sloths live in Central and South America. There are the three-toed sloths, which are members of the genus Bradypus. There’s the two-toed kind, known as Choloepus. They’re both suspensory. It means they spend their time not just in trees but suspended from trees.
Being suspensory is next-level tree-dwelling, and both tree sloths are so well adapted to it today. They can barely walk on the ground at all. But there aren’t any fossils of suspensory sloths, including those of Bradypus or Choelopus. It makes it hard for us to understand their evolutionary story. These two surviving tree sloths don’t seem to be very closely related at all.
Choloepus is probably a member of Megalonychidae, which included some pretty big, bad-ground sloths like Megalonyx. They lived in North America as much as 10 million years ago. But Bradypus may be the oldest and most primitive member of all sloths. Its lineage split off from other fossil and modern sloths over 30 million years ago. It’s so strange that some paleontologists put it into its own family, all by itself.
So, they come from different ancestral groups, but today’s modern sloths both hang from trees! It means that being suspensory must have evolved twice independently in each lineage! Over millions of years, sloths have shown an ability to gradually adapt to environments as diverse as the ocean, hard-rock caves, and tropical forests.
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