Picture a sloth beside the Pacific Ocean, then watch it wade into the shallows and disappear beneath the surface to feed. This is not a cartoon. Millions of years ago, a remarkable group of extinct sloths called Thalassocnus adapted to life along the western coast of South America.
Modern sloths belong in rainforest canopies, but their family history is far stranger and more varied than the six or seven living species suggest. Fossils from Peru, Chile and Argentina reveal sloths that spent much of their time in or beside the sea. Across several species, their skeletons changed in ways that helped them stay underwater, move through the shallows and graze on marine plants.
The result was one of evolution's loveliest oddities: a sea-going sloth filling a role rather like a small sea cow.
What was Thalassocnus?
Thalassocnus was a genus of extinct ground sloths that lived during the Miocene and Pliocene epochs. Researchers currently recognise five species. Fossils have been recovered from marine deposits along the Pacific coasts of Peru and Chile, with further material known from Argentina.
A 2025 peer-reviewed study indexed by PubMed described a nearly complete skeleton of Thalassocnus natans from Chile's Atacama Region. It is among the most complete aquatic-sloth specimens yet reported from the country. That matters because a fuller skeleton lets palaeontologists compare the skull, limbs, spine and other bones as parts of one animal rather than relying on scattered fragments.
These animals were not direct ancestors of today's tree sloths. They were members of the much broader sloth family tree, which once included an astonishing variety of ground-dwelling forms. Thalassocnus represents the family's only known move into an aquatic lifestyle.
How do we know these sloths lived in the water?
Fossils do not preserve a video of an animal swimming, so scientists look for several clues that point in the same direction. One of the clearest is bone density.
The California Academy of Sciences explains that researchers used CT scans to examine fossils from all five known species. Their bones were much denser than those of living sloths. Heavy bones can act rather like a diver's weight belt, reducing buoyancy and making it easier for a large plant-eater to remain submerged while feeding.
The legs and feet changed too. The Natural History Museum of Los Angeles County reports that later species developed flatter, more plantigrade hind feet. Earlier ground sloths tended to place their feet differently, but the changing ankle and foot shape may have made movement in water easier.
No single feature proves the whole story. Dense bones, changing limbs, marine sediments and feeding wear become persuasive when considered together.
The change happened step by step
The five species are especially useful because they preserve different stages of adaptation. Earlier members of the group retained more features suited to life on land and may have gathered plants close to shore. Later species show stronger aquatic specialisation.
This was not a land animal suddenly turning into a seal. The transition unfolded over millions of years. As generations spent more time feeding in coastal water, traits that improved stability, access to food or movement underwater could become more common.
That gradual pattern is a useful reminder of what evolution really looks like. Animals do not choose a new design because they need it. Small inherited differences are filtered by survival and reproduction over immense spans of time. A lineage can eventually look startlingly different from its earlier relatives without any single dramatic leap.
What did aquatic sloths eat?
Thalassocnus appears to have been a marine herbivore. The latest research places these sloths within a wider community of plant-eating marine mammals along the ancient Pacific coast. Their likely menu included seaweed and seagrasses in shallow coastal habitats.
That makes comparison with dugongs and manatees useful, although aquatic sloths were their own distinct experiment. Like modern sea cows, they could have cropped marine vegetation while their dense skeletons helped control buoyancy. The Natural History Museum notes that the South Pacific's aquatic sloths did not reach the enormous sizes achieved by some plant-eating marine mammals farther north.
They may nevertheless have played an important ecological role. A large grazer can shape plant beds, recycle nutrients and create feeding patches used by other organisms. Ancient coastal ecosystems were not simply modern seas with a few extinct animals added. They contained combinations of species and ecological roles that no longer exist.
Why are their fossils found in a desert?
The Atacama is now famous as one of the driest places on Earth, which makes the discovery of a sea-going sloth there feel especially surprising. The answer lies in geology. Sediments that formed on or near an ancient seabed were later exposed on land as sea levels and the landscape changed.
Marine fossils in a desert are therefore not misplaced. They are records of an older coastline. The rocks can preserve sloths alongside the remains of whales, sharks, fish and other animals that inhabited the South Pacific millions of years ago.
The newly described Chilean skeleton also shows why field discoveries still matter. Even when scientists already know a genus from other sites, a more complete specimen or a new locality can change estimates of its anatomy, age, range and relationship to neighbouring species.
What happened to the sea sloths?
Thalassocnus eventually disappeared, and researchers are still refining the timing and causes. Changing ocean conditions, habitats and food supplies may all have mattered, but a neat single explanation would go beyond the available evidence.
What remains is an unusually rich evolutionary trail. Across several related species, the bones document a sloth lineage becoming increasingly at home in coastal water. It is rare to see such a distinctive lifestyle emerge through a sequence of fossils.
Modern sloths are excellent swimmers when necessary, but they are rainforest animals, not leftovers from this marine branch. Their extinct relatives simply show how much possibility once existed within the sloth family. Long before today's slow climbers became icons of the canopy, other sloths grazed beneath Pacific waves.
Sources and image credit
- Natural History Museum of Los Angeles County: Dive Into the Weird Miocene World of Aquatic Sloths
- PubMed: 2025 study of aquatic sloths from the Miocene of Chile
- California Academy of Sciences: Aquatic Sloths
- Featured image: artwork by Alex Boersma, via the Natural History Museum of Los Angeles County.