We tend to picture sloths hanging quietly in a tree, looking as though their next appointment is sometime on Thursday.

Their fossil relatives had other plans. Some went into the ocean.

Meet Thalassocnus, a group of extinct sloths that lived along the Pacific coast of South America roughly nine to five million years ago. Fossils from Peru and Chile reveal animals that increasingly adapted to feeding in coastal waters.

A sloth grazing underwater may sound like a scheduling error, but its bones tell a convincing story.

Across successive species, researchers have documented thicker, denser bones. That extra weight would have helped counter buoyancy, allowing these animals to remain submerged while feeding. 

Changes in their skulls and limbs also support an increasingly aquatic lifestyle. Later species appear to have been better equipped to graze on submerged vegetation.

These were distant cousins of living sloths, rather than tree sloths that had simply fallen off a branch. Their evolutionary history records a remarkable shift from terrestrial life towards the sea.

And the modern family still has an unexpected aquatic talent. Living sloths can swim, using their long arms to move through the water. Those limbs may look wonderfully awkward on the ground, but water offers a different set of possibilities.

The fossil record gives us a much broader view of sloth life: animals that climbed, animals that walked on the ground, and animals that ventured beneath the waves.

Somewhere in that family history, a sloth looked at the ocean and found lunch.

Sources and Further Reading

• The Sloths that Once Swam in the Sea — Smithsonian Ocean.
• Little-Known Facts about Sloths — Smithsonian Institution.
• Valenzuela-Toro et al. (2025). Aquatic sloths (Thalassocnus) from the Miocene of Chile and the evolution of marine mammal herbivory in the Pacific Ocean.