With Dense Bones And A Stiff Spine, Giant Sloths Were Designed For The Sea

Source: Hakai Magazine/Brian Switek 

Photo: Kleber Varejão Filho/Unsplash


Over and over again, animals have dipped their toes in the ocean and stayed. Reptiles did it multiple times, becoming majestic sea dragons like the ichthyosaurs, plesiosaurs, and mosasaurs. Whales are an even more celebrated example. How cetaceans went from four-legged landlubbers to streamlined sea blubbers is an iconic example of transcendent evolutionary change. And then there are the giant sloths.

Reconstructed in museum fossil halls, the giant sloths of ages past look about as seaworthy as bricks. Their bones are large, bulbous, and stout, a sturdy framework to hold the immense weight of the herbivorous mammals that could weigh over four tonnes. And yet, in Peru and Chile, in strata between seven and three million years old, paleontologists have found a set of five sloth species, all from the genus Thalassocnus, that together illustrate how giant sloths once shuffled into the sea.

When these shaggy beasts’ fossils were first described in 1995, it was thought that Thalassocnus was a giant wader. The sloth didn’t so much swim as slog in the shallows, holding onto rocks with its enormous claws and slurping up soft vegetation. But recent reanalysis of the sloths’ bones have changed that story.

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Photo: Kleber Varejão Filho/Unsplash

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