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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.
Eli Amson, a paleontologist at the Museum für Naturkunde in Berlin, Germany, is one of the researchers who undertook this osteological revision. After reexamining the fossils, Amson and his team found that Thalassocnus was more at home in the water than previously thought. Some of the changes seen through a succession of five distinct Thalassocnus species are similar to those observed in cetaceans as they evolved to life in the sea. For example, the bones of Thalassocnus, like those of early cetaceans, are osteosclerotic—evolved to be exceptionally dense—and acted as ballast to help the animals maintain neutral buoyancy as they swam in the shallows. The modifications were so extensive that even the thin bones inside the sloth’s nose, called turbinates, became thickened as a result of changes in the rest of the body.
But there are differences in the path along which the sloths evolved, too, that set them apart from that taken by cetaceans. Amson says several features in the sloths’ skeletons point toward a bottom-walking way of swimming in which the sloths punted along, pushing off the bottom from point to point, much like modern hippos do. The sloths weren’t undulating their spines up and down to swim like early whales. Instead, Amson says they bounded along the bottom, using their long tails to steer. The sloths sculled around in the shallows, munching on plants with their flattened molars. These differences in locomotion are reflected in the spine—the Thalassocnus species that evolved later in the transition to the sea had backbones that became more fused and stiff, rather than more flexible.
“Thalassocnus has the potential of becoming a fantastic example to study evolutionary transitions, as many of the traits observed in the lineage offer really compelling cases of gradual adaptation,” Amson says. And the beast also shows that “sloths in the Miocene lived in the trees, on the ground, and in the water,” says San Diego State University marine mammal expert Annalisa Berta. The time was a heyday for these large, furry mammals.
As unusual as a bear-sized, swimming sloth might seem, though, the mammal’s evolution “is not that surprising, I think,” Amson says. While giant sloths weren’t speed demons, they were much more active than their modern counterparts. And that some became adapted to life in the water also isn’t entirely unexpected, especially given vertebrates have repeatedly taken the plunge. In fact, Thalassocnus shares some traits with other marine mammals that also took to the waves to enjoy some sea greens.
Today’s sirenians—manatees and dugongs—are herbivores, and an unusual group of extinct beasts called desmostylians also wandered into the water to munch on vegetation. “Different marine mammal herbivores colonized the oceans,” Berta says, with “desmostylians in the North Pacific, Thalassocnus in the South Pacific, and sirenians more globally dispersed.” This not only makes Thalassocnus an example of transcendent evolutionary change, but of convergence—when different lineages independently arrive at similar body plans or behaviors.