Buzz
Modern sloths are far from graceful in the water. When today’s two- and three-toed sloths enter the water, they clumsily paddle in a manner reminiscent of a doggy paddle. However, their ancient ancestors were a different breed altogether. Evolution had sculpted these prehistoric sloths into aquatic specialists. Over millions of years, one group of giant sloths transitioned into marine environments.
The typical depiction of a giant sloth is a large, slow-moving creature munching on leaves in an Ice Age forest. However, the marine sloths—known as Thalassocnus—inhabited the coastal regions of Peru approximately 8 to 4 million years ago. This area lacked the dense forests sloths are often associated with. Instead, the desert met the ocean, and it was in these waters that the sloths sourced their sustenance.
Since their initial discovery in Peru’s Pisco Formation in 1995, scientists have uncovered five distinct species of Thalassocnus, each thriving in the same coastal habitat in succession. Found alongside marine creatures, their fossils indicate that these sloths were well-adapted to life in the shallow waters. From the earliest to the most recent species, Thalassocnus remained a quintessential coastal sloth.
How did Thalassocnus achieve what no other sloth had? In a study published in the Proceedings of the Royal Society B, paleontologist Eli Amson and his team at the Muséum national d'Histoire naturelle, Paris, uncovered the key to Thalassocnus's success: its bones held the secret.
Sloths are known for their unusually dense bones compared to other mammals, and this trait was even more pronounced in the extinct giant sloths. The successive species of Thalassocnus took this to an extreme. By examining the ribs and limb bones of the first four Thalassocnus species, Amson and his colleagues discovered that their bones grew increasingly dense over time. In cross-section, the bones nearly lost their hollow centers, a condition known as osteosclerosis. Additionally, some bones exhibited pachyostosis, a thickening that allowed the sloths to accumulate more bone tissue.
These bone adaptations are not unique to sloths. Paleontologists have observed similar features in other semi-aquatic mammals. Early whales and sea cows, which were transitioning to fully aquatic lifestyles, also displayed these traits. These skeletal changes were not abnormalities but evolutionary adaptations that provided ballast for buoyancy control.
While air-filled lungs are essential for preventing drowning, they can hinder deep diving. Large, air-filled lungs create buoyancy, making it harder for animals to stay submerged. Denser, swollen bones helped early whales, sea cows, and giant sloths like Thalassocnus achieve neutral buoyancy, allowing them to dive more efficiently and conserve energy underwater.
Despite its adaptations, Thalassocnus was no Olympic swimmer. It retained the bulky build of its terrestrial ancestors. Instead of evolving into a sleek swimmer, Thalassocnus likely adopted a strategy similar to modern marine iguanas. With its heavy bones acting as anchors, the sloth used its large claws to grip the seabed and graze on soft plants while drifting in the waves. Essentially, it was a sloth that feasted on seaweed by the shore.
