While sloths are known as slow-moving tree dwellers, an extinct lineage from South America defies this common perception: the aquatic sloth, Thalassocnus. This unique genus showcases how some ground sloths adapted to life in the ocean, swimming the coastal waters of what is now Peru and Chile.
The World of Thalassocnus
Thalassocnus inhabited the Pacific coast of South America from the late Miocene epoch, around 7 million years ago, through the end of the Pliocene epoch, approximately 1.5 to 3 million years ago. Their fossilized remains have been primarily unearthed in the Pisco Formation of Peru, a region that was a coastal desert during their existence. Fossils have also been found in Chile and Argentina. This ancient coastal ecosystem was rich in marine life, with evidence of whales, dolphins, and seabirds found alongside Thalassocnus remains.
From Land to Sea
The transition of Thalassocnus from a terrestrial ground sloth to an aquatic one is documented through a series of five distinct species, each showing increasing marine adaptations. One notable adaptation is osteosclerosis, a condition where bones become unusually dense and heavy. This increased bone mass acted as natural ballast, helping them remain submerged while feeding on the seafloor, much like modern manatees. The earliest species showed less osteosclerosis, with later species exhibiting progressively thicker and more compact bones in their limbs and ribs.
Their large, powerful claws, inherited from their ground sloth ancestors, were repurposed for an aquatic lifestyle. These claws likely served to anchor Thalassocnus to the seabed, providing stability against ocean currents while grazing. They may also have been used for digging up food or clinging to rocks during strong waves. Changes in the skull and snout further indicate their aquatic specialization, with the snout becoming wider and more elongated for consuming marine plants. The internal nostrils also migrated deeper into the head, aiding breathing while submerged.
A Marine Herbivore’s Lifestyle
Thalassocnus was a herbivore, primarily feeding on seagrass and marine vegetation in shallow coastal waters. Early species, such as T. antiquus, likely foraged near the shore, possibly eating plants washed ashore or grazing in very shallow depths, less than 1 meter. Evidence from their teeth, which show scratch marks from sand ingestion, supports this nearshore feeding behavior.
As the lineage evolved, later species became more specialized marine grazers. These forms show a distinct transverse movement in their jaws and a lack of sand-induced tooth wear, indicating they fed exclusively in deeper water. They likely walked along the seafloor, using their powerful claws to dig up seagrasses by the roots, similar to beavers or platypuses. Strong lips also assisted in ripping out vegetation, a behavior seen in sirenians.
The End of an Era
The disappearance of Thalassocnus at the end of the Pliocene epoch is linked to significant environmental shifts. A global cooling trend led to colder ocean temperatures along the Pacific South American coast. This cooling directly impacted the warm-water seagrass meadows, their main food source, causing a widespread decline.
The final formation of the Isthmus of Panama, around 3.5 million years ago, also played a role in these environmental changes. This land bridge connected North and South America, profoundly altering global ocean currents. It redirected warm equatorial Atlantic currents northward, initiating a series of ice ages and further contributing to the cooling of the Pacific waters where Thalassocnus lived. The dense bones and limited fat layer, adaptations that aided their bottom-feeding lifestyle, would have made thermoregulation challenging in colder waters, particularly given the naturally low metabolic rate of sloths.