Thalassocnus

Thalassocnus is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast. It is monotypic within the subfamilyThalassocninae. The five species—T. antiquus, T. natans, T. littoralis, T. carolomartini, and T. yuacensis—represent a chronospecies, a population gradually adapting to marine life in one direct lineage. They are the only known aquatic sloths, but they may have also been adapted to a terrestrial lifestyle. They have been found in the Pisco Formation of Peru, the of Argentina, and the Bahía Inglesa, Coquimbo, and formations of Chile. Thalassocninae has been placed in both the familiesMegatheriidae and Nothrotheriidae.

Thalassocnus evolved several marine adaptations over 4 million years, developing dense and heavy bones to counteract buoyancy, internal nostrils that migrated farther into the head to facilitate breathing while completely submerged, a wider and more elongated snout to consume aquatic plants, and a head that angled progressively farther and farther downwards to aid in bottom feeding. The long tail was probably used for diving and balance similar to the modern day beaver (Castor spp.) and platypus (Ornithorhynchus anatinus).

Thalassocnus probably walked across the seafloor and dug up food with its claws. They probably could not do high-powered swimming, relying on paddling if necessary. Early Thalassocnus were probably generalist grazers eating seaweed and seagrasses close to shore, whereas later species specialized on seagrasses farther off the coast. They were probably preyed upon by sharks and macroraptorial sperm whales such as Acrophyseter. Thalassocnus were found in formations with large marine mammal and shark assemblages.