Thalassocnus

Thalassocnus Temporal range: Late Miocene – Late Pliocene (Huayquerian–Uquian) ~7–3 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓ [1]
T. natans skeleton in its hypothesized swimming pose, Muséum national d'histoire naturelle, Paris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Family:	†Nothrotheriidae
Subfamily:	†Thalassocninae
Genus:	†Thalassocnus de Muizon & McDonald, 1995
Type species
†Thalassocnus natans de Muizon & McDonald, 1995
Other species
†T. antiquus de Muizon et al., 2003 †T. carolomartini McDonald & Muizon, 2002 †T. littoralis McDonald & Muizon, 2002 †T. yuacensis de Muizon et al., 2004

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 subfamily Thalassocninae. 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 Tafna Formation of Argentina,^[2] and the Bahía Inglesa, Coquimbo, and Horcón formations of Chile. Thalassocninae has been placed in both the families Megatheriidae^[3] and Nothrotheriidae.^[4]

Thalassocnus evolved several marine adaptations over 4 million years, such as dense and heavy bones to counteract buoyancy, the internal nostrils migrating farther into the head to help with breathing while completely submerged, the snout becoming wider and more elongated to consume aquatic plants better, and the head angling 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.