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, and the Bahía Inglesa, Coquimbo, and Horcón formations of Chile. Thalassocninae has been placed in both the families Megatheriidae and Nothrotheriidae.
Thalassocnus
T. natans fossil in Museum of Natural History, Lima
Life restoration of T. natans in its hypothetical swimming pose without fur
Partial T. cf. natans humerus
Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. Ground sloths varied widely in size, with the largest, belonging to genera Megatherium and Eremotherium, being around the size of elephants. Ground sloths are a paraphyletic group, as living tree sloths are thought to have evolved from ground sloth ancestors.
Ground sloth
Fossil Eremotherium skeleton, National Museum of Natural History, Washington, DC.
Paramylodon harlani, Texas Memorial Museum, University of Texas at Austin
A Tamandua anteater in an upright defensive stance similar to those presumed to have been adopted by ground sloths, per trackways preserved in New Mexico
