Thalassocnus is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast.

antiquus, T. natans, T. littoralis, T. carolomartini, and T. yuacensis—represent a chronospecies, a population gradually adapting to marine life in one direct lineage.

Thalassocnus were ground sloths that lived from the Late Miocene to the end of the Pliocene—Late Huayquerian to Early Uquian in the SALMA classification—and all five species were discovered in different horizons of the Pisco Formation in Peru.

[7] A total of three species has been identified with certainty in Chilean formations, T. carolomartini, T. natans, T. antiquus while the presence of T. yaucensis is judged likely.

[8] In 1995, the genus Thalassocnus was formally described with the species T. natans, with a partial skeleton, MUSM 433, by paleontologists Christian de Muizon and H. Gregory McDonald.

[5] In 1968, taxonomist Robert Hoffstetter placed undescribed sloth remains into the family Megatheriidae, possibly belonging to the now-defunct subfamily Planopsinae, mainly based on similarities with the ankle bone and femur.

[14][11][9] Nothrotherium Eremotherium Megatherium T. antiquus T. natans T. littoralis T. carolomartini T. yaucensis Thalassocnus is the only aquatic xenarthran—a group that includes sloths, anteaters, and armadillos—though the ground sloth Eionaletherium from the Miocene of Venezuela may have adapted to nearshore life, as well as Ahytherium from the Pleistocene of Brazil.

The lower jaw progressively elongated and became more spoon-shaped, possibly mimicking the function of the splayed incisor teeth in ruminants.

[16] In later species, to adapt to feeding underwater, the soft palate of the mouth separating the trachea from the esophagus was more developed, and the internal nostrils between the nasal cavity and the throat were farther inside the head.

[14][10] The latest species, T. carolomartini and T. yuacensis show some evidence of having a short trunk similar to tapirs and elephant seals.

The vertebral centra segments progressively became shorter in length, making the spine more stable, probably an adaptation for digging efficiency.

However, the atlanto-occipital joint, which controls neck movement, was stronger than it is in other sloths, which was probably an adaptation for bottom feeding to keep the head in a fixed position.

The length of the tail, proportionally longer than other ground sloths, was probably a diving adaptation similar to modern day cormorants (Phalacrocorax spp.)

[16] Indicated by the large fossae, or depressions, on the shoulder blades, elbow, and wrist, the later Thalassocnus species had strong arm muscles.

[16] The decreasing width of the legs in later species and the reduction of the iliac crests of the pelvis indicate less reliance on them for support—relying more on buoyancy—and the animal probably preferred to sit semi-submerged in the water while resting.

[14][9][16][19] The legs also became more flexible, with later species able to, at maximum extension, have the femora reach a horizontal position parallel to the torso.

[19] The thick and dense bones of later species (pachyosteosclerosis) allowed the animal to combat buoyancy and sink to the seafloor, similar to modern day manatees.

[16][20] Based on δ18Op values obtained from Thalassocnus bones and teeth, the genus appeared to have had an elevated metabolic rate relative to other marine mammals, a feature likely retained from its terrestrial ancestors.

[14][16] The later species fed entirely on the seafloor, similar to sirenians (manatees and dugongs) and the extinct desmostylians, becoming specialist bottom feeders of seagrasses.

They probably fed on Zosteraceae seagrasses, which are known from the Pisco Formation, specifically the marine eelgrass Zostera tasmanica, which is now only known from Australia.

The later species probably dug up seagrasses to the roots with their claws like the beaver and platypus, though, like sirenians, also used strong lips to rip out grasses.

[10] They, along with the other marine mammals of the Bahía Inglesa Formation (specifically at Cerro Ballena), could have been killed by harmful algal blooms.

[14] Thalassocnus may have exhibited sexual dimorphism, indicated by the variation of individual fossils of T. littoralis and between two skulls of T. carolomartini.

The size difference in the premaxillae are reminiscent of the developed upper lips or proboscis in males of modern mammals like the elephant seal (Mirounga spp.).

[25][26] Other vertebrates include the seals Acrophoca and Hadrokirus, sea turtles such as Pacifichelys urbinai, the gavialid Piscogavialis, cormorants, petrels, and boobies (Sula spp.).

[10][16] As seagrass specialists, the later species of Thalassocnus had evolved negative buoyancy to allow them to feed at the sea floor, similar to modern dugongs.

Negative buoyancy requires dense bones and a limited layer of blubber, which would make thermoregulation difficult for them in cooler water temperatures, particularly in view of the low metabolic rate of xenarthrans.