Aquatic marine sloth (Thalassocnus)

Thalassocnus is an extinct genus of apparently semiaquatic (for the geologically oldest species) or fully aquatic (for the geologically most recent species) marine sloth from the Miocene and Pliocene of South America. Fossils found to date have been from the coast of Peru and Chile. They were apparently grazers of sea grass and seaweed. The various species of this genus provides the best-documented case of gradual adaptation to a secondarily aquatic lifestyle. This is documented both at the morphological level, such as a progressive flattening of the radius and at the microanatomical level, which shows a progressive increase in compactness of the long limb bones and ribs. They may have used their powerful claws to anchor themselves to the sea floor to facilitate feeding, similar to the behavior of the marine iguana.

The older species, T. antiquus, T. natans and T. littoralis, show indications of partial adaptation to grazing, with little evidence of transverse mandibular movement while eating, and abundant dental striae indicating ingestion of sand from feeding on vegetation stranded on beaches or in shallow (less than 1 m deep) water. The later species, T. carolomartini and T. yaucensis were apparently specialized grazers that fed in deeper water; they display distinct evidence of transverse mandibular movement and lack dental striae.