The most distinct feature of Thalassocnus natans is its skull. The bone of the snout, called the premaxilla, is elongated unlike that of other sloths from the same family and flares out a little at the tip (McDonald and de Muizon 2002). The underside of the bone, i.e. the side inside the mouth, is rough, indicating that it had a strong upper lip or some sort of thick pad (McDonald and de Muizon 2002).
The ribs of T. natans are compact, exhibit osteosclerosis (the non-pathological increase in bone density), and are amedullary (decrease in spongy bone) (Amson et al. 2014). This is one of the main adaptations seen when terrestrial mammals move into an aquatic mode of life. It also shows evidence of increasing bone density in the fore and hind limbs (Amson et al. 2014). These heavy and dense bones act like ballast, which would have helped T. natans to feed in the water.
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