The skull of Thalassocnus natans is oriented straight in front of the neck, much like that of a seal or a whale. Other terrestrial herbivores have a skull that is positioned at an angle. This has been suggested as an adaptation for aquatic life (McDonald and de Muizon 2002). The tip of the snout probably had a large and powerful lip, much larger than that seen in other nothrotheres. It might have used this lip to grasp aquatic vegetation like a manatee would (McDonald and de Muizon 2002). The narrow snout might have helped it to close off the windpipe more easily while it was feeding underwater (McDonald and de Muizon 2002). The back of the skull has attachments for large neck muscles and it has been suggested that these muscles helped keep the animal’s neck stable while it swam or fed (McDonald and de Muizon 2002).
The ribs of T. natans are compact and 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.
It also had long claws on its hands that would have helped it grab rocks while it foraged for food.
- Amson, E., C. de Muizon, M. Laurin, C. Argot, and V. de Buffrenil. 2014. Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru. Proc Biol Sci 281:20140192.
- McDonald, H. G., and C. de Muizon. 2002. The cranial anatomy of Thalassocnus (Xenarthra, Mammalia), a derived nothrothere from the Neogene of the Pisco Formation (Peru). Journal of Vertebrate Paleontology 22:349-365.
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