Since then, four other species of Thalassocnus have been unearthed from various coastal environments of Peru and Chile. These sea-dwelling sloths all had features in their bones that resemble other aquatic animals like otters and beavers. Features in their jaws suggest that they grazed on seaweed and sea grasses from shallow coastal waters, likely less than one meter (3.3 ft) deep.
On top of it all, their bones yield another clue to their aquatic lifestyles. When a land-dwelling animal adapts to life in the sky or sea, their bone density tends to change. The bones of flying creatures become filled with air sacs to reduce their weight, while shallow-diving species went the opposite direction.
Such an evolutionary trend is called bone mass increase (BMI), which involves the increase of the bone’s robustness (pachyostosis) or density (osteosclerosis). In short, aquatic mammals like otters, sea cows, and even platypus tend to have thickened, hardened bones compared to their terrestrial relatives.
A similar case is reflected in the bones of the marine sloths. In a study published in May 2018, postdoctoral researcher Eli Amson detailed his observation of their bone thickness. By examining slices of the bones, known as thin sections, he noticed the thickening of the sloths’ bones including the roof of the skull.
The skull thickening is particularly notable, since every study on BMI so far has mostly looked into body parts related to locomotion. Amson’s study shows, at least in the sloths’ case, that BMI isn’t restricted to the animals’ limbs.
“In shallow diving 'slow swimmers', such as manatees and aquatic sloths, the skeleton is more compact and acts like the weight-belt of human divers, to allow them to be neutrally buoyant passively,” Amson said in an email to Earth Archives.
Interestingly, the different species of Thalassocnus he observed showed a constant trend of BMI throughout their history. By comparing bones from T. natans (7 Ma), T. littoralis (6 Ma), and T. carolomartini (5 Ma), it was clear that the sloths became more and more aquatic before they finally went extinct around 2.5 million years ago.
This trend is also followed by the reduction of air-filled spaces surrounding the nasal cavity, or sinuses. Such reduction has also been observed in the evolutionary history of bears and whales, but it’s not yet fully understood how it would’ve affected the animals’ lifestyles.
As with the whales, it seems like they took their own path to live underwater. “Deep-divers, and more general active swimmers, can actually show the opposite trend. Extant [living] cetaceans (whales and dolphins), for instance, have osteoporotic (less compact) bones. They actively regulate their buoyancy,” Amson said.
After their extinction, the sloths’ rich fossil record is what sets them apart from other swimming mammals. No other group left a similarly detailed fossil record of ever-increasingly aquatic lifestyle in a succession, but it’s expected that they must have underwent a similar phase in history.
For now, we have to be content that the bones Thalassocnus left behind gave us the chance to observe their evolution in such great detail.
A life reconstruction of Thalassocnus in its natural habitat by Oliver Demuth
Amson, E., Billet, G., & Muizon, C. de. (2018). Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure. Proc. R. Soc. B 2018 285 20180270; DOI: 10.1098/rspb.2018.0270. Published 9 May 2018
Amson, E., Argot, C., McDonald, H. G., & Muizon, C. de. (2015a). Osteology and functional morphology of the axial postcranium of the marine sloth Thalassocnus (Mammalia, Tardigrada) with paleobiological implications. Journal of Mammalian Evolution, 22(4), 473–518. http://doi.org/10.1007/s10914-014-9280-7
Amson, E., Argot, C., McDonald, H. G., & Muizon, C. de. (2015b). Osteology and functional morphology of the forelimb of the marine sloth Thalassocnus (Mammalia, Tardigrada). Journal of Mammalian Evolution, 22(2), 169–242. http://doi.org/10.1007/s10914-014-9268-3
Amson, E., Argot, C., McDonald, H. G., & Muizon, C. de. (2015c). Osteology and functional morphology of the hind limb of the marine sloth Thalassocnus (Mammalia, Tardigrada). Journal of Mammalian Evolution, 22(3), 355–419. http://doi.org/10.1007/s10914-014-9274-5
Muizon, C. de, & McDonald, H. G. (1995). An aquatic sloth from the Pliocene of Peru. Nature, 375, 224–227. http://doi.org/10.1038/375224a0
Muizon, C. de, McDonald, H. G., Salas, R., & Urbina, M. (2004). The evolution of feeding adaptations of the aquatic sloth Thalassocnus. Journal of Vertebrate Paleontology, 24(2), 398–410. http://doi.org/10.1671/2429b