The Swimming Sloth

Image by Bill Parsons, from here.

Sometimes it seems that there is nothing that is so bizarre that some organism does not do it. Life at temperatures that approach boiling? No problem. Male-only species? We can do that. Or is this what you had in mind? But sometimes something comes along that makes one think, "Diversity of life is all well and good, but that's just stoopid". Case in point - Thalassocnus, the marine sloth.

Like the recently covered Odobenocetops, Thalassocnus hailed from what is now the west coast of South America. Indeed, as the Miocene to Pliocene time range of Thalassocnus includes that of Odobenocetops, it is entirely possible, if not probable, that the two of them could have bumped into each other on a regular basis. Though it persisted for some four or five million years with five species described from successive time periods, Thalassocnus seems to have always had a quite restricted distribution. It has only been recorded from the Pisco Formation on the southern coast of Peru (de Muizon et al., 2003).



Though the idea of a marine sloth seems at first quite illogical, sloths are actually quite adept swimmers. The coastline along what is now the Pisco Formation was a barren desert at the time that Thalassocnus inhabited it (de Muizon et al., 2004a), which would have meant that the only food source available to the herbivorous sloths would have been seagrasses and seaweeds growing offshore. Earlier species of Thalassocnus show a high degree of wear on their teeth caused by chewing lots of sand with their food (de Muizon et al., 2003b), suggesting that Thalassocnus started out collecting plant matter washed onshore, or feeding in close enough that swimming would have brought sand into suspension. As time progressed, Thalassocnus would have ventured further and further out. Later species show almost no sign of sand-induced wear, indicating that by that time they were feeding entirely aquatically at some distance from the shore.

In fact, the most fascinating thing about Thalassocnus, other than its sheer existence, is the almost perfect evolutionary series it offers. Each of the five species is known from a different level of the formation, and while cladistic analysis was not entirely able to eliminate other options (de Muizon et al., 2003), the most likely explanation seems to be that only one species inhabited the relatively restricted distribution at a time, with each successive species giving rise to the next anagenetically (for those not in the know, 'anagenesis' refers to change within a single lineage, while 'cladogenesis' refers to species splitting off from each other). It has been suggested that anagenetic change is more likely to be purely adaptive than cladogenetic change, and Thalassocnus does not disappoint in this regard. Each successive species shows a higher degree of adaptation for an aquatic lifestyle and aquatic feeding, as shown for instance in the figure below from de Muizon et al. (2004a):



This figure shows the lower jaws of all five Thalassocnus species from the oldest (T. antiquus) on the left to the youngest (T. yaucensis) on the right. Note the progressive lengthening and change in shape of the end of the jaw, which de Muizon et al. (2004b) suggest was related to the development of thick muscular lips for collecting seagrasses. Later species also showed adaptations for a grazing rather than browsing feeding mode.

One aquatic adaptation that is notable by its absence is that even the later species of Thalassocnus show no sign of pachyostosis, thickened and heavy bones as found in other aquatic bottom-feeders such as sirenians. This is a curious deficiency in regards to Thalassocnus' probable lifestyle. If you have ever tried to sit or lie on the bottom of a pool, you would have noticed that staying submerged requires continuous and quite strenuous effort, as the body tends to float to the surface. Most bottom-feeding aquatic vertebrates evolve denser builds to remove this requirement. De Muizon et al. (2004b) suggest that instead of evolving greater density, Thalassocnus used the enlarged claws it retained from its sloth ancestry to anchor itself to the bottom. They compare it in this regard to the seaweed-grazing marine iguana (Amblyrhynchus cristatus), which also has enlarged claws. Such claws would have also aided Thalassocnus (as they do the iguana) in clinging to rocks when entering and exiting the water, even in the presence of heavy surf.



Thalassocnus must have been a remarkable animal to see in life (image above from here). However, this brings me to question what exactly it did look like in life. Sloths have a reputation for being decidedly shaggy animals, and most extinct ground sloths are reconstructed with similarly long, flowing hair. However, such a pelt seems poorly suited for an aquatic lifestyle, and so unlikely for Thalassocnus. Many aquatic mammals, such as cetaceans and sirenians, have almost entirely lost their hair in favour of an insulating fat layer. Others, such as sea lions and the sea otter (Enhydra lutris), have evolved a dense pelt. Personally, I find it easier to imagine a sloth with a pelt, albeit a shorter one than its terrestrial relatives, than an entirely naked sloth, and point out in my support that the pelted marine mammals include a higher proportion of those that divide their lives between land and sea. Still, barring remarkable good fortune, either option must remain entirely hypothetical when dealing with an extinct animal.

REFERENCES

Muizon, C. de, H. G. McDonald, R. Salas & M. Urbina. 2003. A new species of the aquatic sloth Thalassocnus (Mammalia, Xenarthra) from the Late Miocene of Peru. Journal of Vertebrate Paleontology 23 (4): 886–894.

Muizon, C. de, H. G. McDonald, R. Salas & M. Urbina. 2004a. The youngest species of the aquatic sloth Thalassocnus and a reassessment of the relationships of the nothrothere sloths (Mammalia: Xenarthra). Journal of Vertebrate Paleontology 24 (2): 387–397.

Muizon, C. de, H. G. McDonald, R. Salas & M. Urbina. 2004b. The evolution of feeding adaptations of the aquatic sloth Thalassocnus. Journal of Vertebrate Paleontology 24 (2): 398–410.