Doing the crawl?

The life aquatic has lured many animal groups back into its liquid embrace. Marine iguanas, penguins, whales and dolphins, sea-cows and manatees, seals and sea-lions all returned to the sea from land (and air!) adapted forms. It makes a lot of sense- the littoral is a pretty rich zone that surely tempted those that could take advantage of it. From there it is but a short evolutionary jump back into the blue. Many animals are already halfway there, with one hoof in each camp. Think of the hippo, the beaver, the desman, the otter. Given the wide variety of creatures that have done this, it can be fun to imagine future forms. Bat-penguins? River-antelopes? Water-wombats?

Flights of fancy like this have a long pedigree. Darwin himself, in The Origin mentions:

“In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.”

Sea-bears? Why not!

As usual, nature is not only weirder than we imagine, but weirder than we can imagine. Perhaps the most unlikely group ever to have flirted with the aquatic lifestyle was only recently recognised from Pliocene fossils: Thalassocnus sp. Thalassocnus is, and I can’t really believe this either, a giant marine sloth.

Thalassocnus yaucensis by Justine Jacquot-H, image from Amson et al. JME (22) p473-518

The first clues to this puzzle were due to some flukes of taphonomy. The Pisco Formation in Peru is a richly stratified Miocene and Pliocene site with amazing preservation of marine creatures including articulated whales, dolphins, crocodiles, fish, sharks, and dugongs. And giant sloths. Now, you don’t usually expect to find giant sloths in a strictly marine site. But these sloths were quickly identified as something pretty special indeed. Detailed study has shown that not only were these behemoths at home in a marine shore environment, they had picked up some pretty cool adaptations along the way.

The giant, swimming, sloth, Thalassocnus. At home beneth the waves. (Image by Jan Freedman)

Neater still, the layers of the Pisco Formation contains thalassocnine sloths from multiple timepoints between the Miocene and Pliocene, showing how some of those characters changed through time. It seems that this shallow prehistoric sea was prone to occasional algal red blooms that regularly poisoned everything, leaving articulated skeletons behind. Through these fossils, you can actually see evolution happening. In the ribs and limbs you find a gradual thickening of the cortical bone; an increase in density that helped to counteract the natural buoyancy of an air-breathing mammal. There is also a gentle elongation of the premaxillae and mandible symphysis, creating a long and wide snout, better adapted to munching water-weeds. The earlier species of thalassocnine have lots and lots of striae on their weird, peg-like sloth teeth- indicating that they were taking in a lot of sand while they were feeding, probably due to wading in shallow water where any movement would stir up lots of sediment. The later species don’t have this. Suggesting that they were feeding further out to sea, where sand wasn’t an issue.

Time-calibrated phylogeny of some Xenarthra showing the relationship between thalassocnine sloths and cortical bone density, taken from Amson et al. Proc Roy Soc B v281

Mandibles of 5 species of Thalassocnus, from oldest to youngest LtoR, taken from de Muizon et al. JVP (24:2)p287-397

Premaxillae of 5 species of Thalassocnus, from oldest to youngest LtoR, taken from de Muizon et al. JVP(22:2)p398-410

The thalassocnine sloths appear to have died out in the Pliocene. Thankfully, this is one extinction that had nothing to do with us! The Late Pliocene is when it seems the Isthmus of Panama finally closed, having major effects on ocean currents and water circulation. It seems likely that this major geological event would have had a catastrophic effect on the gentle swimmers, grazing on the nutritious sea-grasses and kelps off the coast of ancient Peru.

Written by Ross Barnett (@DeepFriedDNA)

Further Reading:

Amson, E., C. Argot, H. G. McDonald, and C. de Muizon. “Osteology and Functional Morphology of the Axial Postcranium of the Marine Sloth Thalassocnus (Mammlia, Tardigrada) with Paleobiological Implications.” Journal of Mammal Evolution 22 (2015): 473-518.[Full Article]

———. “Osteology and Functional Morphology of the Forelimb of the Marine Sloth Thalassocnus (Mammalia, Tardigrada).” Journal of Mammal Evolution 22 (2015): 169-242.[Full Article]

Amson, E., C. de Muizon, M. Laurin, C. Argot, and V. de Buffrenil. “Gradual Adaptation of Bone Structure to Aquatic Lifestyle in Extinct Sloths from Peru.” Proc Roy Soc B 281, no. 1782 (May 7 2014): 20140192.[Full Article]

Canto, J., R. Salas-Gismondi, M. Cozzuol, and J. Yañez. “The Aquatic Sloth Thalassocnus (Mammalia, Xenarthra) from the Late Miocene of Noeth-Central Chile: Biogeographic and Ecological Implications.” Journal of Vertebrate Paleontology 28, no. 3 (2008): 918-22.[Full Article]

Darwin, Charles. On the Origin of Species by Means of Natural Selection. London,: J. Murray, 1859.[Full Book]

de Muizon, C., H. G. McDonald, R. Salas, and M. Urbina. “The Evolution of Feeding Adaptations of the Aquatic Sloth Thalassocnus.” Journal of Vertebrate Paleontology 24, no. 2 (2004): 398-410.[Full Article]

———. “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, no. 2 (2004): 387-97.[Full Article]

McDonald, H. G., and C. de Muizon. “The Cranial Anatomy of Thalassocnus (Xenarthra, Mammalia), a Derived Nothrothere from the Neogene of the Pisco Formation (Peru).” Journal of Vertebrate Paleontology 22, no. 2 (2002): 349-65.[Full Article]