Thalassocnus Sea Sloth

SWIMMING SLOTHS

(THALASSOCNUS)

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When you think of marine mammals, sloths are probably the last thing you’d expect to take to the water. But from the late Miocene (7-8 million years ago) to the late Pliocene (3-1.5 million years ago), several species of giant ground sloths in the genus Thalassocnus did indeed evolve to take advantage of the aquatic environment. The older species (that is, the ones that evolved first) were probably waders, spending most of their time on the shore and venturing into the shallows to crop sea grass. Wear on their teeth indicates that they got a lot of sand in their mouths when they ate, which is what you’d expect from an animal trying to eat in the turbulent surf. Later species of Thalassocnus swam out into deeper water to get their meals, evident in the reduced sand abrasion on their teeth (deeper and therefore calmer waters would have had little to no suspended sediment to get into the sloth’s mouth). These latter animals were probably similar to marine iguanas from the Galapagos, which dive to scrape seaweed off nearshore rocks.

So why would a sloth take to the water in the first place? Because Thalassocnus evolved in the dry, nearly barren desert that extends along the Pacific coast of Peru. With so little terrestrial flora, the sloths had to turn to the sea for sustenance.

While researching this piece I found myself with a dilemma—should I portray Thalassocnus with or without fur? All other depictions I’ve seen show the animal with thick, shaggy fur like a modern arboreal sloth. However, emerging from the water with all that soaking wet hair would quickly sap the animal’s body heat. Modern hairy aquatic animals such as otters and polar bears have a thick layer of down hairs covered by oily, water-repellant guard hairs to hold in heat. While this works well for an animal that spends most of its life in water-- such as a sea otter-- or in a cold climate-- such as a polar bear-- it probably wouldn’t have worked as well for Thalassocnus. Even the younger, deeper-grazing sea sloths would have come out of the water frequently (none of them show major modifications for a swimming habit, indicating that they spent the majority of their lives on land) and in the coastal desert, where temperatures typically range between 60-100 degrees Fahrenheit, such a bulky sweater of a pelt would have quickly caused them to overheat.

There are, however, some only partially aquatic animals with fur such as capybaras, beavers, muskrats and platypi. Thus it isn’t unreasonable to suppose that a simple wader that spends the majority of its time on land-- such as the late Miocene Thalassocnus natans-- may have had hair like modern sloths and its contemporaries on the South American savannah.

After much debate I decided to split the difference and depict two different species of Thalassocnus at the opposite ends of the evolutionary line. T. natans, a wader from the Miocene, is shaggy while T. yaucensis from the later Pliocene is hairless with a layer of insulating blubber much like a manatee.

Information for this piece came from the following papers:

De Muizon, Christian; McDonald, Gregory H.; Salas, Rodolfo; Urbina, Mario "The Evolution of Feeding Adaptations of the Aquatic Sloth Thalassocnus" Journal of Vertebrate Paleontology Vol. 24 No. 2 (2004): 398-410.

De Muizon, Christian; McDonald, Gregory H; Salas, Rodolfo; Urbina, Mario "The Youngest Species of the Aquatic Sloth Thalassocnus and a Reassessment of the Relationships of the Nothrothere Sloths (Mammalia: Xenarthra)" Journal of Vertebrate Paleontology Vol. 24 No. 2 (2004): 387-397.

De Muizon, Christian; McDonald, Gregory H "The Cranial Anatomy of Thalassocnus (Xenarthra, Mammalia), a Derived Nothrothere from the Neogene of the Pisco Formation (Peru)" Journal of Vertebrate Paleontology Vol. 22 No. 2 (2002): 349-365

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Thalassocnus ©2010 John Meszaros.  All Rights Reserved