Thalassocnus
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Thalassocnus Temporal range: Late Miocene–Late Pliocene |
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| Thalassocnus skeleton in its hypothetical swimming pose, Muséum national d'histoire naturelle, Paris | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Subclass: | Theria |
| Infraclass: | Eutheria |
| Superorder: | Xenarthra |
| Order: | Pilosa |
| Family: | †Megatheriidae |
| Subfamily: | †Thalassocninae |
| Genus: |
†Thalassocnus de Muizon & McDonald, 1995 |
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Thalassocnus is an extinct genus of apparently semiaquatic (for the geologically oldest species) or fully aquatic (for the geologically most recent species) marine sloth from the Miocene and Pliocene of South America. Fossils found to date have been from the coast of Peru and Chile. They were apparently grazers of sea grass and seaweed. The various species of this genus provides the best-documented case of gradual adaptation to a secondarily aquatic lifestyle. This is documented both at the morphological level, such as a progressive flattening of the radius, and at the microanatomical level, which shows a progressive increase in thickness (pachyostosis) and compactness (osteosclerosis) of the long limb bones and ribs, providing ballast. They may have used their powerful claws to anchor themselves to the sea floor to facilitate feeding, similar to the behavior of the marine iguana.
Thalassocnus were semi-marine sloths that grew to be slightly larger than the average human. The first finding of a Thalassocnus fossil was in Peru, at the Pisco Formation site. It was also here where the different species of Thalassocnus were identified. As time passed, a gradual thickening of the cortical bone was found in the ribs and limbs of Thalassocnus; an increase in density that helped to counteract the natural buoyancy of an air-breathing mammal. Later species also displayed elongation of the premaxillae and mandible symphysis, creating a long and wide snout, better adapted to consuming water plants. The earlier species of Thalassocnus have numerous striae on their teeth, possibly indicating intake of sand when feeding in shallow water. Later species such as T. carolomartini and T. yaucensis, were apparently specialized grazers that fed in deeper water; they display distinct evidence of transverse mandibular movement and lack dental striae.
T. antiquus
T. natans
T. littoralis
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