|Mounted cast of the type specimen, American Museum of Natural History|
Thalassomedon is among the largest elasmosaurids, with a total length of 10.86 metres (35.6 ft) for the holotype. There is a larger skull however suggesting a much larger animal, potentially up to 11.7 metres (38 ft). The neck is also very long; it comprises 62 vertebrae and is about 5.9 metres (19 ft) - over half of the total length. The skull is 47 centimetres (19 in) long, with 5 centimetres (2.0 in) long teeth. The flippers were about 1.5–2 metres (4.9–6.6 ft) long. Stones have been found in its stomach area leading some to theorize that they were used for ballast or digestion. If the latter, stomach action would cause the stones to help grind ingested food.
This genus of plesiosaur lived in North America, approximately 95 million years ago - this places it during the Cenomanian stage. Its closest relative is Elasmosaurus, and both belong to the family Elasmosauridae. There are six specimens of varying states of preservation on display at various museums in the United States.
- O'Gorman, J.P. (2016). "A Small Body Sized Non-Aristonectine Elasmosaurid (Sauropterygia, Plesiosauria) from the Late Cretaceous of Patagonia with Comments on the Relationships of the Patagonian and Antarctic Elasmosaurids". Ameghiniana. 53 (3): 245–268. doi:10.5710/AMGH.29.11.2015.2928.
- Carpenter, K. (1999). "Revision of North American elasmosaurs from the Cretaceous western interior." Paludicola, 2(2): 148-173.