Thalassomedon

Thalassomedon (from Greek, thalassa, "sea" and Greek, medon, "lord" or "ruler", meaning "sea lord") is a genus of plesiosaur, named by Welles in 1943.

Description
Thalassomedon is among the largest elasmosaurids, with the holotype measuring 10.86 m long and weighing more than 4.44 MT. There is a larger skull, however, suggesting a much larger animal, potentially up to 11.6 m. The neck is also very long; it comprises 62 vertebrae and is about 5.9 m - over half of the total length. The skull is 47 cm long, with 5 cm long teeth. The flippers were about 1.5 – 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.

Discovery
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.