Thalassomedon

Thalassomedon |"the-lass-a-med-in"| (meaning "lord of the seas") was a plesiosaur (not a dinosaur, but an extinct marine reptile from the Mesozoic Era that lived in the open oceans and breathed air). Fossils have been found in the USA. The type species, "Thalassomedon haningtoni" (named by Welles in 1943).
Description.
It had a long neck (with 63 vertebrae), a long snout, long, sharp teeth (up to 5 cm long), a short, pointed tail and four flippers, each about 1.5–2 meters (4.9–6.6 ft) long. This reptile was about 40 feet (12 m) long; its skull was about 18.7 inches (47 cm) long.
Stones have been found in the fossil's stomach area, leading some to believe they helped with eating.