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).