Plesiosaurs were dinosaurs

Plesiosaurs were marine reptiles belonging to the clade Sauropterygia, entirely separate from Dinosauria. They evolved flippers from terrestrial tetrapod limbs and had a different body plan optimized for aquatic locomotion. Mosasaurs were marine squamates (related to modern snakes and lizards), even more distant from dinosaurs. While these marine and flying reptiles coexisted with non-avian dinosaurs during the Mesozoic, they represent distinct evolutionary lineages within Archosauria and are not classified as dinosaurs.