- any of various Mesozoic marine reptiles of the superorder Sauropterygia, including the suborder Plesiosauria.
- of or relating to a sauropterygian.
- Any of various extinct aquatic reptiles of the group Sauropterygia of the Mesozoic Era. Sauropterygians were carnivorous, had paddlelike limbs, and included the plesiosaurs. They were distantly related to the dinosaurs.