Plesiosauroidea

Plesiosauroidea ( /ˈpliːsiəsɔər/; Greek: plēsios/πλησιος 'near' or 'close to' and sauros/σαυρος 'lizard') is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods. After their discovery, some plesiosauroids were somewhat fancifully said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.

Plesiosauroidea appeared at the Early Jurassic Period (late Sinemurian stage) and thrived until the K-Pg extinction, at the end of the Cretaceous Period. The oldest confirmed plesiosauroid is Plesiosaurus itself, as all younger taxa were recently found to be pliosauroids. While they were Mesozoic diapsid reptiles that lived at the same time as dinosaurs, they were not dinosaurs. Gastroliths are frequently found associated with plesiosaurs.