Plesiosauroidea

Plesiosauroidea (; Greek: πλησιοςplēsios 'near, close to' and σαυροςsauros 'lizard') is an extinctclade of carnivorousmarinereptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceousperiods. After their discovery, some plesiosauroids were said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.

Plesiosauroidea appeared at the Early JurassicPeriod (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 Mesozoicdiapsid reptiles that lived at the same time as dinosaurs, they did not belong to the latter. Gastroliths are frequently found associated with plesiosaurs.