Plesiosaurus (Greek: πλησιος/plesios, near to + σαυρος/sauros, lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the early part of the Jurassic Period, and is known by nearly complete skeletons from the Lias of England. Although there are a number of modern-day myths surrounding this order of creature, such as the myth of the Loch Ness Monster, these creatures are known to be extinct. It is distinguishable by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order Plesiosauria, of which it is an early, but fairly typical member. It contains only one species, Plesiosaurus dolichodeirus. P. brachypterygius, P. guilielmiiperatoris, and P. tournemirensis were assigned to new genera, Hydrorion, Seeleyosaurus and Occitanosaurus.

Plesiosaurus (Greek: πλησιος/plesios, near to + σαυρος/sauros, lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the early part of the Jurassic Period, and is known by nearly complete skeletons from the Lias of England. Although there are a number of modern-day myths surrounding this order of creature, such as the myth of the Loch Ness Monster, these creatures are known to be extinct. It is distinguishable by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order Plesiosauria, of which it is an early, but fairly typical member. It contains only one species, Plesiosaurus dolichodeirus. P. brachypterygius, P. guilielmiiperatoris, and P. tournemirensis were assigned to new genera, Hydrorion, Seeleyosaurus and Occitanosaurus.