Plesiosaurus (Greek: πλησίος (plesios), near to + σαῦρος (sauros), lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the Early Jurassic.

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.

Other species once assigned to this genus, including P. brachypterygius, P. guilielmiimperatoris, and P. tournemirensis have been reassigned to new genera, such as Hydrorion, Seeleyosaurus and Microcleidus.

It was so-named ("near lizard") by William Conybeare and Henry De la Beche, to indicate that it was more like a normal reptile than Ichthyosaurus, which had been found in the same rock strata just a few years earlier.

Conybeare and De la Beche coined the name for scattered finds from the Bristol region, Dorset, and Lyme Regis in 1821.

[6] The type species of Plesiosaurus, P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of Anning's original finds.

[8] As in other plesiosaurs, the pterygoids of the palate are fused to the basioccipital of the braincase,[8] although the union is not as robust as in the pliosaurs Rhomaleosaurus and Pliosaurus.

[12] The teeth of Plesiosaurus are "simple, needle-like cones" that are "slightly curved and circular in transverse section".

It includes fused clavicles at the anterior end, scapulae (shoulder blades), and large coracoids.

The humerus (upper arm bone) has distinctive curvature, which appears to be a retained primitive feature among sauropterygians.

[17] The hand paddle has five digits; the phalangeal formula is uncertain, but the count for one large individual, from "thumb" to fifth "finger", is 4-8-9-8-6.

The lower hindlimb includes two roughly equal-sized bones, the robust tibia and the semilunate-shaped fibula.

Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to Plesiosaurus.

P. brachypterygius, P. guilielmiimperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion, Seeleyosaurus and Occitanosaurus.

[20] "Pistosaurus postcranium" Pistosaurus Yunguisaurus liae Augustasaurus hagdorni Bobosaurus forojuliensis Anningasaura lymense Stratesaurus taylori Macroplata tenuiceps Avalonnectes arturi Eurycleidus arcuatus Meyerasaurus victor Maresaurus coccai Atychodracon megacephalus Archaeonectrus rostratus Rhomaleosaurus cramptoni Rhomaleosaurus thorntoni Rhomaleosaurus zetlandicus Thalassiodracon hawkinsi Hauffiosaurus spp.

[22] If that is the case then Plesiosaurus would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult.

Unequivocal specimens of Plesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset.

[24] Plesiosaurus is best represented from the "upper part of the Blue Lias, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the Charmouth Mudstone; using the Lias Group ammonite fossil zones, these rocks date to the early Sinemurian stage.