Plesiosaurus

Plesiosaurus Temporal range: Early Jurassic, 199.6–175.6 Ma PreЄ Є O S D C P T J K Pg N
Restored skeleton in Japan
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Superfamily:	†Plesiosauroidea
Family:	†Plesiosauridae
Genus:	†Plesiosaurus Conybeare, 1821
Species:	†P. dolichodeirus
Binomial name
Plesiosaurus dolichodeirus Conybeare, 1824

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 these animals are known to be extinct, every year there are a high number of unconfirmed modern-day reported sightings of creature(s) that are often described to resemble the Plesiosaur, such as the Loch Ness Monster. 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.

Compared to other plesiosaur genera, Plesiosaurus has a small head. The skull is much narrower than long, reaching its greatest width just behind the eyes (the postorbital bar). The anterior portion is "bluntly triangular". In lateral view, the skull reaches its highest point at the rear of the skull table. "The external nostrils overlie the internal nares". They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull. Unlike the nostrils of Rhomaleosaurus, they do not appear to be adapted for underwater olfaction. The orbits (eye sockets) are roughly circular and are positioned about halfway along the length of the skull. They face up and to the sides. Just posterior to the orbits are the supratemporal fenestrae, which are about the same size as the orbits and also roughly circular. Between the four openings is the pineal foramen, and between the temporal fenestrae is a narrow sagittal ridge. As in other plesiosaurs, the pterygoids of the palate are fused to the basioccipital of the braincase, although the union is not as robust as in the pliosaurs Rhomaleosaurus and Pliosaurus. "The palatal bones are thin, but there is no suborbital fenestra."