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Plesiosaurus (Greek:- near to + lizard) was a genus of 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
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 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.
The teeth of Plesiosaurus are "simple, needle-like cones" that are "slightly curved and circular in transverse section". They are sharply pointed with fine striations running from tip to base, and point forward (procumbent). This procumbency becomes more pronounced near the leading end of the skull, where they may be only 10-15° above horizontal. There are 20 to 25 teeth per upper jaw tooth row, and 24 per low jaw tooth row. Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.
Plesiosaurus was a moderately sized plesiosaur that grew to a length of about 3.5 metres (11 ft). There are approximately 40 cervical vertebrae (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae. Some of the dorsals have rugose articular edges, like the cervicals; this feature is typically absent from the caudals.
Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads. Dorsal ribs are thick and have only one head. Sacral ribs are "short, robust, and blunt or knob-like on both ends." Caudal ribs have different morphologies depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt." Plesiosaurus also has gastralia, also known as "belly ribs." Nine or more sets of gastralia are present between the shoulder and pelvis. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.