Plesiosaur

A plesiosaur was an extinct marine reptile in the order Plesiosauria, with a broad body and four paddle-like limbs. Most had long necks, while some lineages evolved shorter necks and large heads.

Sauropterygia: An extinct group of aquatic reptiles that includes plesiosaurs and several earlier marine lineages. Plesiosauria belongs within this broader radiation of marine reptiles.
Plesiosaur locomotion: The swimming behavior and propulsion of plesiosaurs using their four flippers. Their distinctive four-flipper stroke is central to reconstructing how they swam.
Triassic: The geologic period from about 252 to 201 million years ago, preceding the Jurassic. The earliest known plesiosaurs appeared near the end of this period.
Plesiosaurus dolichodeirus: A species of long-necked plesiosaur known from Early Jurassic rocks in England. Its early description helped establish the recognizable long-necked plesiosaur form.
Plesiosauria: An order of extinct marine reptiles divided chiefly into long-necked plesiosaurs and short-necked pliosaurs. This is the formal order containing the animals commonly called plesiosaurs.
Hydrofoil: A surface or body shaped to generate lift as it moves through a fluid. Plesiosaur flippers functioned as underwater wings, producing lift and thrust.
Jurassic: The geologic period from about 201 to 145 million years ago. Plesiosaurs diversified widely through Jurassic seas.
Elasmosaurus: A Late Cretaceous plesiosaur genus known for an exceptionally long neck and small head. Its extreme neck proportions show how far long-necked plesiosaurs could specialize.
Diapsid: A reptile-group designation based on two openings on each side of the skull behind the eye. Plesiosaurs belong to the broader diapsid lineage of reptiles.
Buoyancy: The upward force a fluid exerts on an immersed object. Controlling buoyancy affected how plesiosaurs maintained depth and surfaced to breathe.

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