Plesiosaurs are an order or clade of Mesozoic marine reptiles (marine Sauropsida), belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 203 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution.
Plesiosaurs were among the first fossil reptiles discovered; in the beginning of the nineteenth century, scientists realized how distinctive their build was and they were named as a separate order in 1835. The first plesiosaurian genus, the eponymous Plesiosaurus, was named in 1821. Since then, more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an improved understanding of their anatomy, relationships and way of life. Plesiosaurs had a broad flat body and a short tail. Their limbs had evolved into four long flippers, which were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and the pelvis. The flippers made a flying movement through the water. Plesiosaurs breathed air, and bore live young; there are indications that they were warm-blooded.
Modern evidence suggests that plesiosaurs are elusive piscivores [although some plesiosaur fossils were found with mollusc shells in their stomachs, along with gastroliths (small stones swallowed by many animals to help crush food for easier digestion). One plesiosaur fossil found in South Dakota had 253 gatroliths, weighing a total of 29 pounds, with teeth designed to capture fish and don't posses particularly strong jaws, so for this individual to have interest in eating a feathery theropod like Pyroraptor is almost absurd.