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Plesiosaurs

Plesiosaurs originated at the end of the Triassic (Rhaetian). By the end of the Triassic, all other sauropterygians, including placodonts and nothosaurs, had become extinct. At least six lineages of plesiosaur crossed the Triassic–Jurassic boundary. Plesiosaurs were already diverse in the earliest Jurassic, with the majority of plesiosaurs in the Hettangian-aged Blue Lias belonging to the Rhomaleosauridae. Early plesiosaurs were generally small-bodied, with body size increasing into the Toarcian. There appears to have been a strong turnover around the Early–Middle Jurassic boundary, with microcleidids and rhomaleosaurids becoming extinct and nearly extinct respectively after the end of the Toarcian with the first appearance of the dominant clade of plesiosaurs of the latter half of the Jurassic, the Cryptoclididae during the Bajocian. The Middle Jurassic saw the evolution of short-necked and large-headed thalassophonean pliosaurs from ancestrally small-headed, long-necked forms. Some thalassophonean pliosaurs, such as some species of Pliosaurus, had skulls up to two metres in length with body lengths estimated around 10–12 meters(32-39 ft), making them the apex predators of Late Jurassic oceans. Plesiosaurs invaded freshwater environments during the Jurassic, with indeterminate remains of small-bodied pleisosaurs known from freshwater sediments from the Jurassic of China and Australia.