Hydrorion (meaning 'water hunter') is a genus of plesiosaur from the Toarcian Age of the Lower Jurassic. It is only known from multiple specimens, all discovered in the Posidonia Shale of south-western Germany. The only species of Hydrorion is H. brachypterygius, which was originally described as a species of Plesiosaurus and then Microcleidus.F. v. Huene. (1923). Ein neuer Plesiosaurier aus dem oberen Lias Württembergs. Jahreshefte des Vereins für Vaterländische Naturkunde in Württemberg 79:3-23 It was a relatively small plesiosaur, with the largest specimen measuring long.
See also Timeline of plesiosaur research
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