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Any of various extinct sea reptiles of the genus Ichthyosaurus and related genera, that had a medium-sized to large dolphin-like body with a dorsal fin, four flippers, and a large, crescent-shaped tail. The head had a long beak with sharp teeth, large eyes and earbones, and nostrils near the eyes on top of the skull. Ichthyosaurs were most common and diverse in the Triassic and Jurassic Periods and died out well before the end of the Cretaceous.
any member of an extinct group of aquatic reptiles, most of which were very similar to porpoises in appearance and habits. These distant relatives of lizards and snakes (lepidosaurs) were the most highly specialized aquatic reptiles, but ichthyosaurs were not dinosaurs.