Any extinct aquatic reptile belonging to the order Ichthyosauria, which lived during the Mesozoic era (250–66 million years ago). The earliest forms resembled lizards with flippers and probably swam in an eel-like fashion. Their successors evolved to become progressively more fishlike, with thickened bodies, finlike fore and hind limbs, and crescent-shaped tails. Fossilized skeletal remains show that the eyes were unusually large and adapted for hunting in poor visibility. Ichthyosaurs typically lived on a diet of squid, and some species attained lengths of over 15 m.