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Ichthyosaurs, whose name means "fish lizard" in Greek, were large marine reptiles that lived between 230 and 90 million years ago. They superficially resembled fish or dolphins. Ichthyosaurs shared the seas of Earth with sharks, fish, and other marine reptiles like plesiosaurs and pliosaurs. Ichthyosaurs were first described from fossil fragments dug up in 1699 in Wales.
Ichthyosaurs evolved just 21 million years after the greatest mass extinction in history, and disappeared about 25 million years before the mass extinction that killed off the dinosaurs. Though ichthyosaurs are sometimes incorrectly called dinosaurs, they were not. The fish-like body structure of ichthyosaurs led biologist Stephen Jay Gould to call them his favorite example of parallel evolution.
The evolution of ichthyosaurs into streamlined dolphin-like forms is made even more remarkable by the fact that they evolved from terrestrial reptiles without any body features to work with; not even so much as a small tail fin. The earliest ichthyosaurs were small (about a meter in length) and lacked the long flippers of later ichthyosaurs, swimming instead with an undulating eel-like motion.
Most ichthyosaurs were about 2 to 4 m (6.6 to 13.2 ft) in length, with a porpoise-like head, long snout, and sharp teeth. Some reached 55 ft (17 m) in length, such as Shonisaurus, the state fossil of Nevada, though these were very large and much less typical. The largest was Shonisaurus sikanniensis, the largest known marine reptile at 21 m (69 ft). The largest ichthyosaurs died out from the extinctions at the end of the Triassic period.
Most ichthyosaurs had large, bulging eyes. They ate meat, especially fish and the occasional seabird or juvenile marine reptile. The heydey of ichthyosaurs was in the Triassic, and they evolved around the same time the dinosaurs did. After the Triassic and early Jurassic, their diversity went downhill, and by the mid-Jurassic, all ichthyosaurs belonged to a single clade. Ithyosaurs then went extinct in the Cretaceous, one of the only major groups to die out on its own and not due to the mass extinction at the end of the period.