During Triassic, Jurassic, and Cretaceous times, evolution of reptiles, similar to that of the dinosaurs on land, occurred in the oceans. Between 252 and 227 million years ago, three groups appeared—turtlelike placodonts, lizardlike nothosaurs, and dolphinlike ichthyosaurs. Of these, only ichthyosaurs survived until the Jurassic.
The Jurassic ocean teemed with life. Modern fish groups were well represented, as were ammonites, mollusks, squid, and modern corals. A variety of ichthyosaurs evolved, some giant forms reaching 9 meters in length, but they soon died out and were replaced by modern sharks.
The gap left by the extinction of the placodonts and nothosaurs was filled by long-necked plesiosaurs. Those with a short body and tail and a small head lived in shallow water, while larger forms, called pliosaurs, probably lived in deep water.