As with all life in the Cretaceous, there was a mixture of the older groups and the emergence of modern groups. The Mesozoic “marine revolution” occurred during the Cretaceous period, and included the appearance of new, modern predators that could feed on the older, hard-shelled forms.

Many modern families of marine animals appeared during the Cretaceous period, including the modern crabs, clams, and snails; sharks also evolved into their modern families by the late Cretaceous. Larger animals included mollusks and lobsters. Many bony fishes continued to evolve from much earlier periods than the Mesozoic era.

Marine reptiles still lived in the seas—most of them until the end of the Cretaceous. Mosasaurs, or marine lizards with paddle-like flippers that grew up to 33 feet (10 meters) in length, were around until the Late Cretaceous. Plesiosaurs and ichthyosaurs still swam in most of the oceans, but they died out in the Late Cretaceous. There were also a few marine crocodiles left over from the Jurassic period.