The earliest sea-dwelling reptiles were the nothosaurs and the ichthyosaurs. These creatures were adapted to life in the seas by having a streamlined shape and paddle-like limbs. Both these groups of reptiles are characterised by having a single opening high on the side of the skull behind the eye socket. Animals with this kind of skull are called euryapsids.

Several other groups of marine reptiles are important. The first are the plesiosaurs, a group which may have evolved from the nothosaurs at the end of the Triassic period. Next come the placodonts, a group of four-legged Triassic reptiles that had teeth adapted to crush shellfish. Placodonts may have lived a rather similar life to modern mammals like the walrus, spending time on land and swimming to the bottom of rivers and coastal waters to catch shellfish.

The final group are the mosasaurs, which took the place of the ichthyosaurs in the Cretaceous period. Mosasaurs had diapsid skulls with two openings behind the eye, and so were related to the modern lizards. Like the dinosaurs, the large marine reptiles all died out at the end of the Cretaceous.