The earliest marine reptiles arose in the Permian period during the Paleozoic era. During the Mesozoic era, many groups of reptiles became adapted to life in the seas, including such familiar clades as the ichthyosaurs, plesiosaurs, placodonts, and mosasaurs.
After the mass extinction at the end of the Cretaceous period, marine reptiles were less numerous. Extant marine reptiles include marine iguanas, sea snakes, sea turtles, and some species of crocodiles.
Some marine reptiles, such as ichthyosaurs and mosasaurs, rarely ventured onto land and gave birth in the water. Others, such as sea turtles, and saltwater crocodiles, return to shore to lay their eggs. Some marine reptiles also occasionally rest and bask on land.