Diapsid
Living at the same time as, but not closely related to, dinosaurs, the mosasaurs resembled crocodiles but were more strongly adapted to marine life. They became extinct within 65 million years ago, at the same time as the dinosaurs.
Modern diapsids which have made their own adaptions to allow them to spend significant time in the water include marine iguanas and marine crocodiles. Sea snakes are extensively adapted to the marine environment, giving birth to live offspring in the same way as the Euryapsida (see below) and are largely incapable of terrestrial activity. The arc of their adaptation is evident by observing the primitive Laticauda genus, which must return to land to lay eggs.
Read more about this topic: Aquatic Adaptation