Aquatic adaptation

Several animal groups have undergone aquatic adaptation, going from being purely terrestrial animals to living at least part of the time in water. The adaptations in early speciation tend to develop as the animal ventures into water in order to find available food. As successive generations spend more time in the water, natural selection causes the acquisition of more adaptations. Animals of later generations may spend the majority of their life in the water, coming ashore for mating. Finally, fully adapted animals may take to mating and birthing in water.

Archelon is a type of giant sea turtle dating from the Cretaceous Period, now long extinct. Its smaller cousins survive as the sea turtles of today.

Mesosaurus (and other mesosaurids) were another group of anapsid reptiles to secondarily return to the sea, eschewing shells, and are also long extinct.

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 66 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.

These marine reptiles had ancestors who moved back into the oceans. In the case of ichthyosaurs adapting as fully as the dolphins they superficially resemble, even giving birth to live offspring instead of laying eggs.

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