The ichtyosaurs of the jurassic already possessed the body profile of a fish, these reptiles were built for speed, having the same body shape as tunas and mackerel, which are the fastest fish in the sea.  Many of these reptiles were small animals compared to modern dolphins, but some very large ones have been found, up to thirty feet in length. These reptiles lived a similar life to porpoises and small whales, pursuing fish through the water and breathing air like them.
 
 

Fossils that have been found indicate that these reptiles gave birth to live young, indicating that these reptiles were highly specialized, and almost certainly  were not able to leave the water, and had no need to do so.

By the late cretaceous the icthyosaurs were apparently on the way to extinction. Only a single genus is known from this period, and this is the last of north American ichthyosaurs. These highly adapted reptiles that were successful  for most of the mesozoic, apparently became extinct before the end of the cretaceous. The reasons for their disappearance; even when conditions and food supplies were favourable for them, are unknown, it is speculated that their disappearance might have been caused by competition from plesiosaurs