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
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