Conquering various living environments
The amazing aspect of these mesozoic reptiles were their exploitation of not only the terrestrial surface but their conquest of the air by pterosaurs and their recolonization of the aquatic environment by Ichthyosaurus and Plesiosaurus. The ichthyosaurs were completely adapted to a marine life, like mammal such as dolphins are today. Fossil evidence suggested that egg-laying on land had been abandoned, and that the young were born alive and at sea. The body shape was completely reconverted to that of a fish; the neck telescoped to give a fusiform body shape, the limbs shortened into small steering devices. Locomotion was performed, fish-like, by undulations of the trunk and tail; a fishlike fin was developed on the back (but like that of dolphins, it lacked the skeletal support found in dorsal fins of fishes), but the tail became a powerful swimming organ, in appearance like that of a shark. In this last regard, however, there is a notable structural difference; for whereas in a shark the end of the backbone tilts into the upper lobe of the tail fin, that of the ichthyosuar turns sharply down at the back, with the fin expanding above it. Most ichthyosaurs were presumably fish-eaters, but some feed on ammonites. The plesiosaurs were less extreme in their adaptations and probably were able to wadddle up on to a beach for egg-laying rather like marine turtles do today. They possessed a long neck or long snout or both; the body was short, broad, and relatively flat. Reversion to a truly fishlike means of locomotion was impossible, for the trunk was inflexible and the tail short; instead the limbs were developed into powerful oarlike structures, with which the creatures "rowed" its way through the sea.