By the end of the early triassic the reptiles ruled the continents. Very few habitats were not occupied by a reptile adapted to a special kind of life allowing it to survive in its own fashion. The ichthyosaurs were the most highly adapted of all reptiles for marine life. These reptiles suddenly appeared in beds of middle triassic age.
Ichthyosaurs were the most fish
like of all reptiles. They are classified as reptiles because in fish,
the shoulder girdle and skull are connected, but not in tetrapods. Ichthyosaurs
also have a pair of openings in the skull called temporal fenestra and
this is a feature of diapsids, a group which includes turtles.