During the Mesozoicperiod when dinosaurs ruled life on land, sea reptiles were the absolute rulers of the seas and oceans. Some of these animals were 14 meters in length Plesiosaurs appeared during the Triassic, later they split up into two superfamilies, the Plesiosauroidae that looked liked dolfines with lang necks en snakelike heads, and the Pliosauroidae with there stubby body\'s, short necks en crocodile-like heads.

Both superfamilies lived troughout the Mesozoic period. Plesiosaurs are from origin landanimals, which adapted to live in the sea, there legs changhed in to swimmingpeddals and instead of five they developed ten finger- and toobones. They moved trough the water by flapping there peddals up and down, so it looked as if they flew trough the water as birds .

The pelvis and shoulderbones were deformed to massive boneplates, a unique characteristic for plesiosaurs. Ichtyosaurs were the highest specialised marine reptiles of there time, they were verry similar to modern dolphins. Thanks to their build they could swim  with a speed of  40 km/h.  Ichtyosaurs used tunlike the  plesiosaurs their tail fin for movement.

They were so adapted to water that they couldn\'t go on shore anymore. Ichthyosaurs were viviparous animals and they lived for more then 100 milion years on earth. The earliest fossil records of ichthyosaurs are dated from the Early Triassic and the latest from the Middle Cretaceous, the greatest diversity among these animals appeared during the Jurassic.

Mosasaurs were flesh eating land animals that returned to sea to take over the gape that the ichthyosaurs left after they disappeared. They quickly spread and evolved into twenty different families with more than seventy genera.

There lifespan was a relatively short one, twenty-five million years after their entrance and even before the end of the Cretaceous they were already disappeared from the Earth leaving their place to marine mammals.

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