Plesiosaurs were the greatest ocean-going reptiles of the ]urassic and Cretaceous seas at the time when
dinosaurs ruled the land. There were two groups. The plesiosaurs were long-necked with smallish heads
and hunted smaller prey than the pliosaurs. The pliosaurs had much larger heads and mouths armed
with sharp teeth. All dieq out with the dinosaurs, some 65 million years ago.