S1, E3: Dinosaurs may rule the land, but it's a different story in the oceans. Marine reptiles dominate the seas. A school of female Opthalmasaurus are moving into the reefs around the northern Tethys Sea to give birth. Once born, the young ichthyosaurs must dodge sharks, plesiosaurs like Cryptoclydus, and the monsterous pliosaur Liopleurodon, the largest carnivore ever.