A richly illustrated introduction to the spectacular reptiles that swam the oceans when dinosaurs roamed the land. During the Mesozoic Era, 252 to 66 million years ago, dinosaurs ruled the land, but the ocean deeps were roiling with equally spectacular reptiles - including giant predators. This richly illustrated, authoritative, and accessible book introduces readers to the world of these fascinating marine animals, whose predecessors returned to the seas a few million years after the first vertebrates emerged from the water. As we meet ichthyosaurs, plesiosaurs, mosasaurs, and many others, we learn about the astonishing anatomical, physiological, and behavioural adaptations that enabled these reptiles to become ocean dwellers again. We also learn about their living descendants, including sea turtles and sea snakes. Featuring stunning artwork depicting these prehistoric ocean creatures and photographs of their fossil remains, this book invites readers to discover the enthralling past of marine reptiles in all their extraordinary diversity.