Primitive fishes resembling sharks were swimming in the oceans over 450 million years ago. Their descendants survived successive mass extinctions, including the catastrophe at the end of the Permian period (245 million years ago) when 96 percent of all marine life was extinguished. They saw the dinosaurs come and go, and were still thriving when the mammals returned to the sea. Their long evolutionary history has refined sharks to the rank of near-perfect predators.