Please Explain: Sharks

There are some 440 species of sharks, and the fish have been swimming in the oceans for 420 million years, before dinosaurs existed. On today's Please Explain we'll find out about sharks, from hammerheads to great whites, and look at how they are becoming threatened. We're joined by Melanie L. J. Stiassny, Axelrod Research Curator of Fishes, Department of Ichthyology, American Museum of Natural History; and John Maisey Curator and Axelrod Research Chair, Division of Paleontology, American Museum of Natural History.