Sharks, rays and skates are elasmobranch fish. This group of predatory fish has skeletons made of cartilage instead of bone. They first appeared in the Ordovician Period approximately 450 MYA. Circumstances rarely allow the cartilaginous skeletons to be preserved in the fossil record. Only the teeth are commonly recovered, so much of our assumptions about these animals are based on what we know of their living relatives. The “Megalodon” shark is probably the largest carnivorous fish to have ever lived. This gigantic predator reached a size of more than 50 feet long (15 m), weighing in at about 50 tons (45 tonnes).