Organisms within the class Chondrichthyes are otherwise known as cartilaginous fish. They first appeared on Earth almost 450 million years ago. Cartilaginous fish today include both animals such as sharks, rays, skates, and ratfish. Members of the Chondrichthyes all lack true bone and have a skeleton made of cartilage. Only their teeth, and sometimes their vertebrae, are calcified. Sharks, for example, are a top predator at or near the top of their marine food chains, and they regulate the populations of species below them. All sharks have multiple rows of teeth, and while they lose teeth on a regular basis, new teeth continue to grow in and replace those they lose. Rays and skates, on the other hand, while also being the top predators, have flattened bodies, enlarged pectoral fins attached to the head, spiracles on top of head and are mostly adapted to bottom existence, but some live in open water.