Cartilaginous fish lack true bones and have a skeleton made of cartilage. They include sharks, rays and skates.
They first appeared almost 450 million years ago during the Odovician Period.
Calcification of cartilage requires special conditions and, so, cartilaginous fish form fossils less easily then bony fish. Fossilized shark and ray teeth are common from the Cretaceous Period onwards (i.e. from 144 million years ago).