CoachA fossil is the remains, or the trace, of a living thing of long ago, kept in rock. Some fossils are the hard parts of the body — a shell, a bone, a tooth, wood — turned to stone. Others keep only a shape: a mould is the hollow imprint left in the rock where a shell or a bone has dissolved; a cast is the copy that forms when a mould is filled with new mineral matter or sediment. A trace fossil is a sign of what an animal did — a footprint, a burrow, a trail — and not a part of its body.
