In the broadest definition, a fossil is the remains of a living thing that lived long ago and that has survived down to the present day by being preserved under natural conditions. The fossils that come down to us are parts of an organism, or remains left behind when the living thing concerned was still alive (the latter are known as trace fossils.) They are formed when dead animals or plants are preserved before they completely decay and eventually become part of the earth's sedimentary rock. In order for fossilization to take place, the animal or plant concerned must be buried in a fairly rapid manner—generally by being covered in a layer of silt. This is generally followed by a chemical process, during which preservation is ensured by means of mineral changes that take place in the original tissues.
