A fossil is any trace of past life. The most obvious fossils are body parts, such as shells, bones, and teeth; but fossils also include:
• trace fossils - remains of the activity of living things, such as burrows or footprints;
• chemical fossils - the organic chemicals they form.
Fossilization is an improbable eventuality. It is more probable for some kinds of species than others, and for some parts of an organism than for other parts. After burial in sediment, the fossils slowly transform through time; but the transformed remains, if they are preserved, can still tell us (after expert interpretation) much about the original living form. The study of fossils is called paleontology, while the biological study of fossils is called paleobiology.
An animation illustrates the process of fossilization.
The image on the left shows a trace fossil: the decapod Mecochirus longimanatus has been washed into the hostile environment of a lagoon during a storm, where it sank down and died after a few steps.