Trace fossil

A trace fossil records what a living thing did rather than what it was made of. Examples include footprints and trackways, burrows and borings, bite and chew marks on bones and leaves, nests and eggshells, root casts, and coprolites, which are fossilized droppings. This contrasts with body fossils, which preserve bones, teeth, shells, leaves or other parts of the organism itself. Many trace fossils form when a print or tunnel in soft mud or sand is filled by later sediment, which hardens into a natural cast.

Because traces are almost never moved after they form, they show where and how ancient animals lived. Trackways reveal walking speed, posture and whether animals traveled in groups; burrows show where soft-bodied creatures fed and sheltered, even when the animals themselves left no remains; and coprolites show what was eaten. Traces also give clues to past environments, such as oxygen levels in the sediment and the depth of the water. The main drawback is that the maker usually has to be inferred, since it is rarely found at the end of its tracks.

The study of trace fossils is called ichnology. Simple burrows are among the oldest signs of animals that could move, and the base of the Cambrian Period is officially defined by the first appearance of a particular burrow type.