Fossils are the preserved remains or traces of plants or animals. The conditions needed to preserve fossils occur very rarely in nature, so the fossil record is only a tiny fraction of the history of all living organisms.
The word fossil comes from a Latin word meaning "to dig up." Fossils have not always been recognized as the remains of once living organisms. Not understanding mountain building or plate tectonics, early civilizations have struggled with the mystery of sea shells stranded on mountain tops and in barren deserts.
Trace fossils, orichnofossils, include such things as footprints and trails. Burrows or borings made by animals such as worms, trilobites, and bivalves can also be preserved if they are filled with sediment and hardened into rock.