Remains or traces of prehistoric life that have been preserved are called fossils. Any type of organic matter can become fossilized under the right conditions and fossils may range in size to microscopic bacteria to large dinosaur bones. Items that are harder and denser, such as seashells, bones and teeth are more likely to become fossils than soft tissues.

Since organic matter is naturally broken down by the process of decomposition after an organism dies, fossils will only form under specific conditions which prevent this natural decay.Earth History: What Fossils Tell Us Most fossils are formed when an organism is buried under layers of earth. As the earth is compressed into sedimentary rock, minerals replace the organisms' natural matter to produce a replica of the original item in the rock. Fossils may also be created when organisms are frozen or buried during volcanic eruptions.unmuseum.org: What's a fossil?