Fossils The study of prehistoric life is called_______________ The fossil record forms the basis of the geologic time scale and is important in the______________. Formation of Fossils Shells, bones, petrified trees, footprints, leaves, or worm burrows are all considered fossils. They can form in several different ways including: 1. Original remains: 2. Replaced remains – Remains can be replaced molecule by molecule as in the case of fossil bones, teeth, and shells. 3. Molds and Casts – A dead organism makes an impression into soft sediments 4. Trace Fossils – Evidence of life is preserved in a rock. 5. Carbonaceous Films – ________________are used to date rock layers. The best index fossils are easily recognizable, abundant, widespread, and existed for a brief period. Fossils can give an indication about the _______________of a rock example. For example, the Niagara Escarpment was once a _______________as indicated by the abundant fossilized coral.