1. Fossils

2. Knowing the nature of the life forms that existed at a particular time helps researchers understand past environmental conditions Fossils are important time indicators and play a key role in correlating rocks of similar ages that are from different places

3. How Fossils Form Petrification– small internal cavities and pores of the original structure are filled with precipitated mineral matter Frozen Mummification—dry conditions

4. How Fossils Form Replacement—cell walls and other solid material are removed and replaced with mineral matter

5. How Fossils Form When a shell or other structure is buried in sediment and then dissolved by underground water, it forms a mold If the hollow spaces of a mold are filled with mineral matter it forms a cast

6. How Fossils Form Carbonization—fine sediment encases the remains of an organism, pressure squeezes out liquid and gaseous components, and leaves a thin residue of carbon If the film is lost from such a fossil, a replica of the surface may show detail (impression)

7. Other Types of Fossils Tracks—lithified animal footprints Burrows—tubes in sediment, wood, or rock made by an animal Coprolites—fossil dung and stomach contents Gastroliths—highly polished stomach stones that were used in the grinding of food by some extinct reptiles

8. Conditions Favoring Preservation Rapid burial –If the remains are buried quickly by sediment, they are protected from scavengers and decomposition Possession of hard parts –Shells, bones and teeth

9. Principle of Fossil Succession Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.

10. Index Fossils A fossil that is associated with a particular span of geologic time Rock formations do not always contain a specific index fossil; in these instances groups of fossils are used to establish the age of the bed.