Fossils

Fossils

• 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

How Fossils Form

• Petrification– small internal cavities and pores of the original structure are filled with precipitated mineral matter

• Frozen• Mummification—dry

conditions

How Fossils Form

• Replacement—cell walls and other solid material are removed and replaced with mineral matter

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

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)

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

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

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.

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.