Earth History

Fossils

Fossils are any remains of a dead organism. Some common types of fossils are listed below.

• Tracks
  • organism footprints or locomotion patterns left in soft sediment that became lithified
• Burrows
  • cylindrical holes or tubes left in sediment, wood or rock made by an organism
• Coprolites
  • fossilized dung or stomach contents of organisms
• Gastroliths
  • polished stomach stones used by some extinct reptiles to grind food
• Petrified Fossils
  • small, internal cavities and pores of the organism’s original structure is filled with precipitated mineral material
• Molds
  • the impression left by an organism in buried sediment; the organism is usually dissolved by underground water
• Casts
  • occur when the hollow spaces left behind by a decaying organism is filled with mineral matter
• Carbonization
  • fine sediment surrounds the remains of an organism, as time passes, the pressure squeezes out the liquid and gaseous components leaving a thin residue of carbon
• Amber
  • organisms are trapped in tree sap

The principle of fossil succession states that fossilized organisms succeed one another in definite and determinable order, and therefore any time period can be recognized by its fossil content. This statement enables geologists to determine the relative age of a rock by analyzing the fossils it contains. Index fossils are of particular interest to geologists. Index fossils are geographically widespread fossils, but limited to a short span of geologic time. The presence of these fossils allows geologists to match rock formations of the same age.