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Terri Mathews




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Geology 112




















Fossils 

Learning Objectives



A fossil is the trace or remains of a once living organism.

Fossilization is more likely if the organism has hard parts (bones, shells) and is buried rapidly.  There are different ways that a fossil can be preserved:

A) Direct Fossilization: fossilization of the organism
 

    1) Unaltered Hard Parts: the actual hard part (shell or bone) is simply buried and remains intact.

    2) Altered Hard Parts:

      Replacement: mineral material replaces the tissues of the organism molecule by molecule.

      Permineralization: mineral material is deposited within the cells of the organism.  If the mineral material is dissolved then some of the original organism will remain.

    3) Carbonization: Pressure from overlying rock strata squeeze organic compounds out of an organism leaving a carbon film behind.  Common method for soft organisms.

    4) Mold: the depression left behind by an organism.  Retains detail of the external structure.

    5) Cast: a mold that has been filled in with mineral material.

    6) Other: some rare methods of fossilization: these methods tend to preserve soft tissue and in some cases the skin and hair of an organism.

B) Indirect Fossilization: no part of the organism remains or is represented but there is evidence that the organism existed.
    1) Tracks, Trails, Footprints

    2) Gastroliths: gizzard stones

    3) Coprolite: preserved dung

    4) Artifacts: useful for studying human evolution.


Fossils are important tools for reconstructing Earth's history. They are important for correlation of rock units.

Geologic Range: the time interval between the first and last appearance of a fossil or fossil assemblage.

Endemic Fossil: an organism that had a small geographic distribution.

Cosmopolitan Fossil: an organism that had a wide geographic distribution.

Guide Fossil: an organism that is useful for determining a specific time period.  The best guide fossils have a short geologic range and are cosmopolitan.

Zone, Biozone: a body of rock identified by a fossil.

Assemblage Zone: a body of rock identified by a group of fossils.

 Concurrent Range Zone: two fossil organisms with different geologic ranges but the ranges overlap during a narrow window of geologic time.



Fossils can also help reconstruct past environments:

Paleoecology: The study of ancient ecosystems.  Geologists look at modern ecosystems and the hard parts contained within to understand ancient ecosystems.

An understanding of paleoecology can help geologists determine:

    Ancient Environments
    Paleolatitudes
    Geographic movements
    Presence of Land Bridges