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