Rock Cycle - Past Life (3)
Pre Lab 

  • Comparing different ways a fossil can be preserved.
  • Distinguishing modes of preservation.
  • alteration
  • preservation
  • leaf
  • chicken bone
  • bark
  • shell
  • Fossils by P. Taylor

Students compare and distinguish different types of fossil preservation.

Permineralization of dinosaur bone


Fossils are the remains of plants and animals that lived long ago. The probability that an organism will be preserved as a fossil is very low. Geological processes such as erosion, weathering, sedimentation, and leaching constantly "attack" the fossil, and may destroy it before anyone sees it.

There are two main types of fossil preservation, with  alteration and direct preservation.  Most common is fossil preservation with alteration; the original organic material is partially to fully changed into new material. There are several types of preservation with alteration:

  1. carbonization, a chemical reaction where water transforms the organic material of plant or animal to a thin film of carbon. Nitrogen, hydrogen, and oxygen are driven off as gases, leaving an outline of the organism. Organisms often preserved by carbonization include fish, leaves and the woody tissues of plants.
  2. permineralization or petrifaction takes place in porous materials such as bones, plants and shells. The material is buried; later, groundwater percolates through its pore spaces. A solution, commonly supersaturated in either calcium carbonate or silica, precipitates minerals in the spaces. The original wood or shell like material preserved.
  3. recrystallization occurs when a solution or precipitate changes the internal physical structure of a fossil. Recrystallization changes the microstructure of the original minerals; they often reform as larger crystals. The composition of the mineral does not change, only the crystal structure. For example, many shells originally composed of calcium carbonate in the form of the mineral aragonite recrystallize into the more stable form of calcium carbonate called calcite.
  4. replacement involves the complete removal of original hard parts by solution and deposition of a new mineral in its place. The Petrified Forest in Arizona is an excellent example of this type of preservation. Here the original organic material (wood) has been wholly replaced by silica.

The second type of fossil creation is direct preservation, the preservation of original organic materials. The most common directly preserved fossils are unaltered hard parts of a living organism, like shells, teeth, and bones. This material is unchanged, except for the removal of less stable organic matter. Other examples of this type of preservation include fossil corals, shells, sponges, microscopic fossils and a host of other organisms with hard parts. In rare circumstances, preservation of the soft parts of an organism may occur. An example is an insect which has become "stuck" in tree resin. When the resin fossilizes to form amber, it preserves the insect. 

  1. Show the students different examples of living material, such as chicken bones, leaves, or tree bark. If you have a live animal in the classroom, you may also want to use it as an example. Ask the students what will remain after the organism dies. Lead them into a discussion about why hard parts such as shells or bones are more likely to be found in the fossil record then are the soft parts of an organism.
  2. If you have fossil specimens go over whether they are direct preservation or with alteration.  You may even ask students to bring in a fossil that they may have at home.  
  3. The Eyewitness Book on Fossils by P. Taylor has wonderful pictures of fossils for the students to examine. They will see examples of many of the modes of preservation presented above.

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