Fossils: Types of Fossils

Mummification

This rare form of preservation preserves life form with some tissue or skin intact. Specimens that are preserved this way are very fragile. Natural mummification usually happens in dry and cold places where preservation happens quickly and effectively. Mummification is not truly fossilization.

External Molds

These are imprints of the organism embedded in rocks.

Casts

These are formed when external molds are filled with sediment.

Internal Molds

These occur when sediment fills the shell of a deceased organism such as a bivalve or a gastropod. These remain after the organism's remains decompose to show the internal features of the organism

Petrification/Petrifaction/Silicification

These occur when minerals slowly replace the various organic tissues of an organism. The most common mineral to cause petrification is silicon, but other minerals also work.

Carbonization/Coalification

These occur when over time all parts of the original organism except the carbon are removed from the fossil over time. The remaining carbon is the same carbon that the organism was made of.

Recrystallization

This occurs when original minerals in the fossil over time revert into more stable minerals, such as an apatite shell recrystallizing into the more thermodynamically stable calcite.

Replacement

This occurs when the hard parts of the organism are replaced with minerals over time.

Trace Fossils

fossils that are not part of the organism. These include footprints, burrows, eggshells, and coprolite (fossilized excrement). They give insight into an organism's behavior.

Actual Remains

These are much rarer than other fossil types. These are still intact parts of the organism. Actual remains can be seen preserved in ice, tar, or amber. A good example is mammoth hair, which is often frozen and still preserved.

Tar

When organisms become trapped in tar, due to the oxygen deprived environment, it allows for the rapid burial of body parts which are well preserved. A good example is the La Brea tar pits in Los Angeles.