9.2: Types of Preservation
Preserving a recently deceased organism in the geologic record is challenging. This process typically requires one of the following:
Hard and Soft Parts
Preservation without alteration can happen when particularly robust hard parts of organisms like bones, shells, teeth, etc. are buried and protected from decay and damage. Under very special conditions such as freezing, entrapment in amber, or mummification the soft parts of organisms like hair, skin, or muscle tissue can be preserved.
Preservation with Alteration
Preservation with alteration is the most common way of preserving body fossils and it happens when the original tissue is changed in some way from its original form. Recrystallization happens when the original material is transformed into a more stable crystal form; the most common example of this is the transition from aragonite (iridescent nacre or mother of pearl) into chalky calcite. Carbonization happens when the organism is compressed, and water/gas are driven off so that only a thin carbon film remains. When minerals precipitate out of groundwater to fill in open spaces in wood, bone or other porous tissue the process of permineralization takes place (ex: “petrified” wood formed when microcrystalline quartz fills in woody tissue). And lastly, replacement takes place when one mineral is precipitated in place of another (ex: pyrite replacing calcite).
Casts, Molds, and Impressions
Sometimes even when the tissue itself isn’t preserved, a fossil may be still be there in the form of a cast, mold, or impression. A cast occurs when sediment or minerals fill in a void space left by a decayed or dissolved organism. A mold is the name we use for the actual void. An impression happens when organic material is pressed into sediment and the texture of that material is preserved.