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Fossil Preservation

The photograph above shows bachiopod shell fragments in a thin (30 millionths of a meter) slice of rock, as observed with a microscope. The brachiopod shells are the pink areas. The pink color is due to a chemical stain that was applied to the rock to help identify the mineral calcite. When brachiopod shells form, they have an intricate internal structure. In the photograph, this structure is not seen. Rather, large calcite crystals occupy the area of the original shell The large crystals indicate that the brachiopod shell dissolved and new crystals formed in the area occupied by the original shell. This process is a type of diagenesis.
The photograph above shows bachiopod shell fragments in a thin (30 millionths of a meter) slice of rock, as observed with a microscope. The brachiopod shells are the pink areas. The pink color is due to a chemical stain that was applied to the rock to help identify the mineral calcite. When brachiopod shells form, they have an intricate internal structure. In the photograph, this structure is not seen. Rather, large calcite crystals occupy the area of the original shell The large crystals indicate that the brachiopod shell dissolved and new crystals formed in the area occupied by the original shell. This process is a type of diagenesis.

Diagenesis refers to the chemical and physical changes that occur in sedimentary geologic materials after their initial deposition or formation. During the Silurian Period, reefs in Wisconsin and Illinois consisted of shells and soft mud composed of calcium carbonate. The shells and mud eventually crystallized into rock called limestone. Later in geologic time, salt-rich waters introduced magnesium into the deposits, changing the limestone into a rock called dolostone, which consists of calcium-magnesium carbonate.

Although some fossils have not been changed from their original composition, most fossils have undergone some degree of diagenesis. There are 5 basic ways that fossils form:

Preserved Remains - The original bone, shell, or even soft tissue is preserved and has not been mineralized. Examples include insects in amber, mammoth frozen in ice, and original bone and shell material. The older a fossil the less likely that the material has not been altered.
Permineralization - Occurs when tiny spaces in bone or wood are filled by new minerals. Silicification is the alteration of a shell composed of calcium carbonate into a fossil shell composed of silica (SiO2).
Molds and Casts - Shells may also be dissolved during diagenesis, leaving only their impression in the rock. This is how most Silurian reef fossils of Wisconsin and Illinois are preserved. An external mold is an impression that preserves the appearance of the outer surface of a shell. An internal mold is an impression of the inner surface of a shell. External and internal molds of the same shell are often very different in appearance.
Replacement - The original shell material is dissolved away and a cavity is left behind which is later filled in with a mineral. The fossil will have details of the external ornamentation but the internal structures of the shell has been lost.
Compression Specimen - Organism is compressed under high pressure and an imprint is left in the rock. Common with leave fossils and sometimes softbody animal fossils as with Mazon Creek fossils.  

Photo Gallery

Specimen UC 875, Buthotrephis newlin, is an example of a Silurian fossil alga preserved as a carbonized compression fossil.
© The Field Museum - CC BY-NC
2013 Grainger Mazon Creek Holotype Digitization Project
© The Field Museum - CC BY-NC
An example of an external and internal mold of a Silurian Pentamerid brachiopod. The external mold is an impression that preserves the appearance of the outer surface of a shell. The internal mold shows what the inside of the shell looked like.
© The Field Museum - CC BY-NC
The photograph above shows bachiopod shell fragments in a thin (30 millionths of a meter) slice of rock, as observed with a microscope. The brachiopod shells are the pink areas. The pink color is due to a chemical stain that was applied to the rock to help identify the mineral calcite. When brachiopod shells form, they have an intricate internal structure. In the photograph, this structure is not seen. Rather, large calcite crystals occupy the area of the original shell The large crystals indicate that the brachiopod shell dissolved and new crystals formed in the area occupied by the original shell. This process is a type of diagenesis.
© The Field Museum - CC BY-NC