Fossilization

Fossilization is a rare event
Fossilization of a particular living organism is sedimentary deposits is a rare event. Yet, past life is so boundless in number that there exists, in fact, an enormous quantity of fossils buried in the earth. Finding and accessing those fossils, however, is not so easy. Normally, a fossiliferous layer of sedimentary rock will be sandwiched between layers that contain no fossils. The layers without fossils may be widely separated from those with fossils.

Fossil Formation
Animals or plants become fossilized after burial by sediments. Sediments are particulate matter usually transported by water that eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. Sedimentation is the deposition by settling of a suspended material. For example, seas, oceans, and lakes over time accumulate sediment that originates from both the land and marine environments. Deposited sediments are the source of sedimentary rocks that can contain fossils of the inhabitants of the body of water that were, upon death, covered by accumulating sediment.

As the sediments accumulate on the remains of the organism, the pressure increases, some water may be expelled and chemical changes occur to produce sedimentary rocks such as shale, sandstone and limestone. In harder rocks such as limestone, fossils may be more likely to retain their original appearance, while softer rocks may compress or flatten fossils the fossils (show Utah versus Oklahoma trilobites).

Stratigraphy, a branch of geology, is the study of rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. It is useful to review the basic principles of stratigraphy as set forth some three and one half centuries ago by Nicolaus Steno.

Once they are imbedded, fossils experience the same geological history as the rocks they in which they are contained (show distorted Housia). Rocks may be mashed, twisted and distorted with the contained fossils undergoing the same stresses and strains. Over geological time, temperature and pressure may leave fossils into a morphological state markedly altered from the initial organism, if not fragmentized. Fossils may also have traveled great horizontal distances owing to plate tectonics and great vertical distances owing to mountain building events (e.g., ammonites can be found in rocks at an altitude of over 5000 meters in the Himalaya Mountains).

Fossil examples, that some say are the world's most significant are those in the Burgess Shale, Canada. The Burgess Shale fossils are special because of their great age, and their exquisite preservation.

• There are ways that fossils form:
  mummification - complete preservation in a relatively undamaged state of hard and soft body parts;
  skeletons and shells - these are the most common fossilized hard parts; they may loose their color but are unchanged in their chemical composition;
  petrification and replacement - impregnation of fossils by secondary minerals which usually leads to an increase of weight and hardness of the fossil;
  carbonization - usually formed from woody and chitinous material, which loses its oxygen and nitrogen through decomposition by anaerobic bacteria;
  impressions and traces - impressions left by dead animals that decayed away or traces left by moving animals;
  casts and moulds - where the rock containing the fossils has hardened and the original fossil has been dissolved away, leaving a hole which can often be filled by other sediment.
  

Steno's Principles of Stratigraphy