Fossils

How do fossils form? http://www.youtube.com/watch?v=SEDfRy6DQns For fossils to form, evidence of, or the bodily remains of the organism must be preserved. This will occur if there is either quick burial of soft parts in a protective medium or there is preservation of some kind of hard parts, such as a shell or skeleton. For remains of an organism to be preserved it is important that natural processes of decomposition are delayed or stopped. This could be achieved by burial in soft mud or volcanic ash, by low temperatures, by being in very dry air, by seawater or by covering in tar or resin.

Types of fossils unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap, preservation of animals in permafrost or tar pits). Very rare.

permineralization=petrification (in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized)

carbonization=coalification (in which only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed)

replacement (An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron)

authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).

Moulds As time passes, the organism itself, particularly when composed primarily of softer materials, is eroded and carried away, leaving a negative imprint of its body. This type of fossil is called a mould.

Casts A cast fossil forms when a mold fossil is filled with some form of mineral, usually through the seepage of water depositing the minerals within the mold. In time, the mold is filled and the materials deposited harden, creating a replica of the original fossil.

Tracks, trails, borings

Coprolites – fossilised excrement