Fossil formation, types, and preservation

Fossil Formation

Fossils typically form from organisms that were quickly buried after their death by sediment.
This rapid burial prevents the remains from being consumed by decomposers and protects them from the elements. The sediment eventually hardens into rock and preserves the shape of the organisms.
Permineralisation is a common process in fossilisation where minerals carried by water are deposited around or replace the hard structures (like bones and teeth). Once the minerals harden, a rock-like mould of the organism remains.
Carbon films occur when organisms or their parts are subjected to heat and pressure causing all elements apart from carbon to be driven off. This leaves a residue or film of carbon on the rock surface.
Moulds and casts are another form of fossil. If an organism leaves an impression in the sediment, and that impression becomes rock, this is a mould. If the space of the impression later gets filled with other minerals, this is a cast.
Trace fossils capture the activity of organisms, including tracks, burrows, and marks left by feeding. These fossils can provide valuable information on the behaviour of extinct species.

Types of Fossils

Body Fossils are the remains of plants and animals. They include teeth, bones, shells and leaves.
Molecular fossils are organic compounds or biomarkers that can be attributed to extinct species.
Microfossils are the microscopic remains of fungi, animals and plants. These require a microscope for their investigation.
Ichthyoliths are the remains of ancient fish.

Fossil Preservation

Preservation of fossils often depends on the material of the organism, with hard materials like bone and shell preserving better than soft tissues.
The type of environment also plays a major role in preservation. Anaerobic (oxygen-free) environments, for example, are optimal for preserving finer details as decomposition is slower.
Over geological time, a buried fossil may be subjected to pressure and temperature changes that could modify its appearance and composition.
Fossils are often found in sedimentary rock, as the process of sediment deposition assists in the preservation of fossils.
Key for fossil preservation is quick burial in sediment, lack of oxygen, and minimal disturbance after burial.
Due to selective preservation, the fossil record can be biased towards preserving certain types of organisms more than others, which needs to be taken into account when studying extinct life.