Fossilisation

Fossilisation, or fossilization, is the process by which organic remains are preserved in the geological record after death. It encompasses a variety of mechanisms that either conserve the original material or replace it with minerals, producing fossils of different kinds. Fossils can be body fossils, preserving parts such as bones, shells, or teeth, or trace fossils, such as tracks, burrows, or other evidence of biological activity.

Common modes of preservation include permineralization, where mineral-rich water fills pore spaces and leaves behind a rock-like replica of the original tissues; replacement, in which the original material is dissolved and chemically replaced by new minerals; and recrystallization, where minerals within the remains change form without changing composition. Carbonization preserves a thin film of carbon, typical of plant fossils. Impressions and casts form when an organism leaves an external mold or interior cast in sediment. Other modes include preservation in amber, in tar, or by freezing, and pyritization, where iron sulfide minerals replace tissues under anoxic conditions.

Fossilisation is favored by rapid burial, low oxygen, fine-grained sediments, and the presence of hard parts such as shells or bones, which survive decay longer. Exceptional deposits with unusually detailed preservation are known as Lagerstätten. Preservation biases influence the fossil record, tending to favor organisms with hard parts and those living in conditions conducive to burial. Understanding fossilisation helps paleontologists interpret ancient life, environments, and evolutionary processes, while acknowledging the incompleteness of the geological record.