Fossilisation can be referred as the set of inorganic processes which allow the remains of formerly living organisms to be preserved after its death and buried under sediment. It happen only in dead organisms, usually the hard parts can be preserved (bones, husk, shell, etc), because they can survive the alteration and disintegration or decomposition caused by atmosphere phenomena while Soft parts are rare to be fossilised this because they are vulnerable to predator and decomposition.

This process of preservation is influenced by natural processes (weathering, floods and volcanic eruption) which bury dead organisms in the surface. Some natural process cause death of the species and some of the process bury them in sediments. Volcanic eruption can cause death to many species and it can bury them on its larva, while erosion erodes all the dead particles and deposited them with other material to form sediments. Weathering which cause breaking down of rocks bury dead organisms in the particles and form sediment or rock.

Environmental condition also plays a part in perseveration of fossils this occur in the maintenance of the morphological structure of the species, if not altered by destructive agent they can keep their morphological structure. Fossilisation that happen in terrestrial environment like marine or fluvial environment are well preserved this because depositing agents act more quickly in marine environments than degrading ones. Morphological structure can be disturbed by physical changes that may involve, like flattened or distorted as sediments is hardened into rock.

Factor that influence the preservation of fossils are:

Carbonization which is known for plants where the organic matter are leached out and reduced to carbon film and the rigid organic walls are the one preserved
Sedimentation; most organisms are preserved in this process as the organism buried in mud where some of the remaining are replaced by sulphide or phosphate mineral.
Recrystalisation; this method of preservation destroy microscopic details of the shell but does not change the overall shape, so it can keep morphological structure of the species.

All this methods of preservation helps to keep evidence of unknown species that live in the past which were influential to evolution as it buries the remaining bones of dead species.

Importance of fossils in evolution is that it helps researchers Archaeologist to identify species that live in the past decades and its origin; they can be used to interpret the process of evolution by using the fossilised material.

The fossil of different species can identify the indigenous species in the area, and they also play a part in tourist attraction because some of people would like to see the indigenous creature of the past. Fossils play a part in interpreting evolution because the remaining bones or shell support the findings of the researcher and what was happening in the previous years about different species that evolve.

Reference:

1. Fossilisation; [Internet] [cited 2006-May-04] Available from: http://www.musei.unina.it/Paleontologia/eng/3.2.4.1.htm.
2. Fossilisation; [internet] [cited 2006-May-03] Available from: http://www.museum.vic.gov.au/dinosaurs/sci-form.html.