Forms of preservation

All of the processes that affect an organism during and after its fossilisation are grouped together under the term diagenesis. If all the conditions come together, anything can be fossilised: a feather, a leaf, an organ, a footprint, etc.

Several modes of fossilisation can be observed, illustrated here by a bivalve whose shell is depicted in cross-section.

The original mineral of the shell may be replaced in different ways by another mineral:

• Dead organism
• Crystallised
• External cast: the external cast may bear the imprint of the external ornamentation of the shell.
• Internal cast
• Organism completely preserved
• Epigenesis: the original mineral of the shell is replaced by another mineral.
• Hard parts preserved: separated valves – fragmented valves