Chemical transformation

Over geological time, the original hard parts of an organism will be transformed while lying in the sedimentary rock. Minerals from the surrounding rock slowly impregnate the bones, or shell, of the fossil, changing its chemical composition.

Calcareous skeletons also change chemically. There are two forms of carbonate: aragonite and calcite; aragonite is less stable and becomes rare in older fossils. Calcite may be replaced in some fossils by silica or pyrite. In extreme cases, the calcite may be dissolved away completely, and the space filled in by other material: the fossil then acts as a mould, or cast, for the new material. The remains still reveal the shape of the organism's hard parts.