The most common type of fossil preservation is with alteration. The
original organic material is partially or totally changed into new
material. There are four common types of preservation with alteration
including carbonization, permineralization, recrystallization,
and replacement. These processes involve a chemical reaction with
the organism and its surrounding.
Carbonization is a chemical reaction when water transforms the
organic material of plant or animal to a thin film of carbon. Nitrogen,
hydrogen, and oxygen are driven off as gases, leaving an outline of the
organism. Organisms often preserved by carbonization include fish,
leaves and the woody parts of plants.