In this process of fossilization plant leaves, and some soft body parts of fish, reptiles, and marine invertebrates decompose leaving behind only the carbon. This carbon creates an impression in the rock outlining the fossil, sometimes with great detail.This is the most common method of fossil preservation. Minerals fill the cellular spaces and crystallize. The shape of the original plant or animal is preserved as rock. Sometimes the original material is dissolved away leaving the form and structure but none of the organic material remains. For a detailed and illustrated description see.