Fossils are evidence or remains that show an animal or plant previously existed on the earth. The process of fossilization is a complex one as many conditions must exist for it to occur. Think of it as the perfect storm. There must be the correct ingredients of pressure, moisture, acidity and composition of sediment in order for a fossil to form. There are six ways a fossil can be formed. They are unaltered preservation, permineralization, replacement, carbonization, recrystallization and authigenic preservation. Unaltered preservation occurs when insects or plant parts get trapped in amber. Permineralization is the process where minerals seep in and replace the original organic tissues. The parts of an organism dissolve and are replaced by other minerals in replacement. Carbonization occurs when carbon is the only element left in the organism. Recrystallization is where the original parts turn into larger crystals. Authigenic preservation refers to the molds being dissolved.
Permineralization is an easy concept to understand, so we will use that process as an example. To understand the process of permineralization, we can use an example of a Tyrannosaurus rex dinosaur engaged in a battle with a Brontosaurus dinosaur. The Brontosaurus is badly wounded and retreats to the ocean for safety. Unfortunately, the Brontosaurus meets his demise in the ocean. As Brontosaurus dies, its body begins to sink to the ocean floor. The soft inner parts of the dinosaur will begin to disintegrate away and leave just the skeleton. The sand or sediment then continues to cover the skeleton. The sand surrounding the skeleton begins to thicken and transforms the skeleton into a rock-like substance. The continuous pressure from the ocean floor movement is part of the process which turns the skeleton into hard rock.
The skeleton is then further dissolved by the water and leaves a cavity which retains the original form of the skeleton. This cavity is referred to as a natural mold. The rich minerals from the ocean seep into this mold and begin form a cast. The cast will then have the original shape of the skeleton. A fossil has now been formed.