Fossils are abundant in cliffs of Dover. This article is part of our Guide to Fossils.
The process of fossilization can vary according to the type of tissue and external conditions. Different ways of fossilization is present. For instance permineralization, it is the process by which fossilisation occurs when an organism is buried. In this case the empty spaces found inside an organism gets filled with mineral rich groundwater. The minerals can precipitate from groundwater to occupy these spaces. This process could occur in very small places like within a cell wall or a plant cell. Through small scale permineralization it could form detailed fossils. In the case of permineralisation, the organisms must be covered with sediment soon after death or early during the decay process. The degree to which the remains are decayed determines the later details of the fossil. Certain fossils have only the remains of skeletons or teeth. Others have traces of skin and feathers.
Casts and molds are formed when the original remains of organisms dissolve or are destroyed. The remains fit into the shape of an organism in the rock forming a hole and is an external mold. When this mold is filled with other minerals later, it is called a cast. An endocast forms when sediments or minerals fill the internal cavity of an organism such as the inside of a snail or an animal skull.
Authigenic mineralisation is a special form of cast and mold formation. If the chemistry is right, then the organism could act as a nucleus helping to precipitate minerals like siderite to form a nodule around it. If this occurs prior to a lot of decay then the three dimensional morphological is preserved. The mazon creek fossil beds of USA are some of the best documented examples of such mineralisation.
Replacement and recrystallization is another process which brings about fossilisation. Replacement happens when the shell, bone or tissue gets replaced with another mineral. During certain cases mineral replacement of the original shell can occur gradually. The microstructural features are preserved despite the loss of original material. Shells are said to be recrystallized when the original skeleton is formed as a different one with aragonite to calcite.
Fossils could also be formed through chemical reduction of complex organic molecules which form the tissues of an organism. Fossils are formed through adpression, which is how fossil ferns are formed through compression brought about through chemical reduction of molecules this chemical change is an expression of diagenesis, leaving a carbonaceous film.