Absolute and relative dating of fossils
There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystallization, adpression, carbonization, and bioimmuration.
to dinosaurs and trees, many meters long and weighing many tons.
This chemical change is an expression of diagenesis.
Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression.
An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it.
If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved.
Permineralization is a process of fossilization that occurs when an organism is buried.
For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decay process.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.
In this case the fossil consists of original material, albeit in a geochemically altered state.
Minerals precipitate from the groundwater, occupying the empty spaces.
This process can occur in very small spaces, such as within the cell wall of a plant cell.
In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material.