For this reason, one term covers the two modes of preservation: adpression.
Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, and the isolation of proteins and evidence for DNA fragments.
For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decay process.
The degree to which the remains are decayed when covered determines the later details of the fossil.
If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved.
Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.
If this hole is later filled with other minerals, it is a cast.
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.
The remaining organism-shaped hole in the rock is called an external mold.
The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystallization, adpression, carbonization, and bioimmuration.
Replacement occurs when the shell, bone or other tissue is replaced with another mineral.
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.