Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites).
These types of fossil are called trace fossils (or ichnofossils), as opposed to body fossils.
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.
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.
The observation that certain fossils were associated with certain rock strata led early geologists to recognize a geological timescale in the 19th century.
The development of radiometric dating techniques in the early 20th century allowed geologists to determine the numerical or "absolute" age of the various strata and thereby the included fossils.
Finally, past life leaves some markers that cannot be seen but can be detected in the form of biochemical signals; these are known as chemofossils or biosignatures.
Permineralization is a process of fossilization that occurs when an organism is buried.
In many cases, however, compressions and impressions occur together.