In many cases, however, compressions and impressions occur together.
For instance, when the rock is broken open, the phytoleim will often be attached to one part (compression), whereas the counterpart will just be an impression.
Some fossils are biochemical and are called chemofossils or biosignatures.
This process can occur in very small spaces, such as within the cell wall of a plant cell.
Small scale permineralization can produce very detailed fossils.
to dinosaurs and trees, many meters long and weighing many tons.
A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.
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.A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression.Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil.Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites).