Dating rock layers fossils
is any preserved remains, impression, or trace of any once-living thing from a past geological age.
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.
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.
Some fossils are biochemical and are called chemofossils or biosignatures.
Permineralization is a process of fossilization that occurs when an organism is buried.