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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.
Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, hair, petrified wood, oil, coal, and DNA remnants.
Specimens are usually considered to be fossils if they are over 10,000 years old.
The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils.
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.
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If this hole is later filled with other minerals, it is a cast.