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 rockā€”an impression fossil.

Some fossils are biochemical and are called chemofossils or biosignatures.

The totality of fossils is known as the fossil record.

Paleontology is the study of fossils: their age, method of formation, and evolutionary significance.

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.

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.

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.

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.

Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. In some cases the original remains of the organism completely dissolve or are otherwise destroyed.