The term “fossil” is a broad term for any artifact that gives evidence of a past life form that has been preserved in the Earth’s crust. Fossils can consist of imprints in sedimentary rock, petrified remains, or even an entire specimen preserved in amber, ice, or tar. While most fossils contain the element carbon in some quantity, a particular type known as a carbon film fossil is composed primarily of carbon.
All living things contain carbon, and when a dead organism lays on a rock, an extremely thin layer of carbon is deposited onto the rock over time. As the hydrogen, oxygen and nitrogen in the organism’s body disappear -- usually being dissolved and vaporized under a body of water -- the only remaining material is this layer of carbon. This decaying process is called carbonization or distillation.
A Two-Dimensional Imprint
In contrast to imprint fossils, which can be used to create a three-dimensional cast that is a copy of the organism’s true shape, a carbon film fossil appears as a two-dimensional image imprinted delicately into rock. They are usually black or brown, standing out in contrast to the color of the rock. Carbon film fossils are therefore not as “flashy” or prominent as fossils formed by other methods, but they can sometimes demonstrate intricate surface detail.
Because carbon films are usually left by specimens preserved under a body of water, the most common fossils are of fish, crustaceans, and leaves. These specimens probably sank and adhered to rock under bodies of slow-moving water where they were allowed to settle rather than being ripped or crushed by a current. In the case of leaves, internal components of the leaf such as cell walls and internal cell structures are usually lost, but cells are sometimes filled with mineral-rich water that solidifies to preserve these miniscule features.
Deducing Information from Fossils
Carbon film fossils often occur in tandem with compression fossils, and the combination sometimes raises the possibility of extracting more information than the general shape and morphology of the organism that produced the fossil. For example, analysis of fossilized feathers from the Cretaceous period revealed the structure of the melanosomes that formed the feather, which in turn opens up the possibility of determining the color of the original feather.