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When an organism is buried under many layers of sediment, pressure and heat increase during diagenesis and if the organism lacks a hard skeleton, it will only leave this thin film of carbon residue on rock surfaces.
The soft tissues of organisms are made largely of organic carbon compounds. Sometimes, fossils contain only carbon. Fossils usually form when sediment buries a dead organism. As sediment piles up, the organism's remains are subjected to pressure and heat. These conditions force gases and liquids from the body. A thin film of carbon residue is left, forming a silhouette of the original organism called a carbon film. Plant fossils often occur as a residue or film of carbon.
- Erickson, Jon, An Introduction to Fossils and Minerals: Seeking Clues to the Earth's Past , Facts on File, 2000, p. 95, ISBN 978-0816042364
- Milsome, Clare and Sue Rigby, Fossils at a Glance, Wiley-Blackwell; 2nd ed., 2009, p. 103, ISBN 978-1405193368
- Fossils from the Burgess Shale of the Stephen Formation