The Virtual Petrified Wood Museum.  Dedicated to the Exhibition and Educational Study of Permineralized Plant Material
Home Button
Science Button
Students Button
Fossils Button
Time Button
Tectonics Button
Taxonomy Button
Anatomy Button
Links Button
Contact Button
Bibliography Button
Paleozoic Drop Down Menu
Mesozoic Drop Down Menu
Cenozoic Drop Down Menu
Imprints & Impressions
Picture of Fossil Fish Imprint

Picture of Fossil Fish Compression
Imprint & Compression of Fish
Knightia eocaena
Cenozoic; Paleocene; Eocene
Green River Formation
Kemmerer, Wyoming
Complete Fish 12 cm long
Imprints are really shallow external molds or voids left by animal or plant tissue. When the siltstone pictured above was split into two slabs the organic matter adhered to one side. The top picture represents an imprint in which bones and scales left a shallow external mold. The lower picture is a compression because it possess organic residue left from scales, original bone, and bone reinforced with calcite. Compressions retain original or chemically altered organic material while imprints do not. Fish and leaves are often found as imprints and compressions.

Fossil leaves discovered by splitting bedding planes may reveal two fossils from a single specimen. The side with more organic material is called a compression. The thin carbon layer on a plant compression is known as a phytoleim (Cleal & Thomas, 2009, p. 4). The phytoleim may retain original cuticle, which resists decay. The cuticle is the protective noncellular waxy covering of the epidermis. When removed and studied the cuticle may reveal the arrangement of epidermal cells and stomata, which can sometimes aid in species identification (Tidwell, 1998, p. 27). The side with little or no organic material is called an impression (Tidwell, 1998, p. 27; Taylor, Taylor & Krings, 2009, p. 21; Schopf, 1975, p. 37). Sometimes parts of a specimen are preserved as a compression while other parts are an impression. In this case the term adpression may be used. Adpressions form when the matrix of a fossil is soft and the phytoleim has fallen off in some places (Cleal & Thomas, 2009, p. 4).

Paleobotanists refer to the compression as the part (positive side) and the impression as the counterpart (negative side). The impression in this case shows all the surface details of the compression and represents a leaf imprint (Taylor, Taylor & Krings, 2009, p. 21). The counterparts of Green River fish that represent imprints can be used to make positive laytex casts for further study (Grande, 1984, pp. 119 & 120). If the layer of carbon is lost on the compression through weatherning or further diagenesis then it is also known as an impression (Cleal & Thomas, 2009, p. 4).
Many Mazon Creek nodules do not retain organic material and so both the part and counterpart are referred to as impressions (Janssen, 1979, p. 24; Rich, Rich, Fenton & Fenton, 1996 pp. 4-6). For more information on Mazon Creek fossils read our article on Replacement.

Compressions and impressions are the most common insect fossil. Insects with organic matter are called compressions, while those with no organic matter are referred to as impressions. For the paleontologist that studies insects, impressions are like casts and molds, which may preserve some relief like pleating on wings (Grimaldi & Engel, 2005, p. 43). This is important because wing venation can be used to identify an insect.

Lake deposits are the most common environment in which leaf and insect fossils form. Insects and leaves become trapped in sediments. As the sediments accumulate the insects and leaves may decompose leaving behind imprints. As the sediments compact and hardened into rock the imprints become impression fossils. If organic matter remains then a compression fossil has formed. Even a single specimen can represent both a compression and impression. Many insects found at Florissant, Colorado are found with their bodies fossilized as compressions and their wings as impressions. The body still retains the altered cuticle, while the wings do not have any organic matter remaining.

Picture of Leaf Imprint
Leaf Imprint or Impression
Fort Union Formation
Big Horn County, Montana
3 cm long x 2.5 cm wide

Ginkgo adiantoides
Sentinel Butte Formation
Morton Co., North Dakota

The fossil Ginkgo leaf above represents a part and counterpart. The fossil on the left formed as a compression and represents the part (positive side). The fossil on the right is an impression and represents the counterpart (negative side).


Cleal C.J. & Thomas, B.A. (2009). Introduction to Plant Fossils. United Kingdom: Cambridge University Press.

Grande, L. (1984). Paleontology of the Green River Formation, with a Review of the Fish Fauna [2nd edition]. The Geological Survey of Wyoming, Bulliten 63.

Grimaldi, D. & Engel, M.S., (2005). Evolution of the Insects. New York: Cambridge University Press.

Janssen, R.E. (1979). Leaves and Stems from Fossil Forests: A Handbook of the Paleobotanical Collections in the Illinois State Museum. Springfield, Illinois: Illinois State Museum.

Piccini S. (1997). Fossils of the Green River Formation. Italy: GEOFIN s.r.l.

Rich P.V., Rich T. H., Fenton, M.A., & Fenton, C.L. (1996). The Fossil Book: A Record of Prehistoric Life. Mineola, NY: Dover Publications, Inc.

Schopf, J.M. (1975). Modes of Fossil Preservation. Review of Palaeobotany and Palynology, vol 20: pp. 27-53.

Taylor, T.N., Taylor E.L. & Krings, M. (2009). Paleobotany: The Biology and Evolution of Fossil Plants [2nd Ed]. New York: Academic Press.

Tidwell, W.D. (1998). Common Fossil Plants of Western North America. [2nd Ed]. Washington: Smithsonian Institution Press.

©Copyright 2008 by Mike Viney| Website Use |