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  Imprint & Compression of Fish Knightia eocaena Cenozoic; Paleocene; Eocene Green River Formation Kemmerer, Wyoming Complete Fish 12 cm long |
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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 or compressions. Fossil leaves revealed by splitting bedding planes may also reveal two fossils from a single specimen. The side with more organic material is called a compression. The side with little or no organic material is called an impression. 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). Some authors use the word impression to describe both the part and counterpart found in Mazon Creek nodules (Janssen, 1979, p. 24; Rich, Rich, Fenton & Fenton, 1996 pp. 4-6). However, the term impression is most often reserved for the counterpart or negative side (Tidwell, 1998, p. 27; Taylor, Taylor & Krings, 2009, p. 21; Schopf, 1975, p. 37). 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.
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. | |
 Leaf Imprint or Impression Paleocene Fort Union Formation Big Horn County, Montana 3 cm long x 2.5 cm wide |
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 Ginkgo adiantoides Paleocene Sentinel Butte Formation Morton Co., North Dakota |
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The
fossil Ginkgo leaf above represents a part and counterpart.
The fossil on the left is a compression and represents the
part (positive
side). The fossil on the right is an impression and represents
the counterpart
(negative side). |
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Bibliography |
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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. 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. 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. Tidwell, W.D. (1998). Common Fossil Plants of Western North America. [2nd Ed]. Washington: Smithsonian Institution Press. |
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