Imprint & Compression
Cenozoic; Paleocene; Eocene
Green River Formation
Complete Fish 12 cm long
are really shallow external molds or voids left by
animal or plant tissue. When the siltstone pictured above
into two slabs the organic matter adhered to one side.
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
from scales, original
with calcite. Compressions retain original or
chemically altered organic material while imprints
do not. Fish
and leaves are often found as imprints and compressions.
leaves discovered by splitting bedding planes may
reveal two fossils from a single specimen. The side
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
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
parts are an impression. In this case the term adpression
may be used. Adpressions form when the matrix of
is soft and the phytoleim has fallen off in some
places (Cleal & Thomas, 2009, p. 4).
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
both the part
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.
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
can be used to identify an insect.
deposits are the most common environment in which leaf
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
If organic matter remains then a compression fossil has
formed. Even a single specimen can represent both a compression
are found with their bodies fossilized as
as impressions. The body still retains the altered
cuticle, while the wings do not have any organic matter
Imprint or Impression
Fort Union Formation
Big Horn County, Montana
3 cm long x 2.5 cm wide
Sentinel Butte Formation
Morton Co., North Dakota
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
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