Organisms buried in sediment may decay or dissolve away leaving
a cavity or mold. If the space is
an external cast
can be made. Molds and casts are three dimensional
and preserve the surface contours of the organism. A
mold preserves a negative imprint of the surface, while
of the organism (Taylor, Taylor & Krings, 2009, p.
a shell can be filled with minerals
and then dissolve away. The internal cast that remains
is termed a steinkern, which in German means "stone cast"
(Pothero, 1998, p. 7). Steinkerns are most often represented
molds of mollusks.
The pith cast of Calamites is
the most common plant steinkern. As a Calamites tree
the center of the stem (pith) became hollow, developing
into a tube-shaped air cavity. The pith cast preserves
an impression of the pith
cavities outside surface, which represents the inside
vascular and cortex tissue (Taylor, Taylor & Kring,
2009, p. 23).
Most molds and casts do not contain the actual remains
Shells, bone, and wood often form as molds or casts.
Some trace fossils (ichnofossils), such as tracks and burrows
can form as casts or molds. Tracks and burrows can provide
behavior and biomechanics of an organism while it was alive.
often encapsulate a fossil mold and cast. The siderite
(iron carbonate) nodules of Mazon Creek, Illinois preserve
aged animals and plants as molds and casts (Nudds & Selden,
2008, p. 120). It should be noted that some authors classify
the fossils in Mazon Creek nodules as impressions (Janssen,
1979, p. 24; Rich, Rich, Fenton & Fenton, 1996 pp. 4-6).
Limestone concretions in Ft. Collins, Colorado contain
and casts of
mollusks. As kids, my friends and I collected multiple Inoceramus clams
from a 3-foot diameter concretion. These fossils are found
as molds and casts with the cast filling the mold.
is important to realize that many fossil specimens
represent more than one mode of
preservation. For example, the
cast in a Mazon Creek nodule also represents fossilization
by authigenic mineralization, a type of replacement.
Subtle pH changes created by the decaying body of
carbonate to precipitate. Thus, the organism became
nucleation site for the formation of a siderite nodule.
of the shells found in the limestone nodules of Ft.
shell material, which represents recrystallization.
Aragonite (calcium carbonate) in the shell
recrystallized to calcite, a more stable form of calcium
usually think of casts and molds as exhibiting obvious
three-dimensional character. The thin leaves of plants
or the wings of insects can produce shallow casts
and molds. Shallow casts and molds may take the form
of imprints, or impressions, preserving the three
dimensional character of an organism. Impressions
are a common insect fossil. Even some of the
relief, like pleating on the wings can be preserved.
This is important because the veins on a wing can
to key an
insect to the family
D. & Engel, M.S., 2005, pp 42-45).
Imprints, impressions and many
trace fossils, such as, burrows, insect galleries,
and tracks, may represent types of
molds or casts
if they retain their three-dimensional character. Molds
and casts are important because they can faithfully replicate
the external form of an organism in a three-dimensional
fashion, giving the paleontologist information about surface
& Molds of Trilobites
Bohemia, Czech Republic
Trilobites 2 cm in length
D. & Engel, M.S., (2005). Evolution of the Insects.
New York: Cambridge University Press.
R.E. (1979). Leaves and Stems from Fossil Forests:
A Handbook of the Paleobotanical Collections in the Illinois
State Museum. Springfield, Illinois: Illinois State
Meyer, H.W., (2003). The Fossils of Florissant. Washington: Smithsonian
J.R. & Selden P.A. (2008). Fossil Ecosystems
of North America: A Guide to the Sites and Their Extraordinary
Biotas. Chicago: University of Chicago Press.
Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology.
New York: McGraw-Hill.
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.
T.N., Taylor E.L. & Krings, M. (2009). Paleobotany:
The Biology and Evolution of Fossil Plants [2nd
Ed]. New York: Academic Press.