Fossils: Window to the past
Casts & Molds
What are casts and molds?
Casts and molds are types of fossilization where the physical characteristics
of organisms are impressed onto rocks, especially coarse porous rocks such
as sandstones. Typically, the hard parts of an organism (shells of
mollusks, skeletal structures of coelenterates, bones and teeth of vertebrate,
chitinous exoskeleton of arthropods, trunks of trees, and many sphenophyte)
leave the best impressions. These hard structures are usually composed of calcium
carbonate, calcium phosphate, silica, or chitin, and do not decay as easily. The rigidity of the hard body parts
also allows the sediment to form around the organism. Soft body
parts decay too fast for impressions to form and are not rigid enough for a mold to set around.
The fossilization process
The fossilization process begins when the whole organism or hard body
part is trapped in sediments. Because most of these body parts are typically
composed of substances which are soluble in carbonated water, this entrapment
usually occurs in coarse and porous rock such as sandstones. The porous
nature of the rock enables the carbonated ground water to permeate and
dissolve the original tissue leaving a detailed mold of the organism.
Two types of molds result from this process: external and internal. An
external mold is created with the dissolution of the organic which then
leaves an empty cavity imprinted with the external details of the organism.
An internal mold may form with hollow structures. The "shell"
of the organism is filled with various inorganic materials such as sediment
or crystals. When the shell dissolves, it leaves an impression of the interior
surface of the shells (e.g., muscle scars) on the material.
A cast of the organism can then be made using the two types of molds.
Natural casts are formed when minerals are deposited within the mold.
Casts can also be synthetically created when the molds are filled or covered
with synthetic material, such as latex or plaster of paris, to generate a
replica of the organism. In this manner, cast and mold fossilization
enables us to "recreate" the structure of the organism. However
in the creation of the cast, some details of the skeletal structures are