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 lost.