FOSSILIZATION DEMONSTRATED WITH STYROFOAM COFFEE CUPS

Fossils provide direct or indirect evidence of ancient life in the rock record. Fossils can be divided into eight basic classes. Four classes of indirect evidence are represented by organic activity (plants or animals) and impressions of hard parts (external or internal molds). Four classes of direct evidence are represented by the preservation of unaltered parts (hard or soft) and altered parts (hard or soft). The most common fossil classes are altered hard parts (casts) followed by internal and external molds.

Many students, and some teachers, have trouble when studying fossils because the preservation process and its by-product, the fossil, are commingled. For example, the description pyritized brachiopod, identifies both the fossilization process and a taxonomic identification, but fails to specify the nature of the fossil, cast or mold. An additional problem occurs because three-dimensional internal molds are confused with casts. This happens because one's mental concept of an impression or mold is two-dimensional and not three-dimensional.

Multiple demonstration models utilizing styrofoam coffee cups, Plaster of Paris, food coloring and acetone can clarify these points of confusion. A styrofoam coffee cup simulates an unaltered hard part. By immersing a cup in Plaster of Paris, an external mold of the cup can be formed. Filling the inside of a cup to the rim with different colored Plaster of Paris forms an internal mold. By separating this system, a visual comparison of the nature of these fossils can be made: unaltered hard part and internal and external molds. If acetone is poured on an immersed and filled styrofoam cup, the cup dissolves leaving a void between the internal and external molds. If this model is kept warm, hot wax can be poured into the void to reconstitute a cast of the original cup. Ultimately, this model can be broken or cut to allow students to observe the relationships between external and internal cast surfaces and internal and external molds.