Activity: Types of Fossils

The processes of fossilization occur over a very long time. Fossils of various ages show different stages of these processes.

The shells of living bivalves are composed of calcium carbonate in either of two forms: calcite or aragonite. Holding the crystals of CaCO₃ together is organic material (kind of like “mortar” to the crystal “bricks”) that also gives the shell its color.

Young fossils (those below are from the Pliocene Epoch, approx. 5 million years old) look a lot like those of living species, only “chalkier.” They are often full of sand. Their shells are still composed of calcium carbonate, but the organic material in the shell has degraded slightly. Former color patterns sometimes show under “black light” (ultraviolet light) — try it!

Older fossils either have some shell still attached to a rock-like interior or themselves look more or less like rock (example: Fossil Oyster, Cretaceous Period, approx. 100 million years old).

All traces of the shell have completely disappeared on the oldest fossils. This might be a mold of the inside surface of the original bivalve (like the Devonian bivalve shown here, approximately 400 million years old) or of the outside surface of the original bivalve. Sometimes we have two pieces of rock that have split to reveal the mold of a bivalve on both sides — one of these is called “part” and the other is called “counterpart.”

The processes of fossilization include a long period of alteration, involving (in chronological order) recrystallization of the shell minerals, “permineralization” (during which other minerals seep into and fill voids within the shell), or complete replacement of the shell by minerals. Think of the difference between a newly cut tree trunk and slice of “petrified wood”; the latter has had all of the wood replaced by minerals, turning it to stone!

Black light bulbs are readily available at hardware and novelty stores (especially around Halloween!). Darken your classroom or use a large box to create a “mini darkroom.” Observe a young fossil under natural light and under black light. Those with smoother surfaces are more likely to show color patterns. Why wouldn’t color patterns be preserved in older fossils?

Discuss with your students what charactersCharacter:
A single attribute of an organism. are typically needed to identify a bivalve (such as hinge structure, pallial sinusPallial Sinus:
An embayment in the posterior part of the pallial line that indicates the attachment of siphonal retractor muscles and demarcates that part of the mantle cavity into which the siphons can retract in bivalves., exterior sculpture, periostracum). Are these available or unavailable in these different kinds of fossils? In other words, how are the fossil species similar to, or different from, the modern ones? What might that mean to a paleontologist?