What is a Fossil and How is it Preserved?

(Lansing State Journal, August 20, 1997)

There are several different types of fossils. A fossil may be an actual bone or tooth from an animal that has been preserved throughout the centuries, or it may be a replica of original bony parts that were filled in with minerals from the Earth. A fossil may also be a cast, or impression of the original: such as a tiny sea creature's skeleton that has been carved into stone.

There are very specific conditions which are necessary for the Earth to produce a fossil, but the most important of these is that the specimen must be protected from the wind and the rain. There are several different ways for this to happen. First, a plant or animal may be buried in ash or in sediment (sediment is small pieces of rock and gravel and sand). Second, specimens such as insects may become trapped in liquid amber which hardens with time. Scientists estimate that amber ranges in age from 1.5 million to 300 million years old. Finally, the best potential source for fossils is near a place that is or once was volcanically active.

There are also several ways for a specimen to become fossilized. First, minerals may seep into the pores of a slowly decaying shell or bone and preserve it from further decay. Acids may dissolve a shell leaving an impression or cast of the shell in the rock it was pressed against. Finally, sometimes minerals will fill the mold carved into the rock forming a replica of the original shell. Often, these fossils are part of sides of slowly growing mountains. As these mountains shift, they cause pieces of rock and dirt from their sides to fall into rivers which carry many of the fossils to the ocean where they get covered by sediments. This process protects them from the bacteria that live on dry land so that they do not decay. Most fossils that have been recovered are found under bodies of water or in areas that used to be covered by water.

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