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Hunting for Fossils
How are fossil molds and casts made?
An LSU Museum of Natural Science presentation to accompany the activity
Preserved- Fossils that are unaltered and the original organism stays relatively intact. Soft as well as hard parts can be preserved. (Insect preserved in hardened tree sap- amber)
Replacement- An organism’s hard parts are replaced with minerals like calcite, silica, pyrite, or iron
Permineralization (petrification)- Fossil-type most people recognize. After the original hard parts are buried, water that seeps through the sediment passes through the bone. The water often dissolves the bone and replaces it with mineral deposits ultimately changing it into stone.
Carbonization- Only carbon remains of the specimen- other elements such as hydrogen and nitrogen are removed.
Impression- These fossils are made by organisms that are left in sediments.
Impression fossils are divided into 3 categories:
These are impressions of thin organisms such as feathers, leaves, or fish that have fallen into soft sediment before it hardened.
As the organism decomposes it leaves a carbon ‘film’ of the organisms on the rock.
External mold- imprints of the outside of a fossil. For example, if the original fossil shell was convex, the external mold will be concave.
Internal mold- imprints of the inside of a fossil. They are produced when an organism such as a shell, is filled with sediment that becomes cemented and then the shell dissolves away.
Tracks or marks left by ancient organisms that have been preserved in sedimentary rocks. They show that the organisms were in the area. They include:
Baylor, Byrd. (1980). If You are a Hunter of Fossils. Illustrated by Peter Parnall. New York: Charles Scribner's Sons. (ISBN 0-684-16419-1).
Goldish, Meish. (1989). What is a Fossil? Illustrated by Ivan Dieruf. Milwaukee: Raintree Publishers. (ISBN 0-8172-3535-3).