Fossil Types

Molds
Casts
Trace Fossils
Coprolites
False Fossils


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Fossil Types

Molds

The most common type of fossil, molds show an impression of either the external or the internal structure of an extinct organism. Modls are formed when acidic conditions slowly dissolve away the skeleton of fossil animals preserved in rock. After the original fossil is dissolved, a space forms where the organism used to be. In an external mold, an impression of the outside of the shell occurs. Internal molds are common in bivalves. In an internal mold, an impression of the inside of a shell forms when sediment fills the shell before the shell dissolves. Molds of clams may be washed up along a beach.