Geologic Processes of Evolution

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Carbonization
Petrifaction
Replacement
Recrystallization
Soft-Tissue Preservation
Mummification
Organic Traps
Tar
Peat Moss


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Fossilization Process

Recrystallization

Many animal shells are composed of the mineral aragonite, a form of calcium carbonate that breaks down over millions of years to form the more stable mineral calcite. This method of preservation, called recrystallization, destroys the microscopic details of the shell but does not change the overall shape. Snail shells and bivalve shells from the Jurassic Period (205 million to 138 million years before present) and later are still composed principally of aragonite. Most older shells that have been preserved have recrystallized to calcite.