Why are index fossils useful for dating rocks

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Replacement occurs when the shell, bone or other tissue is replaced with another mineral.

In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material.

Permineralization is a process of fossilization that occurs when an organism is buried.If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved.Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.to dinosaurs and trees, many meters long and weighing many tons.A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.

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