Why are index fossils useful for dating rocks

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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.

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.

This chemical change is an expression of diagenesis.

If this hole is later filled with other minerals, it is a cast.

An endocast, or internal mold, is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it.

This process can occur in very small spaces, such as within the cell wall of a plant cell.

Small scale permineralization can produce very detailed fossils.

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