Index fossil
From Wikinfo
An index fossil is a fossil that is found in rocks of only one geologic period and is therefore used to assign a date to any rock in which it is found. Index fossils may be used to compare the age of strata in different regions, particularly those fossils that may be found over a large area. Marine organisms that are small and are well-preserved and easy to recover are commonly used as index fossils. In particular, conodonts may be identified by experts using light microscopy such that they can be used to index a given sample with good resolution.
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References
- Adapted from the Wikipedia article, "Index_fossil" http://en.wikipedia.org/wiki/Index_fossil, used under the GNU Free Documentation License
