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==Additional sources of information==
* [https://en.wikipedia.org/wiki/System_(stratigraphy) System]
* [https://en.wikipedia.org/wiki/Geologic_time_scale:Terminology Geologic time scale]
* [http://www.geol.umd.edu/~tholtz/G102/102bios.htm Biostratigraphy and the geologic time scale]
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