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Examples of index fossils<ref name="USGS_indexfossils">Index Fossils, from the US Geological Survey. Updated July 31, 1997.</ref>
Template:Paleontology Index fossils (also known as guide fossils or indicator fossils) are fossils used to define and identify geologic periods (or faunal stages). Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Another term, "zone fossil", is used when the fossil has all the characters stated above except wide geographical distribution; thus, they correlate the surrounding rock to a biozone rather than a specific time period.
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