Index fossils are fossils used to identify (and sometimes to define) geologic periods of a geological formation or a rock layer within it. The hypothesis is that the same fossilized species, regardless of location, is from the same geologic time period (or close to it). If a species persisted for a short geological time span, say a a few hundred thousand years, then the formation was ostensibly deposited during that time span. An index fossil is preferably a shorter lived species, since the index fossil will then pin point age more precisely. Because of this, more rapidly evolving life forms make better indexes. Obviously, fossils that are common, easily recognized and widely geographically dispersed are preferable for indexing. The chart above shows some valuable index fossils, including Quanternary age bivalve Pectin gibbus, the ammonite genus Perisphinctes marking the Jurassic, trilobite genus Paradoxides marking the Cambrian, and trilobite genus Bathyurus marking the Ordovician.