What is the name given to a fossil that best defines a rock layer's age?

A fossil that best defines the age of a rock layer is called an index fossil. Geologists use these fossils to identify and compare the relative ages of strata in different locations.

The most useful index fossils usually belonged to organisms that were widespread, abundant, and easy to recognize, but existed for a relatively short span of geological time. If the same distinctive fossil appears in rock layers separated by great distances, those layers may have formed during the same period.

Examples include certain ammonites, graptolites, trilobites, and microscopic plankton such as some foraminifera. Their usefulness varies by geological era and environment, since not every fossil occurs everywhere or survives equally well.

Index fossils establish relative age rather than an exact numerical date. To obtain an absolute age, scientists may use radiometric dating of suitable minerals or volcanic ash layers. Another common mix-up is treating any old fossil as an index fossil; age alone is not enough—the fossil’s distribution and limited time range matter too.