FADs are determined by identifying the geologically oldest fossil discovered, to date, of a particular species.
A given FAD can be used to define a Global Boundary Stratotype Section and Point (GSSP).
[1] For example, the beginning of the Tremadocian Stage of the Ordovician Period is marked by the first appearance of the conodont Iapetognathus fluctivagus in the geologic record.
This occurs in bed 23 of the rock formation known as the Green Point section, located in western Newfoundland, as well as in geologically correlated strata in many parts of the world.
[3] However, diachronous FADs can be problematic for correlating chronostratigraphic units, particularly over longer distances.