First appearance datum

First appearance datum is a term used by geologists and paleontologists to designate the first (geologically oldest) appearance of a species in the geologic record. First appearance datums are determined by identifying the geologically oldest fossil discovered, to date, of a particular species.

First appearance datums are frequently used to designate segments in the geologic time scale. For example, the beginning of the Tremadocian Stage of the Ordovician Period is marked by the first appearance (FAD) 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.

See also

• Biostratigraphy

References

• BIOSTRATIGRAPHIC ZONATION