First appearance datum
First appearance datum (FAD) is a term used by geologists and paleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest fossil discovered, to date, of a particular species.[1]
FADs are frequently used to designate segments in the geologic time scale; these are also called index fossils. 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.[2] However, because FADs are diachronous, establishing chronostratigraphic units based solely on FADs may be problematic.[1]
See also
References
- ^ a b c Landing, Ed; Geyer, Gerd; Brasier, Martin D.; Bowring, Samuel A. (August 2013). "Cambrian Evolutionary Radiation: Context, correlation, and chronostratigraphy—Overcoming deficiencies of the first appearance datum (FAD) concept". Earth-Science Reviews. 123: 135 – via Astrophysics Data System.
- ^ "International Commission on Stratigraphy". stratigraphy.org. Retrieved 2021-04-05.
External links
- Biostratigrahic Zonation
- Global Boundary Stratotype Sections and Points -- lists FADs associated with GSSPs
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