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.
A related term is last appearance datum (LAD), the last appearance of a species in the geologic record.
FADs are frequently used to designate segments in the
geologic time scale
The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...
. A given FAD can be used to define a
Global Boundary Stratotype Section and Point (GSSP).
For example, the beginning of the
Tremadocian Stage of the
Ordovician Period is marked by the first appearance of the
conodont
Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...
''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
Newfoundland and Labrador (; french: Terre-Neuve-et-Labrador; frequently abbreviated as NL) is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region ...
, as well as in geologically correlated
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
in many parts of the world. However,
diachronous FADs can be problematic for correlating chronostratigraphic units, particularly over longer distances.
Use of other data, such as
radiometric data, may be ultimately necessary in order to establish more reliable correlations of chronostratigraphic units.
See also
*
Biostratigraphy
References
External links
Biostratigrahic ZonationGlobal Boundary Stratotype Sections and Points
-- lists FADs associated with GSSPs
Historical geology
Biostratigraphy
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