First appearance datum (FAD) is a term used by
geologist
A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...
s and
paleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
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 geochro ...
. A given FAD can be used to define a
Global Boundary Stratotype Section and Point
A Global Boundary Stratotype Section and Point (GSSP), sometimes referred to as a golden spike, is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. ...
(GSSP).
For example, the beginning of the
Tremadocian Stage of the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
Period is marked by the first appearance of the
conodont
Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...
''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 is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region of Labrador, having a total size of . As of 2025 the population ...
, as well as in geologically correlated
strata
In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...
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
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...
References
External links
*
*
Historical geology
Biostratigraphy
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