First appearance datum (FAD) is a term used bygeologists andpaleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldestfossil discovered, to date, of a particular species.[1] A related term is last appearance datum (LAD), the last appearance of a species in the geologic record.[2]
FADs are frequently used to designate segments in thegeologic time scale. A given FAD can be used to define aGlobal Boundary Stratotype Section and Point (GSSP).[1] For example, the beginning of theTremadocian Stage of theOrdovician Period is marked by the first appearance of theconodontIapetognathus fluctivagus in the geologic record. This occurs inbed 23 of the rock formation known as the Green Point section, located in westernNewfoundland, as well as in geologically correlatedstrata in many parts of the world.[3] However,diachronous FADs can be problematic for correlating chronostratigraphic units, particularly over longer distances.[1][4] Use of other data, such asradiometric data, may be ultimately necessary in order to establish more reliable correlations of chronostratigraphic units.[5]
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