First appearance datum
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First appearance datum is a term used by geologists and paleontologists to designate the first (oldest) appearance of a species in the geologic record. In other words, FADs represent the geologically oldest fossil of a particular species that has so far been discovered.
FADs 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.
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