First appearance datum

First appearance datum (FAD) is a term used by <a href="/facts/Geologist/vGc0tBHB">geologists</a> and <a href="/facts/Paleontology/60n5yldd">paleontologists</a> to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest <a href="/facts/Fossil/by7NzIA1">fossil</a> 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 <a href="/facts/Geologic_time_scale/vBAm0D8Q">geologic time scale</a>. A given FAD can be used to define a <a href="/facts/Global_Boundary_Stratotype_Section_and_Point/B6NDa75W">Global Boundary Stratotype Section and Point</a> (GSSP). For example, the beginning of the <a href="/facts/Tremadocian/d3lTcsDl">Tremadocian</a> Stage of the <a href="/facts/Ordovician/BjMrLIEA">Ordovician</a> Period is marked by the first appearance of the <a href="/facts/Conodont/wqA7V7cz">conodont</a> Iapetognathus fluctivagus in the geologic record. This occurs in <a href="/facts/Stratigraphy/o3J6Ij3R">bed</a> 23 of the rock formation known as the Green Point section, located in western <a href="/facts/Newfoundland_(island)/PdfPl9ca">Newfoundland</a>, as well as in geologically correlated <a href="/facts/Stratum/w5D4VAwa">strata</a> in many parts of the world. However, <a href="/facts/Diachronism/5YsNqbYJ">diachronous</a> FADs can be problematic for correlating chronostratigraphic units, particularly over longer distances. Use of other data, such as <a href="/facts/Radiometric_dating/bDFjdPqV">radiometric data</a>, may be ultimately necessary in order to establish more reliable correlations of chronostratigraphic units.

First appearance datum open-in-new

First appearance datum