The geologic time scale is the scientific reconstruction and representation of Earth’s history from its rock record. The geologic time scale has its origins in the nineteenth-century recognition that through the application of the laws of superposition and fossil succession, a reproducible relative age order of rock strata could be assembled. The geologic time scale consists of two conceptual frameworks, one linked to the observable rock record of superposed sedimentary strata and their embedded physical, chemical, and fossil assemblage characters and the other tied to the theoretical construct of absolute time (Gradstein 2012). This rock versus time duality of the geologic time scale is manifested in the two distinct nomenclatures for chronostratigraphic and geochronologic age.
In the Phanerozoic and the latest Precambrian, chronostratigraphic units are constructed from fossiliferous strata. The appearance, absence, acme, or coexistence of fossil.
