The standard stratigraphic nomenclature is a chronostratigraphic system based on palaeontological intervals of time defined by recognised fossil assemblages (biostratigraphy).
The aim of chronostratigraphy is to give a meaningful age date to these fossil assemblage intervals and interfaces.
Because igneous rocks occur at specific intervals in time and are essentially instantaneous on a geologic time scale, and because they contain mineral assemblages which may be dated more accurately and precisely by isotopic methods, the construction of a chronostratigraphic column relies heavily upon intrusive and extrusive igneous rocks.
This process requires a considerable degree of effort and checking of field relationships and age dates.
[3][4] Chronostratigraphic units are geological material, so it is correct to say that fossils of the species Tyrannosaurus rex have been found in the Upper Cretaceous Series.