Global Boundary Stratotype Section and Point
For more recent periods, a Global Boundary Stratotype Section and Point (GSSP), largely based on research progress in geobiology and improved methods of fossil dating is used to define such boundaries. In contrast to GSSAs, GSSPs are based on important events and transitions within a particular stratigraphic section. In older sections, there is insufficient fossil record or well preserved sections to identify the key events necessary for a GSSP so GSSAs are defined based on fixed dates.