Biological events and the Precambrian time scale
For stratigraphic purposes, time scales are constructed on the basis of either geophysical and geochemical, or biological processes and events. Biological evolutionary processes provide the main markers linking the Phanerozoic time scale with fossiliferous rock sequences. Progress in the study of Precambrian fossils makes it necessary to assess present and future possibilities of their use as time markers and correlation indicators for the Precambrian. The low grade of diversification attained by organisms during the first one-half or two-thirds of Precambrian time may reduce the possibilities of biostratigraphic dating of early and middle Precambrian sediments to insignificance, but enough is known about late Precambrian plants and animals to suggest that careful investigation of their stratigraphic distribution and potential use as time markers will repay the effort.