These assemblages have been used to define biostratigraphic zones. The base of each zone is defined by the first appearance of its characteristic species. The top of a zone is automatically defined by the base of the next zone. Two or more zones grouped together form a geologic stage. Stages are linked to form a geologic series.
Series combine to form a geologic system or period.
The zone is therefore the fundamental building block of the geological time scale. Consequently, discussions of geological history which ignore the timing provided by fossils and fossil zones, do so at their own peril. They had a high rate of evolution during the Cambrian, Ordovician and Silurian periods. The composition of the faunas changed many times so that many zones are recognized. At the same time, the above organisms were abundant and widespread.