Throughout the majority of the Paleozoic Era, including the Ordovician Period, the most important fossil index taxa used by biostratigraphers belong to two main groups: graptolites and conodonts.
Both of these, which are now extinct, are believed to have lived in open ocean settings as plankton (graptolites) and nekton (conodonts).
In addition to biostratigraphic and sequence stratigraphic zonation, also diagramed is the relative scale provided by the composite standard section (CSS).
This represents a composite stratigraphic section for all of the Middle to Upper Ordovician rocks of central and eastern North America based on graphic correlations techniques (see Sweet, 1984 for more information).
The diagram clearly shows the relative position of key graptolite and conodont species within each representative time sub-unit of the Middle to Upper Ordovician.
In the rollover image, the key graptolite and conodont taxa present in the Trenton Limestone are in white.
Moreover as clearly shown above, the upper Mohawkian Series is subdivided into three stages (Ro.=Rocklandian, Ki.= Kirkfieldian, and Shermanian) based on biostratigraphic zonation.
The roll-over diagram above highlights key taxa useful for biostratigraphic studies of the Upper Mohawkian Series.
Note that in most cases, the boundaries of graptolite and conodont biozones do not correspond directly with time-rock series and stage boundaries.
This is a reflection of the practical difference between using the biostratigraphic distribution of fossils for chronology versus time-rock classification schemes which are based often on the lithologic expression of a rock succession at a given set of geographically constrained outcrop exposures.