TrilobiteFossils are excellent indicators of geologic time and are the basis for the science of biostratigraphy. The word "biostratigraphy" indicates the use of fossils to define or describe layers of sedimentary rock and provides relative ages for the rock layers.

Every fossil species has a stratigraphic range based on the first and last appearances of that species in the rock record. The range of a single species or a combination of overlapping ranges of two or more species are used to define a biostratigraphic zone, which is a small subdivision of the geologic record.

Biostratigraphic zones are commonly defined by a single fossil group. Nevertheless, several fossil groups may be incorporated into a composite zone.

In the Illinois Basin, microfossils, in particular, Foraminifera, spores, and pollen, have been the most useful for providing relative age dates, but several invertebrate fossil groups, including trilobites, graptolites, echinoderms, cephalopods, brachiopods, and corals have been useful for selected time intervals.

Extensive paleontological collections at the Illinois State Geological Survey (ISGS) contain significant data for future biostratigraphic use. Currently the ISGS is converting microfossil data into a useable biostratigraphic database.

Biostratigraphy is the primary method for defining the subdivisions of geologic time and provides a time reference for interpretive studies of surface and subsurface rock sequences. Widely used by the oil industry in the Cenozoic, Mesozoic, and Paleozoic Eras to correlate sedimentary rock units and to determine the relative age of these units, biostratigraphic methods are important in the search for petroleum.

At the ISGS, biostratigraphy has been used to correlate Pennsylvanian coal units within the Illinois Basin and with adjacent basins. Biostratigraphy has also been used to correlate economic industrial mineral rock units in the Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian Systems in the Illinois and adjacent basins.

By knowing the age of the rock strata, geologists can predict the presence of an economic rock unit in areas that have not yet been drilled.