biostratigraphic

Biostratigraphy is a branch of stratigraphy that uses fossil evidence to establish relative ages of rock layers and to correlate strata across geographic regions. The central idea is the principle of faunal succession: fossil assemblages change through time in a recognizable and repeatable way, allowing rocks with the same fossil content to be assigned to the same interval of geological time. The core units are biozones, which are intervals of rock characterized by a particular fossil taxon or a particular assemblage of taxa. Biozones can be defined by first and last appearances of species (range zones), by continuous ranges of a single species (interval zones), or by co-occurring suites of fossils (assemblage zones). Index fossils are species with short, geographically widespread ranges that make effective markers for age correlations.

Biostratigraphic analysis typically relies on microfossils such as foraminifera and conodonts, as well as macrofossils like ammonites, brachiopods, and calcareous nannoplankton. The method supports correlations within and between sedimentary basins and provides a framework for constructing the geological time scale. When possible, biostratigraphy is integrated with other dating methods, including radiometric ages and magnetostratigraphy, to obtain numerical ages and to calibrate regional stratigraphy.

Limitations include diachronous appearances, reworking of fossils, ecological provincialism, and sampling biases. Despite these, biostratigraphy remains essential for petroleum geology, paleogeography, and paleoenvironmental reconstructions, enabling researchers to interpret Earth's history through fossil succession and stratigraphic continuity.