Biostratigraphy is a branch of stratigraphy that uses the fossil content of rock layers to establish relative ages and correlate strata across different geographic regions. Fossils, particularly those of species that were widespread but only existed for a short geological period (index fossils), are crucial for identifying and dating rock units. Biostratigraphy is essential in paleontology, geology, and petroleum exploration.
Reference: Gradstein, F. M., Ogg, J. G., Schmitz, M. D., & Ogg, G. M. (2012). The Geologic Time Scale 2012. Elsevier.