As far back as 1856, it was apparent that accurate age determinations could be made by examining the vertical and horizontal ranges of two or more fossils (Opell 1856). Ranges are created from first and last appearance data of fossil successions (FAD and LAD). Ranges form the basis of zones, from which biohorizons are identified. From these horizons, biostratigraphers can observe and interpret the data within the scope of their study. Modern statistics continue to improve on the quantification and interpretation of the observed data.

Common dinoflagellate found in oil bearing sediment.