Graptolites in biostratigraphy: the primary tool for subdivision and correlation of Ordovician, Silurian, and Lower Devonian offshore marine successions

Planktic graptolites are the fossils of primary choice in the biostratigraphical subdivision and correlation of Ordovician and Silurian offshore marine sedimentary successions worldwide. Their widespread use in biostratigraphy continues until their ultimate disappearance from the fossil record in the Lower Devonian. The rapid evolution and dispersal, morphological diversity and complexity of graptolite rhabdosomes, combined with the high numerical abundance of preserved specimens and the wide geographical distribution of many species, have made graptolites the optimal biozone fossils in outer-shelf and deeper marine facies. Although the maximum utility of graptolite biostratigraphy and correlation is linked to the widespread facies of graptolitic black shales, less-detailed subdivision and correlation may occasionally be applied also in relatively shallow-marine, well-oxygenated settings dominated by limestones. Indeed, these fossil occurrences facilitate correlations between disparate biofacies. Assemblage biozones, widely utilized in graptolite biostratigraphy, have been further refined by the use of various types of interval biozones, defined by their characteristic assemblages, bounding biohorizons, and typical name-giving index taxa. Further improvement of biostratigraphical resolution and correlation is achieved by integration of graptolite data with the conodont and chitinozoan fossil record. Implementation of quantitative biostratigraphy, based on the lowest and highest occurrences of as many species as possible recorded from a large number of sections, has been made feasible through computer-assisted methods, such as constrained optimization (e.g., CONOP). Combining graptolite biostratigraphical data with radiometric dates has given rise to high-resolution time scales and chronostratigraphical correlation, but many more radiometric dates are required to calibrate the graptolite biozonation, especially in the Silurian.