The Silurian Period

The Silurian Period (443.1–419.0 Ma) was a time of general convergence of continental plates, strong fluctuations in global sea level, and the early stages of colonization of land. The base of the Silurian System is defined at the level of the first appearance of the graptolite species Akidograptus ascensus at Dob’s Linn, Scotland. Silurian time can be finely resolved using integrated graptolite, conodont, and isotope biochemostratigraphy. The Silurian time scale is based on a CONOP9 composite of graptolite range data derived from 837 stratigraphic sections and 2651 graptolite taxa, with interpolated radioisotope dates, spanning the Ordovician into the Lower Devonian. There is a succession of at least seven globally recognizable positive carbon-isotope excursions, most of which are associated with important bioevents and environmental changes indicated by other geochemical proxies. These data show that the Silurian was a time of dramatic changes in climate, ocean chemistry, and biodiversity.