Chapter 20 - The Ordovician Period

Abstract

The Ordovician Period (486.9–443.1Ma) encompasses two extraordinary biological events in the history of life on the Earth. The first, the “Great Ordovician Biodiversification Event,” is a great evolutionary radiation of marine life and the second is a catastrophic Late Ordovician extinction. Understanding the duration, rate, and magnitude of these events requires an increasingly precise time scale. The Ordovician time scale is based on the subdivision of a Lower Paleozoic CONOP9 composite graptolite range chart derived from 837 stratigraphic sections and 2651 graptolite taxa with interpolated radioisotopic dates. Thirty-seven new radioisotope dates are used in the scaling of the new Ordovician time scale. The base of the Ordovician Period is defined at the level of the first appearance of the conodont Iapetognathus fluctivagus at the Green Point Newfoundland section. Its top, the base of the Silurian Period, is set as the level of the first appearance of the graptolite Akidograptus ascensus at Dob’s Linn, Scotland. For the first time an independently time-scaled CONOP9 composite conodont range chart is presented to facilitate the application of the time scale to carbonate facies sections.