THE ORDOVICIAN TIMESCALE, 2020 (Invited Presentation)

The Ordovician Period encompasses two extraordinary biological events in the history of life on Earth. The first, the “Great Ordovician Biodiversification Event” or GOBE, 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 timescale. As part of an eight-year cycle of revision for the entire geologic timescale, ages for the Ordovician and its subdivisions have been rescaled and updated for 2020. Compared to GTS 2012, the GTS 2020 Ordovician scale is more reliable, the result of a biostratigraphic composite based on substantial new data and almost double the number of radioisotope dates, a majority of which have considerably less analytical uncertainty than in GTS 2012. The new Ordovician timescale is based on the subdivision of a global Lower Paleozoic CONOP9 composite graptolite range chart derived from 837 stratigraphic sections and 2651 graptolite taxa with interpolated radioisotopic dates. Thirty-seven new dates are used in the scaling of the 2020 Ordovician timescale. Unchanged from 2012, 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. However, the Cambrian-Ordovician boundary age is now estimated at 486.9 ± 1.5 Ma and the Ordovician-Silurian boundary at 443.1 ± 0.9 Ma. The new best estimate for the duration of the Ordovician Period is 43.8 million years. The largest differences from the GTS 2012 timescale are a 1.5 Myr older Tremadocian base (C/O boundary), a 1.3 Myr older Dapingian base, and a 2.1 Myr older Darriwilian base. The estimated duration of the Hirnantian Stage has increased from 1.4 Myr to 2.2 Myr. Several new and expanded sections on chemostratigraphy, cyclostratigraphy, and understanding uncertainty in timescale construction are included in the chapter. Finally, for the first time an independently time scaled global CONOP9 composite conodont range chart is included to facilitate the application of the timescale to carbonate facies sections.