The Tonian and Cryogenian periods (ca. 1000-635.5 Ma) witnessed important biological and climatic events, including diversification of eukaryotes, the rise of algae as primary producers, the origin of Metazoa, and a pair of Snowball Earth glaciations. The Tonian and Cryogenian will also be the next periods in the geological time scale to be formally defined. Time-calibrating this interval is essential for properly ordering and interpreting these events and establishing and testing hypotheses for paleoenvironmental change. Here, we briefly review the methods by which the Proterozoic time scale is dated and provide an up-to-date compilation of age constraints on key fossil first and last appearances, geological events, and horizons during the Tonian and Cryogenian periods. We also develop a new age model for a ca. 819-740 Ma composite section in Svalbard, which is unusually complete and contains a rich Tonian fossil archive. This model provides useful preliminary age estimates for the Tonian succession in Svalbard and distinct carbon isotope anomalies that can be globally correlated and used as an indirect dating tool.
Keywords: eukaryotic evolution; fossil record; geochronology; geological time scale; stratigraphy.
© 2018 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society and the Royal Society of Biology.