Science 25 July 2008:
Vol. 321. no. 5888, pp. 550 - 554
DOI: 10.1126/science.1155814 

Reports
Did Cooling Oceans Trigger Ordovician Biodiversification? Evidence from Conodont Thermometry
Julie A. Trotter,1,2* Ian S. Williams,1 Christopher R. Barnes,3 Christophe Lécuyer,4 Robert S. Nicoll5

The Ordovician Period, long considered a supergreenhouse state, saw one of the greatest radiations of life in Earth's history. Previous temperature estimates of up to 70°C have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time. We present a very different global climate record determined by ion microprobe oxygen isotope analyses of Early Ordovician–Silurian conodonts. This record shows a steady cooling trend through the Early Ordovician reaching modern equatorial temperatures that were sustained throughout the Middle and Late Ordovician. This favorable climate regime implies not only that the oxygen isotopic composition of Ordovician seawater was similar to that of today, but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period.

1 Research School of Earth Sciences, The Australian National University, Mills Road, Canberra ACT 0200, Australia.
2 CSIRO Petroleum Resources, North Ryde, NSW 1670, Australia.
3 School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 2Y2, Canada.
4 Laboratoire CNRS UMR 5125 Paléoenvironnements and Paléobiosphère, Université Claude Bernard Lyon 1, Campus de la Doua, F-69622 Villeurbanne, France.
5 Department of Earth and Marine Sciences, Australian National University, Canberra ACT 0200, Australia.

* To whom correspondence should be addressed. E-mail: julie.trotter@anu.edu.au