1. R. A. Berner is in the Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA.
2. J. M. VandenBrooks is in the School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
3. P. D. Ward is in the Department of Biology, University of Washington, Seattle, WA 98195, USA.