1. Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA
2. All Souls College, University of Oxford, Oxford OX1 4AL, UK
3. Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK
4. Geology Department, Pomona College, Claremont, California 91711, USA
5. Peabody Museum of Natural History, Yale University, New Haven, Connecticut 06520, USA