1. Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
2. Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, PO Box 37012, Washington, DC 20013-2012, USA
3. Section of Ecology, Behavior and Evolution, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0116, USA