1. Department of Environmental Science and Policy, George Mason University, Fairfax, Virginia 22030, U.S.A.
2. Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, U.S.A
3. Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, Fairfax, Virginia 22030, U.S.A