1. Geochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709
2. Instutito Mexicano del Petróleo, Eje Central Lazaro Cardenas No. 152, 07730 México D. F., México
3. Department of Geology and Geophysics, University of California, Berkeley, CA 94720, and Osservatorio Geologico, Col di Gioco, Apiro (MC), Italy
4. Department of Geology, University of California, Davis, CA 95616
5. Department of Geology, Florida International University, Tamiami Trail and Southwest 107th Avenue, Miami, FL 33199
6. Department of Sedimentary Geology, Free University, de Boelelaan 1085, 1081 HV Amsterdam, Netherlands
7. Department of Geophysics, Stanford University, Stanford, CA 94305