1. Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan 5400, Argentina.
2. Department of Organismal Biology and Anatomy and Committee of Evolutionary Biology, University of Chicago, Chicago, IL 60637, USA.
3. Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina.
4. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.
5. Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA.
6. Department of Geology, University of California, Davis, CA 95616, USA.
7. Department of Geology, Miami University, Oxford, OH 45056, USA.