Geochronology: Age of Mexican ash with alleged `footprints' Renne, Paul R. ; Feinberg, Joshua M. ; Waters, Michael R. ; Arroyo-Cabrales, Joaquin ; Ochoa-Castillo, Patricia ; Perez-Campa, Mario ; Knight, Kim B. Abstract Publication: Nature Pub Date: December 2005 DOI: 10.1038/nature04425 Bibcode: 2005Natur.438E...7R