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Renne, Paul R.; Feinberg, Joshua M.; Waters, Michael R.; Arroyo-Cabrales, Joaquin; Ochoa-Castillo, Patricia; Perez-Campa, Mario; Knight, Kim B. (1 December 2005). "Age of Mexican ash with alleged 'footprints'". Nature. 438 (7068): E7–E8. doi:10.1038/nature04425. PMID16319838.