Ammonoids and their biozonation across the Santonian-Campanian boundary in north-eastern Coahuila, Mexico

Fossil-rich shelf carbonates of Santonian-Campanian age are superbly exposed at Tepeyac near Jiménez, northern Coahuila, north-eastern Mexico. In this paper we describe 24 species of ammonoids assigned to 12 genera from this section. The 330 specimens were mostly recorded in situ and allow for a detailed biozonation of the section. We propose the first formal biozonation across the Santonian-Campanian boundary in the Gulf Coast region. The section yields abundant Parapuzosia assigned to P. (P.) seppenradensis and may thus correspond to one of the historical sites from which Böse and Cavins documented this giant ammonite ca. 90 years ago. The faunal assemblage provides evidence for repeated and increasing faunal exchange with Europe and southern latitudes, but also includes intervals of faunal cut-off and endemic evolution. The abundance of fossils, the excellent outcrop conditions, and biozonal completeness favor the use of the Tepeyac section as a potential reference section for the Santonian-Campanian boundary in the Americas. Christina Ifrim. Institut für Geowissenschaften, Ruprecht-Karls-Universität, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany current address: SNSB Jura-Museum, Burgstr. 19, 85072 Eichstätt, Germany, ifrim@snsb.de Wolfgang Stinnesbeck. Institut für Geowissenschaften, Ruprecht-Karls-Universität, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany. wolfgang.stinnesbeck@geow.uni-heidelberg.de alternative address: Universidad Autónoma de Coahuila, Centro en Geociencias Aplicadas, Nueva Rosita, México.