Maastrichtian shallow-water ammonites of northeastern Mexico

Abstract

In northeastern Mexico, extensive deltaic and prodeltaic sediment complexes developed during Maastrichtian times, known as the Difunta Group and Escondido Formation in Coahuila and the Northwest of Nuevo León, and the Cárdenas Formation in San Luis Potosí. These sediments contain abundant and diverse invertebrate assemblages, among them the last ammonites in the area, prior to their extinction. Sphenodiscus (S. lobatus, S. pleurisepta) and Coahuilites sheltoni are the dominant elements of this paucispecific fauna and clearly favored shallow water coastal environments. Ammonites other than sphenodiscids are rare and restricted to occasional Baculites ovatus and a single Pachydiscus (Pachydiscus) neubergicus. This early (and possibly early late) Maastrichtian assemblage differs notably from the coeval ammonite fauna of the distal prodeltaic and open marine Méndez Formation in eastern Nuevo León. The latter assemblage is far more diverse (23 species referred to 18 genera) and includes, in addition to typical Gulf of Mexico faunal elements, many Tethyan, cosmopolitan, and even southern high latitude Indopacific species previously unknown from Mexico. Faunal differences between the two nearby outcrop areas are thus clearly related to paleobathymetry and not to paleobiogeography, being Baculites ovatus the only conspecific element.