Inoceramids and biozonation across the Turonian – Coniacian boundary (Upper Cretaceous) at El Rosario, Coahuila, northeastern Mexico

Abstract

The Rosario section in northern Coahuila, northeastern Mexico, contains a complete record of sediment across the Turonian–Coniacian boundary. Here we describe the inoceramids and the biozonation based on these bivalves. Inoceramus longealatus, Mytiloides herbichi, M. incertus, M. scupini, Didymotis costatus, Cremnoceramus waltersdorfensis waltersdorfensis, C. waltersdorfensis hannovrensis, C. deformis erectus, and C. crassus inconstans, were identified at El Rosario. The uppermost Turonian Mytiloides scupini and C. waltersdorfensis w. zones and the lower Coniacian C. crassus inconstans zones are well represented and are much more expanded than in sections of the US Western Interior and Europe. The index for the base of the Coniacian, C. deformis erectus, is also present, although few data exist to date from the critical Turon ian–Coniacian boundary level. Ammonites are rare, mostly endemic and provide a much lower resolution across the Turonian–Coniacian boundary than inoceramids. We suggest that various acmes observed across the Turonian–Coniacian boundary are driven by local or regional rather than by global causes. The Rosario section provides an expanded sediment and complete inoceramid record across the Turonian–Coniacian boundary.