Cenomanian (early Late Cretaceous) ammonoid faunas of western Europe. Part I: Biochronology (unitary associations) and diachronism of datums

Abstract

Biochronological data of Cenomanian ammonoid faunas of Western Europe are analyzed for three well-documented basins: the Vocontian Basin in southeastern France, the Mnster Basin in Germany, and the Anglo-Paris Basin. A taxonomically standardized data set is constructed for these three basins and includes more than one hundred species and is analyzed with the deterministic Unitary Association Method. Data for each basin are first processed separately, thus yielding three regional biochronological zonations. Then, the three regional sequences are processed together as a three- section data set for the construction of the inter-basin sequence. The latter comprises 24 unitary associations grouped in eight zones for the Cenomanian Stage. These results take into account species that co-occur in time and space (real coexistence), those that only co-occur in time (virtual coexistence), and those that do not co-occur in space nor in time (exclusion of chronological significance). At the paleogeographical scale of Western Europe, this new zonation is in good agreement with the widely used standard zonation, but its resolution is three times higher. This demonstrates the benefits of applying the Unitary Association Method, even to a taxonomic group which is traditionally acknowledged as one of the leading group in dating Mesozoic marine rocks. It also highlights the variable completeness and resolution of the faunal record through space and time, and reveals that a substantial number of the taxa (47%, inclusive of some indexes) have a diachronous distribution across these three Cenomanian basins. However, even with such an enhanced time resolution, no preferred orientation of the diachronous taxa emerges, suggesting that none of these basins seemed to have functioned as a paleobiogeographical source or sink. The absence of any preferred direction of the diachronism of the datums is most likely explained by two, non-mutually exclusive causes: either a random distribution of the incompleteness of the faunal record and/or a common paleobiogeographical history of the faunas in the three basins.