Are the bivalves from the Cenomanian stratotype representative for the Cenomanian stage?

A total of 178 bivalve species names are listed on the CD that accompanies the recently published ‘Stratotype Cénomanian’ volume (Morel 2015). This list is based on a preliminary revision of the literature. A comprehensive revision of the Cenomanian bivalve fauna of Sarthe, assessing type and original material, is lacking. Obviously, the listed bivalves are derived from a variety of different lithostratigraphic units and, respectively, facies as well as faunal associations. As a result, any statement on the biostratigraphy and palaeoecology of the bivalve fauna can only be provisional.

With a few exceptions, e.g. the Inoceramidae, bivalves are generally better suited for assessing palaeoecology and palaeoenvironments than for biostratigraphy. However, detailed studies at family or genus level have been carried out only for a few Cretaceous bivalve groups, but produced valuable results with regard to the spatial and chronostratigraphic ranges of individual species (e.g. Dhondt 1971, 1972, 1973a, b, 1976). The question whether the bivalves from the Cenomanian stratotype are representative for the Cenomanian thus actually can be split into two. (1) Are similar facies, with similar bivalve assemblages, widely distributed during the Cenomanian? (2) Are these bivalves biostratigraphically significant?

Revision of Cenomanian bivalve faunas from around the Mid-European Island (i.e. the Rhenish and Bohemian massifs) in Bavaria and Saxony (Germany; Schneider et al. 2013, Niebuhr et al. 2014) has revealed assemblages of rather different composition from those of the stratotype. Assemblages from the Danubian Cretaceous Group (Bavaria) are dominated by epifaunal and semi-infaunal taxa. Some assemblages from Saxony have more infauna, but still many burrowing bivalves are absent. Likewise, the bivalve fauna of the chalk lacks significant components of the fauna from Sarthe, e.g. the majority of the trigoniids and the rudists. On the other hand, the bivalve faunas from the Late Albian Blackdown Greensand and from Cenomanian strata in southern England, which are dominated by arenaceous facies, show great similarity.

We assess these differences based on faunal lists from our own studies and the literature. We try to specify which taxa are biostratigraphically significant. Furthermore, we try to identify bivalve groups or associations, which are facies-specific, and are missing in places due to a lack of suitable environments. Finally, considering the outstanding preservation of fossils from Sarthe, the issue of diagenetic bias is addressed.