The Cenomanian marine reorganization

The middle Cretaceous is usually associated with numerous climatic and oceanic perturbations, and a minor to intermediate extinction event at the Cenomanian-Turonian (CTB) boundary. Amongst marine vertebrate palaeontologists, the CTB is mostly known as the extinction of the last ichthyosaurs (Reptilia), after a long period of decline (e.g. Bardet, 1992; Bardet, 1994; Sander, 2000; Lingham-Soliar, 2003). However, new data from France, Canada and Russia indicates that the diversity of late Early Cretaceous ichthyosaurs was far higher than previously thought, both in terms of taxonomical and ecological diversity, but the Cenomanian ichthyosaur diversity remains extremely low. This suggests that the ichthyosaur extinction is far more severe than initially thought and took place during the whole duration of the Cenomanian, in a diachronic fashion. Additionally, a compilation of the current data on the diversity of other marine vertebrates groups shows that the Cenomanian is a peculiar stage within the Cretaceous, with the radiation burst of marine squamates, such as dolichosaurs and mosasauroids (Bardet et al., 2007; Bardet et al., 2008), as well as chondrichtyans, polycotylid plesiosaurs and teleost fishes (e.g. Cumbaa et al., 2010; Schultze et al., 2010). This profound reorganisation of the marine ecosystems (coeval with the onset of the “Chalk sea”) was probably driven by external, physical factors given the diversity of biotic responses. The numerous potential causes for that major reorganisation renders the identification of its precise mechanisms difficult.