European ammonoid diversity questions the spreading of anoxia as primary cause for the Cenomanian/Turonian (Late Cretaceous) mass extinction

Ammonoid diversity patterns show that the spreading of oceanic anoxia is not the initial and major kill mechanism for the Cenomanian/Turonian mass extinction as usually suggested. In the Anglo-Paris Basin and the Vocontian Basin, the drop of ammonoid species richness starts around the middle/late Cenomanian boundary, i.e. 0.75 myr before the occurrence of anoxic deep-water sediments. The stepwise extinction of first heteromorphs and then acanthoceratids is incompatible with the rise of the oxygen minimum zone. Moreover, shelf environments of these basins remained well oxygenated during the Cenomanian/Turonian boundary interval. Thus, we stress that other causative mechanisms initiated the ammonoid extinction even if anoxia subsequently participated in the demise of marine ecosystems.