Insights from Ammonoid Biodiversity Patterns on the Cenomanian/Turonian Boundary Mass Extinction (Late Cretaceous)

The Cenomanian/Turonian boundary witnessed one of the ten most severe biotic crises of life's history with the extinction of approximately 26% of marine animal genera. It was also a period of high organic matter burial coupled with a high positive excursion in the carbon isotope record, which have been widely recognized to reflect anoxic conditions (Oceanic Anoxic Event 2). Hence, the Cenomanian/Turonian boundary has been considered as a typical example of global extinction caused by the spreading of anoxic waters. However, recent studies question the extent and origin of this mass extinction. This study analyses the biodiversity patterns of ammonites in order to better understand the relationships between biotic and environmental changes during the Cenomanian-Turonian interval. It is based on a recently compiled ammonoid database relying on a homogenized taxonomy and a revised biochronology. It focuses on three major areas: Europe, Tunisia, and the Western Interior. Changes of ammonoid diversity (species richness, origination/extinction, turnover, survivorship) highlight that the mass extinction of the Cenomanian/Turonian boundary appears to be restricted to Europe when considering ammonoids only. The three studied areas have clearly different patterns of ammonoid biodiversity during the late Cenomanian: only European data document an actual decrease of species richness, while species richness increases in Tunisia and remains comparatively stable in the Western Interior. Although there is extensive evidence for widespread anoxia, observed ammonoid patterns are better explained by the exceptional late Cenomanian high sea level and the concomitant climate changes.