The Cenomanian–Turonian boundary mass extinction (Late Cretaceous): New insights from ammonoid biodiversity patterns of Europe, Tunisia and the Western Interior (North America)

Major biological crises are attributed to a variety of factors, whose respective contributions and interactions are often difficult to disentangle. The Cenomanian–Turonian boundary was a period of high organic matter burial coupled with a high positive excursion in the carbon isotope record. This event has been widely recognized to reflect anoxic conditions (the Oceanic Anoxic Event 2) and is concomitant with one of the major mass extinctions of the Phanerozoic. It has been considered a typical example of global extinction caused by the spreading of anoxic waters. However, recent studies question this anoxia kill mechanism in Europe both from palaeontological and sedimentological arguments. Hence, this study analyzes the biodiversity patterns of ammonoids from three major areas (Europe, Tunisia, and the Western Interior) in order to better understand the relationships between ammonoid biodiversity patterns and abiotic factors during the Cenomanian–Turonian interval. The biodiversity patterns of ammonoids (species richness, origination/extinction, turnover, poly-cohort survivorship, and taxonomic distinctness) highlight that the mass extinction of the Cenomanian–Turonian boundary is restricted to Europe when considering only ammonoids. Only Europe documents an actual decrease of species richness during the late Cenomanian, which results mainly from decreasing origination. In Tunisia, where the onset of anoxic waters is synchronous with Europe, species richness increases during the late Cenomanian and reaches its highest values in the lower Turonian. The Western Interior records relatively high species richness during the late Cenomanian with only a single minor extinction event. Furthermore, major changes in biodiversity patterns of ammonoids occurred around the middle–upper Cenomanian boundary, i.e. ca. 0.75 myr before the onset of the OAE2. Although there is extensive evidence for widespread anoxia during the Cenomanian–Turonian boundary interval in deep sea environments, the biodiversity patterns of ammonoids in Europe, Tunisia, and the Western Interior rule out global anoxia as a direct causal mechanism for changes in ammonoid diversity. These biodiversity patterns also question the global scale character of the so-called Cenomanian–Turonian mass extinction. Observed biodiversity patterns of ammonoids strongly support the global warming of the late Cenomanian as evidenced by the northward migration of the Tethyan Realm. Changes in ammonoid diversity are compatible with the exceptional high sea level occurring at that time and with concomitant regional climate changes.