The extinction of ichthyosaurs is a facet of a major Cenomanian turnover in marine ecosystems

Untangling geosphere-biosphere interactions is crucial to understand the biodiversity and how it might evolve in a changing environment. But little is known about the possible impact of environmental changes on animals occupying the highest levels of the marine trophic chains. The Cretaceous period offers a fantastic opportunity to investigate such relationship, being characterized by a globally warm but changing climate and a diverse assemblage of aquatic reptiles that underwent profound modifications during this period, ultimately permitting the diversification of the animals dominating nowadays marine ecosystems. The origin, magnitude and nature of these turnovers are poorly understood and have rarely been investigated in a global canvas. Especially, how and why ichthyosaurs, the ‘fish-shaped’ marine reptiles, went extinct at the beginning of the Late Cretaceous has been a mystery for decades. Previous assessments of their diversity suggested ichthyosaurs were already on the decline since the end of the Jurassic; their final extinction was therefore regarded as anecdotal. Several theories have proposed unique biological drivers to this event, including a break in the food chain or competition with other marine vertebrates.
The reassessment of the taxonomy, phylogeny and paleoecology Cretaceous ichthyosaurs from Eurasia tells a much different story. Ichthyosaurs were ecologically and taxonomically diverse in several Eurasian ecosystems up to the latest Early Cretaceous. This revision also reveals that their extinction is diachronic, being staggered over four phases that span the entire Cenomanian stage. Detailed comparison with other groups suggests the multiphased extinction of ichthyosaurs is not an isolated event, as was previously assumed, but correlates with profound, multiphased turnovers among other marine animals, such as microplankton, rudists, ammonoids, and pythonomorphs. The diversity and contemporaneity of the biotic responses suggest worldwide physicochemical drivers for this profound reorganization of the marine ecosystems. The extinction of ichthyosaurs therefore appears as one of the facets of a much wider event that affected most of the marine ecosystems during the Cenomanian.