'Fish' (Actinopterygii and Elasmobranchii) diversification patterns through deep time

Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility

Despite their profound adaptations to the aquatic realm and their apparent success throughout the Triassic and the Jurassic, ichthyosaurs became extinct roughly 30 million years before the end-Cretaceous mass extinction. Current hypotheses for this early demise involve relatively minor biotic events, but are at odds with recent understanding of the ichthyosaur fossil record. Here, we show that ichthyosaurs maintained high but diminishing richness and disparity throughout the Early Cretaceous. The last ichthyosaurs are characterized by reduced rates of origination and phenotypic evolution and their elevated extinction rates correlate with increased environmental volatility. In addition, we find that ichthyosaurs suffered from a profound Early Cenomanian extinction that reduced their ecological diversity, likely contributing to their final extinction at the end of the Cenomanian. Our results support a growing body of evidence revealing that global environmental change resulted in a major, temporally staggered turnover event that profoundly reorganized marine ecosystems during the Cenomanian.

The extinction of the ichthyosaurs had previously been attributed to increasing competition or to the loss of their main prey. Here, Fischer et al. analyse phylogenetic and ecological patterns of ichthyosaur diversification and extinction, and find that the decline of the group is more likely due to climatic volatility.