Ichthyosaurs became extinct in the Late Cretaceous about 90 million years ago – that’s roughly 30 million years before the mass extinction event that wiped out all non-avian dinosaurs at the end of the Cretaceous. According to a new Nature Communications study published this week, the extinction of these dolphin-shaped marine reptiles occurred in two phases driven by climate instability and reduced evolutionary rates.

Researchers have previously proposed that the earlier demise of ichthyosaurs – after their 157-million-year reign – was because of an increase in competition with other marine reptiles or a drop in their food resources, soft cephalopods in particular. But recent studies suggested that ichthyosaurs were richly diverse up to just a few million years before their extinction, so the overall cause of their disappearance is still a mystery.

To investigate, a team led by University of Oxford’s Valentin Fischer estimated ichthyosaur diversity over time by studying their evolutionary relationships. They assembled this so-called phylogenetic dataset using 88 characters from 36 ichthyosaur groups, including three-fourths of all known species from the Late Jurassic and Cretaceous. The team also estimated ecological diversity in terms of diet using a series of measurements on skulls and teeth, and they examined the wide range of differences in ichthyosaur body shapes and forms. They then correlated these results with environmental data ranging from ocean chemistry to sea level change. 

Ichthyosaurs were highly diverse throughout the Early Cretaceous, but they were evolving slowly. These findings support climate change as a main driver of changes in marine ecosystems, and they fit with the hypothesis that ichthyosaurs were outcompeted by other marine reptiles and fishes. Additionally, the team identified an earlier extinction event that occurred about 100 million years ago. The earlier event eliminated most of their ecological diversity, and a second event about 94 million years ago finished them off, Fischer explains to IFLScience. 

Ichthyosaurs faced an abrupt two-phase extinction linked to reduced evolutionary rates and environmental volatility. The beginning of the Late Cretaceous was a peculiar period of numerous climatic and oceanic disturbances, the authors write, with no polar ice, extremely high sea levels, unique sedimentation, strong anoxia, and high temperatures and carbon dioxide concentrations.