Despite their success, the entire group declined steadily near the end of the Cretaceous. For no clear reason, they were all gone long before the mass extinction event that infamously wiped out most dinosaurs on land.
To shine a light on this enigmatic decline, a group of multi-national European researchers studied the diversity of ichthyosaurs and their ecology. According to the study, ichthyosaurs were actually well-adapted to various niches and evolved different body shapes before their eventual demise. However, the rate of their evolution considerably slowed down for a prolonged time, especially when compared to their diversification back in the Triassic.
This failure to adapt, combined with the instability of the environment, likely caused the group’s decline. By the end of the Jurassic, another group of marine reptiles known as plesiosaurs started pushing them aside. In the Late Cretaceous, specifically the Cenomanian age, an extinction event finished off ichthyosaur sub-groups that specialized on soft-bodied preys and the generalists. Only six million years later, at the end of the Cenomanian age, a second extinction event ended the large predatory ichthyosaurs. This two-step extinction event spared only a single group called Platypterygius, which also died out shortly after.