The extinction of the iconic dolphin-like marine reptiles known as ichthyosaurs was driven by climatic instability and reduced evolutionary rates, suggests a new study in Nature Communications. Ichthyosaurs became extinct roughly 90 million years ago, in the Late Cretaceous period, about 28 million years before the end Cretaceous mass extinction which wiped out the dinosaurs and made way for mammals to dominate food webs.

Although increased competition with other marine reptiles or a drop in food resources have been proposed as possible causes of this early demise, recent studies have suggested that ichthyosaurs were diverse up to a few million years before their extinction, and the overall cause of their disappearance has therefore remained unclear up to now.

Valentin Fischer and colleagues use phylogenetic-based methods to estimate ichthyosaur diversity over time and correlate their results with environmental data, such as ocean chemistry and sea level change. They find that ichthyosaurs were highly diverse throughout the Early Cretaceous and that the last ichthyosaurs are characterized by reduced evolutionary rates. The authors’ data support climate changes as a main driver of marine ecosystem changes and are not consistent with the hypothesis that ichthyosaurs were outcompeted by other marine reptiles and fishes.

In addition, the study identifies an earlier extinction event (about 100 million years ago) as having reduced ichthyosaur diversity. The authors therefore propose that ichthyosaurs suffered from a two-phase demise.