"Sea dragons" from the time of dinosaurs were one of the earliest victims of climate change, researchers have claimed.
Shark-like ichthyosaurs disappeared tens of millions of years before the end-Cretaceous extinction some 65 million years ago that marked the end for dinosaurs and the beginning of the age of mammals.
Until now why they went extinct has puzzled paleontologists because their demise could not be linked to an obvious geological or geochemical event such as a large meteorite or massive volcanic eruption.
Increased competition with other marine predators and a decline in their principal source of food was also thought to have played a part in their demise.
But the University of Oxford led-study found they failed to evolve quickly enough to cope with massive climate change.
Before their extinction, ichthyosaurs were in fact highly diverse, both in terms of body shape and ecological role.
At the time the Earth's poles were essentially ice-free and sea levels were much higher than today.
But changes in sea surface temperatures and levels, coupled with their inability to adapt, saw them die out while numerous bony fishes and sharks evolved.
The study was able to reconstruct the evolution of the ichthyosaurs during the last 120 million years of their lifetime.
Assistant Professor Dr Valentin Fischer at the University of Liege and the University of Oxford said: 'We analysed the extinction of this crucial marine group thoroughly for the first time.
"We compared the diversity of ichthyosaurs with the geological record of global change, emphasising the dynamics of these datasets.
"Ichthyosaurs were actually well diversified during the last chapter of their reign, with several species, body shapes and ecological niches present.
"However, their evolution was much slower than earlier in their history.
"Additionally, they were seemingly negatively affected by the profound global changes going on during the Cretaceous, as their extinction rate correlates with environmental volatility.
"Although the rising temperatures and sea levels evidenced in rock records throughout the world may not directly have affected ichthyosaurs, related factors such as changes in food availability, migratory routes, competitors and birthing places are all potential drivers, probably occurring in conjunction to drive ichthyosaurs to extinction."
This new work supports a growing body of evidence suggesting that a major, global, change-driven turnover profoundly reorganised marine ecosystems at the beginning of the Late Cretaceous, giving rise to the highly peculiar and geologically brief Late Cretaceous marine world.
Ichthyosaurs disappeared in the course of this turnover, while numerous lineages of bony fishes and sharks evolved.
The extinction of ichthyosaurs thus appears to be one aspect of a larger event, the study published in the journal Nature Communications said.