Scientists have long puzzled over the reasons for the demise of the ichthyosaur, a marine reptile group that disappeared tens of millions of years before the mass extinction event that wiped out the dinosaurs. But a new study reveals "intense climate change" could be to blame.

The once numerous ichthyosaurs, or "sea dragons", are similar to today's dolphins, but disappeared before the end-Cretaceous extinction that made way for the era of mammals. The causes for the reptile's extinction have remained speculative and range from a lack of diversity to increased food competition. However, the research published Wednesday suggests ichthyosaur was unable to adapt quickly enough to global changes in climate around 100 million years ago.

"Ichthyosaurs were actually well diversified during the last chapter of their reign, with several species, body shapes and ecological niches present," said Valentin Fischer of the Universities of Liege and Oxford. "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."

Fundamental changes to oceans

Fossil remains showing the first amphibious ichthyosaur found in China

Around 90 million years ago Earth's climate was extreme: the poles were ice-free and sea levels were much higher than today. The oceans were also warmer and lacking in oxygen, causing huge changes to the marine ecosystem. The consequences of these changes likely led to a drastic reduction in the group's biodiversity ultimately leading to its extinction, according to the research published in journal #link:http://www.nature.com/ncomms/2016/160308/ncomms10825/full/ncomms10825.html:Nature Communications#.

"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," said Fischer.

The study supports growing evidence of a major global shift that "profoundly reorganized marine ecosystems" at the beginning of the Late Cretaceous, during with ichthyosaurs disappeared and various bony fishes and sharks emerged, according to the study.

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