Climate change sealed the fate of ichthyosaurs, marine reptiles that ruled the oceans for 157 million years, suggests an analysis of fossils.
The dolphin-like animals died out some 30 million years before the mass dinosaur extinction at the end of the Cretaceous 66 million years ago.
Vertebrate palaeontologist Dr Valentin Fischer, who led the research, published today in Nature Communications, said that the extinction of ichthyosaurs, which were extremely well adapted to oceanic life, was a long-standing enigma.
A number of hypotheses have previously been proposed to explain the ancient marine reptiles disappearance, including increased competition from other marine reptiles and fish and the decline of their main food source, squid-like belemnites, said Dr Fischer of the University of Oxford.
Over time, these pressures would have reduced species diversity, allowing relatively minor events to tip them into extinction.
"These theories were at odds with the recent understanding of the ichthyosaur fossil record, which suggests they were actually quite diverse prior to their extinction," he said.