Prehistoric global warming wiped out the ichthyosaur, a toothy, dolphin-like reptile that disappeared from Earth's oceans tens of millions of years before the last dinosaurs.
Palaeontologists have long scratched their heads over the abrupt disappearance of these apex predators, sometimes called sea dragons, after an impressive 157-million-year deep-sea reign.
They were a successful family of marine reptiles - widespread and with many genetically diverse sub-species, which is generally a portender of future success.
Some scientists have thought that ichthyosaurs may have been beaten in a competition for food and living space by rival reptiles or fish, or that their prey itself had gone extinct.
On Tuesday, a European team of researchers said they had solved the mystery by comparing the fossil history of ichthyosaurs with geological records of climate change.
The creatures were wiped out in two phases, they said, finally disappearing at the beginning of the Late Cretaceous period, about 100 million years ago, when their physical evolution could not keep up with planetary change.
"At that time, the Earth's poles were essentially ice-free, and sea levels were much higher than today," said a statement by University of Oxford researchers who took part in the study.
Rising temperatures and sea levels likely affected food availability, migratory routes, competitor numbers and birthing places, said the team, "probably occurring in conjunction to drive ichthyosaurs to extinction".
The last of the land dinosaurs disappeared about 65 million years ago, followed by the rise of the mammals.
The research was published in the journal Nature Communications.