What happened to the huge underwater dinosaur known as the ichythosaur, which ruled the seas for 150 million years before vanishing about 90 million years ago?

Scientists are pointing to a culprit that is all too familiar to us now: climate change, according to a University of Oxford statement.

A giant asteroid smashing into the planet 65 million years ago is the main theory for how the dinosaurs died out en masse, but some of them disappeared earlier seemingly without explanation, including the ichthyosaur.

This dolphin-like giant lizard lived from the early Triassic period to the late Cretaceous period, but then disappeared mysteriously and scientists hadn’t been able to figure out why.

But now, researchers have published a study in the journal Nature Communications that indicates the climate change in the late Cretaceous resulted in great changes to marine ecosystems, which turned out to be too much to bear for the ichthyosaur.

Researchers went through museum collections and past literature on fossils of the ichythosaur, putting together a dataset and found that the dinosaur endured two extinction events — an initial event in the late Cretaceous that slashed their numbers, and one several million years later that ended their existence.

At first, scientists thought that losing a specific type of food called the belemnite, a squid-like creature, was the reason. But ichthyosaurs were quite diverse as a group according to the analysis, so they likely had a more diverse diet.

That resulted in more tests, which helped scientists narrow it down to climate change, as extreme variations in the climate in the late Cretaceous resulted in environmental change with fluctuating temperatures, sea levels, and oxygen and carbon dioxide. Scientists noticed that plankton and many cephalopods and reef animals were disappearing at the same time.

“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,” Dr Valentin Fischer, of the University of Liège, Belgium, and the University of Oxford, UK, said in the statement.