Cold spelt end of dinosaurs

That asteroid 65 million years ago in the Cretaceous period was probably the "final straw".

But Australian experts say up to half of all dinosaurs were gone by then, because the climate had got too cold for them to bear.

Fossil evidence from the Drumheller valley in Alberta, Canada, covering 7m years before the asteroid hit, shows that average temperatures dropped from 25°C to 15°C.

Many cold-blooded reptiles such as crocodiles, turtles and many large plant-eating dinosaurs died out as the climate cooled.

Oxygen isotope readings from fossils show the temperatures at which they formed, so scientists can track the climate change over time.

They found there was also a decrease in annual rainfall over the period where species died out.

But, David Eberth of the Royal Tyrell Museum of Palaeontology, who carried out the research, said it was unclear why dinosaurs were so dramatically affected.

Body temperature

Dr Angela Milner, associate keeper of palaeontology at the Natural History Museum, told BBC News Online: "A lot of people have suggested that climate change is the reason dinosaurs started to decline.

"But this research, using isotopes records, shows categorically there was quite a big temperature drop."

Dr Milner said cold-blooded dinosaurs would have been able to bear a colder climate for a short time because their size would have meant they could keep their body temperature constant.

But they would have been unable to cope for very long.

Dr Milner added: "In popular perception, dinosaurs died out because of the asteroid. But the actual evidence from the fossils doesn't support that in the way some people like to think.

"But that may well have been the final straw that broke the remaining camels' backs."

The research is featured in Chemistry and Industry.

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