An ancient increase in oxygen levels helped North American dinosaurs to flourish, scientists say.
A new technique for measuring levels of the gas in early rocks shows the levels leapt by nearly a third in a couple of million years.
Scientists say this possibly set the scene for the Chindesaurus and Sauropods to expand into the tropics of North America and elsewhere.
They presented the findings at the Goldschmidt Geochemistry conference in Barcelona.
Researchers explained that they have developed a new technique for releasing tiny amounts of gas trapped in ancient carbonate minerals.
The gases are then channelled directly into a mass spectrometer, which measures their composition.
Scientists tested rocks from the Colorado Plateau and the Newark Basin that formed at the same time about 1000km apart on the supercontinent of Pangaea.
They found that that over a period of around three million years – which is very rapid in geological terms – the oxygen levels in the atmosphere jumped from around 15% to around 19%.
They use the 21% oxygen in today’s atmosphere as comparison.
Lead researcher, Professor Morgan Schaller of the Rensselaer Polytechnic Institute, New York, said: “We really don’t know what might have caused this increase, but we also see a drop in CO2 levels at that time.
“We expect that this change in oxygen concentration would have been global change, and in fact we found the change in samples which were 1000km apart.
“What is remarkable is that right at the oxygen peak we see the first dinosaurs appearing in the North American tropics, the Chindesaurus.
“The Sauropods followed soon afterwards. “
He added that while they cannot say whether it was a global development, the dinosaurs did not rise to ecological dominance in the tropics until after the End-Triassic extinction.
Prof Schaller added: “What we can say is that this shows that the changing environment 215 million years ago was right for their evolutionary diversification, but of course oxygen levels may not have been the only factor.”
Chindesaurus was an upright carnivorous dinosaur around 2m long and nearly 1m high.
It was found extensively in North America, with origins in the North American Tropics, it was a characteristic late Triassic dinosaur of the American Southwest.
The Sauropods, which appeared soon after Chindesaurus, were the largest animals ever to live on land.
Commenting on the research, Professor Mike Benton, University of Bristol, said: “The first dinosaurs were quite small, but higher oxygen levels in the atmosphere are often associated with a trend to larger size.
“This new result is interesting as the timing of oxygen rise and dinosaur appearance is good, although dinosaurs had become abundant in South America rather earlier, about 232 million years ago.”
At the time the gases were trapped, the Colorado Plateau and the Newark Basin were part of Pangaea, and located near the equator.
The rocks containing the oxygen and carbon dioxide were dated by measuring the radioactive decay of uranium which was found in the samples.