Flying dinosaurs were not driven to extinction by birds as once thought but continued to evolve for millions of years, a British researcher says.

Katy Prentice of the University of Bristol says pterosaurs continued to evolve even after true birds appeared, becoming more and more specialized through their 160 million years on Earth.

"Usually, when a new group of animals or plants evolves, they quickly try out all the options. When we did this study, we thought pterosaurs would be the same," Prentice said. "Pterosaurs were the first flying animals -- they appeared on Earth 50 million years before Archaeopteryx, the first bird -- and they were good at what they did. But the amazing thing is that they didn't really begin to evolve until after the birds had appeared."

Prentice's study shows that pterosaurs remained conservative for 70 million years, and then started to experiment with all kinds of new modes of life as birds emerged and became successful.

The airborne dinosaurs were not pushed to extinction, as had been suggested, but responded to the new flyers by becoming larger and trying out new lifestyles.

"Pterosaurs were at the height of their success about 125 million years ago, just as the birds became really diverse too," Marcello Ruta, who supervised Prentice's study, said. "Our new numerical studies of all their physical features show they became three times as diverse in adaptations in the Early Cretaceous than they had been in the Jurassic, before Archaeopteryx and the birds appeared."

Pterosaurs disappeared 65 million years ago during the mass extinction that killed the dinosaurs but in their day they had been a fair match for the birds, and the two groups divided up aerial ecosystems between them and avoided conflict, the researchers said.