Frogs flourished after dinosaurs croaked

A massive asteroid strike that wiped out the dinosaurs millions of years ago created room for frogs to colonize the Earth, said a study Monday that shows how frogs became among the most diverse vertebrates in the world.

As many as 10 types of frogs are believed to have survived the mass extinction some 66 million years ago, which erased three-quarters of life on Earth, said the report in the Proceedings of the National Academy of Sciences.

Of these survivors, just three major kinds of frogs went on to diversify and populate the planet. Some 6,700 known frog species exist today.

Nearly nine in 10 -- 88 percent -- of modern frogs can trace their roots back to these three lineages of hardy ancestors.

"Frogs have been around for well over 200 million years, but this study shows it wasn't until the extinction of the dinosaurs that we had this burst of frog diversity that resulted in the vast majority of frogs we see today," said study co-author David Blackburn, associate curator of amphibians and reptiles at the Florida Museum of Natural History.

"This finding was totally unexpected."

Until now, scientists believed that most modern frog species emerged at a steady pace between 66 million and 150 million years ago.

But the latest research shows frogs burst onto the scene more like an "explosion," as the tiny amphibians swept into habitats left vacant by other creatures.

- Largest genetic study to date -

For the study, researchers in China and the United States compiled the largest set of frog genetic data ever evaluated.

Genetic samples were taken from 156 frog species and combined with previously published data on 145 more species.

Past studies looked at five to 12 genes, while the current one examined variations in 95 genes, offering a much more detailed look at how individual species relate to one another.

Researchers also studied fossil records to determine when different kinds of frogs likely diverged from one another.

Researchers found "evidence of not one but three explosions of new frog species, on different continents, and all concentrated in the aftermath of the mass die-off of most dinosaurs and many other species about 66 million years ago," said the report.

Two of the three surviving lineages -- Microhylidae and Natatanura -- came out of Africa. The third, Hyloidea, spread throughout what became South America.

"These frogs made it through on luck, perhaps because they were either underground or could stay underground for long periods of time," said co-author David Wake of the University of California.

"This certainly draws renewed attention to the positive aspects of mass extinctions: They provide ecological opportunity for new things."

Similar evolutionary events happened with birds, said co-author David Hillis, professor of integrative biology at University of Texas, Austin.

"We know that the mass extinction event wiped out most of the dinosaurs, except for a few bird species, which then exploded in diversity and became one of the dominant groups of land animals," he said.

"As we look at more and more groups of life, we see the same pattern, and that turns out to be the case for frogs as well."

- Finding new habitats -

In particular, the demise of dinosaurs and most birds created new niches for frogs in the late Cretaceous Period.

Some climbed into trees for shelter, leading to the evolution of the now ubiquitous tree frogs.

"We think there were massive alterations of ecosystems at that time, including widespread destruction of forests," said Blackburn.

"But frogs are pretty good at eking out a living in microhabitats, and as forests and tropical ecosystems rebounded, they quickly took advantage of those new ecological opportunities."

Some frogs also adapted by producing young without a tadpole stage -- today the standard for about half of all frog species.

Modern frogs face a host of threats to their survival, including habitat destruction, human encroachment and climate change.

"I think the most exciting thing about our study is that we show that frogs are such a strong animal group," said lead author Peng Zhang, a researcher at Sun Yat-sen University in Guangzhou, China.

"They survived from the mass extinction that completely erased dinosaurs and boomed back quickly."

source: AFP