Dinosaur extinction set the stage for frog evolution

Turns out, the global catastrophe that delivered the coup de grace to the dinosaurs was actually good news for frogs as they took a leap forward to survival after the mass extinction. An international team of researchers presented a new tree of life for frogs that helps solve longstanding riddles about relationships and sheds light on the history and pace of frog evolution.

Unexpectedly, their analyses showed three major lineages of modern frogs, about 88 percent of living species, appeared simultaneously, evolving on the heels of the extinction event that marked the end of the Cretaceous Period and the beginning of the Paleogene 66 million years ago. Previous research suggested a more ancient origin of many of these modern frog groups.

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“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 co-author David Blackburn from the University of Florida. “This finding was totally unexpected.”

The speed at which frogs diversified after the asteroid or comet impact that triggered a massive die-off of most plant and animal life suggests that the survivors were probably filling up new niches on Earth, Blackburn said.

“We think there were massive alterations of ecosystems at that time, including widespread destruction of forests,” he said. “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.” Frogs rose to become one of the most diverse groups of vertebrates, with more than 6,700 described species. But sparse genetic data has hindered scientists from reliably tracing their evolutionary history and the links between frog families.

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Researchers from Sun Yat-Sen University, the University of Texas at Austin and the University of California, Berkeley sampled a core set of 95 nuclear genes from 156 frog species, combining this with previously published genetic data on an additional 145 species to produce the strongest-supported evolutionary tree, or phylogeny, to date. The tree represents all 55 known families of frogs and generates a new timeline of frog evolution.

The researchers then used fossil records to translate genetic differences between frog lineages into dates at which they likely diverged from one another. When the analyses pointed to a simultaneous evolution of the three major frog clades, Hyloidea, Microhylidae and Natatanura, the researchers initially eyed the finding with skepticism, said Peng Zhang, a corresponding study author and professor in the department of biochemistry and molecular biology at Sun Yat-Sen University in China.

“Nobody had seen this result before,” he said. “We redid the analysis using different parameter settings, but the result remained the same. I realized the signal was very strong in our data. What I saw could not be a false thing.” While the extinction event opened new opportunities for frogs, notably leading to the evolution of tree frogs worldwide, it snuffed out many frog lineages, particularly in North America, Blackburn said.

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The study also indicated that global frog distribution tracks the breakup of the supercontinents, beginning with Pangea about 200 million years ago and then, Gondwana, which split into South America and Africa. The data suggested that frogs likely used Antarctica, not yet encased in ice sheets, as a stepping stone from South America to Australia.

Blackburn is eager to use the new phylogeny as a roadmap for the fossil record, particularly for frogs that occurred in the Cretaceous. The study is published in the Proceedings of the National Academy of Sciences.

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