The results, published online today in the journal Proceedings of the National Academy of Sciences, are a surprise, because previous studies of frog evolution pinpointed the blossoming of the main frog lineages today to about 35 million years earlier, in the middle of the Mesozoic era. Today’s frogs, comprising more than 6,700 known species, as well as many other animal and plant species are under severe stress around the world because of habitat destruction, human population explosion and climate change, possibly heralding a new period of mass extinction. "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," study co-author David Blackburn, associate curator of amphibians and reptiles at the Florida Museum of Natural History on the University of Florida campus, said in a statement. 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. The team 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. Perhaps 10 frog groups survived the mass extinction, suggest Blackburn and his colleagues, but only three lineages – the Hyloidea, Microhylidae, and Natatanura – flourished in changed climates and habitats, and thrived by occupying vacant ecological niches. Another important adaptation, Wake says, was to produce offspring without a tadpole stage, known as direct development, which is how about half of all frog species now reproduce: “The majority of the frogs that thrive now are thriving because of direct development of eggs in terrestrial situations.”

While the ability to stay underground for long periods probably enabled frogs to survive the K-Pa event, Wake credits the combination of direct development and use of arboreal habitat with “a great deal” of frog prosperity subsequently, with frog numbers exploding to become one of the most diverse and widely dispersed vertebrate groups on the planet. In an effort to learn more about frog evolution, researchers David Blackburn and Peng Zhang put together a new phylogenetic tree of these amphibians (phylogenetic trees are like family trees, but at the species level, and driven by genetic data) . “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. 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.
Researchers discover that the diversity of frogs is a direct result of the extinction of the dinosaurs.