A massive asteroid strike that wiped out the dinosaurs millions of years ago created room for frogs to colonize the Earth, according to a study on how frogs became among the most diverse vertebrates in the world.
The asteroid that crashed into the planet about 66 million years might have been the end of the line for much of life on Earth, but for a few frogs it allowed a great leap forward.
Genetic analysis by an international team of researchers from the US and China indicates that 88% of existing frog species are descended from just three frog families that survived and prospered following the mass extinction event known as the Cretaceous-Paleogene (K–Pg) boundary, which wiped out an estimated three-quarters of animals including non-avian dinosaurs and most large mammals.
“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,” says study co-author David Blackburn, of the Florida Museum of Natural History. “This finding was totally unexpected.”
Quite a lot of frogs were also wiped out by the K–Pg extinction event or failed to adapt to its aftermath. 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.
Taking to the trees was particularly beneficial. As the planet’s vegetation recovered, flowering plants (or angiosperms) proliferated. “That’s when trees evolved to their full flowering,” says co-author David Wake, of the Museum of Vertebrate Zoology at the University of California, Berkeley. “Frogs started becoming arboreal.” Trees not only provided sanctuary from non-climbing predators but also ground cover via fallen leaves along with attracting food sources, such as insects.
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.
The analysis, published in the Proceedings of the National Academy of Sciences, was based on gene sequencing data from 301 frog species. Though this is but a fraction of the 6,775 frog species so far identified, it covered all 55 frog families and importantly included a greatly expanded sequencing of 156 frog species from 44 of those families. That sequencing, led by Yan-Jie Feng of Sun Yat-Sen University in Guangzhou, used 95 nuclear protein-coding genes, rather than the five, six or 12 of previous studies.
Peng Zhang, also of Sun Yat-Sen University, says the most exciting thing about the team’s study is that it shows frogs are such a strong animal group. “They survived from the mass extinction that completely erased dinosaurs and boomed back quickly,” he says. “However, frog species are declining nowadays because humans are destroying their habitats. Does that mean humans are making a huge extinction event even stronger than this one? We need to think about it.”