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How a Mass Extinction Event Caused a Boom in Most of the Frog Species Alive Today

New research suggests that the mass extinction event that occurred 66 million years ago—when the dinosaurs were wiped out by an asteroid that collided with Earth—spurred something of an explosion of new frog species.
A frog climbing a glass wall.

There are three orders in the class Amphibia, collectively comprising nearly 7,700 species. Of those three, the order Anura, which includes frogs and toads, represents 6,787 of the world's known amphibian species, spread across 55 families.

It's believed that frogs diverged from the other orders of amphibians—Caudata, or salamanders, and Gymnophiona, or caecilians (snake-like amphibians with no limbs who live mostly underground, which is why most people haven't heard of them)—some time during the Palaeozoic Era or early Mesozoic Era, before the super-continent Pangaea broke up into the major land masses we are familiar with today.

Based on fossil records and the available genetic data, scientists have generally estimated that modern frog species first began to appear at a steady pace between 150 million and 66 million years ago. But new research published in the Proceedings of the National Academy of Sciences suggests that the timeframe was actually much tighter than that.

The mass extinction event that occurred 66 million years ago, during the Cretaceous Period at the end of the Mesozoic—when the dinosaurs were wiped out by an asteroid that collided with Earth—actually spurred something of an explosion of new frog species. Nearly 90 percent of the short-bodied, tailless amphibians roaming our planet right now first appeared in the years following the cataclysmic event that caused all dinosaurs but birds to go extinct, according to the study.

An international team of scientists with the University of Texas–Austin, the University of California–Berkeley, the Florida Museum of Natural History, and China's Sun Yat-Sen University analyzed genetic samples that they gathered from 156 frog species together with previously published data on 145 others. The team studied variations in 95 genes, whereas most studies look at just a dozen or less, in order to construct the most complete frog family ever created.

The researchers say they found evidence that there were three separate booms of new frog species that occurred on three different continents following the mass die-off of the dinosaurs. While most frogs alive at the time were also wiped out, the researchers theorize that, with so many other species having disappeared, there were suddenly an abundance of new ecological niches that the surviving frogs could fill. Moving into all of those different habitats essentially jumpstarted the evolutionary process and allowed for rapid frog diversification.

"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," David Hillis, a professor of integrative biology at UT–Austin and a co-author of the PNAS study, said in a statement. "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."

Hillis and team determined that three clades in particular—Hyloidea, Microhylidae, and Natatanura, which collectively comprise 88 percent of Anura species—underwent rapid diversification during a period known as the Cretaceous-Paleogene boundary, which marks the end of the Cretaceous Period and the start of the Paleogene Period, the first period of the Cenozoic Era. The researchers also discovered that frog families and subfamilies containing arboreal species originated "near or after" the Cretaceous-Paleogene boundary.

"These results suggest that the [Cretaceous-Paleogene] mass extinction may have triggered explosive radiations of frogs by creating new ecological opportunities," they write in the PNAS study.

The team's findings bolster those of a 2015 study by another team of scientists at UT–Austin who used computer simulations to show that mass extinctions can accelerate evolution as animals spread into and adapt to new habitats. "Even destruction can be leveraged for evolutionary creativity," Joel Lehman, a co-author of that study, said in a statement.

This story originally appeared at the website of global conservation news service Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.