No creature could have prepared for the disaster. When an asteroid struck the Earth 66 million years ago, the world was shaken up as earthquakes reverberated out from the impact site and falling debris from the collision heated the air to the equivalent of an oven on broil. Three years of “impact winter” followed; temperatures plummeted, and photosynthesis nearly ceased. And yet, through it all, many frogs survived what the mighty Tyrannosaurus and Triceratops could not.
The end-Cretaceous mass extinction wiped out roughly 75 percent of known fossil species virtually overnight. Not only did all the non-bird dinosaurs go extinct, but mass extinctions also decimated lizards, mammals and various other organisms. But frogs fared better than average. Despite the fires, darkened skies, acid rain, chilly temperatures and other impact consequences, frogs don’t seem to have experienced a mass extinction 66 million years ago. “The high resilience of frogs to the last mass extinction has been a mystery,” says Chinese Academy of Sciences biologist Anderson Feijó. But now Feijó and co-authors may have found an answer, which they published earlier this year in Climate Change Ecology. The frogs that survived the worst day on Earth may have not been too big, nor too small, but just the right size.
Frogs alive today come in various sizes. The smallest, announced in 2012, is only about a quarter of an inch long and can sit comfortably on a dime, while the appropriately named goliath frog can get to be more than a foot long. But during the end of the Cretaceous, Feijó and co-authors found, most frogs were medium-sized—about three inches from snout to rump—and were not at either extreme.
But why would being not too big or not too small make such a large difference? Study co-author Catharina Karlsson notes that medium-size frogs fared better during the end-Cretaceous extinction, and through time, than extreme-size frogs. Quirks of amphibian physiology might explain why medium-sized frogs are so resilient. Frogs need to stay moist to survive, for example. Very small frog species can absorb water fast, but they also dry out quicker in hotter climates, the researchers point out, whereas larger frogs can hold on to water better but take longer to rehydrate if they dry out. If a habitat’s climate rapidly changes, then frogs at either ends of the size scale face new challenges, whereas medium-size frogs hit a sweet spot where they are able to absorb water fast enough while also holding on to it.
Further study will be needed to test this proposal, says Royal Tyrrell Museum of Paleontology curator James Gardner, who was not involved in the research. “Our understanding of the amphibian record across the K-Pg boundary [or the rocks that record the before and after of when the asteroid hit] remains patchy,” he notes, for reasons including the limited localities where rocks from directly before and after the impact are found, the rarity of amphibian skeletons and the difficulty of identifying those fossils reliably. “It’s an intriguing idea,” Gardner notes, but we need many more fossils to identify whether body size—or some other factor—was decisive in why frogs survived the end-Cretaceous extinction so swimmingly.
The researchers didn’t just look at the catastrophe that struck 66 million years ago, though. They surveyed fossil frog records ranging from about 220 million years ago until the present to check the pattern. Overall, time and again, medium-size frogs seemed to survive better than their tiny or gargantuan relatives through shifting climates. “They make a compelling case that, for various biological reasons, medium-sized frogs living today seem to have the optimum body size for handling environmental stress,” Gardner says. And in a world where humans are quickly altering the global climate, these results may give conservationists clues as to which frog species might be most at risk.
A little more than 50 percent of very small or very large frog species will be in a “risk area” by 2100, almost four times as many at present, the researchers conclude. Even though many frog species have ways to cope with dry habitats—such as hibernation, cocoons made of mucus and adaptations in their skin—the climate is likely to shift too quickly to allow extreme-sized species to adapt. Their options are to move or go extinct, whereas medium-sized frogs likely have wider tolerances for what they can withstand before feeling the climatic squeeze.
“From local to global scales, as well as through evolutionary history, medium-sized frogs seem more resilient to climate changes and are the only ones found in more extreme environments,” Feijó says. Giants like Beelzebufo from ancient Madagascar weren’t the ones that made it, but species that might not have seemed all that remarkable at first glance did. Even though paleontology often focuses on and celebrates the extreme, sometimes the more average-looking creatures are the ones able to leap out of extinction’s jaws.
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