Frogs Are So Vulnerable, How Did They Survive When T. Rex Didn’t?

Amphibians have been with us since the Devonian period, about 370 million years ago, when they evolved from fish ambitious enough to have a go at making it on land. Today, most of their members belong to the order Anura (frogs and toads), which is thought to have evolved in the Early Triassic, about the same time as the dinosaurs.

That means amphibians have survived three of the “Big Five” mass extinctions, as well as the Capitanian event, which some paleontologists think should make a big six, and many smaller crises for life. The Anura have made it through two mass extinctions.

Yet you wouldn’t think that to look at the world today. We don’t know how many species of frogs and toads have gone extinct in the last century, but it’s certainly a lot. The chytrid fungus alone is thought to have wiped out dozens of species, with hundreds more barely hanging on. The situation is so bad that ecologists are building frog saunas to slow the damage. Meanwhile, frogs disappear elsewhere because the pH of the water has changed a little too much, or the temperature is too hot.

It’s not the profile you would expect of an animal that made it through the wall of heat, clouds of ash, and years of famine that followed the asteroid impact. Nor is it consistent with getting through the Triassic-Jurassic extinction event, most likely caused by supervolcanoes, when almost as large a proportion of species died as at the end of the Cretaceous.

We may never know how Kermit made it, but that doesn’t mean palaeontologists can’t have some ideas.

Tougher than dinosaurs

They like a catastrophe

Melanie During

Melanie During, an expert on the Cretaceous-Tertiary extinction event, told IFLScience that one overlooked fact about frogs is that many of them are “pioneering species,” among the first back after a big change.

During gave the example of construction sites, which she said are “bound to attract frogs you never see anywhere else. Often it will stop building,” while the frogs might be moved. “They like a catastrophe,” During added. “They pick a damaged area on purpose because they can’t compete, so as soon as competition arrives they disappear.” 

This doesn’t apply to all the members of Anura; Australians have learned just how well cane toads can compete. Nevertheless, During said, “It wouldn’t surprise me if frogs were the first to rebound” after the asteroid. 

That still leaves the question of how frog ancestors made it through the worst of the catastrophe. While During doesn’t know, she suggests cave-dwelling species might have survived the initial infrared blast, which she says was the most devastating part of the event, at least for thousands of kilometers either side of the Atlantic Basin.

Creatures that live on insects probably also found it easier than most to get food during the period when ash choked the skies and nothing grew. 

On the other hand, the impact also devastated the ozone layer, letting in excess ultraviolet (UV) radiation for decades. That part would have been particularly tough for frogs. “They produce blood in their livers, not their bone marrow, so they are way more vulnerable to UV,” During told IFLScience. Some frogs today live largely underground, so During speculates it was those with a similar lifestyle that survived the ozone-depleted years. They would have found “an enormous number of niches afterwards,” she added.

To gain a better idea of what happened, we would need to find fossils of frogs from soon after the disaster, whose features might reveal something of their ancestors during the crisis. Something similar has occurred with birds in the form of evidence that the survivors were seed-eating species.

“The main problem is fossilization at the time,” During told IFLScience. “North America is severely overstudied and yet I have tripped over T-rex, hadrosaur bones etc,” from before the impact. “As soon as you cross the boundary you get mammalian teeth or jaws or whatever, but everything is very fragmentary.” Finding delicate frog bones in that context may be too much to hope for.

It is thought the impact was less devastating in the Southern Hemisphere because, as During herself proved, the impact occurred in the northern spring. Distance may also have protected Australia and Antarctica. That would make any Australian fossils from the start of the Paleogene period particularly precious, but unfortunately no rocks of the right age are known. During also noted the few species present shortly after the impact were also less likely to become fossils. “Don’t underestimate the hunger of survival,” she warned.

Extinctions of yore

Surprisingly, we may know more about how amphibians survived the Permian-Triassic mass extinction, known as the Great Dying, than the more recent event. The first frogs appear in the fossil record not too long after this, the largest extinction by proportion of species lost in the planet’s history.

A study published earlier this year revealed the amphibians of the early Triassic were generalist feeders, highly adaptable to changes in prey as conditions shifted, and also probably needing less food than many competitors.

The reasons for the Great Dying are still debated, but the authors of the study show the amphibian survivors favored freshwater habitats, which were probably more stable than conditions on land.

“One of the great mysteries has been the survival and flourishing of a major group of amphibians called the temnospondyls,” lead author Aamir Mehmood from the University of Bristol said in a statement. “These were predatory animals that fed on fishes and other prey, but were primarily linked to the water, just like modern amphibians such as frogs and salamanders. We know that climates then were hot, and especially so after the extinction event. How could these water-loving animals have been so successful?”

Many animals couldn’t live in the tropics in that heat, but Mehmood and colleagues found evidence that temnospondyls from one hemisphere migrated to the other, demonstrating a capacity to cross the hottest regions. The authors think they may have done this during short, relatively cool periods, but these would still have been hot compared to most of Earth’s history.

Temnospondyls didn’t do so well as dinosaurs, and the ancestors of mammals diversified through the Triassic, but many biologists think frogs are their descendants, although this is still disputed.