How Amphibians Crossed Continents: New Evidence that Frogs Swam Across Oceans

Amphibians can be found all across the planet--on six continents and in almost every type of ecosystem. Yet exactly how amphibians managed to spread so far has long puzzled scientists. Now, researchers have taken a look at the more than 7,000 known species of amphibians to find out why they can be found all over the world.

Using DNA sequence data, the researchers constructed the first-of-its-kind comprehensive diagram of the geographic distribution of amphibians. In fact, the new diagram showed the movement of 3,309 species between 12 global ecoregions. In addition, the phylogeny includes about half of all extant amphibian species from every taxonomic group.

Scientists have long believed that the distribution of extant lineages of amphibians has been driven by two major processes: vicariance and dispersal. Vicariance occurs when a population is separated following a large-scale geophysical event. This allows species to essentially "hitch a ride" from one continent to another as they break up. In contrast, dispersal occurs when species move across land bridges or short distances across oceans.

In this case, the researchers found that continental movement likely can explain the majority of patterns of distribution of extant species of amphibians. Yet during the Cenozoic Era about 66 million years ago, dispersal also contributed. Not only that, but long-distance over water dispersal also likely occurred. For instance, one group of frogs found in both Australia and New Guinea that originated around 61 to 52 million years ago is deeply nested within a group of amphibians that exist only in South America. This species is about 120 million years too late to have walked to Australia.

"You wouldn't think that frogs would be able to swim all the way there, but that seems like one of the more likely explanations for how you could have such a young group nested within South America and have it somehow get to this other continent," said Alex Pyron, one of the researchers, in a news release.

The findings reveal a little bit more about amphibian distribution. Not only that, but they also provide evidence that long-distance oceanic dispersal was, in fact, possible.

The findings are published in the journal Systematic Biology.