There are nearly 7,000 known species of amphibians, but for the first time scientists have attempted to figure out how some of these types managed to cross continents throughout time.

Using DNA sequence data, researchers at the George Washington University pieced together the 300-million-year-old storyline of amphibians such as frogs, toads, salamanders and caecilians.

Lead study author Alex Pyron and his team succeeded in constructing a first-of-its-kind comprehensive diagram of the geographic distribution of amphibians, showing the movement of 3,309 species between 12 global ecoregions. The phylogeny - or diagram of evolutionary relationships - includes about half of all extant amphibian species from every taxonomic group.

"There have been smaller-scale studies, but they included only a few major lineages and were very broad," Pyron said in a statement. "What we needed was a large-scale phylogeny that included as many species as possible. That allows us to track back through time, not only how different species are related, but also how they moved from place to place."

The findings indicate, contrary to popular scientific opinion, that certain groups of amphibians may have swam long distances from one landmass to another within the past few million years.

Biologists have long believed that two processes, known as vicariance and dispersal, were drivers of the distribution of various amphibious species.

Vicariance occurs when a population is separated following a large-scale geophysical event - for example, the splitting of the ancient supercontinent Pangaea. Scientists hypothesized that amphibians hitched a ride from one continent to another.

Pyron and colleagues also found that dispersal during the Cenozoic Era (66 million years ago to the present), likely across land bridges or short distances across oceans, also contributed to their distribution.

However, based on a surprising finding, the research team believes that these amphibians were expert swimmers and swam their way to other continents - even though this was thought impossible due to their salt intolerance.

For instance, one group of frogs found in Australia and New Guinea (pelodryadine hylids) that originated around 61 to 52 million years ago are strongly related to a group of amphibians that exist only in South America. By the time pelodryadines originated, all major continental landmasses occupied their present-day positions, with South America and Australia long separated from Antarctica.

"They're 120 million years too late to have walked to Australia," Pyron explained.

"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," he concluded.

The study's findings were published in the journal Systematic Biology.