Salamanders show Americas joined much earlier than thought

The humble salamander may provide evidence to support a controversial claim that the continents of North and South America were joined together much earlier than previously thought, scientists say.
The two continents are generally believed to have been joined together around three million years ago by the formation of a land bridge - what is now Panama - that sealed up the sea channel between them.
However, a new study of salamanders in South America by Dr Kathryn Elmer of the University of Glasgow, has found evidence that challenges these assumptions and supports a controversial claim by Smithsonian scientist Carlos Jaramillo that most of the Isthmus of Panama was formed around 23 million years ago.

The fusion of both land masses led to a two-way migration of animals called the 'Great American Biotic Interchange', where animals that had previously evolved separately moved between the two continents, increasing the biodiversity in both regions.
The relative dearth of species of salamander in South America - around 30 - compared to Central America, where there are more than 300 species, is usually attributed to the relatively short time the tiny lizard-like amphibians have had to make their way south down the Isthmus of Panama.
However, using DNA analysis, Elmer found that salamanders in South America had much greater genetic divergence from their Central American cousins than should be expected if salamanders migrated across a three million year-old land bridge.

The study published in journal BMC Evolutionary Biology suggested salamanders colonised South America in the Early Miocene epoch, more than 20 million years ago, preceding the Great American Biotic Interchange by many millions of years.

"Unlike the mountains and forests of nearby Central America, the biodiversity of salamanders in the Andes and Amazon is quite low. Why are there so few species of salamanders in South America?" Elmer said in a statement.
"When we began this study, all the literature said the Panama land bridge was three to four million years old. We struggled because our genetic data for salamanders were totally incompatible with the established geology," Elmer added.

"Then in 2011, studies from geologists started challenging the age of the Isthmus, saying it must be much older. This was welcome news to the salamander evolutionary biologists," Elmer said.
"Salamanders are poor dispersers and they are unlikely to cross salt water gaps," said co-author David Wake, from the University of California, Berkeley.
"Thus the new data, which show that even within what once was considered a single species, divergence is sufficiently great that there must have been an earlier land connection, is bringing home the near certainty of a land connection earlier, and of course this is what some controversial evidence from geologists such as Jaramillo is suggesting as well," Wake said.