Atlantis of the Iguanas

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Douglas Palmer
1999-06-02 14:00
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Submarine volcanoes in the eastern Pacific were once sun-drenched islands that could have been home to iguanas and other creatures now found only on the Galápagos Islands. The discovery of this drowned archipelago, reported in this month's Geology, may answer a nagging question about the evolution of the Galápagos's famous iguanas.

The uniqueness and diversity of the Galápagos fauna were first noticed in 1835 by Charles Darwin; the observations played an important role in the development of his theory of evolution by natural selection. Darwin suggested that the islands' 13 species of finches and two species of iguanas had evolved from ancestral populations that had lived on the South American mainland. However, a genetic analysis of the iguanas in 1992 cast doubt on this origin: It suggested that the two iguana species must have diverged from their mainland cousins at least 13 million years ago--about 9 million years before the volcanic Galápagos Islands rose from the sea.

Now an international team of scientists may have solved this puzzle. The researchers, led by Hans Mrazek from the University of Greifswald in Germany and colleagues from Geomar, a marine geosciences research institute in Kiel, were imaging and sampling a 250-kilometer-long stretch of ocean floor that includes a series of extinct volcanoes that run from the Galápagos to the mainland. Dredging up rock samples, they found that the lavas and volcanic ashes contain gas bubbles and fractures, both typical of lava which pours out at the surface rather than on the seabed. Sonar images of the seafloor revealed the nearby Quepos plateau, whose top must have been leveled by waves and surf, the researchers say. And radioactive dating indicated that the island volcanoes formed 14 million years ago--sufficiently long enough ago to allow time for speciation to occur on the islands, says the team.

Here's the scenario: As a nearby ocean ridge spread, the crustal plates drifted apart. The oldest islands slowly disappeared as they cooled and sank, now lying a kilometer below the ocean surface. The iguanas, the researchers say, then swam or drifted on logs to new volcanic islands, including the Galápagos, that were being pushed up by the hot crust near the spreading ridge.

"Considering Darwin's passionate interest in geology ... he would have been delighted with this new evidence," says Richard Darwin Keynes, emeritus professor of physiology at Cambridge University, editor of Charles Darwin's Beagle Diary--and great great grandson of the zoologist.

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