Sometime about 90 million years ago, Asian turtles hit the road for North America. Although researchers thought that these reptiles had crawled around the globe via Russia and Alaska, new findings suggest that they may have taken a shortcut--over a series of islands now submerged under the Arctic Sea.
The conclusions are based on an unusual turtle fossil. While doing geophysical fieldwork in the Canadian Arctic, researchers led by geophysicist John Tarduno and colleagues from the University of Rochester in New York stumbled upon the large, flat-shelled fossil on Axel Heiberg Island, just west of northern Greenland. The team determined that the turtle lived in fresh water because it lacked webbed feet and that it resembled a warm-weather species called Aurorachelys gaffneyi that originated in Mongolia about 120 million years ago.
Scientists had assumed that Aurorachelys migrated to North America over a land bridge connecting Russia to Alaska between about 100 million and 65 million years ago, according to fossil finds in places such as Montana and California. But the age and location of the new fossil, reported in the current issue of Geology, suggests that the Asian turtles instead took a direct route over the Arctic Ocean.
How did a freshwater turtle get across such a vast sea? Scientists suggest that volcanoes belching CO2 into the atmosphere created supergreenhouse conditions 90 million years ago. As a result, the North Pole was ice-free and at least 34°C warmer than it is today, with a climate resembling northern Florida's. The heat would have contributed to more rainfall in the north, resulting in massive river runoff that capped the Arctic Ocean with a layer of fresh water.
But the turtles couldn't spend all of their time in the water. And that's where the volcanoes come in again. Tarduno suggests that the same widespread volcanism that increased CO2 worldwide also gave rise to a ridge underneath the Arctic Ocean. The heat pushed parts of the ridge above sea level, creating an archipelago that allowed the turtles--and probably many other species--to island-hop from Asia to North America. Eventually, as this oceanic crust cooled, the islands sank below sea level.
The turtle fossil is a "fantastic find" that's undoubtedly related to the Asian species, says vertebrate paleontologist Walter Joyce of the University of Tübingen in Germany. The new route seems like a plausible explanation for its migration, he says. Oceanographer Kathryn Moran of the University of Rhode Island, Narragansett, agrees that the route makes sense, but she says that scientists should drill on the ridge to make sure that it's volcanic--and thus could have created an archipelago that many years ago.