No one knows exactly what caused the sudden change. Theories range from tectonic shifts that affected ocean circulation to decreasing levels of carbon dioxide in the atmosphere.
The new findings—which add to evidence released last month that focused on the prehistoric Antarctic crustaceans at the site—will appear in today's issue of the Proceedings of the National Academy of Sciences.
The discovery of the ancient plant and animal life provides a unique glimpse at a more hospitable Antarctica.
"The contrast couldn't be more striking. It's like comparing Patagonia [in southern South America] today with the surface of Mars," study co-author Marchant said. "To study these deposits is akin to strolling across the Dry Valleys 14.1 million years ago."
Experts have also compared the Antarctica of 14 million years ago to the present-day climate of the Tierra del Fuego mountains in South America, which have seasonally ice-free glacial lakes.
Lewis, of North Dakota State, said that, based on a study of pollen found in the lake deposit, there may have been southern beech trees similar to those that grow in South America, in New Zealand, and on the Australian island of Tasmania.
The lakes would have had thick mats of green moss, he said, and swarms of blackflies and other insects would have populated the region.
"It may be a pretty small group of plants and animals that were able to survive there. It is right on the fringe," Lewis said. "It's entirely possible that we are looking at the last few survivors."
The newfound remains still contain organic tissue.
"You get this freeze-dried leaf and you put in water and it still unfurls, opening up as it rehydrates," Marchant said. "At that point it looks as fresh as a museum specimen."
The ability to artificially rehydrate the organisms suggests that they were never been naturally rehydrated. In other words, in 14 million years the site never warmed enough to thaw the specimens.
"It is one of the most dramatic and long-lasting changes that one can imagine," Marchant said. "I don't know of any other place on Earth where such an enduring change has been documented."
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