Just how the ostracods were introduced into the lake 14 million years ago—when it likely measured about 330 feet (100 meters) by 200 feet (60 meters)—remains a mystery.
The creatures could have been relics that survived on the continent after it broke away from South America almost 30 million years ago, Ashworth said.
(See a prehistoric time line.)
Or migratory birds wading into the lake could have had the organisms tangled in their leg feathers.
The permanently frozen ground also likely helped preserve the fossils, Ashworth added.
Ostracods no longer live in Antarctica today. The closest place they are found is about 620 miles (1,000 kilometers) in subantarctic islands, he said.
The research team found hundreds of the ostracod shells in various stages of their growth cycles.
"It tells you this was a healthy community," Ashworth said.
Also found at the site were mosses and pollen from tundra vegetation that lived during the same period.
"What the ostracods and mosses are telling you is that this was a time that was significantly warmer than present Antarctica," Ashworth said.
(See pictures of life on today's tundra.)
Warmer temperatures would have been essential for the ostracods' survival, with summer temperatures well above freezing for months at a time, Ashworth said.
Today, summer temperatures at the site average between -4 to 13 degrees Fahrenheit (20 to 25 degrees Celsius.
Point of No Return
The findings illuminate how minor climate changes suddenly become major when a threshold is crossed, he said.
"We think that climate change 14 million years ago caused the extinction of ostracods and other animals and plants on the Antarctic continent," he said.
Since that point of no return, conditions have not warmed enough in Antarctica to allow the reintroduction of life, Ashworth said.
(Related: "Greenland Ice Shows Rapid Climate Flips, Study Says" [June 19, 2008].)
"We believe that that [glacial-lake] system has remained basically frozen for the past 14 million years," Ashworth said.
Not everyone is convinced, however.
"No question, it is a fantastic find," Reed Scherer, a micropaleontologist at Northern Illinois University in DeKalb, said of the ostracod discovery.
Scherer, who has conducted extensive research in Antarctica, was not affiliated with this study.
But he pointed out that the preservation of the ostracods may not necessarily mean they had been in continuously frozen tundra for the past 14 million years, he said.
Some assume that because the samples have not been disturbed that the Dry Valley site has remained frozen, he said.
"There's logic to that argument, but I don't see the alternative [of further temperature fluctuations] as ruled out," Scherer said.
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