By Jonathan Amos
Science reporter, BBC News
Exquisite detail has been preserved in the fossils
Tiny fossils have helped refine the timing of the climate shift that gave rise to Antarctica's remarkable Dry Valleys, a landscape akin to Mars.
The famously ice-free terrain enjoyed more benign, tundra-like conditions 14 million years ago - but then flipped to the intensely cold setting seen today.
Scientists tell a Royal Society journal that ancient lake-living shrimp-like creatures can pinpoint the big switch.
The ostracods would not have coped with a harsh, dry environment, they say.
Dr Adam Lewis, from North Dakota State University, US, explained: "Our dating says the lake existed 14 million years ago, and within about 250,000 years of that lake existing and holding those ostracods, all the glaciers in the surrounding area stopped melting and they become cold-based and began to evaporate.
"So after about 13.8 million years ago, there's no water - it's bone dry, dry-frozen," he told BBC News.
Antarctica's Dry Valleys, with their barren gravel-strewn floors, are said to be the closest place on Earth to Mars.
Valley floors are littered with the mummified corpses of dead animals
The air that falls into the region from the continent's elevated interior has extremely low humidity. There is so little precipitation in the valleys that they are technically regarded as deserts - as is much of Antarctica.
Animals such as seals which wander into this landscape and get lost will die and mummify.
The team found a range of fossils in sediments on the slopes of Mount Boreas, on the edge of McKelvey Valley. These include mosses, diatoms and beetles. But it is the 1mm-long ostracods which catch the eye.
They are exquisitely well preserved, with their soft tissues visible in three dimensions.
Dr Mark Williams, from the University of Leicester, UK, said: "We've got the legs and the mouth parts, and the reproductive organs; and we can even see micron-scale hairs on the legs.
"This is a first of its kind from the Antarctic; it's unusual in the fossil record in general - it's special.
"Notwithstanding the significance of the fossil preservation, the presence of lake ostracods at this latitude, 77 degrees South, is also of great note. With modern ostracod distribution, the most southerly ones are at about 60 degrees South."
The team says the fossils therefore represent a precise marker, indicating the switch from conditions which one might see in Northern Canada and Iceland today, where summer warmth brings a melt, liquid water and a flourish of life - to the more severe, arid conditions we recognise in the Antarctic today.
"This also helps us understand the whole evolution of Earth's climate system because you've got this huge climate jump that takes place about 14 million years ago when the oceans reorganise, Antarctica freezes over - a whole host of things change right at that point.
"What we're doing with these ostracods is to say: 'that's it, that's the point'."
The fossil research is detailed in the journal Proceedings of the Royal Society B.