"There might be some modern amphibians with small palatal teeth, but nothing as proportionately massive as [Kryostega]," Sidor said.
The new species will be detailed in an upcoming issue of the Journal of Vertebrate Paleontology.
During Kryostega's time, the continents were still congealed in one supercontinent called Pangaea. (See an illustration of Pangaea breaking up.)
Antarctica itself was located farther north and attached to South Africa, South America, and Australia.
The continent was also much warmer back then, crisscrossed by large rivers and primeval forests.
(Related: "Tiny Fossils Reveal Warm Antarctic Past" [July 25, 2008].)
The first dinosaurs had not yet appeared, but their ancestors, the dinosauromorphs, roamed the Earth, along with other reptiles and the reptile-like ancestors of mammals.
Kryostega was a top predator in Antarctica, Sidor said.
It likely stayed close to water and dined on fish and other amphibians.
"However, like crocodiles, if land-living animals strayed too close to the river's edge, I expect that it would have been able to drag them in," Sidor said.
Sebastian Steyer is an ancient-amphibian expert at the National Museum of Natural History in Paris, France. He was not involved in the study.
The new fossil is a rare and important find, Steyer said, because it sheds light on life during a little-known era of Antarctica's history.
The only other Antarctic creatures known to live during the middle Triassic include one or two other large amphibians, and a burrowing reptile-like mammal.)
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