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A 2-metre-tall flightless bird may have been the top predator in what is now Antarctica 50 million years ago.

Two 8-centimetre fossil claws found on Seymour Island, near the tip of the Antarctic Peninsula, belonged to a large family of similar birds that palaeontologists call terror birds, according to an analysis by at the National University of La Plata in Argentina and at the National Museum of Natural History in Montevideo, Uruguay.

Formally called phorusrhacids, terror birds were the apex predators in what is now South America from 53 million years ago to 100,000 years ago, when they went extinct.

The sharply pointed toe bones were enough to identify the fossils as terror birds, says Acosta Hospitaleche.

Like their distant cousin Tyrannosaurus rex, terror birds evolved a combination of fearsome claws on their feet and massive jaws with sharp teeth to kill their prey. But unlike the bulky T. rex, terror birds evolved lean bodies to run down prey, weighing in at around 100 kilograms.

The fossils date from a time when the world was much warmer than it is now. Antarctica鈥檚 climate then was like New Zealand鈥檚 today, says Acosta Hospitaleche.

The Seymour Island site is rich in fossils of small mammals and flying birds, but the terror birds are the only carnivores yet found. Some palaeontologists have speculated that terror birds might have scavenged rather than capturing prey, but Acosta Hospitaleche says that the ones in Antarctica were large enough to hunt most animals in the area.

鈥淚t鈥檚 not unexpected that you could find things [animals] that are known from South America [in Antarctica],鈥� says at the French National Centre for Scientific Research.

Terror birds evolved their large size very quickly after dinosaurs went extinct, he says, and fossils of them or close relatives have been found on all continents except Asia. Yet only in South America did they retain the top predator niche for a long time, and Buffetaut hopes the Antarctic find will help palaeontologists explain their uneven spread.