Fossilised bones uncovered in the US state of South Carolina belonged to the largest flying bird in history, with a wingspan of 6.4 metres, equal to that of a very small plane, a new study says.
The wingspan of Pelagornis sandersi was twice as wide as that of biggest modern-day flying bird, the royal albatross, said the research published in the Proceedings of the National Academy of Sciences on Monday.
Coupled with its long beak and sharp bony teeth, the enormous wings probably helped the bird hunt food as it glided over water about 28 million years ago.
However, it might have needed some help getting airborne, given its wings were too long to flap easily from the ground.
Scientists believe it may have made a running start downhill, or used air gusts - much like a hang glider - to make its way aloft.
Once in the air, study author Dan Ksepka, of the National Evolutionary Synthesis Centre in Durham, North Carolina, said the bird could probably soar for several kilometres without ever flapping its wings. "That's important in the ocean, where food is patchy," Ksepka said.
P. sandersi lived after the dinosaurs became extinct but before the first humans are known to have inhabited North America.
The bird's wing and leg bones along with its complete skull were first discovered in 1983 near Charleston, South Carolina, during excavation work for a new international airport. "The upper wing bone alone was longer than my arm," said Ksepka, recalling that a mechanical digger was called in to help unearth the bones.
The bone measurements suggest that the bird's wingspan was between 6.06 and 7.38 metres, according to the PNAS article.
The fossils of P. sandersi shed light on the flying ability of a remarkable bird.
But they also raise new questions about the group of bony toothed seabirds known as pelagornithids, which disappeared about 2.5 million years ago.
These ancient birds were "remarkably efficient fliers" that were found across all seven continents, making "the cause of their ultimate extinction all of the more mysterious", said the study.