A prehistoric fossil unearthed almost 30 years ago has finally been identified, and it's nothing less than the biggest bird to have every soared through Earth's sky, paleontologists say.

Excavated in Charleston, S.C., in 1983, the fossil is that of a bird possessing a wingspan of more than 20 feet, twice the span of the wandering albatross, the largest flying bird in existence today, they say.

The fossil has been identified as a previously unknown pelagornithid species, Pelagornis sandersi, from a group of giant seabirds looking like massively overgrown seagulls, the researchers report in the Proceeding of the National Academy of Sciences.

The sheer size of the creature, which appeared in the Paleocene era after the extinction of the dinosaurs around 55 million years ago, is perhaps its most impressive feature, the researchers say.

"Anyone with a beating heart would have been struck with awe," says paleontologist Daniel Ksepka of the Bruce Museum in Greenwich, Connecticut. "This bird would have just blotted out the sun as it swooped overhead. Up close, it may have called to mind a dragon."

The giant birds, weighing between 50 and 90 pounds, lived on every continent including Antarctica until they went extinct about 3 million years ago for unknown reasons, the researchers say.

The fossil shows the bird had short legs and was probably somewhat ungainly while walking of land, however its immense wings would have allowed to glide effortlessly and stay aloft for long periods of time despite its huge size, they say.

The fossils skull of the creature, unearthed during work to enlarge Charleston International Airport, is nearly complete and in excellent condition, paleontologists say, and important wing bones, along with leg bones, the wishbone and the shoulder blade, have also been recovered.

The wingspan and mass of the giant creature seemingly defies aerodynamic theory, but like the bumblebee it appears not to have noticed.

"Pelagornis sandersi surpassed theoretical mass limits for flapping flight and wingspan limits for soaring flight based on previous models," Kspeka says, "but the extremely elongated wings and reduced hind limbs indicate it was a volant (flying) bird."

"New data suggest that they were remarkably efficient flyers, which together with their global distribution across all seven continents and long temporal range, makes the cause of their ultimate extinction all the more mysterious," he says.

Their method of flight, with emphasis on gliding rather than wing flapping, would have been similar to the extinct flying reptiles known as pterosaurs that lived in the time of the dinosaurs and had the greatest wingspans of any known flying creatures, approaching 36 feet.