南加州发现世界上最大的巨鸟化石

Scientists have identified the fossilized remains of an extinct giant bird that could be the biggest flying bird ever found. With an estimated 20-24-foot wingspan, the creature surpassed size estimates based on wing bones from the previous record holder -- a long-extinct bird named Argentavis magnificens -- and was twice as big as the Royal Albatross, the largest flying bird today. Scheduled to appear online the week of July 7, 2014, in the journal Proceedings of the National Academy of Sciences, the findings show that the creature was an extremely efficient glider, with long slender wings that helped it stay aloft（在空中） despite its enormous size. The new fossil was first unearthed in 1983 near Charleston, South Carolina, when construction workers began excavations for a new terminal at the Charleston International Airport. The specimen was so big they had to dig it out with a backhoe（挖沟机）. "The upper wing bone alone was longer than my arm," said author Dan Ksepka of the National Evolutionary Synthesis Center in Durham, North Carolina.

Now in the collections at the Charleston Museum, the strikingly well-preserved specimen consisted of multiple wing and leg bones and a complete skull. Its sheer size and telltale（迹象，指示器） beak allowed Ksepka to identify the find as a previously unknown species of pelagornithid, an extinct group of giant seabirds known for bony tooth-like spikes that lined their upper and lower jaws. Named 'Pelagornis sandersi' in honor of retired Charleston Museum curator Albert Sanders, who led the fossil's excavation, the bird lived 25 to 28 million years ago -- after the dinosaurs died out but long before the first humans arrived in the area.

Researchers have no doubt that P. sandersi flew. It's paper-thin hollow bones, stumpy（粗短的） legs and giant wings would have made it at home in the air but awkward on land. But because it exceeded what some mathematical models say is the maximum body size possible for flying birds, what was less clear was how it managed to take off and stay aloft despite its massive size.

To find out, Ksepka fed the fossil data into a computer program designed to predict flight performance given various estimates of mass, wingspan and wing shape. P. sandersi was probably too big to take off simply by flapping its wings and launching itself into the air from a standstill, analyses show. Like Argentavis, whose flight was described by a computer simulation study in 2007, P. sandersi may have gotten off the ground by running downhill into a headwind or taking advantage of air gusts to get aloft, much like a hang glider.

Once it was airborne, Ksepka's simulations suggest that the bird's long, slender wings made it an incredibly efficient glider. By riding on air currents that rise up from the ocean's surface, P. sandersi was able to soar for miles over the open ocean without flapping its wings, occasionally swooping down to the water to feed on soft-bodied prey like squid and eels.

"That's important in the ocean, where food is patchy," said Ksepka, who is now Curator of Science at the Bruce Museum in Greenwich Connecticut.

Researchers hope the find will help shed light on why the family of birds that P. sandersi belonged to eventually died out, and add to our understanding of how the giants of the skies managed to fly.