Some primitive birds boasted four wings, before evolution led them to ditch their hind feathers in favor of webbed or scaly feet, scientists in China said on Thursday.
Previous research had uncovered the existence of bird-like dinosaurs with hind limb feathers, but evidence has remained slim in birds, which are widely believed to have evolved from dinosaurs.
And even though the latest discovery documents new evidence of feathered feet in early birds, the question remains whether the plumes were actually a help or a hindrance in flight.
The Chinese scientists behind the study, published in the US journal Science, said the 11 newly described fossil specimens offer evidence the leg feathers were used as a part of a four-winged system for flying.
Researchers found the new trove of data by poring over fossils at China's Shandong Tianyu Museum of Nature, said lead researcher Xing Xu, a well-known dinosaur researcher.
The 11 birds come from five species and were relatively robust: larger than a crow but smaller than a turkey, according to Xu, a professor at the Key Laboratory of Vertebrate Evolution and Human Origin, Chinese Academy of Sciences.
One key specimen was a Sapeornis, a bird that was not previously believed to have hind feathers.
However, fossils held in the museum showed the creature with a fan of feathers on each heel, some as long as five centimeters (two inches).
"We believe they were related to flight," Xu told AFP, describing the finding as "exciting" because ancient birds, with their delicate skeletons, have left behind few fossils for researchers to examine.
The birds described in the study come from the Cretaceous period and flew some 121-125 million years ago, existing alongside dinosaurs, he said.
The back wings took the form of leg plumes and may have helped the creatures maneuver in the air while the arm wings flapped or stretched wide to soar, according to the study.
The arrangement of the feathers, along with their stiff vanes, suggests they were "aerodynamic in function, providing lift, creating drag, and/or enhancing maneuverability, and thus played a role in flight," said the study.
Researchers are continuing to hunt for details on the possible color of these feathers, and are crafting models to show exactly how they may have been used in flight, Xu said.
But other experts are not so sure that the foot-feathers were used to fly, but may have served other purposes like attracting mates.
"No one thinks that these animals flapped their legs," said Kevin Padian, professor of Integrative Biology at the University of California, Berkeley.
Padian was not involved in the study but was among the experts who reviewed it before publication.
"Some say that the leg feathers would have increased lift, but there is no evidence for this: to increase lift the feathers would have to be arranged in such a way as to form a competent, planar airfoil, and no one thinks (or has shown) that this was the case," Padian told AFP.
"On the other hand it is indisputable that such feathers would create drag," he added.
"In fact, the authors neither perform nor cite any research in support of any hypothesis that these feathers contributed to any sort of flight," Padian wrote.
Nevertheless, Padian applauded the research, calling it a "great study" because it shows how leg feathers changed over time among bird-like dinosaurs and primitive birds.
Nowadays, there are some modern birds that have retained flashy leg feathers, but those tend to be exotic chickens or pigeons that are bred for the rarity, not birds that have evolved that way, said Xu.
Some, like the golden eagle, have retained feathered feet but their fluffy pedalers are for insulation, not flight.
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More information: "Hind Wings in Basal Birds and the Evolution of Leg Feathers," by X. Zheng et al, Science, 2013. www.sciencemag.org/content/339/6125/1309