The earliest birds may have had wings on both their arms and legs, according to paleontologist Xiaoting Zheng and his team at the Institute of Geology and Paleontology in Shandong, China. In a paper published in the journal Science, the researchers report that 11 fossil specimens of Cretaceous birds show evidence of leg feathers, some of which appear to be flight feathers.

Most modern birds have legs covered in scales. Where leg feathers do exist, for example, in eagles, vultures and some chickens, they are thinner and much less complex than arm feather. Birds cannot use them for flight. Instead, their leg feathers help these birds keep warm.

However, some birdlike dinosaurs, including Microraptor, did have feathered back limbs. In 2003, Chinese researchers discovered Microraptor fossils showing that one species of the tiny dinosaur, now named Microraptor gui, had flight feathers on its legs, arms and tail.

Until now, scientists did not know if leg feathers were lost before the first birds appeared.

Zheng and his colleagues studied 11 basal bird specimens housed at the Shandong Tianyu Museum of Nature (STM). Some of these had large, veined feathers that stood out from their leg bones.

One specimen of the genus Sapeornis had a leg feather more than 50 millimeters long.

The team claims that the feathers’ stiff veins, curved shafts  and positioning almost perpendicular to the leg bone suggests that they had an aerodynamic function. They could have made maneuvering easier, provided lift or created drag.

However, other scientists argue that there is no evidence that these Cretaceous birds used their large leg feathers for flight.  The birds could have used them for insulation or for display.

To determine the validity of their hypothesis, Zheng’s team plans to study more specimens and use models to test how well the wings could have served as flight aids.

The researchers think that modern birds lost their back feathers in order to make it easier for them to move about on the ground. They then developed two specialized locomotion systems – one for flying and one for walking or running.

Some ground-dwelling dinosaurs had signs of rudimentary flight feathers.  However, scientists think that dinosaurs such as Microraptor, which had better developed flight systems, probably lived in trees.  If birds developed scaled feet in order to facilitate ground locomotion, this would indicate a movement from life on the ground to life in trees and then back to the ground again.