From the Daily Mail:
Researchers have uncovered new evidence that the first birds had four wings instead of two.
A team from China say they flew rather like a biplane - with wings on their legs providing a second boost.
The team analyzed 11 bird skeletons from between 150 and 100 million years ago, and believe they eventually shed the second set so they could use their legs more effectively.
Modern birds have two types of feathers: vaned feathers that cover the outside of the body, and the down feathers that grow underneath them.
The Chinese team found that one type of vaned plumage, also known as pennaceous feathers, was neatly preserved in skeletal fossils of these specimens, along each creatures’ hind limbs.
These findings suggest that this four-winged condition preceded the two-winged body plan, and that birds have gradually lost the feathers on their hind limbs over evolutionary time.
However, some experts have questioned the results, and claim the feather may not have been used for flying, but as part of a mating display.