Look at the foot of almost any bird and you’ll view feathers extending the thigh but scales covering everything from the ‘knee’ downwards. There space a couple of exceptions—some bird of food look like they’re wearing baggy pants and golden eagles have fluffy foot feathers because that insulation. But for the many part, life birds have actually naked lower legs.

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It wasn’t always this way. We understand that birds progressed from tiny two-legged, meat-eating dinosaurs the were extended in basic fuzzy feathers. Those on your arms eventually ended up being longer and also flatter, evolving from hollow tubes into flat asymmetrical vanes. They revolutionized from “dino-fuzz” into flight feathers, and also their eight transformed right into wings.

Meanwhile, it’s tempting to think that the feather on your hind foot gradually ended up being smaller and also gave method to scales. But that’s not just how it happened. For a start, we know that some little dinosaurs had long feathers on your legs as well as their arms. And now, 11 newly analysed fossils tell united state that some at an early stage birds mutual the very same feature. This specimens suggest that several of our feathered friend had 4 wings.

It to be an ornithologist dubbed William Beebe who very first suggested that early birds can have passed with a four-winged gliding stage on their method to evolving true flapping flight. That remained in 1915 and though Beebe’s idea to be fanciful, there wasn’t much solid evidence behind it.

Then, in 2003, the prolific Chinese dinosaur-hunter Xing Xu discovered an really four-winged dinosaur. He referred to as it Microraptor gui. Xu witnessed the outlines that feathers clearly splaying indigenous the creature’s legs and its arms. These were clearly traces the long, flat and also asymmetric plumes, lot like those that save today’s flying birds aloft. While that lived, Microraptor most likely looked like a starling wearing flares. Xu suggested that it may have actually used that is leg wings to aid it glide, while rather later suggested that it might have flown favor a biplane.

Xu go on to find other dinosaurs with long leg feathers, such as Anchiornis, Pedopenna and also Xiaotingia. Because that a time, it looked like these feathers disappeared before true birds landed on the scene, yet Xu is now back with 11 new fossils the discount that idea.

The specimens include varieties like Sapeornis, Confuciusornis, Cathayornis, and Yanornis. Every one of them are beforehand birds, perched ~ above primitive branches of the group’s family members tree. Every one of them resided in China throughout the Cretaceous period. And all of them had four wings, with long feathers on their legs.

You can see lock in the images throughout this post—dark shadows protruding native the skeletal of the lower leg. In some of the specimens, the foot feathers present a stiff, curved main rod (or “rachis”) v symmetrical vanes sticking out from one of two people side. Castle protrude indigenous the bones at appropriate angles and also seem to kind a huge flat surface.

Xu thinks the these feathers might have aided the owners to fly. They might have developed extra elevator or maybe aided the birds to turn more easily. But other scientists who work on the evolution of flight are no convinced. “ has basically just taken a punting that because the feathers were stiff, lock were probably aerodynamic in function,” claims Michael Habib native the university of southerly California. “It is a little of a weak argument.”

Habib thinks that the lengthy asymmetric leg feathers of Microraptor most likely did pat some role in gliding or flying, but the smaller plumes of other baggy-legged varieties “might have actually merely to be there due to the fact that of a developmental quirk”. If some genes are producing huge feathers on the front limbs, “it can not take much to tweak a set onto the hind limbs too,” that says.

Kevin Padian native the college of California, Berkeley agrees. That points the end that no one has actually done any kind of proper test to show if the leg feathers were associated in flight. Castle would absolutely have produced drag, but they can only have noted lift if they sat in a flat sheet like the wing of contemporary birds. Xu claims that lock were, yet Padian states that the feathers could just have been flattened into a plane as they became fossilised. “It hasn’t been shown that this is yes, really an aerodynamically knowledgeable wing,” he says.

Nonetheless, both Habib and Padian worship Xu’s work. “It’s a an excellent study due to the fact that it establishes that leg feather were commonly distributed,” says Padian. From starts as little outgrowths, foot feathers became dramatically bigger in few of the dinosaur teams on the evolutionary line leading to birds. They ultimately shrank away again prior to disappearing entirely and also being replaced by scales.

Of course, like any type of evolutionary story, this one could be falsified or complicated by the following cool discovery. Xu says that if he uncovered early bird or feathered dinosaurs with extensive scales on your feet, that would spell problem for his hypothesis. “But personally, i am rather confident v our scenario,” he says.

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Why walk the foot feathers, having an initial become large, ultimately disappear? Xu thinks the it was because the birds set their two pairs of four towards various ends—the front pair because that flying and also the hind pair because that walking or running. In ~ the same time, they might have relocated from life in the tree to life top top the ground, or near water. Under all these scenarios, long leg feather would have actually just got in the way, and also were shortly lost.

Something similar may have happened in various other flying animals. For example, the earliest paris insects tend to have 4 wings, while some of the most competent flyers like, well, flies, only have two. The second pair has evolved into a pair that gyroscopes referred to as halteres. “In the beforehand evolution of flight, different pet groups always try to usage as much surface together possible,” states Xu. “Once the significant flight organ is fine developed, the pet just fires the other organs.”

Xu’s 11 specimens all come from personal collectors and also had been housed in ~ the Shandong Tianyu Museum that Nature for roughly a decade. The museum contains over 2,000 specimens of at an early stage birds, many of which maintain beautiful traces of feathers, skin and more. In fact, the museum’s treasure trove of riches is so substantial that it has turned right into a backlog. There’s just too much an excellent stuff there to go through. “It take it a while for me to realize how essential these specimens are,” says Xu. “These days, we are working difficult to extract brand-new information from these wonderful specimens and hopefully have the right to produce much more interesting outcomes in future.”

Reference: Zheng, Zhou, Wang, Zhang, Zhang, Wang, Wei, Wang & Xu. 2013. Hind wing in Basal Birds and the evolution of leg Feathers. Scientific research http://dx.doi.org/10.1126/science.1228753