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Sapeornis fossils, with long leg feathers. From Zheng et al, 2013. Science/AAAS

The Rise and Fall of Four-Winged Birds

Look at the leg of almost any bird and you’ll see feathers covering the thigh but scales covering everything from the ‘knee’ downwards. There are a couple of exceptions—some birds of prey look like they’re wearing baggy trousers and golden eagles have fluffy foot feathers for insulation. But for the most part, living birds have naked lower legs.

It wasn’t always this way. We know that birds evolved from small two-legged, meat-eating dinosaurs that were covered in simple fuzzy feathers. Those on their arms eventually became longer and flatter, evolving from hollow tubes into flat asymmetrical vanes. They transformed from “dino-fuzz” into flight feathers, and their arms transformed into wings.

Meanwhile, it’s tempting to think that the feathers on their hind legs gradually became smaller and gave way to scales. But that’s not how it happened. For a start, we know that some small dinosaurs had long feathers on their legs as well as their arms. And now, 11 newly analysed fossils tell us that some early birds shared the same feature. These specimens suggest that some of our feathered friends had four wings.

It was an ornithologist called William Beebe who first suggested that early birds might have passed through a four-winged gliding stage on their way to evolving true flapping flight. That was in 1915 and though Beebe’s idea was fanciful, there wasn’t much strong evidence behind it.

Then, in 2003, the prolific Chinese dinosaur-hunter Xing Xu found an actual four-winged dinosaur. He called it Microraptor gui. Xu saw the outlines of feathers clearly splaying from the creature’s legs as well as its arms. These were clearly traces of long, flat and asymmetric plumes, much like those that keep today’s flying birds aloft. While it lived, Microraptor probably looked like a starling wearing flares. Xu suggested that it may have used its leg wings to help it glide, while others later suggested that it could have flown like a biplane.

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

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Confuciusornis. From Zheng et al, 2013. Science/AAAS

The specimens include species like Sapeornis, Confuciusornis, Cathayornis, and Yanornis. All of them are early birds, perched on primitive branches of the group’s family tree. All of them lived in China during the Cretaceous period. And all of them had four wings, with long feathers on their legs.

You can see them in the images throughout this post—dark shadows protruding from the bones of the lower leg. In some of the specimens, the leg feathers show a stiff, curved central rod (or “rachis”) with symmetrical vanes sticking out from either side. They protrude from the bones at right angles and seem to form a large flat surface.

Xu thinks that these feathers might have helped the owners to fly. They could have produced extra lift or maybe helped the birds to turn more easily. But other scientists who work on the evolution of flight are not convinced. “[Xu] has basically just taken a punt that because the feathers were stiff, they were probably aerodynamic in function,” says Michael Habib from the University of Southern California. “It is a bit of a weak argument.”

Habib thinks that the long asymmetric leg feathers of Microraptor probably did play some role in gliding or flying, but the smaller plumes of other baggy-legged species “might have merely been there because of a developmental quirk”. If some genes are producing large feathers on the front limbs, “it might not take much to tweak a set onto the hind limbs too,” he says.

Kevin Padian from the University of California, Berkeley agrees. He points out that no one has actually done any proper tests to show if the leg feathers were involved in flight. They would certainly have created drag, but they could only have provided lift if they sat in a flat sheet like the wings of modern birds. Xu claims that they were, but Padian says that the feathers could just have been flattened into a plane as they became fossilised.  “It hasn’t been shown that this is really an aerodynamically competent wing,” he says.

Nonetheless, both Habib and Padian praise Xu’s work. “It’s a great study because it establishes that leg feathers were widely distributed,” says Padian. From beginnings as small outgrowths, leg feathers became dramatically bigger in some of the dinosaur groups on the evolutionary line leading to birds. They eventually shrank away again before disappearing entirely and being replaced by scales.

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Scenario for the evolution of leg feathers. From Zheng et al, 2013. Science/AAAS

Of course, like any evolutionary story, this one could be falsified or complicated by the next cool discovery. Xu says that if he discovered early birds or feathered dinosaurs with extensive scales on their feet, that would spell trouble for his hypothesis. “But personally, I am quite confident with our scenario,” he says.

Why did the leg feathers, having first become large, eventually disappear? Xu thinks that it was because the birds set their two pairs of limbs towards different ends—the front pair for flying and the hind pair for walking or running. At the same time, they might have moved from life in the trees to life on the ground, or near water. Under all these scenarios, long leg feathers would have just got in the way, and were soon lost.

Something similar may have happened in other flying animals. For example, the earliest flying insects tend to have four wings, while some of the most competent flyers like, well, flies, only have two. The second pair has evolved into a pair of gyroscopes called halteres. “In the early evolution of flight, different animal groups always try to use as much surface as possible,” says Xu. “Once the major flight organ is well developed, the animal just fires the other organs.”

Xu’s 11 specimens all came from private collectors and had been housed at the Shandong Tianyu Museum of Nature for roughly a decade. The museum contains over 2,000 specimens of early birds, many of which preserve beautiful traces of feathers, skin and more. In fact, the museum’s treasure trove of riches is so huge that it has turned into a backlog. There’s simply too much good stuff there to go through. “It took a while for me to realize how important these specimens are,” says Xu. “These days, we are working hard to extract new information from these wonderful specimens and hopefully can produce more interesting results in future.”

Reference: Zheng, Zhou, Wang, Zhang, Zhang, Wang, Wei, Wang & Xu. 2013. Hind Wings in Basal Birds and the Evolution of Leg Feathers. Science

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