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Chinese Study Provides Evidence That Early Birds Had Four Wings Instead Of Two

March 15, 2013
Image Caption: Sapeornis fossils with long leg feathers. From Zheng et al, 2013. Credit: Science/AAAS

Lawrence LeBlond for redOrbit.com – Your Universe Online

When you inspect the legs of most birds you will find everything from the knee down is scaly rather than feathery. There is an exception to this rule however. Some birds of prey, such as eagles, have more feathering below the knee extending down to the feet. As for those with scaly legs, it is a remnant of their ancestry, when birds evolved from small two-legged dinosaurs millions of years ago.

For the most part, experts understand this evolutionary process. With forearm construction becoming longer and flatter, they evolved from hollow tubes into flat asymmetrical vanes and eventually became wings. But new evidence coming from China could shake things up a bit. It seems that the scaly hindquarters of our feathered friends were not always leggy in some primitive birds.

Chinese scientists have made a detailed analysis of nearly a dozen four-winged fossil specimens that existed around 130 million years ago. Reporting in the journal Science, this finding provided the first “solid evidence” that some primitive bird species adopted a four-wing body plan instead of the modern fore-wing, hind-leg rule.

The Chinese team of researchers, led by Xiaoting Zheng of China´s Linyi University, said these early birds flew more like a biplane, with hind wings providing added boost in the air. He said that these primitive birds eventually evolved to the more efficient modern look of today.

The 11 birds analyzed come from five different species and most were larger than a modern crow but smaller than a turkey. The fossils were donated by China´s Shandong Tianyu Museum of Nature.

Previously, several fossils have shown that some dinosaurs had large feathers on both their fore and hind limbs, but no examples of a four-winged body plan had been described until now.

Modern birds have two types of feathers: vaned (pennaceous or contoured) feathers that cover the outside of the body, and down feathers that cover the body underneath. The Chinese team found that one type of vaned plumage was neatly preserved in the skeletal fossils they analyzed, along the creatures´ hind limbs.

This discovery suggests that four wings were the norm for at least a short while in the evolutionary scale, and that birds eventually lost the feathers on the hind limbs over time, returning to a more hind-leggy nature.

Xiaoting is not the first scientist to theorize the four-winged display.

EARLIER EVIDENCE

Ornithologist William Beebe in 1915 first suggested that early birds may have passed through a four-wing gliding stage on their way to evolving a true two-winged flying style. His early theory was quite fanciful, but didn´t gain much credence from the scientific community, largely since there was no evidence to back up his claim.

Nearly 90 years later, in 2003, the prolific Chinese dinosaur hunter Xing Xu found an actual four-winged dinosaur he named Microraptor gui. The outlines of the feathers clearly showed signs of wing-like splaying in both its fore and hind limbs. Of course, the feathery wing-like structures were more typically observable as flares rather than actual wings, and may have helped the dinosaur glide like a bi-plane, rather than fly.

Xing, who is also part of the new research team, had gone on to discover other dinosaurs with long leg feathers, including the Anchiornis, Pedopenna and Xiaotingia, which is named after Xiaoting Zheng. That dinosaur only contains one species, zhengi, also named for the Chinese paleontologist.

Xing later theorized that four-winged dinosaurs lost their back feathers before evolving into the two-winged birds of today. However, the 11 specimens that he, Xiaoting and their colleagues recently analyzed offers up rethinking on that earlier belief.

The new specimens include species like sapeornis, confuciusornis, cathayornis and yanornis. All are primitive birds from between 145 and 100 million years ago (during the Cretaceous period) and all had four wings, with long feathers on the hind wings.

DOUBTS

Xing believes the early wings helped in flight, probably by allowing them to turn more easily or giving them more lift. But other scientists who are involved in the evolution of flight are not convinced by Xing´s work.

“[Xing] has basically just taken a punt that because the feathers were stiff, they were probably aerodynamic in function,” Michael Habib from the University of Southern California, told National Geographic´s Ed Yong. “It is a bit of a weak argument.”

Habib said it is possible that the hind feathers, like those found in Microraptor gui, did play some role in gliding or flying, but the smaller plumes of other baggy-legged (feathered) 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 explained.

Kevin Padian from the University of California, Berkeley agrees.

Padian noted that nobody has actually completed any proper tests to show if leg feathers were involved in flight. He said these leg feathers would have likely created drag, but may have provided lift if they sat in a flat sheet like the wings of modern birds. Xing has claimed that these feathers were flat and shear-like, but Padian said they were likely flattened as they became fossilized.

Still, Padian applauded the work of Xing and his colleagues. “It´s a great study because it establishes that leg feathers were widely distributed,” he said.

While the findings are pretty darn cool, Xing noted that it´s possible their work could later be falsified or complicated by another new discovery. Xing said that if he discovers early birds or feathered dinosaurs with extensive scales on their feet, it could spell doom for his hypothesis. “But personally, I am quite confident with our scenario.”

WHAT HAPPENED TO THE FEATHERS?

The more pertinent question today is: Why did the leg feathers eventually disappear?

Xing said he believes this occurred because the birds set their two pairs of limbs towards different ends — the front pair was adept for flying and the hind pair would have been better for walking or running. Also, they may have moved from life in the trees to life on the ground or water. Under any of these scenarios, hind-leg feathers would have been more problematic than useful; and eventually they were lost.

Xing said this could be true of other flying animals. He noted that the earliest flying insects tend to have four wings, while some of the most capable modern insects, like flies, only have two wings. The second pair 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,” said Xing of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP). “Once the major flight organ is well developed, the animal just fires the other organs.”

Mark A. Norell, a paleontologist at the American Museum of Natural History (AMNH) in NYC, who was not involved with the work but has been shown the fossils on recent trips to China, said: “The work is most interesting, but I would like to see a denser sampling” before a firm conclusion can be made.

“We´ve known for a while, from Microraptor, about specimens with feathers down to their toes, and with feet already resembling those of modern birds.”

Norell and others in the field said that the rich fossil beds of China have opened a window into the evolution of early flight in dinosaurs and the evolution of modern birds. Until more specimens can be found to confirm this theory, it can only be surmised how long it took for birds to transition from four to two wings.

The Archaeopteryx, a 150-million-year-old specimen from Germany, is sometimes referred to as the first bird. That specimen likely had feathers on its forelimbs, but more recent fossil finds challenge whether it was a bird-like dinosaur or a dinosaur-like bird.

To cover their bases, the Chinese team wrote that, only until now, no examples of the unusual four-wing structure “have so far been reported in [primitive] birds.”

“These days, we are working hard to extract new information from these wonderful specimens and hopefully can produce more interesting results in future,” concluded Xing.


Source: Lawrence LeBlond for redOrbit.com - Your Universe Online



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