A group of researchers studying bird fossils in a Chinese museum have discovered evidence that some of the earliest birds that developed had large feathers on their legs, suggesting that they actually had four wings.
The idea of birds having four wings was originally proposed back in 2003, when the first fossils were found that showed evidence of wings on the the birds' hind legs. Scientists named this species Microraptor.
"When we published that ten years ago there was some suspicion whether the fossil was faked," says study co-author Xing Xu from Linyi University in Shandong province, according to New Scientist magazine. However, more four-winged fossils have been found since then, laying to rest the possibility of them being bogus.
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The researchers examined more fossil specimens at the Shandong Tianyu Museum of Natural History, in Pingyi County, in eastern China, and found that 11 of the specimens, from four different groups of early birds — genera Sapeornis, Yanornis and Confuciusornis, and the Enantiornithes group — showed evidence of having had feathers on their hind legs. Their findings suggest that these feathers were large enough, and stuck out at enough of an angle, for them to be aerodynamic, and therefore they may have been a second set of wings.
These latest fossils were dug up from sediments laid down in the Early Cretaceous period, sometime between 100 million and 150 million years ago — a time when dinosaurs were still the dominant form of life on the planet, but also when mammals and birds began to develop.
There have been two schools of thought on how birds developed the ability to fly — the 'ground-up' school and the 'tree-down' school. The ground-up idea says that the ability came from animals trying to jump higher, either to reach food or escape becoming food. The tree-down idea says that it developed as animals jumped from tree to tree or from tree to ground.
However, these fossils, and the fossils of other early birds and feathered dinosaurs, show that it is more likely a combination of the two schools. According to Xu, these fossil specimens show that it was the ground-dwelling animals that developed flight first, since their fossils show earlier development of flight feathers. After that, flight developed further in tree-dwelling species, as later fossils (like from Microraptor) show clawed feet, which would be good for grasping the branches of trees. Fossils from after that show smaller and fewer leg feathers, and more of the scales that modern birds have on their legs.
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Although this new study is compelling, not everyone in the scientific community is convinced.
"Unfortunately no one has performed any kind of adequate functional or aerodynamic test," said Kevin Padian, a paleontologist at UC Berkeley. According to him, rather than these leg feathers helping flight, they could have created drag, which would make flight more difficult.
To help resolve these issues, Xu and his research group intend to examine more fossils from the collection, and also develop models to test the wings' aerodynamics.
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