Four wings bad, two wings better - for early birds

Before birds could fly on two wings, they had to ditch their training wings. This idea, first put forward a decade ago, is gaining credence with the discovery of 11 early bird fossils that seem to suggest that birds ? which evolved from feathered dinosaurs ? once had four wings.

In 2003, a group of Chinese researchers excavated six fossils of flying dinosaurs with wings on their hind legs. They called the creatures Microraptor.

"When we published that ten years ago there was some suspicion whether the fossil was faked," says Xing Xu from Linyi University in Shandong province. Since then, more four-winged dinosaurs have been found, but doubt remained about whether they were ancestors of modern birds, or whether they simply died out rather than evolved, says Xu.

Now Xu and colleagues have described another find supporting their claim. In a collection at the Shandong Tianyu Museum of Nature, they came across nearly 2000 complete early bird fossils excavated 10 to 15 years ago. These included a series of bird skeletons that have large feathers on their hind legs, which Xu thinks were likely to have been for flight rather than display. All 11 specimens lived in the Early Cretaceous, about 120 million years ago, and seem to fall into several primitive bird groups.

The find means the branch on the tree of life that contains modern birds is now surrounded by animals that look as if they had four wings, leaving little room for four-winged dinosaurs to be an evolutionary dead end, says Steve Salisbury of the University of Queensland in Australia, who wasn't involved in the work.

Getting off the ground

Traditionally, there have been two schools of thought about what drove the evolution of modern bird flight. The "ground-up" school believe it occurred when small animals tried to jump higher off the ground. The "tree-down" school believe flight evolved in animals jumping from trees.

But the array of early bird and feathered dinosaur fossils hints at a more complicated story: it seems it was a mix of the two. The very beginnings of flight probably evolved in ground-dwelling dinosaurs, says Xu, because they show early signs of flight-feathers.

After that, it looks as if flight further developed in tree-dwellers because dinosaurs that appear to have more advanced flight systems, like Microraptor, have features, such as sharply curved claws, suggesting they lived in trees.

But the story doesn't end there. The new fossils suggest that modern bird flight ? with just two wings ? developed when birds shifted again, back to living on the ground.

Birds are unique in having two completely separate locomotion systems, says Xu. They use their rear limbs for walking and their front limbs ? or wings ? for flying. But it was only when birds started running on the ground that these systems became specialised.

"If an animal has big feathers on its legs and feet, it's definitely something that's not good for fast running," says Xu. He says that one of their specimens seems to suggest this. It lived on the ground, and seems to be in a transitional stage, with smaller hind feathers and the appearance of hind scales, like those of modern birds.

Kevin Padian at the University of California, Berkeley, thinks the find is important, but is not convinced the hind feathers in these animals were actually wings. "Unfortunately no one has performed any kind of adequate functional or aerodynamic test."

Salisbury is more enthusiastic. He says the asymmetrical, airfoil shape of the feathers is good evidence they evolved for flight. "If it's just display or warmth, having them with an aerodynamic shape is hard to explain."

However, he isn't certain that Xu has the whole evolutionary story right. It's hard to tell from the animals' features where they dwelled. "A goat can get in a tree," he jokes.

Xu says his team is now going to analyse more specimens in the collection as well as build models to test the aerodynamics of the wings.

Journal reference: Science, doi: 10.1126/science.1228753

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