The first flying dinosaurs took to the air in a similar way to a World War I bi-plane, a study shows.
Microraptor was a small, feathered dinosaur
A fresh analysis of an early feathered fossil dinosaur suggests that it dropped its hind legs below its body, adopting a bi-plane-like form.
This contrasts with earlier reconstructions showing the dinosaur maintaining its wings in a tandem pattern, a bit like a dragonfly.
Details appear in Proceedings of the National Academy of Sciences journal.
The ancestors of modern birds are thought to have been small, feathered, dinosaurs.
Microraptor gui, which lived 125 million years ago, was one of the earliest gliders. It appears to have utilised four wings, as it had long and asymmetric flight feathers on both its hands and feet.
An initial assessment of Microraptor fossils from China suggested the animal spread its legs out laterally and maintained its wings in a tandem pattern, in a similar manner to dragonflies.
Now, researchers Sankar Chatterjee and R Jack Templin offer an alternative hypothesis.
Their evaluations of the limb joints and feather orientation indicate that a tandem wing design would neither have achieved suitable lift, nor enabled Microraptor to walk on the ground easily.
The evolution of bird flight may mirror aviation history
Instead, the scientists report that its hind legs were positioned below the body, in a bi-plane fashion.
Dr Chatterjee, from Texas Tech University in Lubbock, US, explained that two lines of evidence had led the team to this conclusion.
Firstly, the researchers argue, dinosaurs and birds move their legs in a vertical plane, not sideways as the tandem flight pattern requires.
Secondly, the feathers on Microraptor's hind legs are asymmetrical; one of the two vanes that extend either side of the shaft is narrower than the other.
Aerodynamically, the narrow leading edge of these feathers should face forward in flight, against the direction of airflow. This would have given the flying reptiles lift.
In the tandem pattern, these would have faced sideways.
"We had no other choice but to go for the bi-plane configuration," Dr Chatterjee told BBC News.
A computer flight simulation using this design showed that Microraptor would undulate up and down, an ideal approach for gliding between trees.
The similarities may reflect a common solution to the same problem (Image: Jeff Martz)
The research might also shed light on a contentious debate over the evolution of bird flight.
Some researchers argue that this evolutionary development occurred from the ground up. Others contend that small, feathered dinosaurs were already living in treetops and developed flight in order to get from one tree to another.
This "trees down" model is the one favoured by Chatterjee and Templin.
Alternatively, the bi-plane-like phase could just represent a failed evolutionary experiment.
The Wright stuff
If one accepted the evolutionary importance of the bi-plane formation, there were striking parallels between bird flight and the development of aircraft, said Dr Chatterjee.
Archaeopteryx, regarded as the earliest fossil bird, has what could be described as a monoplane design.
The shift from a bi-plane to a monoplane design could have been facilitated by a much broader wingspan which would have provided increased lift. This mirrors historical developments in aviation.
"We see that the Wright brothers came up with a design for which there was no precedent in nature at the time," said Dr Chatterjee.
"This shows us that if there is a problem in engineering, sometimes there are only one or two possible solutions."