A biplane glider Microraptor gui from China, compared with the Wright 1903 Flyer.© Jeff Martz

Meet Microraptor gui, the dinosaur for whom one pair of wings just wasn't enough. A reanalysis of its fossil remains suggests that the flying beast had two pairs of wings and held one pair above the other, 125 million years before the Wright brothers pioneered the design for human flight.

When M. gui was discovered in 2003, palaeontologists realized that it was the proud possessor of birdlike flight feathers on all four of its limbs. The discovery reignited the debate over the origins of feathers and flight (see 'Fossil boosts trees-down start for flight').

But Sankar Chatterjee, a palaeontologist at Texas Tech University in Lubbock, argues that the researchers who discovered the creature in northeastern China didn't quite grasp its mode of flight.

Soar point

Many experts assumed that M. gui 's hindlimb feathers would have formed a second pair of wings that would have worked in tandem with the forelimbs, much as a dragonfly's four wings flap together.

But Chatterjee and his colleague, aeronautical engineer Joe Templin, argue that the dinosaur's rear wings could not pivot outwards, meaning that this action would have been impossible.

Instead, they say, the creature held its second pair of wings underneath its forelimbs to add stability to its gliding flights, like the stacked wings of a biplane. As they told the this week's annual meeting of the Geological Society of America in Salt Lake City, Utah, computer simulations suggest that this is the most aerodynamically stable posture.

Winging it

A skeleton of the 125-million-year-old Microraptor gui reveals traces of feathers in the wings and legs.© Dr. Xing Xu

Microraptor gui was alive at the time when birds' ancestors were taking their first tentative leaps towards full flight. "It's intriguing to think that perhaps avian flight, like aircraft evolution, went through a biplane stage before the monoplane was introduced," Chatterjee says.

Others urge caution in interpreting the fossils, however. "If this was indeed how the feathers were oriented the animal would look like a biplane," agrees Matthew Wilkinson, who studies prehistoric reptile flight at the University of Cambridge, UK. But he adds that the rear 'wings' could also have been held vertically under the body, acting as a rudder rather than generating lift.

Chatterjee counters that Microraptor could not have stayed stable with its hind feathers in a vertical orientation. "If you put it vertically the poor guy would just end up flat on the ground," he says.

One thing on which the experts do agree is that M. gui was a good glider. According to Chatterjee and Templin's model, the dinosaur would have been well suited to swooping from tree to tree, without ever needing to take off from the ground.

Sadly, M. gui might not tell us much about today's flyers: a recent reworking of its family tree suggests that it and its relatives lived in a different lineage. "It seems that it's not directly linked to modern birds," Wilkinson says.