An early four-winged, feathered dinosaur appears to have been a good glider, researchers say, providing clues about the origin of flight in birds.
Scientists at the University of Kansas and China's Northeastern University collaborated to build gliding models based on the hawk-sized Microraptor gui, which lived 120 million years ago.
"We've done the scientific work and flight tests to show that Microraptor was a very successful glider," stated David Burnham of the University of Kansas.
Fossils of the Microraptor were first found by Chinese farmers in 2000. A fossil found in 2003 was detailed enough that paleontologists could count the feathers on its wings, Burnham said. More than 20 Microraptor fossils have been found.
Unlike birds, Microraptor had feathers on both its front limbs and rear limbs, so scientists have debated how the dinosaur would have positioned its wings as it glided.
"The controversy was that these animals couldn't spread their hind wings to glide," said Burnham. "But we've been able to articulate the bones in their hip socket to show that they could fly."
In the study, published in Proceedings of the National Academies of Sciences, researchers led by David Alexander of the University of Kansas built a model of the dinosaur, based on a cast of one of its fossils, out of plywood, balsa wood, carbon fibre and bird feathers.
They then attached the hind wings in three different positions and tested how well the model could glide in each configuration.
They found that the most stable wing position was a sprawling posture similar to that of modern gliding mammals, such as flying squirrels and the colugo, a tree-dwelling animal from Southeast Asia.
A competing theory, advanced by other paleontologists in 2007, suggested a "biplane" wing configuration, reminiscent of the Wright Brothers' early airplanes. The University of Kansas team found that this would have required an impossibly heavy head to maintain the animal's centre of gravity.
The researchers also said the 18-centimetre feathers on Microraptor's hind wings would have made it clumsy on land, suggesting it spent most of its time in the trees.
They said this supports the "tree-down" model for the origin of flight in feathered dinosaurs and birds, which suggests that tree-dwelling dinosaurs glided from branch to branch, leading to the evolution of flight.
Some paleontologists favour a "ground-up" model, theorizing that flight first emerged in feathered, ground-dwelling dinosaurs that hopped on two legs.