Microraptor was a Feathery Biplane

The ancestors of modern birds are thought to have been small, feathered, dinosaurs, the theropods. One of these small feathered dinosaurs is Microraptor gui, a feathered dromaosaur that lived 125 million years ago in what is now China. According to the evidence, Microraptor gui was one of the earliest gliders. But unlike modern birds, it appears to have utilized four wings, like a biplane, because it had long and asymmetric flight feathers on both its hands and feet.

According to initial interpretations, Microraptor flew with its legs spread out laterally, such that its four wings were configured like a dragonfly's wings. However, upon further study, Sankar Chatterjee and R Jack Templin offer an alternative hypothesis for early avian flight that is consistent with limb and feather orientation such that it produced lift while preserving a normal theropod flight posture (see below).

They propose that the "Z-conformation"; with the hindlimbs tilted upward, slightly behind the forelimbs, and the tail tilted upward to control pitch (D, below) produces the most lift and the least drag. They also compare the aerodynamics of a cylindrical tibio-tarsus versus a tibio-tarsus with caudal feathers, revealing that the feathered tibio-tarsus produces less drag (E, F, G, below).

"We had no other choice but to go for the bi-plane configuration," said Chatterjee.

Chatterjee and Templin point out that the tibio-tarsus aerodynamics of a gliding Microraptor are similar to a modern day raptor pouncing on prey from the air and carrying it (H, below).

They then compare Microraptor to a typical staggered biplane of the 1920s, noting that the biplane aircraft had a large additional drag due to wires, struts, etc. between the two wings, which eventually made the biplane obsolete except for a niche application; whereas such drag-induced structures were absent in Microraptor. Life reconstruction of the M. gui (IVPP V13352) in a dorsal view showing the morphology and distribution of hindlimb feathers (J; left side) and orientation of the hindlimb (J; right side) during gliding.

Using this anatomical model, the evidence indicates that Microraptor was not capable of ground or running takeoff, because it lacked the supracoracoideus ("wishbone") pulley to elevate its wings. Moreover, running takeoff would damage its ventral metatarsal wings. Instead, Chatterjee and Templin show how Microraptor efficiently glided between perches that could have been 40 meters distant using phugoid gliding (curve 1, below).

This research also sheds light on a contentious debate over the evolution of bird flight. Some researchers argue that this evolutionary development occurred from the ground up (the "cursorial model"). Others contend that small, feathered dinosaurs were already living in treetops and developed flight in order to get from one tree to another (the "trees down model").

Chatterjee and Templin favor the "trees down" model.

Alternatively, this biplane-phase of flight, the first time it has ever been seen in nature, could just represent a failed evolutionary experiment.

This study appears in the peer-reviewed journal, Proceedings of the National Academy of Sciences.

Sources

Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui, by Sankar Chatterjee and R. Jack Templin Proceedings of the National Academy of Sciences (doi:10.1073/pnas.0609975104) [PDF]

Contributing story.