THE FEATHERED DINOSAUR MICRORAPTOR: ITS BIPLANE WING PLANFORM AND FLIGHT PERFORMANCE

Microraptor gui, a small, four-winged theropod dinosaur from the Early Cretaceous of China, provides strong evidence for an arboreal-gliding origin of avian flight. The long digits, sharp claws of the fore- and hind limbs, and the swivel wrist joint equipped the animal both for climbing trees and perching in them. Microraptor shows the presence of wing-like structures on both fore- and hind limbs. The reconstructed tetrapteryx wing planform of Microraptor, where the hindwing with asymmetric flight feathers on metatarsals supported by a laterally extended leg, conflicts with known theropod limb joints that entail a parasagittal posture of the hindlimb. Here, we offer an alternative planform of the hindwing of Microraptor that is concordant with its feather orientation and normal theropod hindlimb posture. Our analysis suggests that Microraptor held its hindwing under its body in a Z shape during gliding, where the forewing forms the dorsal wing and the metatarsal wing forms the ventral one. As a result, both wings of Microraptor resemble staggered biplane configuration during gliding. The ventral wing and horizontal tail surfaces offered inherent stability during gliding. The femoral feathers could be used as a drag brake, especially during steep gliding pounces toward prey. The feathers on the tibia were probably positioned backward in a vertical plane for streamlining. Similar but shorter tibial feathers are known in Archaeopteryx and modern raptors that would streamline the legs just before an aerial attack or during a flight with captive prey. The flight performance of Microraptor based on a computer simulation model (using mass = 0.95 kg; wing span = 0.94 m; aspect ratio = 6.7; total wing area = 0.13 m²; wing loading = 70.6 N/m²; and gliding speed = 12 - 15 m/sec) shows that its biplane wings were adapted for undulatory phugoid gliding between trees, but not for powered flapping flight. The metatarsal asymmetric feathers were lost in Sinornithosaurus, Archaeopteryx, and more derived birds to assume monoplane like configuration. It is intriguing to contemplate that perhaps avian flight, like the aircraft evolution, went through a biplane stage before monoplane was introduced.