PADIAN, K.*; DIAL, K.P.; University of California, Berkeley; University of Montana, Missoula: Was there a “four-winged” transitional stage between dinosaurs and birds?

The recent discovery of long leg feathers on the first known bird, the Jurassic Archaeopteryx, a related small non-avian Cretaceous theropod, Microraptor, and a basal Cretaceous enantiornithine bird has fueled speculation that the leg feathers were used in a four-winged gliding stage antecedent to the evolution of powered flight in birds. Proponents argue that the asymmetry of Microraptor�s feathers shows that they were used in flight, and that its claws show that it was arboreal. This scenario has already been adopted uncritically in some quarters, both professional and popular. However, it has not yet been adequately documented or tested. Arboreality is a moot issue because most small tetrapods can climb trees and other heights, and claw geometry alone does not dictate arboreal or cursorial habits. Feather asymmetry or symmetry should be treated with caution when interpreting flying ability. Asymmetrical, curved feathers do not confer flight: they resist torsion and reduce drag; the long leg feathers on raptors are not used to gain lift. Moreover, forelimbs that bear only symmetrical feathers, as in developing ground birds, confer substantial and functional aerodynamic force. Reconstructions of an all-out 4-winged gliding stage appear plausible but require a distension of the hip joint that is difficult to justify anatomically. An assessment of the center and distribution of mass, the configuration and area of the airfoil, and of estimated gliding performance is needed to test claims of gliding ability. On the other hand, a simpler alternative model would allow long leg feathers to help slow the descent of a small animal leaping from a height, whether or not the descent was also slowed by stalling oscillations of the feathered forelimbs. There is as yet no evidence that long leg feathers played any role in the evolution of powered flight.