|The biplane-maniraptor: four-winged Microraptor gui. Photograph courtesy of NOVA.|
The discovery of winged legs has fueled the debate as to whether avian flight evolved from the ground up, or from the trees down. The issue is not so cut-and-dry as one might expect, though with more and more fossil evidence, we are discovering more clues as to how flight first evolved among dinosaurs. I support the theory that, while wings evolved from the ground up, the power of flight originated in the trees. Thor Hanson's Feathers provides a great, comprehensive chapter on the entire argument of wings, flight, and this argument.
|Wings evolved in many flightless families, and had (and have) many functions besides flight. From Heers & Dial, 2012.|
|Sapeornis chaoyangensis, an avialian from Yixian and Jiufotang Formations of China. The greatly-reduced leg feathers can been seen around its ankles. Photograph courtesy of Science/AAAS.|
Clearly, birds didn't just do away with maneuverability. Birds have mastered the art of flying more than any other animal, and can turn on a dime even in midair. What, then are they utilizing instead of these leg feathers in order to maintain, and improve, their aerial acrobatics?
|The remiges (flight feathers on the arm) and rectrices (flight feathers on the tail) |
of a Cape May warbler. Photograph by Christopher Hansen.
Whether or not the evolution of the modern avialian tail caused, or was caused by, the loss of leg feathers remains to be seen, but I believe this it to be a valid hypothesis. Perhaps a future review of avialian fossils will reveal more crucial clues about the evolution of tails, which, so far as I have seen, have been largely overlooked when discussing the evolution of flight.
Sheridan, Kerry. “Early Birds Had Four Wings, Not Two, Study Reports.” 2013. Accessed March 15. http://phys.org/news/2013-03-early-birds-wings.html.
Zheng, Xiaoting, Zhonghe Zhou, Xiaoli Wang, Fucheng Zhang, Xiaomei Zhang, Yan Wang, Guangjin Wei, Shuo Wang, and Xing Xu. 2013. “Hind Wings in Basal Birds and the Evolution of Leg Feathers.” Science 339 (6125) (March 15): 1309–1312. doi:10.1126/science.1228753.
For more about dinosaurs with leg feathers, see:
The feathered dinosaur that didn't force anyone to rethink anything about bird evolution (February 5, 2013)