From biplanes to jets: the evolution of bird wings

When the diminuitive gliding microraptorine Microraptor was described, it was an oddity: it sported two winged arms, which was becoming more and more acceptable for non-avian dinosaurs, as well as two winged legs.

The biplane-maniraptor: four-winged Microraptor gui. Photograph courtesy of NOVA.

More recently, several other genera have been found with well-developed leg wings, as well as winged arms. But what use do wings on your legs serve? The nature of these wings, absent in any feathered flyer today, was debated and argued over for many years; it is now accepted that these wings provided their owners with stability and increased maneuverability while gliding from tree to tree.

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.

As always, beautifully preserved Chinese fossils reveal that these leg wings were relatively commonplace among gliding maniraptors, though they had never been found in birds. That is, up until now. A review of basal avialian fossils from China reveal that a number of early Cretaceous species sported largely reduced leg feathers, revealing some interesting details about the evolution of flight (Zheng et. al, 2013).

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.

Because the leg feathers of these specimens are so reduced, clearly they had to start in a much more developed state. It appears that birds, too, utilized the biplane method of locomotion in their earliest days: as they evolved from gliders to true flyers, these leg wings became more of a hindrance, and slowly they began to evolve smaller and smaller feathers. As the winged arms of these early birds became more and more advanced, slowly losing their claws, becoming more and more aerodynamic, and containing the characteristic powerful muscles needed for flight, the feathers on the legs became smaller and smaller.

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.

While we commonly associate flight feathers with wings, these same feathers can be found in the tails of many birds. These feathers, called rectrices (singular: rectrix), allow birds to brake and steer while flying. Perhaps, as the leg feathers of early birds became more and more reduced, their tails became more advanced, providing them with a more aerodynamic, yet still very effective, way to control their flight.

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.

References

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)