Tuesday, May 4, 2010

Bird and Pterosaur Wing - Continued (3)

Now that we have looked at the pteroid bone which is the pterosaur's thumb, let's look at the corresponding thumb in the modern bird. It is called the "alula".

"The alula, or bastard wing, is a small projection on the anterior edge of the wing of modern (and some ancient) birds. The alula is the freely moving first digit, a bird's "thumb," and is typically covered with three to five small feathers, with the exact number depending on the species. Like the larger flight feathers found on the wing's trailing edge, these alula feathers are asymmetrical, with the shaft running closer to anterior edge."

"In most situations, the alula is held flush against the wing; however, it can be manipulated. When flying at a slow speeds or landing, the bird moves its alula slightly upwards and forward, which creates a small slot on the wing's leading edge. This functions in the same way as the slats on the wing of an aircraft, allowing the wing to achieve a higher than normal angle of attack – and thus lift – without resulting in a stall.[1] During stretching of the wing down toward the ground, the alula is abducted from the wing and can be clearly viewed."

"The presence of an alula has been confirmed in several now-extinct ancient relatives of modern birds, including Eoalulavis hoyasi (an enantiornithine from the mid-Cretaceous, 115[2] mya) and the earlier Protopteryx fengningensis. Since these species are not closely related to modern birds, either the alula evolved twice, or it did so more than 130 mya."

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