The Alula at Dawn

By Tom Waters|Wednesday, January 01, 1997
RELATED TAGS: DINOSAURS
The oldest known bird, Archaeopteryx, looks marvelous as a fossil flattened into 150-million-year-old limestone. But it probably moved much less gracefully. Its long tail and unsophisticated wings have led many paleontologists to conclude that it was a rather goony flapper, with little gliding ability and poor control during takeoffs and landings. The transition to the masterful flight that modern birds display has for decades been mostly a matter of speculation. But last August José Sanz and colleagues at the Autonomous University of Madrid, working with Luis Chiappe of the American Museum of Natural History in New York City, reported the discovery of a 115-million-year-old fossil that marks the oldest known record of modern flight.

The new bird was a goldfinch-size creature that waded or possibly even swam in the shallows of a Cretaceous Period lake near Cuenca, Spain. Fossilized shrimp in its belly attest to its diet. In most ways the bird’s anatomy is typical of its family, the Enantiornithes, which was the dominant group of the Cretaceous and went extinct 70 million years ago. All of them had a fat, fleshy tail and oddly fused ankle bones, and most had teeth. But in one way the Cuenca fossil is not typical at all: its feathers are impeccably preserved.

Feathers are very rare in the fossil record, says Chiappe, and even more rare are fossils where you get the feathers and bones together. Still more rare are fossils where the feathers are in the right position. Usually, the feathers are around the body but you don’t get to see the actual insertion in the bone.

But because the fossil from Cuenca reveals the precise insertion of the wing feathers, Sanz, Chiappe, and their co-workers were able to determine that the bird’s first digit had a tuft of feathers called an alula. While easy to overlook, the alula is essential to a modern bird’s aerodynamics. A bird approaching the ground needs to slow down in order not to crash, and it does that by rotating its wings to a steeper angle. This maneuver has a potential pitfall: since air can no longer glide smoothly over the top of the wing, it breaks up into turbulent eddies. The imbalance of low-pressure turbulent air behind the bird’s wings and smooth-flowing high-pressure air in front creates more drag than the bird wants, and it stalls and drops to the ground--unless it happens to have some anatomic turbulence-buster.

That’s the alula. When pressure drops on top of the wing, Chiappe explains, the bird automatically lifts the alula, which means it is lifting its first digit. This creates a slot between the main wing and the alula. Air slips through the slot and glides smoothly over the top of the wing, and the bird slows without stalling.

Archaeopteryx did not have an alula, which suggests to Chiappe that it couldn’t land on a tree branch or snatch prey on the ground. When it came to Earth, it hit the ground running. But this new fossil bird (which the researchers have dubbed Eoalulavis--dawn alula bird) suggests that the crucial change in flight was not a recent one. Modern birds descend from a branch separate from that of the Enantiornithes. Since it now appears that both major groups of birds had identical alulas, the simplest explanation is that they inherited them from their common ancestor. Given that the earliest Enantiornithes lived about 140 million years ago, Earth has been graced with graceful landings for a long time.
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