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Dino-Era Bird Flew With Four Wings, Study Says

James Owen
for National Geographic News
September 28, 2006
 
Four wings were better than two for the earliest birds, which became airborne by spreading both their feathered arms and legs, a new study says.

The research suggests that the ancestors of modern-day birds first took to the skies by gliding down from trees.

This study is based on well-known fossils from Germany of Archaeopteryx, the most primitive known bird (related news: "Dinosaur-Era Bird Could Fly, Brain Study Says" [August 2004]).

The fossils were re-examined by paleontologist Nick Longrich from the University of Calgary in Canada, who found that the flying dinosaur's leg feathers have an aerodynamic structure and likely acted as lift-generating "winglets."

Longrich said that his research "puts forward some of the strongest evidence yet that birds descended from arboreal parachuters and gliders, similar to modern flying squirrels."

The study supports the "tree down" theory for the origins of avian flight.

This theory suggests that the immediate ancestors of birds were tree-dwelling dinosaurs that developed the ability to glide and paved the way for self-propelled flight.

The competing "ground up" hypothesis argues that species of terrestrial dinosaurs gave rise to birds by running at high speeds and evolving rudimentary wings that lifted them off the ground.

Stripped Fossil

Fossils of the dino-era bird Archaeopteryx, which lived some 150 million years ago, were first discovered in 1861. Eight further specimens were subsequently unearthed.

While researchers at the time noted feathers on the creature's hind limbs, these were generally thought to be used for insulation or display, not for flying.

Longrich examined the structure and function of Archaeopteryx's hind-limb plumage using the fossil record, including the so-called Berlin specimen.

Found in 1877 and now on display at the Museum of Natural History in Berlin, the fossil bird had unusually long feathers around its legs.

Some researchers broke ranks more than a century ago to suggest that Archaeopteryx's leg feathers did play a role in flight.

But, Longrich says, the issue was later forgotten when much of the hind limb plumage of the Berlin specimen was destroyed.

"The specimen was prepared sometime in the late 19th or early 20th century," the paleontologist said. "Most of the hind limb feathers were removed from the right leg to expose the bones of the legs and pelvis."

All that remains of those feathers are photographs, a cast, and a counter slab of limestone bearing an imprint of the feathers.

"What remains of the plumage on the main slab is not nearly as impressive," Longrich added.

Still, closer analysis of the fossil revealed to Longrich that the feathers have features needed for flying, such as curved shafts and asymmetrical vanes, which would have acted as airfoils to help generate lift.

Writing in the current issue of the journal Paleobiology, Longrich notes that several living species use their hind legs as airfoils to glide from trees, including types of lizards and frogs.

The use of all four limbs while airborne is likewise seen in tree-dwelling animals such as flying squirrels (wallpaper: flying squirrel).

Setting the Stage

But, the researcher said, the main reason Archaeopteryx was initially overlooked as a four-winged bird was because the idea seemed "just too improbable, too foreign."

"Everybody knows that birds don't have four wings, so we overlooked them even when they were right under our noses," he said.

That changed when several fossils of a new four-winged dinosaur were discovered in China in 2002, prompting researchers to reassess Archaeopteryx's leg feathers.

Chinese fossil experts suggested that the species, Microraptor gui, used its plumed limbs and a long, feather-fringed tail to glide between trees (read "Four-Winged Dinosaurs Found in China, Experts Announce" [January 2003]).

"I never would have considered the idea of four-winged birds," Longrich added.

"But after the Chinese four-winged dinosaurs shattered my preconceptions, I was willing to say, Why the heck not?"

Archaeopteryx may mark a transitional evolutionary stage between Microraptor-like dinosaurs and modern birds, Longrich says.

"The other possibility is that Archaeopteryx represents the ancestral condition," he added, with one branch of the family tree leading to advanced birds with reduced hind wings.

"The other branch—Microraptor and allies—went off in the other direction and evolved even larger hind wings."

The new study apparently undermines the "ground up" theory to explain the origins of bird flight.

"'Ground up' seems to be more popular these days," Longrich said. "I think the reasoning goes that birds descend from dinosaurs, dinosaurs were terrestrial, so birds must descend from terrestrial animals.

"However, a lot of people feel that an arboreal origin makes a lot more sense, and not all paleontologists are so quick to rule out the idea that birds could come from arboreal dinosaurs."

Joel Cracraft, curator of birds at the American Museum of Natural History in New York, said Longrich's research "will be studied and talked about in all discussions dealing with the evolution of avian flight."

He said the study "nails" the idea that Archaeopteryx had an airfoil on its hind limbs and agrees that it suggests the primitive bird was a good glider.

But, Cracraft added, "whether [the study] also nails the 'tree down' hypothesis is another matter, mainly because the dichotomy between 'tree down' and 'ground up' is too simplistic for my mind."

"Clearly components of flight evolved in [running] dinosaurs," he added. "Our knowledge should increase as more fossils are studied."

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