Prehistoric dino-bird wore feathery pants, say scientists. But why?

A new fossil of an early bird, Archaeopteryx, displays evidence of long feathers around the animal's legs, researchers say. But these feathers may not have been for flight.

A pantaloon-wearing dino-bird sounds like a cartoon character. But researchers say its attire merits more than a chuckle.

This 150-million-year-old fossil complicates researchers' understanding of the evolution of flight. The new specimen, discovered in a collection in Germany, displays a nearly complete set of feathers, according to a new study published in Nature.

"For a long time, people have assumed that the evolution of feathers went hand in hand with the evolution of flight," said study author Oliver Rauhut of the Bayerische Staatssammlung für Paläontologie und Geologie in Munich, Germany. But now Rauhut and his coauthors say feathers evolved separately from flight.

The fossil is an Archaeopteryx, an animal often considered the first bird or a transitional animal between ancient dinosaurs and modern birds. But this is the first fossil of its kind to have nearly complete plumage preserved.

Why would a bird need feathery pants?

Researchers found clear evidence of long feathers extending along the hind legs of this bird, dubbed "trouser" feathers. As the feathers didn't extend to the animal's feet, researchers say these could not be evidence of a second set of wings, as many had thought before.

Last year researchers in China found fossils of feathered dinosaurs with long feathers up and down their entire hind limbs. These researchers suggested that birds went through a four-winged stage while developing flight before using just two wings.

As the Chinese specimens were more primitive than Archaeopteryx, Dr. Rauhut says you would expect to see feathers covering the entire legs of the bird too.

The newly discovered leg feathers also more closely resemble the feathers found on Archaeopteryx's body than on its wings.

Rauhut and his colleagues compare them to leg feathers commonly found on modern birds of prey, such as eagles or hawks. These raptors often have feathers that grow tightly on their legs. These feathers primarily protect the bird's legs from prey struggling in their grasp. But these feathers also act like parachutes, supporting the bird in slow flight or landing. Rauhut says the trouser feathers could have been used similarly by the Archaeopteryx.

Then again, the pants could also be completely unrelated to flight. Rauhut points out that modern fast-running birds like ostriches use feathered limbs for maneuverability. The feathers actually help the bird balance. The Archaeopteryx could also have used the leg feathers for display, camouflage, or protection of young.

Rauhut says it is likely these feathers performed an important display function. "The display structures are always important for mating. That is, of course, how you get along as a species."

"It seems that that might have been even the primary function that feathers were for," Rauhut says of display. When it was time to fly, "They already had these structures, and could then use them to develop aerial locomotion."

Could Archaeopteryx take to the sky?

Although the trouser feathers didn't necessarily help Archaeopteryx fly, both the structure of the wings and the length of the bird's feathery tail contribute to aerodynamics, says Rauhut.

The structure of this new specimen's wings is similar to that of modern birds. Furthermore, the researchers found aerodynamic feathers on the sides of the tail, which would have thus had a job in aerial locomotion, says Rauhut.

The tip of the tail was missing in the other Archaeopteryx fossils, as the rock was broken or cracked there. But this new fossil tells a tale of a long, forked tail, similar to that of a swallow although less dramatically split. These features would certainly have been conducive to flight, says Rauhut. But "there is still debate about how well it could fly."

Is this the original bird?

This fossil is the eleventh Archaeopteryx to be found since 1861, when the first was found. As the first specimen was found in Germany soon after Charles Darwin published his "On the Origin of Species," the Archaeopteryx, known as the urvogel or "first bird" was quickly hailed as a transitional animal between the dinosaurs and modern birds.

But paleontologists today are divided about the precise relationship of Archaeopteryx to modern birds. Some researchers say that it is truly deserving of the name “first bird,” while others say it is merely one step in the evolutionary process towards modern birds.

Furthermore, other researchers think Archaeopteryx may not even have a place in the lineage of modern birds. Instead, it may be a cousin, perhaps like chimpanzees are to Homo sapiens, who diverged from a common ancestor about 6 million years or so ago. But Rauhut is confident that his research places Archaeopteryx as an ancestor of modern birds.

Is Archaeopteryx a bird or a dinosaur?

"What is a bird?" says Rauhut. "Thirty years ago that question seemed very easy to answer. If it had wings and feathers then it was a bird." But as paleontologists began finding fossils of feathered dinosaurs that couldn't fly, "that question has become more and more difficult."

"Scientifically speaking, we restrict the term aves, the latin name for birds, to modern birds and their direct ancestors," says Rauhut. "In that respect Archaeopteryx would not be a bird," he admits, because it is not the ancestor immediately preceding modern birds. But traditionally Archaeopteryx has been considered the "primary bird" so Rauhut calls the animal a bird.

Technically speaking, birds are dinosaurs. Modern birds are the only surviving members of the taxonomic group dinosauria. So, regardless of whether it is a bird, Archaeopteryx is certainly a dinosaur. The distinction lies somewhere along the transition from reptilian features to avian ones.

Unanswered questions

The role that Archaeopteryx's feathers play has been difficult to study for lack of feathers in the other ten specimens. "One of the problems is that, obviously, feathers are not part of the hard tissue, so they're very rarely preserved in the fossil record," says Rauhut. "We have to work with relatively scratchy evidence."

"There are still a lot of open questions," says Rauhut. He and his colleagues will look at the bones of the new specimen to better understand the physiology of the animal.

"What we need is more finds," says Rauhut. A big question remaining for researchers is how well Archaeopteryx could fly, if at all. "Without having a time-machine and actually looking at the animal flying, that's going to be extremely difficult to answer."