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First Proof: Ancient Birds Had Iridescent Feathers

John Roach
for National Geographic News
August 26, 2009
 
Just like modern-day starlings, some ancient birds had glossy black feathers with a metallic, glimmering sheen, scientists report in a new study.

The discovery is based on 40-million-year-old fossils of an unidentified bird species that were stored at the Senckenburg Museum in Frankfurt, Germany for up to 30 years. The fossils represent the first evidence of ancient iridescence in feathers.

Iridescence is caused by an interaction of light with the material that the light hits. The color changes depending on the angle of observation, like the rainbow sheen on an oil slick.

The research moves scientists a step closer to determining the true colors of extinct creatures, said study co-author Richard Prum, an ornithologist at Yale University in New Haven, Connecticut. (Read more about the search for prehistoric color at National Geographic magazine's Blog Central.)

Most of what scientists know about the hues of extinct species are best guesses, based on factors such as the color of related living species, Prum said.

"We are eagerly hoping to be able to work on some of the Chinese dinosaur feathers to try to reconstruct the colors of the feathered dinosaurs," said Prum, who received funding for his work from the National Geographic Society. (The National Geographic Society owns National Geographic News.)

Pigment Capsules

The new research builds on Prum's previous discovery of pigment-containing, organic structures called melanosomes in a hundred-million-year old black-and-white-striped feather from Brazil.

(Related: "Dino-Era Feathers Found Encased in Amber.")

Previously scientists had thought that the microscopic color capsules were bacteria that had consumed the feather during fossilization, Prum said.

The melanosome discovery "led us to start looking at feathers that were extraordinarily well preserved, and that led us to the [newly studied] fossils," he said.

Found in shale oil deposits taken from the Messel Pit near Darmstadt, Germany, the fossilized feathers contain melanosomes arranged in a tightly packed, smooth pattern—similar to the feathers of modern-day starlings and grackles.

The feather fossils were found separate from the skeleton, so the scientists are uncertain what species of bird they belonged to.

However, the feather size indicates the bird was larger than a pigeon. Only a few birds of that size are known from the Messel fossils, which will help to narrow it down.

(See photos of rodents, jewel bugs, and other ancient creatures found in the Messel Pit.)

"[The arrangement of melanosomes] implies that this guy had a black plumage with a very glossy metallic, coppery, greenish, or bluish sheen to it," said Prum, whose new research appears online today in the journal Biology Letters.

The exact details of the iridescent color depend on how light would have reflected off the melanosomes and the layer of keratin, or protein, just above them. The keratin layer decomposed during fossilization.

"What you see is a beautifully smooth surface made of the melanosomes packed together," Prum said.

"And that beautiful smooth surface is the kind of uniform layer that characterizes the melanin distribution within an iridescent feather. We don't see that in a crow or other plain black bird."

True Colors

Matthew Shawkey is a biologist who studies the evolution of feather color at the University of Akron in Ohio.

The evidence for iridescent-producing melanosomes is convincing, Shawkey said, but he urged caution in using the pigment-containing capsules to assign colors to extinct creatures.

(Explore a bizarre-dinosaur interactive.)

"The important thing to do is get a better basis of comparison by looking at a broader range of colors" in living birds, said Shawkey, who was not involved in the research.

For example, he said, researchers should try to determine if all known black birds have a comparable melanosome shape and size that differs from brown birds, and so on.

If so, the physical structure of melanosomes could then be used to assign basic colors such as black and brown with "a high probability" to fossil feathers, he said.

Fossil iridescence, he added, is trickier because of the interaction between the melanosomes and the missing keratin.

But certain patterns of melanosome arrangement may be associated with different types of iridescence.

"That would enable us to say that these are probably not only iridescent, but very bright iridescent or a kind of simple sheen."
 

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