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.
"[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."
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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