Posted by tags: researchblogging.org, melanosomes, plumage color, feather color, fossil preservation, birds, dinosaur, Jakob Vinther
When looking at paintings and reconstructions of fossil birds and dinosaurs, people often ask “how do you know what color they were?” Well, we didn’t. However, a new paper was just published in Biology Letters that explores the possibility of deciphering the actual color of fossilized plumage and makes a startling discovery: scientists can identify at least some of the original colors in ancient feathers.
In sharp contrast to mammals, whose colorations are really very boring, birds are colorful — many species are stunningly so. But colors are expensive and wasteful to produce if they can’t be used to communicate a particular message that can be seen by the intended recipient. In fact, birds evolved colors to send signals to other birds. They also evolved the visual structures in their eyes necessary to perceive those colors and they developed behaviors designed to draw attention to their plumage coloration. Which leads one to ask; what colors were ancient birds and feathered dinosaurs?
Paleontologists have found feather fossils in roughly 50 rock formations ranging from 200 million years old to about 2 million years old. Yale University graduate student, Jakob Vinther, is examining fossilized bird feathers and during his studies, he noticed that some feather fossils have dark and light stripes. These organic “fingerprints” in fossilized feathers have often been assumed to result from carbon contained inside ancient bacteria that were fossilized as the feathers decayed, but Vinther compared the microstructures in these stripes to those found in modern day bird feathers and discovered that they are almost identical to melanomes.
Melanosomes are intracellular organelles that contain the melanins, a class of organic compounds that are the most common of all light-absorbing pigments found in protists, plants and animals. These pigments are the source for a variety of colors ranging from yellows to black.
“Feather melanin is responsible for rusty-red to jet-black colors, and a regular ordering of melanin even produces glossy iridescence,” observes Vinther.
Melanosomes are surrounded by a lipid membrane and are generally shaped like a sausage (figure 1);
Figure 1: Cretaceous feather ultrastructure compared with that in a living bird. (a) Feather from the Crato Formation, Early Cretaceous, Brazil (Leicester University, U.K., Geology Department, LEIUG 115562) showing colour bands; margins of colour bands are similar to those found in living birds and barbules are clearly preserved (i). (b) Dark bands, composed of aligned eumelanosomes, contrast with (c) light areas that reveal only the rock matrix. (d) A broken barbule from a modern Red-winged Blackbird (A. phoeniceus, Aves: Icteridae, Yale Peabody Museum 1047) reveals eumelanosomes aligned along the barbule enclosed in a keratin matrix. Scale bars, (a) 3 mm, insert 1 mm; (b) 1 μm; (c) 10 μm; (d) 1 μm. DOI: 10.1098/rsbl.2008.0302 [larger view].
To do this work, Vinther and his colleagues used a scanning electron microscope to analyze a striped feather fossil found in 100 million-year-old Brazilian rock. The dark bands of the feather contained pigment-bearing structures that looked similar to the melanosomes found in a black feather from a red-winged blackbird, Agelaius phoeniceus, whereas the light bands revealed only the surface of the rock.
Were these structures present in other fossilized feathers from different times? To answer this question, Vinther and his colleagues used the same technique to analyze 55 million year old feathers from a bird fossil originally unearthed in Denmark. They found similar sausage-like structures in the feathers surrounding the skull of this bird (figure 2);
Figure 2: (a) Skull of undescribed bird from the Fur Formation, Early Eocene, Denmark (Danekræ 200, MGUH 28.929), preserving feathers and the eye as an organic film. (b,c) Details of the feather region showing aligned eumelanosomes. (d) Detail of the eye showing elongate and oblate eumelanosomes. (e) TEM of a section through the retina of a modern bird (Caprimulgus vociferous, Caprimulgidae). Scale bars, (a) 10 mm; (b) 1 μm; (c) 5 μm; (d) 1 μm; (e) 5 μm. DOI: 10.1098/rsbl.2008.0302 [larger view].
Interestingly, they also discovered melanosomes in the fossil bird’s eyes, and found they are similar to melanosomes in the eyes of modern birds, suggesting that scientists might also be able to identify colors in fossilized animal eyes.
Vinther and his colleagues think they can identify brown, red, buff and even iridescent colors. Certainly, other studies of modern birds have revealed that a variety of plumage colors result from distinct arrangements of their melanosomes, so it is plausible to think that scientists could identify more ornate plumage colors in ancient birds and dinosaurs.
“Birds frequently have spectacularly colored plumage which are often used in camouflage and courtship display,” Vinther noted. “Feather melanin is responsible for rusty-red to jet-black colors and a regular ordering of melanin even produces glossy iridescence. Understanding these organic remains in fossil feathers also demonstrates that melanin can resist decay for millions of years.”
The next step is to investigate the structure of fossilized feathers from an ancient bird thought to be closely related to a living bird with varied and colourful plumage.
“With luck the microstructure of feathers from different parts of the fossil will vary corresponding to different original colors,” notes coauthor Derek Briggs, a paleontologist at Yale University.
This feature of melanin would allow scientists to begin deciphering a variety of behaviors that accompany plumage colorations, thereby adding detail to our knowledge of the lives led by fossil birds and dinosaurs.
“We might .. be able to tell whether they have sexual dimorphisms — whether males were more spectacularly coloured than females,” Vinther speculates.
Sources
Vinther, J., Briggs, D.E., Prum, R.O., Saranathan, V. (2008). The colour of fossil feathers. Biology Letters, -1(-1), -1–1. DOI: 10.1098/rsbl.2008.0302.
Yale University press release (quotes).
July 10, 2008
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