Modern blue-colored birds and an ancient bird both have similar color-creating microstructures
Sven Traenkner/Senckenberg Research Institute and Nature Museum in Frankfurt
A tree-dwelling bird that lived 48 million years ago probably had blue plumage, researchers say. Scientists inspecting a fossil of Eocoracias brachyptera say they have, for the first time, identified the remnants of the color in a fossil.
The researchers examined 72 feather samples from modern birds of many different colors, and 12 samples of organic material carefully collected from the fossilized plumage of E. brachyptera. Then, the team analyzed the shape and size of a type of pigment-containing cellular structure called a melanosome found within the feathers. Melanosome shapes have been linked to particular hues in feathers and fur, helping paint a picture of ancient animals. Sausage-shaped melanosomes are thought to contain black pigment, for instance, and rounder meatball-shaped pods contain reddish-brown pigment (SN: 6/22/19, p. 14).
Blue is one of the trickier colors to achieve, though. Blue, green and iridescent feathers, like a hummingbird’s, are called structural colors because producing those colors requires a particular setup within the barbs of the feather. That setup includes a spongy, air pocket–filled layer of keratin overlying a layer of black pigment–containing melanosomes.
Keratin isn’t generally well-preserved in fossils, but melanosomes often are. So Babarović and his colleagues analyzed whether they could distinguish the shapes of melanosomes in blue-colored feathers from those of other colors.
Melanosomes of the blue-colored modern birds, as well as of E. brachyptera, did indeed have a unique shape, the researchers report June 26 in Journal of the Royal Society Interface. Those melanosomes were long (about 1,400 nanometers) and relatively wide (about 300 nanometers), larger and chubbier than melanosomes found in black feathers, and unlike those linked to reddish-brown or iridescent colors.
But the microstructure’s shape was similar to pigment-bearing melanosomes linked to the color gray, the team found. That may mean that blue and gray are evolutionarily linked. The overlap does make it difficult to know whether an ancient bird was true blue or, as is more common in modern birds, gray. But once blue has evolved within a particular family group, the color tends to continue to show up in other family members. Many of E. brachyptera’s modern relatives, such as kingfishers and kookaburras, have blue feathers, making it much more likely that the ancient bird did, too.
In bird feathers, melanosomes involved in the production of different colors come in different shapes and sizes. Black, brown and gray colors are produced by pigments in the melanosomes alone. Structural colors, whether iridescent or blue or green, are produced in two steps: Light is refracted by an air pocket–filled layer of keratin within the barbs of a feather, and an underlying layer of melanosomes absorbs the rest of the scattered wavelengths of light. But there’s one area of overlap: The melanosomes for those noniridescent structural colors are similar in size and shape to the gray pigment melanosomes.
“It’s something that hasn’t been explored as much,” says Klara Norden, an evolutionary biologist at Princeton University, who was not involved in the new study. “No one’s really looked at noniridescent structural colors before at a large scale, because we’ve never had this dataset before. It’s really exciting to have this study out there that shows the shape of these melanosomes.”
Matthew Shawkey, an evolutionary biologist at Ghent University in Belgium, notes that the difficulty in distinguishing between blue and gray, without knowledge of a fossil’s family tree, puts limits to how useful the finding can be for identifying other ancient birds’ colors.
Still, Shawkey says, “it’s a neat study, and an unexpected one.” Since the melanosomes underlying the blue-feather structures also contain black pigment, “I wouldn’t have expected them to look different” from the melanosomes involved in the production of black color, he says. “That was surprising.”
F. Babarovic et al. Characterization of melanosomes involved in the production of non-iridescent structural feather colours and their detection in the fossil record. Journal of the Royal Society Interface. Published online June 26, 2019. doi: 10.1098/rsif.2018.0921.
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