Two tiny wings entombed in amber reveal that plumage (the layering, patterning, coloring, and arrangement of feathers) seen in birds today already existed in at least some of their predecessors nearly a hundred million years ago.
A study of the mummified wings, published in the June 28 issue of Nature Communications and funded in part by the National Geographic Society's Expeditions Council, indicated they most likely belonged to enantiornithes , a group of avian dinosaurs that became extinct at the end of the Cretaceous period. (Read more about the evolution from dinosaurs to modern birds.)
While the fact that many, if not nearly all, dinosaurs were feathered has been generally accepted since the 1990s, our knowledge of prehistoric plumage until now has come from feather imprints in carbonized compression fossils and individual feathers fossilized in amber. (See dinosaurs in their feathered glory.)
But while feather imprints in compression fossils may show arrangement, they generally lack very fine detail and rarely preserve information on color, while individual feathers in amber cannot be associated with the animal they originally came from. (How did feathers evolve?)
The two new samples, weighing in at only 0.06 and 0.3 ounces (1.6 and 8.51 grams), contain bone structure, tracts of feathers, and soft tissue. They are the first Cretaceous plumage samples to be studied that are not simply isolated feathers, according to study co-author Lida Xing of the China University of Geosciences.
"The biggest problem we face with feathers in amber is that we usually get small fragments or isolated feathers, and we’re never quite sure who produced [them]," says co-author Ryan McKellar, curator of invertebrate palaeontology at Canada's Royal Saskatchewan Museum. "We don’t get something like this. It’s mind-blowingly cool."
An x-ray micro-CT analysis revealed that both samples appeared to belong to juveniles, based on bone size and stage of development. Similarities in bone structure and proportion, as well as some plumage characteristics, suggest that they may belong to the same species.
Skin, muscle, claws, and feather shafts are visible in both samples, along with the remains of rows of primary asymmetrical flight feathers, secondary feathers, and covert feathers. All are similar in arrangement and microstructure to modern birds.
Although the feathers appeared black to the naked eye, microscopic analysis revealed that the flight feathers were mostly dark brown, while the covert feathers ranged from a slightly paler brown to silver or white bands.
A Bountiful Fossil Source in a Troubled Area
Most fossils in Burmese amber come from mines in the Hukawng Valley in Kachin state, northern Myanmar. The valley is currently under the control of the Kachin Independence Army, which has been in intermittent conflict with the state for more than 50 years.
Due to the conflict, the mining and sale of Burmese amber is mostly unregulated, with the majority of the material sold to Chinese consumers who prize it for jewelry and decorative carvings.
Xing and his research team collected the fossilized wing samples used in the study in a well-known amber market in Myitkyina, the capital of Kachin state.
For scientists, the appeal of Burmese amber lies in the fact that it most likely contains the largest variety of animal and plant life from the Cretaceous period, according to David Grimaldi, curator of invertebrate zoology at the American Museum of Natural History.
"About 70 percent of the Burmese amber is barren, but the other 30 percent features phenomenal biodiversity," says Grimaldi. "Never, ever would I have predicted this level of diversity."
Precious to Science, Impure to Consumers
Because the majority of Burmese amber is used in jewelry and carvings, most fossilized inclusions, such as insects and plant life, are considered impurities that reduce the value of the finished piece. The fossils may be partially or completely destroyed during polishing. The relative darkness of the inclusions within the dark amber can also make them hard to spot before the sample are cut or polished, notes McKellar.
But feathers in Burmese amber are prized for their rarity and beauty, and are cut and polished to highlight the aesthetic value of the prehistoric plumage. Little consideration is given to maintaining as much of the specimen as possible.
The smaller of the two fossilized wing samples in the current study was nicknamed "Angel" by Xing's team because a jewelry designer originally intended to fashion it into a pendant called "Angel's Wings." When the researchers analyzed the fossil, they observed truncated wing surfaces directly on the amber surface that suggested it had been chipped off of a larger amber inclusion that may have originally included the entire early bird specimen.
Still, what Xing saw in the micro-CT analysis made his heart race, he recalls: not just single feathers, but multiple feathers associated with the bony structure of an almost 100-million-year-old wing.
"It is a real angel," the researcher proudly concludes.
Lida's future plans include establishing an institute where researchers on the ground in the Myitkyina markets can check amber coming from the mines for Cretaceous inclusions, and keep those with research value in Myanmar to be studied in partnership with local universities.
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