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Ancient Melanosomes, Beta-Keratin Found in Fossilized Feathers of Early Cretaceous Bird

An international team of paleontologists has found evidence of beta-keratin and melanosome preservation in a 130-million-year-old specimen of the Early Cretaceous bird Eoconfuciusornis.

Eoconfuciusornis zhengi. Image credit: Nobu Tamura / CC BY 3.0.

Eoconfuciusornis zhengi. Image credit: Nobu Tamura / CC BY 3.0.

Eoconfuciusornis, a genus of crow-sized primitive birds from the Early Cretaceous Dabeigou and Huajiying Formations of China, dating from 135 to 125 million years ago, are the earliest birds to have a keratinous beak and no teeth, like modern birds.

Previous studies argued that the feathers of these and other early birds and dinosaurs preserved small, round structures interpreted to be melanosomes – pigment-containing organelles that, along with other pigments, give feathers their color.

However, without additional evidence, it was not possible to prove that these structures weren’t just ancient bacteria that had coated the feather during decomposition and fossilization.

“Microbodies associated with feathers of both non-avian dinosaurs and early birds were first identified as bacteria but have been reinterpreted as melanosomes,” the authors said.

“Whereas melanosomes in modern feathers are always surrounded by and embedded in keratin, melanosomes embedded in keratin in fossils has not been demonstrated.”

“We provide multiple independent molecular analyses of both microbodies and the associated matrix recovered from feathers of a new specimen of the basal bird Eoconfuciusornis from the Early Cretaceous Jehol Biota of China.”

New specimen of Eoconfuciusornis collected from the Early Cretaceous lake deposits in Hebei, northern China. Image credit: Xiaoli Wang.

New specimen of Eoconfuciusornis collected from the Early Cretaceous lake deposits in Hebei, northern China. Image credit: Xiaoli Wang.

The team, led by Dr. Yanhong Pan of the Chinese Academy of Sciences, examined feathers from the 130-million-year-old Eoconfuciusornis specimen.

The researchers used both scanning and transmission electron microscopy to get microscopic details of the feathers’ surface and their internal structure.

They also utilized immunogold labeling – in which gold particles are attached to antibodies that bind to particular proteins in order to make them visible in electron microscopy – to show that filaments within the feathers were keratin.

Finally, the team mapped copper and sulfur to these feathers at high resolution.

Sulfur was broadly distributed, reflecting its presence in both keratin and melanin molecules in modern feathers.

However copper, which is only found in modern melanosomes, and not part of keratin, was only observed in the fossil melanosomes.

These findings both support the identity of the melanosomes and indicate that there was no mixing or leaching during decomposition and fossilization.

“This study is the first to demonstrate evidence for both keratin and melanosomes, using structural, chemical and molecular methods,” Dr. Pan said.

“These methods have the potential to help us understand – on the molecular level – how and why feathers evolved in these lineages.”

“This study represents a breakthrough in the study of ultrastructures of fossil feathers and has provided the methods to apply to the controversial issue of whether the microbodies in many feathered dinosaurs and early birds are really melanosomes, and sheds new light on molecular preservation within normally labile tissues preserved in ancient fossils,” added team member Prof. Zhonghe Zhou, from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China.

The findings were published online this week in the Proceedings of the National Academy of Sciences.

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Yanhong Pan et al. Molecular evidence of keratin and melanosomes in feathers of the Early Cretaceous bird Eoconfuciusornis. PNAS, published online November 21, 2016; doi: 10.1073/pnas.1617168113