Pigments stick around in fossil feathers

The finding has big implication for interpretations of fossil color, says Ryan Carney. The work backs up past proposals that Anchiornis (below) had some dark black feathers. (Credit: Quinn Dombrowski/Flickr)

Can scientists really figure out the coloration of long-lost animals? It could be possible, according to chemical evidence that the fossilized remains of a bird-like dinosaur still harbor melanosomes and the pigment they produced.

In the journal Scientific Reports, paleontologists correlate the distinct chemical signature of animal pigment with physical evidence of melanosome organelles in the fossilized feathers of Anchiornis huxleyi, a bird-like dinosaur that died about 150 million years ago in China.

Over the last several years, scientists have hotly debated the idea that melanosomes, which produce melanin pigment, are preserved in fossils. Microscopic traces that to some scientists seem to resemble melanosomes, appear to skeptics as similar-looking bacteria instead. The new study resolves the debate, says coauthor Ryan Carney, a graduate student at Brown University, by adding a powerful second line of evidence: chemistry.

bird-like dino fossil
A fossilized Anchiornis huxleyi, a bird-like dinosaur, carries evidence of pigment and the subcellular organelles that made it. (Credit: Thierry Hubin/RBINS)

“We have integrated structural and molecular evidence that demonstrates that melanosomes do persist in the fossil record,” says Carney, who helped design and write the study. “This evidence of animal-specific melanin in fossil feathers is the final nail in the coffin that shows that these microbodies are indeed melanosomes and not microbes.”

The finding has important implications for the interpretation of both past and future studies on fossil color, Carney says, and substantiates prior proposals that Anchiornis had some dark black feathers.

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In the new study, led by Johan Lindgren of Lund University in Sweden, the team used electron microscopes to observe what appear to be rod-like melanosome structures and imprints within the barbules of feathers all over the body.

That morphological evidence alone, however, would not advance the debate, so in addition the team performed two different kinds of chemical analyses to see if they could detect animal eumelanin pigment. They used both time-of-flight secondary ion mass spectrometry and infrared reflectance spectroscopy to discern the molecular signature of melanin in the samples.

They compared those observed signatures with the signatures of modern-day animal eumelanin. The melanins were virtually identical, except for minor contributions from sulfur in the fossil, Carney says.

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The researchers also analyzed the observed spectral signatures to compare them with melanins produced by various microbes, just to make sure that the pigments were not from any other source. The closest spectral agreement remained with an animal source, however.

“This is animal melanin, not microbial melanin, and it is associated with these melanosome-like structures in the fossil feathers,” Carney says.

Furthermore, no other types of molecules from potential microbes were detected.

The Swedish Research Council, the Crafoord Foundation, and the National Geographic Society funded the research.

Source: Brown University