Ancient Pigments Leave Metal Clues

A 120 million-year-old fossil of one of Earth’s first birds still has a chemical fingerprint of feather pigments that researchers have detected using X-ray spectroscopy. Because plumage is important for avian camouflage, communication, and sexual reproduction, the pigmentation patterns of these ancient animals provide biologists with important clues to avian evolution.

Researchers led by Roy A. Wogelius at the University of Manchester detected traces of copper and zinc in a beautifully preserved fossil of Confuciusornis sanctus. Using several synchrotron-based X-ray spectroscopies—including a fluorescence-based one that maps metals’ spatial distributions and others that probe the identity and coordination environment of metals—researchers concluded that the copper and zinc were formerly part of a dark organometallic pigment found in the bird’s quills. From this and other data, the team mapped the distribution of dark eumelanin pigment in the ancient bird’s plumage (Sci ence, DOI: 10.1126/science.1205748).

This is the first time scientists have been able to detect specific pigment chemicals from an ancient animal’s soft tissue on a fossil. The new technique complements another recently developed strategy to infer color from the shapes of pigments left on fossils, comments Matthew Shawkey, a paleobiologist at the University of Akron.

“A few years ago, you would have been labeled as a mad speculator to even attempt to identify colors in fossil organisms,” comments Mike J. Benton, a paleobiologist at the University of Bristol, in England. “But now we have a battery of high-tech tools” to study coloration in fossils of birds and dinosaurs, he says.

The idea that it could be possible to interpret color pigmentation from fossils was kicked off in 2008 by a paper in Biology Letters (DOI: 10.1098/rsbl.2008.0302). That’s when a team of researchers at Yale University led by Jakob Vinther and Derek E. G. Briggs used a scanning electron microscope (SEM) to show that structures previously detected on a fossil were actually melanosomes, which are sacs of leftover feather pigment. Until that time, researchers had believed these structures were remnants of bacteria that had feasted on fossilized dinosaur feathers.

By comparing the morphology of ancient fossils with those of modern animals, the Yale team argued that the shape and distribution of pigments in melanosomes could be used to infer color patterns in ancient feathers of birds and dinosaurs. For example, they suggested that sausage-shaped pigments were eumelanin, a dark pigment still found in brown human hair today. Later they suggested that sacs containing spherical pigments indicated the color of the ancient feather was likely a reddish orange.

By January 2010, Benton and his colleagues at the Chinese Academy of Sciences had applied that logic to propose that a 120 million-year-old feathered dinosaur called Sinosauropteryx had orange tail feathers (Nature, DOI: 10.1038/nature08740). One week later Shawkey and the Yale group used the same logic to show that a feathered dinosaur called Anchiornis huxleyi, which lived 160 million years ago, had a red head and a black and white body (Science, DOI: 10.1126/science.1186290).

Although the use of SEM to identify the color of pigment sacs on fossils revolutionized paleobiology, Wogelius says, the technique has a serious downside: It can damage precious fossils. Unlike SEM, his new X-ray technique is noninvasive and thus can be used to examine priceless fossils without harming them. The X-ray technique can also resolve pigment patterns, even if the morphological evidence is gone. “Sometimes geology is not so kind and the melanosome structures are lost,” he explains. “But the trace metals persist because they are not biodegradable.”

Wogelius is now developing the technique to study older fossils. He’s also trying to figure out what other information can be garnered from leftover metals in fossils. And he wants to use X-ray spectroscopy on fossils to “tease out the chemistry of extremely slow reactions. The chemical reactions in these fossils have been running for 10⁷ years. I’d like to see what we can learn about breakdown reactions under very dry conditions.”

First color and then chemical kinetics—who knows what other ancient secrets these fossils might bequeath.