A new fossil discovery shows that the lush ancient forests of China 120 million years ago were home to brilliantly colored extinct birds with iridescent feathers. This unexpected breakthrough reported by an international team in the journal eLife is the result of advances in ultrafine microscopic sectioning of fossil feathers, a new approach to the microscopic study of the three-dimensional arrangement of pigment packages inside fossil feathers, and the key application of powerful supercomputer modeling of light interactions with feather structures to reconstruct the specific iridescent colors that the feathers produced.

We take for granted the world of birds around us filled with species exhibiting a rainbow of colorful feathers from the reds, blues, and yellows of warblers and tanagers to the metallic heads of hummingbirds and dazzling tails of peacocks. By contrast, most fossil pigment based reconstructions of fossilized feathers from early in bird evolution point to feathers with a much more restricted color palette with black, white, gray, and rusty colors. However, new fossil data from the feathered crest on the head of a long extinct, toothed forest bird called Shangyang shows that even some of the earliest birds were as eye catching as those of today.

This expansion in the known range of color expression in the ancient past derives from a sparrow-sized fossil bird specimen excavated from 120 million year old rock deposits of the Cretaceous Period in Liaoning Province in Northeastern China. The fossil preserves the skeleton, including its toothed jaws, and feathers from across its body, wings, and importantly, the head. The long feathers on top of the head formed a crest with feathers twice as tall as the skull. Importantly, this particular fossil bird also preserves two very long tail feathers that most likely indicate that it was a male. In other species of early birds, this pair long tail feathers occurs commonly in males. This extinct crested bird is a species of the genus Shangyang that is part of the extinct group called enantiornithines, the dominant group of birds in forests around the world during the Cretaceous.

These iridescent feathers help to alter our thinking about the evolution of early bird feathers, their function, and even their growth. Concerning this study, lead author Dr. Zhiheng Li of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing says, “When I saw the first Scanning Electron Microscope images of the fossilized pigment packages called melanosomes inside the tiny samples we took from the feathers of the Shangyang crest, I could see that they were rod or sausage shaped. That shape is full of a pigment called eumelanin that produces a black color in feathers. If we were following standard techniques to study fossil feathers, that is where we would stop and declare that the fossil feathers were black or dark colored.” Eumelanins are well known in the feathers and feather-like structures of early birds, dinosaurs, and even pterosaurs, and are present in the skin and hair of mammals, including us.

However instead of ending their investigation with that dark pigment observation, the researchers continued to ultrathin section the fossil feather samples in different directions to see how the rod-shaped melanosomes are arranged in 3D at an extremely fine scale inside the feathers. “After looking at the melanosomes in multiple directions, we found an unexpected regular hexagonal arrangement of the pigment packages at the micron scale with some crosslinks that we have never seen before, indicating something unusual about this feather and its color,” continues Dr. Li.

To help figure out what this unique pigment arrangement might mean for the color of the ancient feathers, the paleontologists turned to the physics of light and supercomputers. Using the unique microscopic 3D dataset from the feathered crest, Dr. Jinsheng Hu and Dr. Mao Ye of Beihang University in Beijing built a computer model of the microscopic packets of pigments inside the fossilized feathers and then applied a computer modeling technique called Finite Difference Time Domain that is used to understand how light interacts with living bird feathers to produce colors, including iridescence. After many hours of simulating reconstructions of the interaction of light of different wavelengths at various angles with the feather microstructure on a supercomputer, the research team was able for the first time to demonstrate that these ancient bird feathers were iridescent, but also what specific colors the feathers would have produced.

Despite the fossil feather having the pigment packets that would typically produce only dark colors, it is their unique hexagonal arrangement inside the feather and the spacing of the cross-linked pigment packets that instead scattered the light in a coherent manner to produce iridescent coloration. The physics of light scattering to produce a color is how the sky looks blue during the day (with air being clear, not blue) and blue or green eyes looking blue or green despite lacking any pigment of those colors.

In this case, the supercomputer simulations show that the feathered crest would have been red, yellow, green, and vibrant blue (with peaks of intensity at light wavelengths 687, 578, 512, and 475 nm). The exact color a viewer would see in that sequence would depend on the angle of light hitting the feather and where the viewer is positioned. The ability to change the perceived feather color based on the angle of light is the same physical mechanism used by hummingbirds and in peacock tails where the color of a feather seems to change or even shift from black to vibrant as the head or tail moves. The fossil feather would have behaved in the same way appearing black at a very low angle and vividly bright at a higher, more straight-on angle, just as we see in modern iridescent feathers.

Interestingly, the presence of iridescent feathers in an extinct enantiornithine bird represents an independent evolution of this colorful trait separate from the diverse iridescent feathers present across birds today. One of the ways that its independent evolutionary origin can be determined is through the position and arrangement of the melanosome pigment packets. In living bird iridescent feathers, the melanosomes are placed in the finest branches of the feathers, called the barbules. However, in the fossil of Shangyang, the pigments are instead in the barbs (the second finest branches), and they are much more densely arranged than what we see in living bird feathers. That greater density might have made the colors in this extinct bird even more vibrant. Although these iridescent feathers were derived independently from those of living birds, the physics of iridescent feathers is the same, and the likely evolutionary drivers and biological needs that led to iridescent feathers in the Cretaceous were probably those that we would recognize among living colorful birds.

In addition, this complex arrangement of melanosomes inside of feathers is produced by a carefully controlled set of processes acting at the cellular level. The melanosomes present in feathers actually are produced in the skin of birds and are transported into feathers while they are growing. The growing feather cells transport them from cell to cell to their destinations in the feather, and then carefully arrange these pigment packets, aligning them and fixing their spacing to produce the resulting coloration. Even though this bird died millions of years ago, we can infer the presence and see the result of this refined set of cell interactions that produced this complex pigment arrangement preserved inside an ancient feather.

The iridescent feathers appear to be restricted to the enlarged feather crest on top of the head in Shangyang with darker feathers on other parts of the body. Coauthor Dr. Thomas Stidham (Austin College and IVPP) elaborates, “Today, iridescent feathers in birds tend to be in obvious or prominent locations on the body, not hidden away. Those splendent feathers are not for camouflage; they are about seeing and being seen.” Dr. Li continues, “Light is necessary for the color and color changing aspect of iridescence to work. Iridescent feathers frequently are used for signaling, and the birds know how to orient their feathers and bodies with respect to the sun and the viewer to achieve the greatest effect. This fossil crest likely would have been consciously raised or lowered, and the head angle changed to leverage the iridescent coloration function to its greatest extent for intimidating rivals and attracting females, since this likely was a male bird with its two elongate tail feathers.”

Dr. Stidham adds, “This tiny bird fossil shows us again that the beauty of evolutionary history and the fossil record can be far more sensational and unexpected than what cinematic magic presents. In other words, my students would say, ‘it's a fossil bird with rizz.’”

Looking into the future Dr. Li hypothesizes, “We suspect that if our approach is used on the currently published dinosaur and early bird feathers thought to be only drab or black colored, we will find that feather iridescence was more common and that the animals inhabiting these ancient environments were much more colorful than currently thought. The next Jurassic Park movie might need a colorful makeover.”

Link for the paper: https://elifesciences.org/articles/103628

Figure: Reconstruction of the fossil bird Shangyang sp. features an enlarged head-crest rendered in iridescent colors. Microscopic images reveal the fine scale packing of melanosomes in a feather sampled from the head. (Picture Credit: Xu Yong & Li Zhiheng, IVPP)