A Flash of Color: Discovering Dinosaur Pigments

In 2010, a remarkable study was published which allowed us, for the first time, to see the world of the dinosaurs in color. Until then, “what color were dinosaurs?” had been an unanswerable question: reconstructions were usually based on modern reptiles like crocodiles, and depicted them in subdued shades of green and brown. Today, science tells us that not only were many dinosaurs covered in feathers, but these were decorated with gaudy colors and patterns.

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Sinosauropteryx                                  photo from Wiki commons

When the first feathered non-avian dinosaur, Sinosauropteryx, was discovered in China in 1996, it caused a sensation–clearly visible around the bones was a halo of fuzzy filaments that looked like downy feathers. It was a crucial point in favor of the hypothesis that birds were the direct descendants of small carnivorous theropod dinosaurs, and that these early dinosaur ancestors probably also had feathers. There were, however, critics who argued against the dino-bird connection, and their alternative explanation for the fuzz surrounding Sinosauropteryx was that they were not feathers at all, but collagen fibers that had unraveled from the connective tissue after death and been preserved in place.

It wasn’t until years later that a method was proposed to decide the matter. In bird feathers, color is produced by a number of specialized cells, with different types producing different colors. The common pigment melanin is stored in little packets called melanosomes. Fortunately for scientists, melanosomes are not only very tough structures, but they form part of the actual intercellular tissue structure, and can therefore be replicated in fossils–and this was confirmed when paleontologists found melanosomes in the extraordinarily-well-preserved fossil of an ancient squid’s ink sac. In a well-preserved dinosaur fossil, it was then reasoned, these melanosomes might also be seen under a microscope. And when the filaments from Sinosauropteryx were examined, they were indeed found to contain fossilized melanosomes. They were real feathers, not just collagen fibers.

But this also opened up another opportunity that paleontologists had never had before. In living birds, different concentrations of melanin produce different colors, ranging from black to reddish-orange, and each color comes from a differently-shaped melanosome. Oblong sausage-shaped melanosomes produce black colors, while long skinny melanosomes produce grey and round spherical melanosomes produce orange. Some animals, in addition, make use of microscopic ridges in the surface of their feathers or hair, which acts as a diffraction grating to scatter sunlight and produce a rainbow-like iridescence, like a CD or DVD. And all of this could be seen under a microscope.

So when a series of extraordinarily well-preserved fossils began appearing in China in the 1990s which contained feathers, both from primitive birds and from their dinosaur ancestors, it presented a unique opportunity. Under an electron microscope, the melanosomes themselves could be examined, and although none of the original pigments would remain, the shape of the melanosomes that once contained them revealed what color the feather had been in the living animal. It opened up an entirely new window on the ancient world.

The first such study was published in 2010. Microscopic examination was carried out on the fossilized feathers of Sinosauropteryx. By noting the distinctive shape of the pigment cells in each feather and by mapping these colors over the body, paleontologists could reconstruct what the animal probably looked like. The findings were startling. Far from being the drab creatures of previous reconstructions, they were brightly colored in vivid patterns. The feathers on Sinosauropteryx’s body had large numbers of spherical melanosomes, indicating that they were reddish-orange in color. The tail had bands of feathers where there were lots of these melanosomes, alternating with bands where there were none, indicating that it probably had white and orange rings around its tail, like a lemur or raccoon.

Soon other species were examined. Confusiusornis, a Chinese bird, had individual orange, black and white downy feathers all over its body, producing a sort of salt-and-pepper effect: it also had a bristle-like feathery crest running from its neck down its back and along the tail. Anchiornis had a black body and wings decorated with white patches, and a reddish-orange head with a crest of feathers–making it look somewhat like a woodpecker. Archaeopteryx was plain black, while Microraptor shimmered with a blueish-black iridescence, like a grackle.

These reconstructions are still guesses, however. The feathers may also have had other pigments, such as carotene or anthocyanin, which produce yellow, orange and red colors. It is also assumed that feathers which show no melanosomes had no pigment and were therefore white, but this may not have been so–they might have had other pigments which were not preserved as part of the tissue structure. But enough was revealed to indicate that at least some of the feathered dinosaurs had eye-popping color patterns that would have been easily visible.

And this, in turn, gave an indication of their behavior. For years, paleontologists had debated about the original purpose of feathers. Since the downy fuzzy coating on the non-avian feathered dinosaurs was not suited for flight, scientists assumed they were used either as insulation to trap heat and maintain a high body temperature, or as displays for breeding or territorial disputes. The answer now became clear. While the “dino-fuzz” may still have played a role as insulation for thermoregulation, their bright colors and pattern contrasts also indicated that they acted as visual displays, forming crests and ridges similar to those seen on many birds today. Since the 1990’s, new discoveries in China and elsewhere have revealed that dozens of different dinosaurs, mostly in the Theropod group but also in some others, had feathers on their body and/or their forearms. Indeed it is possible that feathers are an ancestral trait that goes right back to the origins of the dinosaurs, and that all of them may have had feathers on at least part of their body.

Research carried out in the 2010’s has revealed that while reptiles have a low variety of melanosomes, a much wider variety is found in birds and mammals–and dinosaurs. This indicates that melanosomes (and therefore skin colors) underwent a sudden explosion of diversity at about the time that mammals, pterosaurs, dinosaurs and birds all evolved from their common reptilian forefathers. It also happens that all of these animals are warm-blooded, and some chemical links have been found between melanin and the metabolic processes that regulate body heat. It has therefore been hypothesized that melanosomes originally appeared as part of the evolution of a warm-blooded metabolism, and were co-opted to serve as color-pigments.

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