Few things are universal throughout our culture, but a fascination with dinosaurs comes close. Whether it comes from a childhood visit to a natural history museum or that first appearance of Brachiosaurus towering overhead in Spielberg’s 1993 film, Jurassic Park, almost everyone shares a sense of awe at the notion that such great and massive creatures once roamed the earth. We also share in the curiosity about what they were really like. Many artists have created their own interpretations of the physical forms of dinosaurs – but like life itself, those interpretations have evolved over time.

In the past few decades, our mind’s eye view of dinosaurs has shifted repeatedly and dramatically. Scientists originally envisioned dinosaurs as slow and plodding beasts, with tails drooping to the ground. It was widely believed that long-necked sauropods like Brachiosaurus spent most of their time wading through bodies of water just to support their enormous weight. Starting in the 1960s, paleontologists such as Robert Bakker and John Ostrom began to realize that dinosaurs were, for the most part, actually quite active – even warm-blooded [1]. Their discoveries gave rise to our modern image of dinosaurs as agile predators or startlingly speedy plant-eaters, like those seen in movies such as Jurassic Park.

The most recent revolution in our understanding of dinosaurs is the discovery that many species had feathers. In fact, we now know that dinosaurs were the very first feathered creatures, and that modern birds are actually the descendants of a specific lineage of dinosaurs that includes Velociraptor, called the Dromaeosauridae (Figure 1). Indeed, recent fossil discoveries show that many predatory dinosaurs were feathered, including at least some tyrannosaurs, like the recently discovered Yutyrannus huali [2].

Figure 1. Our view of Velociraptor as it appeared before (left) and after (right) we realized it was covered in feathers. Click to enlarge. (Image credits: Wikimedia Commons; Users Petruss and Matt Martyniuk)

Our modern conception of dinosaurs, therefore, consists of active reptiles that often come with feathers. But because pigments rarely fossilize, this picture has remained a monochrome one – until very recently. In the past few years, several groups of paleontologists have developed a way to reconstruct the colors of fossilized feathers, and among their first subjects were several species of small dinosaur.

Melanosomes create color

The big clue to revealing the color of dinosaur feathers lies in a set of unassuming, blimp-shaped structures called melanosomes. Like mitochondria or chloroplasts, melanosomes are membrane-bound organelles that reside inside cells. Melanosomes are so-named because they produce melanin, the pigment that gives us our skin and hair color – and that gives color to feathers as well. Varying the types of melanin and melanosomes present can create a dizzying array of coloration.

In 2009, a group of Yale University biologists showed that a set of 50 million year old fossilized bird feathers actually contained the remains of their melanosomes [3]. The scientists used a powerful magnification technique called scanning electron microscopy to produce images of the preserved melanosomes. At the same time, another team of paleontologists in China discovered a species of dinosaur that appeared to be fully covered in feathers, which they named Anchiornis huxleyi [4].

The two groups of scientists began working together and quickly came to a startling realization. Although Anchiornis lived more than 100 million years earlier than the birds whose fossilized melanosomes the Yale group had studied, its feathers also contained the clearly-preserved impressions of melanosomes [5]! What’s more, when they sampled feathers from various parts of the body, they found melanosomes of varying shapes (Figure 2). Some were long and rod-shaped, while others were short and almost spherical.

Each of these shapes resembles a type of melanosome that is found in modern animals and leads to a specific colorations in these animals. Rod-like eumelanosomes are known to produce black or gray shades, while short and wide phaeomelanosomes create red and brown hues. One familiar example is human hair, which normally has a lot of eumelanosomes – except in the case of redheads, whose hair primarily contains phaeomelanosomes.

Figure 2. Two different melanosome samples from Anchiornis huxleyi. Note that these are the impressions left behind by the melanosomes, hence their appearance as “holes” in the sample. (Left) Rod-shaped eumelanosomes produce black and gray colors. (Right) Short and wide phaeomelanosomes produce red and brown. From [5].

Because melanosome structure seems to have changed little over the course of feather evolution, the scientists were able to compare the shape and distribution of the melanosomes in the Anchiornis feathers to those of living birds [5]. By doing so, they found that the fossilized melanosomes broadly sorted themselves into three groups, which correlated with gray, black, and red colors in living birds. Using this information, the scientists were able to completely reconstruct the coloration pattern of Anchiornis, which turns out to have been a primarily gray creature with striking black and white bands on its limb feathers and rusty red feathers decorating its head (Figure 3).

Figure 3. Anchiornis huxleyi, fully reconstructed [5]. Note the gray feathers of the body, contrasted with the black and white bands on the wings and the rust-colored plumage on the crest and sides of the head. Click to enlarge.

Of course, there are other visual characteristics besides color. We can make out some, like shape and texture, from the physical appearance of fossilized materials. Others, like the way light plays off a surface, are difficult to determine just by looking at a feather or skin imprint. Fortunately, it turns out that melanosomes can provide clues for answering some of these questions as well.

Organization at a subcellular level creates iridescence

Our intrepid band of biologists from Yale and the Beijing Museum of Natural History found that melanosomes have additional properties beyond just their shape and number. Their orientation within a material also affects how a feather looks – specifically, its level of iridescence. Iridescence is one of those words that sounds like what it means: a glossy shimmering of colors over a surface. We’re familiar with iridescence from the shifting colors on soap bubbles and pearls, or from the blazing “eyes” on the tail feathers of peacocks.

It turns out that dinosaur feathers, at least in some cases, may have something in common with the brilliant displays of their distant, modern descendants. That same group of scientists looked at another dinosaur called Microraptor, a cousin of Velociraptor that is known for, bizarrely, having four wings [5]. That’s right – all four of Microraptor’s limbs show feather patterns characteristic of wings (Figure 4).

Figure 4. The fossilized remains of Microraptor, with the impressions of broad wing feathers visible on both the front and back limbs. Click to enlarge. From [5].

Closer examination of Microraptor’s feathers revealed that its rod-shaped eumelanosomes were arranged end-to-end in parallel, forming organized sheets [6]. This came in distinct contrast to the often-haphazard arrangement of melanosomes in the feathers of Anchiornis. Comparing the eumelanosome sheets of Microraptor to those of current birds revealed that this type of organization often appears in the iridescent feathers of birds like the Brazil duck (Figure 5).

Figure 5. The brilliantly iridescent wing feathers of the Brazil duck are visible as it takes flight. Click to enlarge. (Image Credit: Dario Sanches)

In addition, the overall shape and size of the Microraptor eumelanosomes also matched that of the Brazil duck. The scientists could therefore conclude that Microraptor’s feathers were iridescent as well (Figure 6).

Figure 6. The final reconstructed form of Microraptor [6], displaying its characteristic four wings and the glossy iridescence of its black feathers. Click to enlarge.

An evolving view of dinosaurs

As more paleontologists begin applying the advanced imaging techniques pioneered in these studies, we can expect the visual color palette of dinosaurs and other extinct organisms to expand quickly. Work is already underway to colorize Archaeopteryx, the first fossilized bird ever found and, famously, a key piece of evidence pointing towards the shared lineage of birds and dinosaurs. One study has already been published, but it focuses on a single Archaeopteryx feather (spoiler alert: it’s black), not the entire bird [7].

These and other discoveries are upending decades of conventional wisdom regarding how dinosaurs looked in life. In doing so, they are providing an apt demonstration of how science adapts to include new data. Science is a process, after all, and its conclusions are only as good as the most recent data. Once upon a time, we believed that dinosaurs were sluggish, scaly monsters. Today, bolstered by the wealth of information produced by scientists like Quanguo Li, Jakob Vinther, and their colleagues, whose findings were discussed above, we’ve learned that dinosaurs were lively, vigorous creatures of astounding diversity. Some cared for their young with all the attention of a mother hen. Others roamed the prehistoric landscape in enormous herds. Still others were the top predators of their day and hunted in packs.

And many were feathered. How many, and what hues were displayed on their plumage, remain to be seen. In the meantime, we can enjoy the vast array of colors displayed all around us on the last surviving dinosaurs: modern birds.

Peter Yang is a graduate student in the Biological and Biomedical Sciences program at Harvard University.

References

Useful Tip: Use Google Scholar to access PDF versions of the source articles cited below for free!

[1] Welsh, Jennifer. “Were dinosaurs warm-blooded? The bones point to yes.” Christian Science Monitor. 2012 June 27. http://www.csmonitor.com/Science/2012/0627/Were-dinosaurs-warm-blooded-The-bones-point-to-yes.-video

[2] Xu X et al. (2012). “A gigantic feathered dinosaur from the lower cretaceous of China.” Nature 484(7392): 92-95.

[3] Vinther J et al. (2009). “Structural coloration in a fossil feather.” Biology Letters 6: 128-131.

[4] Xu, X et al. (2009). “A new feathered maniraptoran dinosaur fossil that fills a morphological gap in avian origin.” Chinese Science Bulletin 54 (3): 430–435.

[5] Li Q et al. (2010). “Plumage color patterns of an extinct dinosaur.” Science 327: 1369-1372.

[6] Li Q et al. (2012). “Reconstruction of Microraptor and the evolution of iridescent plumage.” Science 335: 1215-1219.

[7] Carney RM et al. (2012). “New evidence on the colour and nature of the isolated Archaeopteryx feather.” Nature Communications 3:637.

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