Dinosaur books have become more colourful affairs of late, with the dull greens, browns and greys of yesteryear replaced by vivid hues, stripes and patterns. This has largely been a question of artistic licence. While fossils may constrain an artist's hand in terms of size and shape, they haven't provided any information about colour. But that is starting to change.
The fossils of some small meat-eating dinosaurs were covered in filaments that are widely thought to be the precursors of feathers. And among these filaments, a team of Chinese and British scientists have found the distinctive signs of melanosomes, small structures that are partly responsible for the colours of modern bird feathers.
Melanosomes are packed with melanins, pigments that range from drab blacks and greys to reddish-brown and yellow hues. Their presence in dinosaur filaments has allowed Fucheng Zhang to start piecing together the colours of these animals, millions of years after their extinction. For example, Zhang thinks that the small predator Sinosauropteryx had "chestnut to reddish-brown" stripes running down its tail and probably a similarly coloured crest down its back. Meanwhile, the early bird Confuciusornis had a variety of black, grey, red and brown hues, even within a single feather.
Zhang's discovery also launches another salvo into a debate over the very nature of "feathered" dinosaurs. Beautiful fossils, mainly from China, show that several species of dinosaur had feathers akin to the flight-capable plumes of modern birds. Species like Caudipteryx and the four-winged Microraptor had true feathers with asymmetric vanes arranged around a central shaft.
Other species, such as Sinosauropteryx, Sinornithosaurus, Beipaiosaurus, and even distant relatives like Tianyulong and Psittacosaurus, had far simpler filaments ("dino fuzz") covering parts of their skin. These are more controversial. Many groups view these filaments as proto-feathers, the evolutionary predecessors to the fine plumage of birds. But a small minority still interpret them as degraded strands of collagen, a protein that strengthens animal skin.
But if Zhang is right, it may be game over for the collagen hypothesis. His study shows that melanosomes have been preserved in the same way in the filaments of Sinosauropteryx as they were in the true feathers of early birds. It's evidence of a link between the two structures.
Under the extreme magnification of an electron microscope, Zhang looked at the filaments of the dinosaurs Sinosauropteryx and Sinornithosaurus, as well as the true feathers of the early bird Confuciusornis. The microscopes revealed a number of small structures all less than a micrometre long. In shape and size, they are identical to the melanosomes of modern birds. There include two broad categories. The phaemelanosomes are almost spherical in shape and produce phaeomelanin, a reddish-brown or yellow pigment, while the eumelanosomes are more rod-like and produce black-grey eumelanin.
Melanosomes are remarkably resilient structures. They're more resistant to both chemical and physical decay than the keratin of feathers and hair. Nonetheless, critics will undoubtedly argue that these microscopic structures weren't melanosomes at all but bacteria, whose cells have since been replaced by sediment. Zhang is prepared for that, and has put forward several lines of evidence to answer his sceptics.
For a start, they appear inside the feathers and filaments, rather than sitting outside them in a thin film. They're also found in the same parts of the fossils as they are in modern feathers. They're arranged in the same pattern - the eumelanosomes, in particular, are "densely packed and strongly aligned". In some cases, the fossil feathers are clearly striped by bands of pigment and the alleged eumelanosomes only turn up in the dark ones, just as you'd expect. There's no reason for bacteria to have been so evenly spaced - they would more likely have formed a uniform film over the entire structure.
Zhang is ruthless in his interpretation, saying that the melanosomes make it "unequivocally" clear that these filaments were not strands of collagen. Instead, they "are probably the evolutionary precursors of true feathers". And now we're starting to have an idea of what colour these precursors were.
This isn't the first time that fossilised melanosomes have been used to work out the colour of extinct species. In 2008, Jakob Vinther from Yale University found these pigmented structures in the fossil feather of a Cretaceous bird, and concluded that it probably had black and white stripes. Last year, Vinther reported that the layers of melanosomes in another fossil feather would have given it an iridescent sheen, much like the gloss on the wing of a starling. Carl Zimmer covered the story for the New York Times.
Both of Vinther's papers talk about the possibility of using the same technique to reconstruct dinosaurs in accurate Technicolour. Now, it seems that this hope is becoming a reality. But while Vinther is convinced by the melanosomes that Zhang has found, he's more skeptical about the inferences about colour. "Saying that Sinosauropteryx was rufous-red, based on one sample is a stretch," he says. We don't even know how melanosome distributions in modern birds lead to specific colours. "Without this knowledge quantified, we cannot predict specific colors in fossils, maybe except black. So we are still far from putting colors on dinosaurs."
However, Vinther remains optimistic. "I can only say that the future is promising. Eventually we will have dinosaurs in technicolor. We are working seriously on that currently."
Working out what colour dinosaurs were isn't just a matter of nerdy curiosity. The colours of a bird's feathers affect its ability to keep cool, camouflage itself from predators or prey, and communicate with mates and rivals. In this way, a better understanding of dinosaur colours could give us more clues about how these most evocative of animals lived.
Mike Benton, a co-author on the new study, says, "In Sinosauropteryx at least, essentially the most primitive dinosaur with feathers, the possession of an orange and white striped tail says 'display' of some kind, but whether for warning or pre-mating contests, we couldn't say."
Reference: Zhang et al. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature doi:10.1038/nature08740
Images: Sinosauropteryx painting by Chuang Zhao and Lida Xing; Confucisusornis images from Institute of Vertebrate Palaeontology and Paleoanthropology, Beijing and University of Bristol; Sinosauropteryx images by Nanjing Institute and University of Bristol; Zebra finch images by University of Bristol.
More on feathered dinosaurs:
- Evidence that Velociraptor had feathers
- Beipaiosaurus was covered in the simplest known feathers
- Tianyulong - a fuzzy dinosaur that makes the origin of feathers fuzzier
- Microraptor - the dinosaur that flew like a biplane
Fascinating. I don't comment here nearly enough, but as a social scientist who's consistently interested in natural science issues, only lacks the time (and obviously some expertise) to follow them, this kind of reporting is incredibly beneficial. Thank you so much.
EEE! Things like this make me so happy. It's the kind of awesome science that I ate up as a kid.
Cool! But we shouldn't get too far ahead of ourselves. Melanosomes are only one way in which birds produce color in their feathers. Have any of these authors considered the possibility of carotenoids, or whether the sort of structural colors that bluebirds or hummingbirds show would leave fossil traces?
It seems quite likely that we'll have an idea of patterns (still really neat) but with less certainty of which shades those patterns are cast in.
There's cool, there's way cool, and then there's this! :o)
Very exciting! Or should I say, 'beautiful'?
These melanosomes were only found in the fossils of feathered dinosaurs, right? I wonder if researchers will be able to find them in skin traces of nonfeathered dinosaurs as well.
Wow, you wouldn't think we would find out the colour of dinosaurs until we successfully bred one. So many surprising things to be found with patience and a microscope.
Love the enthusiastic responses!
Psweet - Benton says that, yes, there are some colour-producing pigments that won't be represented in this way, including yellow-orange carotenoids or green-purple porphyrins. However, if you check out one of the links in the later paragraphs (under "Vinther reported") you'll see that your point about iridescence and structural colours has been addressed in a similar study.
We won't find blue either, I'm guessing, since that's not a pigment at all. But this is so cool!
Maybe it's just me, but in both sets of electron microscope images above, it's the eumelanosomes that appear more rod-like and the phaemelanosomes that look more spherical. No?
So fossil experts here claim that bacteria can produce fossil shapes, but in meteorites from Mars experts claim that bacteria can't produce fossil shapes?
Jimbonius - you're right, I mixed up the shape descriptions. Now fixed.
Thanks, Ed. I guess we're closer than I thought.
Rod forms are merely spheres with columns between the hemisphere shells, whether in bacteria, melanosomes or buckyballs vs end-capped nanotubes. Geometrically, it is a transition from pentameric icosahedral symmetrical hemi-spheroidal end caps to hexameric columns. The hexa-skin shape may produce a change in light transmission compared to the round shape.
It is said that powered flight in modern birds requires asymmetrical feathers, but I think that flight with symmetrical feathers was possible, using them in a different way, more as gravity-powered glide-flight arboreal raptor ambush style. Having long striped tails is convergently typical of arboreal stem-climbers which hop up branches (especially whorled conifer-like trees) rather than swinging and clinging. /geek