Saturday, April 14, 2012

The colour of dinosaurs

ResearchBlogging.org
When I was young, like many young people, I was obsessed with dinosaurs. I felt gyped when I found out that the drawings of dinosaurs in my books weren't based on any real knowledge of dinosaur coloration. The colours used in the dinosaur artwork were just guesses that were extrapolated from the colours of modern reptiles.

Recently, some of the speculation about the colour of dinosaurs has been resolved. In the last decade or so there has been an explosion in the number of dinosaurs that palaeontologists have identified as having feathers (one spectacular example was revealed just last week). In 2008, a paper1 was published providing details of a technique that could help to determine the colours of fossil bird feathers. Then, in early 2010, another paper2 in Nature used the technique to determine what colour dinosaur feathers might have been.

The structures that give mammalian fur and avian feathers some of their colour are called melanosomes. And, it’s the shape and the arrangement of the melanosomes that help determine the colour. Excitingly, the 2008 and 2010 papers showed that these could be preserved in ancient fossil feathers. So, by examining the shape and distribution of the melanosomes in the fossilised feathers palaeontologists could get and idea of what colour the feathers were.

It didn’t take long for people to start speculating about the possible function of coloured feathers. Indeed, one of the authors of the 2010 Nature paper was prepared to speculate when interviewed, that early feathers were for display and only later were they selected for insulation and flight. It was, however, only a minor point in their paper. I am skeptical about their claim that display came first, although I agree that selection for flight came much later.

One of the elements that their 'display-first' hypothesis rests on, is that the protofeathers of Sinosauropteryx (the dinosaur they examined melanosomes in) are only present in the tail and in a crest along the dorsal surface of the body. They argue that such limited coverage of proto-feathers suggests that they had a limited role in thermoregulation. But, neither of these points is strictly true. 

A Sinosauropteryx fossil from the Jehol region of China showing the distribution of well preserved protofeathers along the back and tail (figure taken from Chen, P., Dong, Z., and Zhen, S. (1998)3).
The paper3 that describes the first two specimens of Sinosauropteryx provides evidence that protofeathers were present over most of the body, but were poorly preserved on the animal's sides. Although the protofeathers are most prominent along the back and tail, several small patches were also present elsewhere suggesting a much broader distribution. The paper goes on to argue that the length, density and likely distribution of the protofeathers suggests they would be were most likely for insulation, not display.

Last month a new paper4 provided further evidence of colour in fossil feathers, and speculation about the structure of feathers for flight, display or insulation was given new life. This time there was evidence that the feathers of Microraptor, the stunning four-winged dinosaur, were iridescent. Not strongly iridescent like the throat feathers of hummingbirds or the tail feathers of peacocks, but weakly iridescent like the glossy plumage of crows and ravens.

An artists reconstruction of Microraptor showing a possible four-winged flight pose for gliding, and it's long midline tail feathers (taken from Li et al. 20124).
I had been taught that iridescence was all down to the physical structure of the feather, which isn't preserved in fossils, and had nothing to do with melanosome pigmentation, which can be preserved. But, a little reading later and I found that science had moved on! Iridescence requires both the physical structure of the feather and particular arrangements of melanosomes. 

As iridescence increases, black melanosomes become arranged in a more orderly way. But, even a well ordered melanosome layer would appear matte black if it wasn't for the structure of a thin keratin layer over the top. Once an ordered layer of melanosomes forms to a thickness of 150 nanometers, no further changes in the melanosomes affect the strength of iridescence, the rest is down to the keratin layer. So, the physical structure of the feather plays the most important role, but an orderly layering of melanosomes is also required5.

The authors of the Microraptor paper were able to infer an iridescent feather colour through two lines of evidence. The shape and the layering of the melanosomes. As far as I am aware, the role that the shape of the melanosome plays in iridescence is unknown. But, in some modern birds the melansomes associated with iridescence have unusual dimensions, and this can be used to distinguish them from other black melanosomes. The authors found that the preserved melanosomes were of the iridescent type and were arranged in a relatively orderly way, strongly suggesting that Microraptor had iridescent plumage.

The authors didn't stop there. The specimen that they had was newly unearthed and provided some interesting additional details about the shape of the tail. It had previously been thought that the tail was quite broad and assisted in flight. But, the new specimen and a re-examination of other specimens, suggested that the tail of Microraptor had two long feathers, or streamers, in the midline right at the back of the tail fan. It also suggested that the tail fan was narrower than previous interpretations.

Two constructions of Microraptor showing different possible gliding positions. Note the size and arrangement of the tail feathers in comparison to the one above (right image is taken from Xu et al  20036, left image is taken from Chaterjee & Templin  20077).
The authors argue that the iridescent feathers and their new interpretation of the tail shape strongly suggested that the tail was more likely to be for display than to assist with aerodynamics. It's true that in many modern birds iridescent feathers and long tail streamers are important in sexual selection. And it would not be surprising if the tail feathers of Microraptor were used for the same purpose. But, I'm not convinced yet.


When traits are sexually selected, the traits are usually much more exaggerated in one of the sexes. Think the tails of peacocks or the bright colours of male guppies. The other sex is more drab or doesn't have the traits at all. In the Microraptor study, the authors examined three different specimens where the tail feathers were well preserved. All three specimens displayed the elongated tail feathers at the end of the tail. 


Closeups of three fossilised Microraptor tails. A and B are previously described fossils, while C is from the newly described fossil. The arrows in A and B point to the elongated midline tail feathers (taken from the supporting online material of Li et al. 20124).

Without a clear demonstration of sexual dimorphism for tail feather length, it's harder to buy the argument that the shape of Microraptor's tail is sexually selected. And unfeathered tails have recently been shown to be important for aerodynamics, so I'm not confident that narrowness of the tail fan rules out an aerodynamic function. The new paper certainly does the sexual selection argument no harm though. And it shows us another amazing fossil from China.

References

1 Vinther, J., Brigs, D. E. G., Prum, R. O., and Saranathan, V. (2008) The colour of fossil feathers. Biology Letters 4, 522 - 525.

2 Zhang, F., Kearns, S. L., Orr, P. J., Benton, M. J., Zhou, Z., Johnson, D., Xu, X., and Wang X. (2010) Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463, 1075 - 1078.

3 Chen, P., Dong, Z., and Zhen, S. (1998) An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China. Nature 391, 147 - 152.

4 Li, Q., Gao, K., Meng, Q., Clarke, J., Shawkey, M., D'Alba, L., Pei, R., Ellison, M., Norell, M., & Vinther, J. (2012). Reconstruction of Microraptor and the Evolution of Iridescent Plumage Science, 335, 1215-1219

5 Maia, R., D'Alba, L., and Shawkey, M. D. (2011) What makes a feather shine? A nanostructural basis for glossy black colours in feathers. Proceedings of the Royal Society B 278, 1973 - 1980.

6 Xu, X., Zhou, Z., Wang, X., Kuang, X., Zhang,. F. and Du, X. (2003) Four-winged dinosaurs from China. Nature 421, 335 - 340.

7 Chatterjee, S. and Templin, R. J. (2007) Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. Proceeding of the National Academy of Sciences 104(5), 1576 - 1580.

Li, Q., Gao, K., Meng, Q., Clarke, J., Shawkey, M., D'Alba, L., Pei, R., Ellison, M., Norell, M., & Vinther, J. (2012). Reconstruction of Microraptor and the Evolution of Iridescent Plumage Science, 335 (6073), 1215-1219 DOI: 10.1126/science.1213780

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