by Matthew Cobb
I spent Friday-Sunday last week at the Royal Society Summer Exhibition, helping out some colleagues from the Manchester University Geochemistry & Paleontology Research Group as they explained their research to the public. I had such fun, and the stuff we were talking about was so interesting. What follows is pretty much the pitch I gave to the public, as someone who wasn’t directly involved in the research. Two WEIT readers came up who had recently graduated from Queen Mary, London – I didn’t catch their names, but hi!
The centre-piece of our display was a 120 MY old fossil of Confuciusornis sanctus, a basal bird from China – it is the earliest known bird with a beak. About the size of a pigeon, it had long twin tail-feathers strongly suggestive of sexual selection. This bird was one of a flock that was caught in a volcanic explosion one Tuesday afternoon (I made that bit up) and ended up fossilised at the bottom of a lake. The fossil itself is pretty neat, with clear impressions of feathers, bones and soft tissues (see image A below):
As a biologist, I’d be happy with the fossil, but my colleagues Roy Wogelius and Phil Manning from Manchester Earth & Environmental Sciences, together with Uwe Bergmann from Stanford and a host of other folk, decided to use a technique they had previously employed on an Archaeopteryx fossil (Bergmann et al., 2010). They put the fossil at the sharp end of the Stanford synchotron and fired a concentrated X-ray beam at the fossil. In response to this procedure, the atoms of the different elements require different energies to fluoresce, so an ‘elemental map’ of the fossil can be built up.
Figure B above shows the signals for three elements: zinc (= green), calcium (= blue) and copper (red). The zinc is present in background levels in the matrix and shows no specific information about the fossil; calcium is present in the bones and beak, and only in the bones and beak. On the one hand, this is trivial (where else would it be?) and on the other it is amazing – it shows that the fossil is not simply a physical impression, there are also chemical traces that are trapped in the rock: atoms of calcium that were once in the bird’s bones.
The red signal of copper is the most audacious to interpret. You can see that it is on the outside of the animal, primarily in the feathers (look at the intense staining in the upper body). However, not all the feathers show this staining – look at the large wing feathers on the bottom left and bottom right of the fossil – they do not show any copper. So the distribution of copper in the feathers is non-random.
Roy and the rest of the team were able to show that this copper is definitely organic, and shows the coordination chemistry that is typical of eumelanin (I am skipping over several complicated stages of chemistry here – details in the references). This would suggest that the distribution of copper could be a biomarker for the presence/absence of melanin in the tissues of the living bird.
To test their hypothesis, they took various feathers and control samples, modern and fossil, and subjected them to the same X-ray fluoresence. The results strongly support their view (I find modern feathers D and E and their X-ray equivalents L and M particularly convincing):
As a result, my colleagues interpreted the black/white distribution of colour thus:
This is cool because it shows the colour (or at least, the black and white distribution) of C. sanctus. Even more interestingly, along with the rest of their work, it clearly shows that fossils are not simply physical imprints, but also chemical traces that, with the right technology, can be interpreted in biological terms: we can work out the biochemistry of cells in an animal that died 120 million years ago. However, it isn’t straightforward: interpreting the distribution of copper was possible because they could demonstrate that it was a good biomarker for eumelanin. The distribution of other elements is not so easy to interpret – the biology is going to be as complicated as the particle physics and analytical chemistry.
There are other techniques for determining colour in fossils – in the same issue of Science that Roy and his colleagues published their findings, Ryan McKellar examined a load of late cretaceous dinosaur and bird feathers that had been trapped in amber. And in 2010 two papers on other fossils of primitive birds and dinosaurs using scanning electron microscopy suggested that the cellular structures involved in pigmentation – melanosomes – could be reliably detected in some specimens (Li et al, 2010; Zhang et al., 2010). The advantage of the technique used by my colleagues is that it relies less upon exceptional preservation, and it gives a whole-organism view. The trick, however, will be to find reliable interpretations of the elemental distribution.
To find out more about the technique, including some examples of its use on archaeological artifacts, including a ‘lost’ score by Cherubini, see Bergmann (2012).
To accompany the exhibition, Phil Manning made an e-book for the iPad, called “Chemical Ghosts”. This is available *free* from the iBook store. He also made an excellent free app, again for the iPad and called “Chemical Ghosts” (there are no Android versions I’m afraid, and they don’t work on the iPhone). At the end of the app, the camera starts up on your iPad (so it doesn’t work if, like me, you have an iPad 1 with no camera…). If you present this image to the camera (probably best to print it out), and get it in focus, you will be amazed what happens. Turn the image to one side and another – this was one of the best bits of the exhibit! People were absolutely amazed. Honest!
UPDATE: Here’s an image of what you see with an iPad with a camera – people in the comments have been asking. This was taken by Phil Manning himself. It’s a 3-D “augmented reality” rendering of the bird. You can move it in three dimensions as you move the cartoon of the bird. It’s really rather impressive (and better than this 2-d snapshot!). Note the realistic shadow!
References (links are only to abstracts I’m afraid, unless you have personal or institutional access):
U. Bergmann, R. W. Morton, P. L. Manning, W. I. Sellers, S. Farrar, K. G. Huntley, R. A. Wogelius, and P. Larson (2010) Archaeopteryx feathers and bone chemistry fully revealed via synchrotron imaging. PNAS 107:9060-9065
Uwe Bergmann, Philip L. Manning & Roy A Wogelius (2012) Chemical mapping of paleontological and archeological artifacts with synchotron X-rays. Annual Review of Analytical Chemistry 5:361-389.
Quanguo Li, Ke-Qin Gao, Jakob Vinther, Matthew D. Shawkey, Julia A. Clarke, Liliana D’Alba, Qingjin Meng, Derek E. G. Briggs and Richard O. Prum (2010) Plumage color patterns of an extinct dinosaur. Science 327:1369-1372
Ryan C. McKellar, Brian D. E. Chatterton, Alexander P. Wolfe, Philip J. Currie (2011) A diverse assemblage of late Cretaceous dinosaur and bird reathers from Canadian amber. Science 333:1619-1622
R. A. Wogelius, P. L. Manning, H. E. Barden, N. P. Edwards, S. M. Webb, W. I. Sellers, K. G. Taylor, P. L. Larson, P. Dodson, H. You, L. Da-qing, U. Bergmann (2011) Trace metals as biomarkers for eumelanin pigment in the fossil record. Science 333:1622-1626.
Fucheng Zhang, Stuart L. Kearns, Patrick J. Orr, Michael J. Benton, Zhonghe Zhou, Diane Johnson, Xing Xu & Xiaolin Wang (2010)Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463:1075-1078