Ruffled feathers in Palaeontology...

That well-known 'palaeontological' journal, The Economist, recently entered the less than subtle debate on the preservation of fossil feathers, with a report called 'Feathers Fly'. This article tells a story that is sadly not new to our field. It seems that some science actively courts controversy, especially when palaeontological plumage is at stake. The rare circumstances that give rise to the preservation of feathers is now being explored by several research groups, including my own. We all share more than a passing interest in the pigments and structures that once bestowed colour upon the now dull fossil plumage that occasionally make it into the fossil hall of fame. How we come to unpick, quantify and identify colour is now being hotly debated.

There seem to be three major camps in this debate (at least). Those who map the morphology of both extant and extinct feathers to compare and contrast the presence, absence and shape of the biological paint-pots (melanosomes) that once might have held pigment (remember that an albino would have no pigment, but still have melanosomes). The second group of folks seem to favour chemistry, given it might well be possible to identify the remnant chemistry of pigments, locked in the degraded keratinous tissues of fossil feathers. The third group, looks at both structure and chemistry.

Biological paint-pots or bacterial contamination, that is the question?

There are some potential problems when you have to base your interpretation of colour solely on morphology (group 1), as there is still debate on the origin/affinity of these structures. Some suggest they might be bacterial in origin and have nothing to do with melanosomes, this is something that only chemistry can really resolve/verify. However, extensive statistics borne from the study of feathers fom extant species of birds supports the first groups approach. This first technique does show huge potential and has been widely published in high-impact journals....here comes the 'but'....the technique relies upon using an environmental scanning electron microscope (ESEM) that often limits the size of sample and the total area of sample to be mapped. To map a whole fossil bird, say the early Cretaceous Confuciusornis, using ESEM would take an operator thousands of hours (if it fits in the ESEM chamber in the first place). Not very practical...and quite mind-numbing. Inevitably, this technique leads to the extrapolation of colour, based upon a fraction of the overall surface area of the fossil available. It also often entails the removal of samples from the surface of stunning fossils...something we try to avoid. Try imaging less than 1% of a zebra skin and reconstructing the colour and patterns...you might get stripes, but there would be a strong chance that white or black dominated your reconstruction.

The  eumelanin pigment molecule with its central copper atom.

Group 2 have been using chemistry to aid in their interpretation of possible pigments preserved in fossil feathers. This is a powerful technique, if it is possible to constrain elements/compounds that are linked to, or are breakdown products of original pigments. Some chemical techniques can only map small areas of fossils and are often not sensitive enough to detect the dilute concentrations of trace-metals that might well be associated with pigments. The x-ray analysers (such as EDAX) on many ESEM's only allow a rough guide as to the presence or absence of elements and are often difficult (if not impossible) to map over large areas. The EDAX is also unable to constrain the oxidation state of elements, which is so critical to the elucidation of their organic or inorganic origins.

ESEM and EDS can provide valuable information, but not to the same resolution or
sensitivity of synchrotron based imaging.

Group 3 takes the approach of using both structural information from ESEM analysis and the chemical mapping of whole fossils, using synchrotron-based imaging. This has the advantage of being able to identify structures that look like melanosomes (via ESEM), but then also verify their possible identity through chemistry (via synchrotron-mapping). This might be the best approach that we have to date, as it allows 100% of a fossils surface to be mapped, but there are many more approaches in development that might aid, refine and further unlock the colour palette of fossils. The synchrotron-based approach also provides the unique ability to undertake the spectroscopic analysis of fossils providing key information on the oxidation state of elements...meaning we can gauge if they were organic in origin, or not.

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It is clear that the different groups are all generating new and exciting data. This is splendid.  As more data gets published, more information becomes available to the wider scientific community.  Hopefully within this growing sink of evidence, more robust hypotheses can be constructed as to the preservation of feathers and the possibility of a whiff of colour.