What colour were fossilised insects?

Ben -   Now that's nearly all we have in our roundup of the British Science Festival that came from Aberdeen this year, but one thing that really caught my eye was the science of brightly-coloured fossilised insects.  When we see a reconstruction of a prehistoric creature, perhaps in a book or on TV, the creator has usually made it very colourful; but how do we know what colour it should be?  Historically, we haven't, and so the creator just made it up.  But Bristol scientist Maria McNamara has made a breakthrough that means she can now work out the colours of ancient fossilised insect remains.

Maria -   Most of the fossil record has no colour preserved at all and 99% of fossils are bones, shells, teeth; and so preserving soft tissues that have colour is very, very rare.  But every so often, if you get the right sort of circumstances, coloured tissues like skin and feathers, and insect cuticles, do get preserved.

Chris -   There's two ways that things can make colour isn't there because there's the kind of colour as in, 'I dye my clothes in a certain colour', but then there's another kind of colour, which is the structure of the surface.

Maria -   Yeah, exactly.  So, the first type of colour you were talking about is pigmentary colours and pigments are chemicals.  These are the colours that we see in our clothes, in our hair, in our skin, and so on; and structural colours; they're produced via an entirely physical process, not involving chemicals at all.  So, what happens is, light comes in to a tissue and is scattered in a coherent way by tiny ordered nanostructures and the light that is reflected, it's reflected using constructive interference so that all the light waves are in phase.  And this really enhances your visual signals.  So it bumps up your colour, it makes it more intense and brighter.  So, these structural colours, although we may not be as familiar with them as pigments, they actually give us the brightest colours in nature.

Chris -   But when they fossilise, are you looking for those tiny nanostructures then that would give that colour, were they still in existence?

Maria -   Exactly, yeah.  So, to find fossilised examples of these structural colours, the first clue is that you find a fossil insect that is blue or green rather than the standard black or brown. And then, when you find one of these coloured fossil insects, what you then do is use very powerful microscopes, electron microscopes, to scan the tissue in detail to look for these nanostructures.  And yes, you know, they are preserved, but they're not preserved exactly as they were in life and that's the key to the whole story.

Chris -   Also, the things you're looking at were, by definition, preserved millions of years ago.  Do you look at modern-day insects and say, "Well, we know that sort of pattern of nanostructures make that sort of colour, so we'll deduce that in our fossil, it must've been that sort of colour?"

Maria -   You could use that approach, but the problem is, we don't have enough fossils for which we have modern analogues of the extant species.  Over millions of years, species diverge.  So, with the fossils, what we found was when we looked at the preserved nanostructures in the tissues, they were giving us a false reading.  They were telling us that the original colours weren't preserved.  So, in order to actually find the missing piece of the puzzle, we decided to do fossilisation experiments.  We made fossils in the lab basically.

Chris -   Must've been a long experiment! It takes millions of years!

Maria -   No, actually, our experiments took 24 hours each.  Basically, what you do is you bump up the temperature and pressure.  You use conditions that you wouldn't get in nature, just to speed up the whole process because, you know, a typical research project lasts 2 or 3 years.  You don't want to spend the whole time gathering your data...

Chris -   So, how do you know then that what you have produced, by altering the conditions, is a faithful reflection (if you excuse the pun) on what would've occurred in that original insect millions of years ago?

Maria -   Well, we know this two ways.  People have done these kinds of burial experiments.  They're called maturation experiments and many, many times before on fossil tissues, and compared the results with what you actually see in fossils, in terms of what the chemistry, and preservation of physical structure.  So we know these experiments are a very good analogue for what we see in the fossil record.  And with our own results, we were actually able to very closely replicate in the experiments what we had in the fossils.  It all matches up perfectly.  We produced a very nice progressive colour change as we increase temperature and pressure, and we were able to explain why this colour change happens.  So, colours change during fossilisation because: number one, the thickness of the layers in the nanostructure changes during the fossilisation process.  But, number two: the chemistry of the tissue changes as well.  This is why colours change.  But we did have one very unexpected result and that was, at the end of our experiments, we decided to just really ramp up the conditions.  [We asked] "how far did we have to go until we manage to turn our structurally-coloured insects black? What we have to do to just obliterate colour completely because this is what we see in the bugs in the fossil record."  And when we did this, eventually, we managed to generate black insects, which are basically what most of the fossil record looks like.  Just out of curiosity, we said, "Let's just look at the structure in these black fossil insects as well."  And when we did that, we still found traces of the colour-producing structure.  So this means that we can now go to the bulk of the fossil record, looking at all of those black fossil insects and, actually, there's a good chance that we'll find evidence of colour there.  So we'll be able to reconstruct original colours and colour patterns for a whole host of different insects.  And so this means that we have the potential to start reconstructing communication strategies in ecosystems and actually tracking how communication strategies, and behaviours, have changed over geological time.  And behaviour is, of course, one of the most elusive aspects of fossil organisms.  So we think we're onto a bit of a winner with this one...

Ben -   Maria McNamara from the University of Bristol.