Ammolite (and Korite) are names given to a semi-precious gemstone that partly constitutes the Magrath ammonites. In 1981 enough high-quality Ammolite was discovered to make mining commercially viable. But their equally commercial bright colours are the result of preservation, the compacting of the shell layer that may have possessed some iridescent properties to begin with. Many shells today have an iridescent layer, containing a multilayer reflector called the nacreous layer. We suspect the Magrath ammonites might also have contained a nacreous layer because other ammonites have been found in a more natural state, also with iridescence.
In Wootton Bassett in Wiltshire, England, ammonites literally pop up out of the ground, for 20 metres below a spring, Jurassic clay in the form of grey mud oozes to the surface in a sort of mud volcano, bringing with it Jurassic ammonites hitching a ride in the eruption. Although 180 million years old, these ammonites are also iridescent, but they are different from those found in Magrath. The Wiltshire ammonites are pristine fossils, unaltered since their initial preservation. Inside the shells are some original organic ligaments, but they also retain their aragonite, a calcium-based mineral and a component of their original shells. It is this aragonite, within the nacreous layer of the shell, which is responsible for the iridescence. Aragonite forms thin layers, each a quarter of the wavelength of light in thickness and all separated by a similar distance. Consequently, the nacreous layer is a multilayer reflector, like those found in metallic beetles and shells today. But as explained in Chapter 3, multiple layers can also provide structural strength, and when strength is the adaptive function, the incidental iridescence is nullified by an opaque, outer covering. Iridescence is a powerful effect, and redundant iridescence would be simply too dangerous to project recklessly into the environment. A camouflaged soldier could not smoke a cigarette in the evening, especially if the light from the cigarette was not also being used as torchlight. So although iridescence is quite eye-catching in these ammonites today, and in specimens from other parts of the world, in the Jurassic the story could have been quite different. The prehistoric seas could have been spared ammonoid iridescence by a dark outer layer of their shells, a layer that has not been preserved. Ammonoids will pop up again later in this book, but now we should consider those fossils whose original colours are displayed today just as they were in environments some fifty million years ago.
The Messel beetles - original multilayer reflectors
There is one particular quarry in Messel, near Frankfurt in Germany, that reveals extraordinarily preserved, articulated skeletons of vertebrates, around fifty million years old, surrounded by complete outlines of their bodies. This quarry also contains insect exoskeletons like no other fossil site - chitin, the primary component of arthropod shells, has been preserved there.
Today the bowl-shaped crater at Messel is fenced off and closely guarded. It is now generally accepted that something special occurred here, but this was not always the case. When the mining that originally created the crater came to an end in the 1960s, the intention was to infill the site with garbage. Then it was that fossils found when quarrying first began were brought to public attention. Almost immediately the United Nations declared Messel a World Heritage site.
Forty-nine million years ago, after the mass extinction that killed off the dinosaurs, Europe was an island and the Messel site lay at the bottom of a lake. Today the rock in the quarry is still damp - it is 40 per cent water. But when the layers of thin sediment are cracked open, they sometimes reveal a little more. Fossils of entire animals, from bats to crocodiles, have been exposed. Preservation is so good in this oil shale that Messel palaeontologists tend to feel more like zoologists. But when the fossils are exposed to air, they must be immediately stored in water, for the rock crumbles if it dries.
Structures such as the feathers of birds have been preserved at Messel as if they had only moments earlier fallen from the sky, but in my biased opinion the greatest treasures of all at Messel - and justification alone for the high security - are the metallic-coloured beetles. Their optical effects are extraordinary. Stag beetles reflect the shimmering blues and greens they displayed while alive. As the shale containing a jewel beetle is broken, the sight of 49-million-year-old iridescent yellows and reds is revealed. And so the list of beetles and colours continues.
Sometime in 1997 I received a parcel from Germany from the palaeontologist Stephan Schaal, a man whose name is almost synonymous with that of Messel. As I had hoped, it contained Messel beetles from a recent excavation. The fossils were stored in water, and the wing cases shimmered with violet, blue and green. Since colour in animals was at the centre of my research, the first question to cross my mind was, ‘What is causing this colour?’ The age of the fossils simply did not register - the beetles looked like zoological museum specimens recently collected from a rainforest expedition. After all, 49 million years is a long period to comprehend or time travel in one’s mind.
To answer my question, I turned to electron microscopy. Small sections of a blue beetle exoskeleton were treated in two different ways. One section was critical point dried - that is, it was dried out in a controlled manner to prevent shrinkage. Although it had retained its structure, the dried section had lost its colour. It had become transparent. To examine the structure in the scanning electron microscope, it was first coated with gold. Then, at 10,000 times magnification, thin layers became evident, with upper layers only partly overlapping the lower layers. The layers were smooth, and there was no sign of a diffraction grating or structures that could cause the scattering of light. But to confirm that this was a multilayer reflector, transmission electron micrographs were needed.
One of the beetle sections was embedded in resin, stained and sliced so thin that it was not visible edge on. Placed on a minute metal grid to provide support, the specimen was imaged in an electron beam. A multilayer reflector was revealed.
To be doubly sure, the dimensions of the reflector were measured and fed into a computer program which re-created the stack of thin layers and predicted the colour of the light reflected in sunlight at 90° to the surface. The predicted colour was blue. The actual colour I saw was blue. Hence the cause of the colour of Messel beetles was a multilayer reflector. The reason the colour had disappeared from the dried specimen also could have been predicted. It emerged that one of the two layer types in the reflector consisted of water - when the water disappeared, so did the colour.
Several specimens of the same beetle species have been found at Messel, and all display exactly the same colours. So we can be confident that 49 million years ago beetles were gracing Europe with spectacular iridescence - last seen flashing when the dead beetles were washed into the Messel lake by floodwater, and were sinking into the depths of history. Re-opening the history book, we learn that light must have been a powerful stimulus to animal behaviour even then. But how far back can structural colours help us take this philosophy?
Fossils of the Burgess Shale - diffraction gratings
In 1966 Kenneth Towe and Charles Harper, palaeontologists at the Smithsonian Institution, published a paper describing the cause of iridescence in 420-million-year-old lamp shells. They found tubular aragonite crystals arranged in layers, with dimensions in the region of the wavelength of light. A layer of juxtaposed tubes may create a diffraction grating on the outside, but a stack of thin layers can also form a multilayer reflector. The lamp shells appeared with a rather faint iridescence or pearly lustre like that of some shells today. Towe and Harper suggested the cause of this optical effect was a combined grating-multilayer structure, and attributed the faintness to variations in spacings, or a degree of randomness in the structure. Further work may be required to confirm these conclusions, but we cannot say for certain that these colours were sparkling in 420-million-year-old waters. Again, considering shells today, the lamp shell iridescence may have been precluded by an opaque outer layer, a layer that was not preserved in the fossil.
True diffraction gratings
are well known to physicists, but before my search, prompted by their discovery in seed-shrimps, they were unknown in nature. Then diffraction gratings began to appear in one animal after another. First there was a lobster found off Hawaii, then a type of shrimp from New Caledonia, again in the Pacific Ocean. But the Indian Ocean was hiding similar treasures, not only within its crustaceans but in bristle worms, comb jellies, jellyfish and peanut worms. Eventually it was discovered that the entire globe contained a vast array of species, from many animal phyla, loaded with diffraction gratings. The world, it turned out, was even more colourful than we had believed it to be, albeit that the newly revealed iridescence was often concealed from view for most of the time.
Part of my work on seed-shrimp iridescence described in Chapter 5 was carried out at the National Museum of Natural History of the Smithsonian Institution. Originally I had found diffraction gratings in some seed-shrimps from Australia and needed to examine as many other species as possible. The world’s expert on this group of animals is Louis Kornicker at the Smithsonian, and it is no coincidence that the best collection of seed-shrimps is found there, too. So it was only natural that I should apply for funding to work in Washington. My application was successful and in 1995 I began working on the Smithsonian collection.
As mentioned in Chapter 1, the Smithsonian also houses probably the best and certainly the most important collection of Burgess Shale fossils anywhere in the world. Now that is a coincidence. The Smithsonian was the home of Charles Doolittle Walcott, who discovered the first Burgess Shale fossils. But other than a general fascination in this ‘wonderful life’ evident among all zoologists, I had no specific interest in the fossils themselves.
Taking a break from work at the Smithsonian, one is spoilt for choice for things to do. Within a few blocks of each other on a single avenue there are several national museums and art galleries. But there was also the Museum of Natural History, and during one late afternoon break I found myself wandering around the fossil galleries.
I discovered a small but excellent exhibit on the Burgess Shale nestling between larger skeletons. This exhibit was worthy of its space because the fossils displayed were complete and detailed examples of the range of life forms for which, in addition to its age, the Burgess Shale was famous. The specimens were also those collected mainly by Walcott in the early 1900s.
Next to each fossil in the exhibit were black and white illustrations showing reconstructions of the animals when they were alive. The drawings were very detailed and really helped one to visualise the living creatures. But the level of detail included something of interest to me specifically. On some reconstructions there was a hint of something quite amazing. On the reconstructed armoured parts of Hallucigenia and Wiwaxia were fine parallel lines. And fine parallel lines were the reason I had come to Washington in the first place.
The day before, I had visited the aviation and space museum which housed some aeroplanes from the 1950s, each with multiple propellers and corrugated wings and fuselage. The corrugations served to increase the strength of the metal structures. Later I was to encounter similar corrugations used to increase structural strength - but in the leaves of a Rocky Mountain plant on my expedition to the Burgess Shale quarry. These leaves were thin and would have collapsed were it not for their corrugated form. This was important to bear in mind. Narrow striations on the Burgess Shale fossils could represent a finely corrugated surface to make them stronger. But it got me thinking. The same rules apply to animals today, although if the striations meet certain size criteria, they cause iridescence - they become diffraction gratings.
Reference to diffraction gratings insinuates microscopically fine corrugations, where a distance approaching the wavelength of light separates neighbouring ridges. Such structures cannot be drawn as lines on paper. No pen is that sharp or precise, and we would not be able to see the lines with the naked eye anyway. But, as I have said, this got me thinking. Maybe the lines figured in the animal reconstructions were merely representatives of diffraction gratings. Fossil preservation is never uniform, and perhaps only some ridges of a grating had been preserved. Then again, maybe the lines figured were complete and did serve to provide strength. If this were the case, the parallel lines illustrated in the Smithsonian exhibit would add nothing to the topic of colour in fossils. But they changed the direction of my thoughts. If animals possess diffraction gratings today, maybe they did so in the past too.
The morning after my first Burgess Shale viewing, I submitted a request to examine the original Cambrian finds. Access to the Smithsonian fossils, and those at Harvard University, was granted, following support from Simon Conway Morris of Cambridge University, Doug Erwin at the Smithsonian and Frederick Collier at Harvard. Then there were some specimens to examine at the Australian Museum back in Sydney. To begin with I employed the most powerful light microscopes available at the Smithsonian Institution. I had not realised that the compact disc case I was using to orientate the specimens under the microscope was titled ‘Handel’s Water Music’ - quite appropriate, as certain onlookers remarked. But it worked. I placed the fossils so they could be viewed from different angles, and structures I had not noticed before became evident. Then I knew exactly what to look at, and the work became serious. I took the fossils to the various underground rooms of different institutions, where vibrations and magnetic fields that could interfere with more powerful microscopes were minimal. And there the microscope cavalry charged in to the project. By the end of my experiments, I had bombarded many species of Burgess animals with a barrage of laser and electron beams, and had imaged the specimens at extremely high magnifications - so high that even single molecules could be observed.
The techniques I used were all harmless to the fossils, which in some cases included original organic material, but there was one further test I wanted to carry out which would have altered the fossils permanently. The scanning electron microscope exacts a thin coat of metal to be applied to any animal surface under observation - a coat that cannot be removed practically. So rather than harming the invaluable fossils, casts were made. Plaster of Paris can be used to make casts of dinosaur footprints, but the Burgess Shale fossils under investigation were small and the diffraction gratings are microscopic. The particles in plaster of Paris are simply too large to fill the grooves of a diffraction grating and produce a detailed cast. But I had learnt of a new technique using acetate, and this enabled fine, elaborate casts to be made. When dry, the casts rather than the fossils were gold-coated and could be examined in a scanning electron microscope.
After the last microscopic tests had been completed, the potentially amazing became a reality. The reactions of several electron microscopy technicians indicated that the results were both positive and special. On the broken surfaces of three species - the bristle worms Wiwaxia and Canadia, and the arthropod Marrella - were remnants of diffraction gratings. Only traces of gratings had been preserved, rather like the few squares that remain in many Roman mosaics, but where they did occur on a single body part, they were always exactly the same size and shape, and were orientated in the same direction. The results were consistent. But the fragmentation had extinguished iridescence in the actual fossils. The fossils were decidedly grey. The mood in my lab, on the other hand, was more colourful.
Figure 6.1 Micrographs of the Burgess bristle worm Canadia at increasing magnification - from x10 to x1,500. The top picture shows the front half of the animal, the middle pictures show details of bristles. The bottom picture shows the surface of a bristle as removed from the rock matrix, revealing the remnants of a diffraction grating with a ridge spacing of 0.9 microns.
Did this really mean that Wiwaxia, Canadia and Marrella would have appeared highly coloured when they lived 515 million years ago? This still seemed unbelievable. To make doubly sure, the original surfaces of Canadia and Marrella were reconstructed in their entirety, based on the remnants that had preserved. This was achieved by carefully positioning two laser beams so they met an
d interfered at the surface of a light-sensitive material and etched out the precise sinusoidal contours of the remnant gratings over the entire material (the model was examined further to confirm this). The reconstructed surfaces were taken out of the dark laboratory and placed in seawater under sunlight, and . . . the colours of three Burgess Shale species shone as spectacularly as they had 515 million years ago. That was the most memorable moment of all. For the first time, the original colour of a Cambrian animal had been uncovered. An almost unimaginable piece of Cambrian history had been revealed.
When a surface has the physical properties - the size and shape - of a diffraction grating, it will cause iridescence in the presence of sunlight. And sunlight would have existed in the environment of the Burgess animals - at least the blue, green and yellow part of sunlight. I applied some simple optical equations to the reconstructions of Wiwaxia, Canadia and Marrella and calculated the directions in which they would have reflected different colours. Because the parts with diffraction gratings were positioned in a variety of orientations, from any direction Wiwaxia, for instance, would have shimmered with all the colours remaining in sunlight. And those colours would have appeared relatively bright like the spectrum of a compact disc. They would have been visible even under the dim light conditions of deeper waters or during dawn and dusk - when pigments become invisible. Interestingly, I photographed the model of Wiwaxia’s spine gratings under ultraviolet light only. Here I used the methods employed previously on the Atlas moth, as described in Chapter 3. Humans are blind to ultraviolet light, so I could see nothing through a camera with an ultraviolet-only filter. But when the ultraviolet-sensitive film was developed, very bright patterns emerged where human-visible colours were absent. The camera could ‘see’ ultraviolet, and I was looking at the camera’s view. So if Wiwaxia had lived where the ultraviolet part of sunlight existed, such as in shallow depths, it would have shone brightly in ultraviolet along with the human rainbow. Unfortunately, we will probably never know the complete spectrum that illuminated the Burgess animals.
In The Blink Of An Eye Page 21
