In The Blink Of An Eye (28 page)

All species still glow in their original, almost fantastic array of colours

HERBERT LUTZ, German biologist, on the colour of 49-million-year-old jewel beetles from Messel, Germany

 

 

 

Today the Museum of Antiquities in Leiden in the Netherlands, houses a statue of the Egyptian god Osiris. This statue is about a foot tall with well-preserved features, and also a fair amount of its original paint - it has seen little sunlight, being mostly preserved within a tomb. Here, Osiris has a blue-green face and wears a red skirt. And another obvious feature of this statue is that it is hollow . . . but why? Without the preserved colour this question would remain unanswered. Numerous statues of Osiris have been excavated but the hollow inside and colouration make this particular representation different.

The interpretation of hieroglyphics, and the preservation of yet more pigments in the form of ancient Egyptian scripts, inform us that blue-green was the colour used to represent the afterlife and red was used for festivity. So now we can interpret this statue of Osiris as being a celebration of the afterlife. From this, and the knowledge that hollow Egyptian figures were filled with papyrus manuscripts, we can infer that our statue once contained a copy of the Egyptian Book of the Dead.

The ancient Egyptians were, in fact, skilled artists. They used colour to represent personality and status, but they knew it would fade with time. Consequently much of their art was sculpted and then painted, so
that at least the physical sculpture would remain long after their death (as was their intention). But they also had gold leaf at their disposal. The cause of the gold effect in this case lies somewhere between a pigment and a structural colour. Gold leaf is a thin layer of metal that reflects a beam of sunlight in a single direction, like a mirror. It reflects all the wavelengths in sunlight except blue, all of which add up to gold. As a physical structure it outlasts the pigments of ordinary paint through time. So gold leaf was used on many Egyptian statues, since the Egyptians were conscious of the short-term prospects of their pigments. And gold leaf is indeed evident in numerous Egyptian artefacts today, as in another statue of Osiris housed in Leiden. The gold in this case is symbolic of eminence.

Chapter 3 demonstrated that colour alone tells us about where and how an animal lives today. Considering the information acquired from the colour of the pigments in the Egyptian statue of Osiris, a question relevant to this chapter now begins to form: ‘Can we bring Cambrian fossils to life in the same manner?' The excellent preservation of gold leaf in the statue of Osiris signals hope of unearthing structural colours of geologically ancient times.

We know that animal body shapes and forms were as complex in the Cambrian as they are today, so perhaps we can also expect Cambrian animals to have been sophisticated in terms of their colour. But we have learnt not simply to predict colour based on animals today. We must find traces of the
original
colours themselves in ancient, extinct animals. And the best place to look is in those fossils that have been preserved under the most favourable conditions. Work in this field is already underway.

Trilobites that lived 500 million years ago, just after the Cambrian, have been found with signs of pink colouration, not something that is easily explained given the type of rock in which they were preserved. It is therefore believed that these randomly arranged pink pigment granules are remnants of a colour that once covered the entire trilobite. That would be interesting. Below the very surface waters, and in the environment inhabited by these trilobites, red light does not exist. Here, pink becomes grey and blends well into the background. So these trilobites may have been coloured for camouflage. But few experiments
have been conducted in this case, and so speculation must end there. And this case of trilobite-pink also represents the end of the road for ancient pigments. Unfortunately, pigments, and also bioluminescent organs, do not take us back to the Cambrian, and so can be of little use to the subject of this chapter. But structural colours are another matter altogether. Could
they
tell us anything about colour in the Cambrian?

As outlined in Chapters 3 to 5, physical devices that cause ‘structural' colours are a significant means of light display today. Like pigments, structural colours rely on a source of incoming light, usually in the form of sunlight, from which certain wavelengths, or ‘colours', are reflected.

Structures can be preserved in the fossil record - at least their shapes and sizes can be, even if the original materials become altered or replaced. Fossils themselves, whether the whole bodies of trilobites or the bones of dinosaurs, are indeed structures. Although on a much smaller scale, it is therefore not surprising that structures responsible for colour can also be preserved within fine sediment - these are, after all, just structures. Obviously micron-sized reflectors could not be preserved in sediment of 1 millimetre sand grains - apart from the obvious physical problems they would be consumed by the bacteria infilling the spaces between grains. This was the reason why we cannot find minute sensory detectors in the Australian Ediacaran (Precambrian) fossils. Shapes of the entire animals can be seen with the naked eye, but under a microscope nothing more than piles of sand grains can be distinguished. Similarly, the chemical components in the embryonic rock must be right to replace organic parts. But there is certainly greater potential for structures to be recorded in the fossil record than for pigments. And if the conditions are right, theoretically there is no lower limit to the size of a structure that can be preserved as a fossil.

Before moving directly to the Cambrian fossils themselves, we should take a look at the methods at our disposal for unearthing ancient hues. We should be aware of the variety of structural colours that may be preserved along with some of the pitfalls one may encounter along the way to the Cambrian.

We know that multilayer reflectors are the most widespread cause of structural colours in animals today. Like pigments, these occur within the bodies of animals, below the surface. Again, the scanning electron microscope is not appropriate here because it can only scan surfaces. So to search for multilayer reflectors, we must look at thin sections of fossil skin or shell - the outer layers of an animal. Some years ago I tried exactly this, using ammonites and ancient beetles as my guinea pigs.

Ammonites are among the few groups of animals whose original, transparent, thin layers have survived in fossils, and colours radiate from some of them today as they may have appeared millions of years ago. But this cannot be assumed for every case of iridescent fossils. There are warnings to heed from opal - all that glitters may not be old, or at least not as old as the animals that have been fossilised.

In Chapter 5 I described my discovery of structural colour in seed-shrimps, almost the first structural colour known in seed-shrimps. A couple of years earlier, while sorting through a large sample of small crustaceans, I had noticed a single flash of colour from one seed-shrimp. There were many other individuals of this species, and all were quite transparent, but while I moved the sample one individual was flashing red one minute, and green and blue the next.

The seed-shrimp was the size of a tomato seed, and the source of the colour much smaller, but it was large enough to be identified under the microscope. The identification also solved the problem of why only one individual should reveal colours. The source of the colour was not a feature of the animal itself, but a tiny opal, and the seed-shrimp had eaten it. The opal lay in the stomach of the transparent animal.

Opal is a form of silica dioxide. It is made up of tiny spheres, around half the wavelength of light in diameter. It reflects light in a complex manner, which has only recently been understood by optical physicists. But it is the physical nature of the structure that provides the optical effect rather than a chemical pigment, and so opal is said to produce structural colours. In fact the bright, iridescent effect of opals is similar to that of the seed-shrimp diffraction gratings.

The original chemicals that make up fossils, at whatever stage in the fossilisation process, can be replaced by other chemicals. Sometimes, the replacement chemicals can be silica dioxide and water, in which case opal is formed in the mould that is the fossil. At Lightning Ridge in Australia, opal miners often excavate dinosaur bones and teeth, and the parts of other animals, which display the characteristic iridescence of opal. These fossils are so well known that most palaeontologists think of them when we mention ‘colour in fossils'. But unfortunately this adds no evidence to the original colours of ancient life - opal has nothing to do with living animals (other than that single seed-shrimp).

Ammonites are the shells of ammonoids, those long-extinct molluscs related to squid as described in Chapter 2. Some ammonites appear coloured, but like opal their hues are non-biological. Particularly striking for their visual effect are the ammonites from Alberta, Canada, which flash spectacular colours as their rocks are cracked open.

In view of the Canadian Rockies lies the small town of Magrath, and the familiar wheat fields and ranches of the Canadian prairies. Seventy-one million years ago, this land was beneath a sea which stretched from the Gulf of Mexico to the Arctic Ocean. And in this sea lived ammonoids - lots of them, ranging from the size of a compact disc to that of a car tyre. Today, one particular ranch near Magrath, of about 800 hectares, is different from the others. Its foundations contain ammonites.

These ammonites were first covered not with sand but with ash from the huge volcanic eruptions - which played a part in the creation of the Rocky Mountains. The ammonites became sealed in a waterproof layer of shale, but this did not prevent quartz, copper and iron from the volcanic ash infiltrating the shells. During the Ice Age, a layer of ice close to 2 kilometres thick covered the region. The weight of this ice served to compress the ammonites and their component chemicals, and ‘Ammolite' was formed.

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.

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.