Investigating Fossils

A History of Palaeontology

Wilson J. Wall

• Publisher: Wiley-Blackwell
• Year: 2021
• ISBN:
  9781119698487
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

INVESTIGATING FOSSILS

INVESTIGATING FOSSILS
A HISTORY OF PALAEONTOLOGY

Investigating Fossils – A History of Palaeontology is a concise and accessible look at changing attitudes to palaeontology in general, and fossils in particular. From the existential and philosophical debates arising from fossils – such as their implications for the age of the Earth – to their role as markers in Darwin's theory of evolution, fossils have been the centre of highly charged debate for over two centuries.

This book, which is aimed at anyone with an interest in the history and philosophy of science, not only describes the process of fossil formation and the history of the discovery of fossils. It goes further, and highlights the continuing importance of fossils to our ever-developing understanding of where the planet and its myriad species have come from.

Painting a vivid, lively portrait of the history and development of palaeontology, Investigating Fossils is a fascinating and informative tour of the recent history – and possible future – of the science of fossils.

1
How are Fossils Formed?

Part of the complex relationship which society has had over the centuries with fossils is at least in part associated with the conceptual problem of exactly how fossils are formed. It was not always assumed that these structures were plant or animal in origin, for a very good reason. From the earliest years of a monotheistic culture, the mortal remains were seen as disposable, epitomised by the Book of Common Prayer of 1662 where the funeral oratory includes the well‐known ‘earth to earth, ashes to ashes, dust to dust’ indicating almost by redundant usage that mortal remains will not survive in any shape or form. So it was naturally assumed that with this authority, everything would disappear, and if nothing remained, those stone‐like inclusions within rocks could not possibly be animal or plant in origin.

Although inadvertently, the Book of Common Prayer reflects something which should be obvious; that fossils are rare. Looking at this from the other direction, it implies that the process of fossilisation is a rare event, and consequently the chances of a specific plant or animal being fossilised are vanishingly small. It took a long time before we understood enough about chemistry that we could have a reasonable idea of how fossilisation takes place.

Fossilisation is a result of a set of conditions which have to be just right to work. It does not necessarily work perfectly every time, and the final product will not always be made of the same material. As we will see later in this chapter, the processes which create fossils vary considerably in detail, which is why fossils also vary so much in their structure and appearance.

The process of fossilisation has to start with the realisation that any living organism is using energy to create a state of order which has to be maintained against the inevitable nature of entropy. Once dead, this process starts to reverse as the organism starts to decay. In some cases, especially vegetable material, decomposition will start with autolysis. It was an understanding of this process in tomatoes that allowed for a genetic modification which considerably increased the shelf life by inserting an antisense copy of the ‘ripening’ gene. The result was the Flav Savr tomato, which appeared for only a few years after 1994. Regardless of autolysis happening, other organisms from large scavengers to bacteria cascade the stored energy of the sun downwards, using it to build themselves up and recycle basic biological materials. In this process, the dead organism reverts to a chaotic state of maximum entropy. Needless to say, this process needs to be halted as soon as possible if any imprint of the dead organism is to be left behind. To cover this process of death and decay through to fossilisation, the word taphonomy was coined by I. A. Efremov (1940). He described this in a paper which gave ideas and supplied explanations for the reasons that remains would move from the biosphere to the lithosphere. The meaning has shifted slightly and broadened out in emphasis so that in the twenty‐first century, taphonomy covers virtually the entire process of death and decay, with or without any final process of fossilisation.

Before the advent of geochemistry, first described by Christian Schönbein in 1838 (Kragh 2008), and for many years afterwards, there was little by way of a clear idea of changes that can take place in the chemistry of rocks and fossils. It was for many years a simple study of chemical composition of rocks, rather than changes in composition of rocks. This lack of clarity of what might be taking place in the fossilisation process meant that any attempt to describe the process was really a descriptive process of observed events. This was the situation when Charles Lyell (1832) was writing Principles of Geology. In grappling with the questions of fossil formation, Lyell expends considerable effort in explaining how various phenomena can result in biological material of all sorts and can become frozen in time. The explanations all stop at the point of ‘inhumation’, but have an interesting historical context, with descriptions of many examples. These range from inundations by rivers and landslips, such as the draining of a lake in Vermont, USA, in 1810, and the burying of villages when the mountain of Piz in Italy fell in 1772, through to blown sand in Africa. The examples cover many different natural causes of burial, by way of explaining how plant and animal material could move in to the geological strata. At the same time, there is no attempt to describe a mechanism by which this buried material could be changed from biological material, essentially organic, to stone, essentially inorganic, while still retaining some structure of the original organism.

There are exceptions to the normal process of fossilisation, which may not at first even appear to be fossilisation in the popular imagination. These are pickling, freezing, amber and tar pits.

Now commonly used for jewellery, amber is an ancient, preserved, product in its own right. This vegetable product is unique in sometimes containing inclusions of plant material from another species or animal material which can be part or whole small species. Commonly, pollen and plant seeds are found embedded in amber, while the most common animal inclusions are insects, although vertebrates such as lizards have been found trapped in amber. There have been fictional works based on the premise that DNA could be removed from one of these trapped organisms. In such a fictional world, this DNA would then be cloned to produce a new version of the original animal. The most well known of these stories is Jurassic Park by Michael Crichton which was published in 1990. This very well thought out story has the quirk of not cloning the animal trapped in amber, but the animal it had fed on. It involved removal of DNA from the gut of an encapsulated mosquito. Supposedly having fed on blood from a dinosaur, the DNA from the mosquito gut was then transferred to a reptile egg which finally hatched as a dinosaur. Sadly, or perhaps not, any DNA in such inclusions would be so badly degraded that it would not be possible to carry the experiment through to the suggested conclusion. Even if all the DNA was present, it would be in short sections, and it is not enough to have the sequence, it has to be in the right order, combined into the correct number of chromosomes. The inclusions within amber, however, have proved very useful in the detailed investigations of arthropod anatomy.

Amber is generally from the Cretaceous period or later, and is mostly composed of mixed tree resins which are soluble in non‐polar solvents such as alcohols and ethers. Also present are some resins which are not soluble in the same solvents or are of very low solubility. Most of the resin is made up of long‐chain hydrocarbons with groups that are eminently suitable for polymerisation. It is the natural process of polymerisation which causes the change from highly viscous liquid to solid. The process carries on within the solid form and eventually produces a substance that we would recognise as the brittle solid, amber. It should not be considered so unusual that inclusions are found within amber as the amount we know of is really quite large and some of the individual pieces far bigger than we can imagine being produced by modern trees. Precisely why this is so remains a mystery, but so far the largest known piece of amber resides at the Natural History Museum in London and weighs 15.25 kg. To produce such a large volume of resin and then to have it preserved is quite extraordinary. It was originally considered that amber was an amorphous material, which considering its origin and chemistry is a quite reasonable assumption. More recently, it has become apparent through X‐ray diffraction studies that in some samples there is a crystalline structure.

The natural process of polymerisation takes place over several years, generally at high temperature and pressure. Just like the formation of all fossils, the process of converting resin into amber is one which is fraught with improbabilities. The original resin has to be resistant to mechanical and biological decay for quite long periods of time, which many plant resins are not, so that there is time for the polymerisation to take place. This will render the resin more resistant to decay or destruction, but does not instantly produce the finished product. These conditions are similar to those thought to be needed for creation of coal, so it is hardly surprising that amber can be found in coal seams.

Initial polymerisation at high temperature and pressure turns the resins into copal. This is a term which originally only applied to resins from South America, but then became a general term for the halfway house between resin and amber. Copal can be used to make a very high quality varnish when mixed with suitable solvents. During the eighteenth and nineteenth centuries, large quantities of copal were consumed specifically to be used as varnish, as it could be applied to any subject that needed a high gloss clear varnish, from carriages to paintings. To complete the polymerisation and to turn the intermediate copal into amber, the pressure and temperature have to be continued. If the pressure is for some reason reduced, but the temperature maintained, the amber, or nascent amber, will break down into its constituent chemicals. In the final stages of polymerisation to make amber, the solvent terpenes are driven off leaving the tree resin as a complex polymer of great resilience.

As one would expect of a product that originates from trees at a time of massive forestation, the distribution of amber is worldwide but heterogeneous in species origin. The majority of amber is generally regarded as being cretaceous or of a more recent in age, which at 142 million years ago, or less, corresponds with the proliferation of flowering plants. Since not all trees produce free resin, it is not so surprising that amber seems to be associated with specific botanical families, of which there are still extant living examples. This is even though the plant families of interest are both ancient and not necessarily flowering. The three family groups that seem to have produced most amber are:

• Araucariaceae, these include the monkey puzzle trees and the kauri trees of New Zealand. They are large evergreen trees which are now almost exclusively found in the wild in the southern hemisphere, but when they were one of the dominant tree species, they were worldwide in distribution. In parts of Turkey, fossilised wood from members of the Araucariaceae is carved and used in jewellery.
• Fabaceae, although most of these legumes are herbs and edible crops, there are some large trees in the family. There is a single tree species in east Africa from which copal is used as incense. They have a widely distributed fossil record, as flowers and pollen as well as leaves.
• Sciadopityaceae, there is only a single species left in this family, the Japanese Umbrella Pine. Although there are no close living relatives, this was a widespread clade with a fossil record extending back more than 200 million years.

It should be emphasised that these are not the trees which originated amber, they are not ‘living fossils’, they are the current species of the lineage that produced most of the amber we know today. With amber being strictly plant in origin, it should not be a surprise that it is a frequent inclusion in some forms of coal, which were laid down from plant material at more or less the same period as amber was being formed.

Although we all have an idea of the colour of amber, having given its name to the shade of orange which we describe as amber, this is only the commonest of the colours associated with it. For example, there is a form of amber which comes from the Dominican Republic that is quite different. In this form, Dominican amber is predominantly blue. The colour is thought to originate from inclusion in the amber of a molecule called perylene. This is a polycyclic aromatic hydrocarbon with the empirical equation of C₂₀H₁₂. Perylene is basically two naphthalene molecules joined by two carbon/carbon bonds. The molecule itself is not blue, but fluoresces shades of blue when illuminated with ultraviolet light, depending upon the wavelength of the ultraviolet radiation. As it is sensitive to a wide range of wavelengths and, of course, ultra violet light is a normal component of daylight, under natural conditions the colour will always appear to be the same. Consequently, the amber will look blue in daylight, but less so, if at all, in artificial light. The unusual inclusion of perylene into Dominican amber implies either a different, possibly unique, species of origin or a considerably modified method of creation.

It is not just by the inclusion of animal material in amber that it is possible to preserve organisms in a near life‐like form without the mineralisation normally associated with fossilisation. Along with the inclusion of animal material in amber, there are also conditions in which large‐scale remains can be preserved for quite long periods of time. One of these which has yielded some quite startling finds is effectively pickling, in some cases with associated freezing. Although, as we shall see, this latter process can be good enough on its own to render stunning levels of preservation of details after death.

The process of pickling involves an organism rapidly finding its way after death into anoxic conditions, as would be expected in a peat bog where the oxygen has been depleted by large‐scale organic decay, usually of plant material. This in itself would cause preservation, although it would depend on long‐term stability of anaerobic conditions to preserve organisms intact. In the composite system of preservation, if the remains move to the next step, which is freezing, then the entire animal may be kept in very good condition for as long as the climate permits it. This can been seen very clearly in mammoths removed from permafrost where very little decay has taken p...