Investigating Fossils. Wilson J. Wall

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.