Читать книгу Investigating Fossils - Wilson J. Wall - Страница 10

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 place over the millennia of entombment in deep frozen condition. This process of preservation by partial chemical treatment followed by freezing could take place almost anywhere that long‐term permafrost can be found.

An extreme example of permafrost preservation was demonstrated at the 47th annual dinner of the Explorers Club held at the Roosevelt Hotel, New York, in 1951. Among the various courses was 250 000‐year‐old mammoth. This was only a taster as supplies were understandably limited, but we do know that it was provided by the Reverend Bernard Hubbard from an animal found at Wooly Cove on Akutan Island. Akutan is one of the Aleutian Islands in Alaska, where glacial permafrost is widespread. A more recent example of permafrost preservation came with the discovery in 2007 of a frozen mammoth calf. This animal, now called Lyuba, is thought to be the best preserved mammoth mummy ever found. Lyuba is a member of the species Mammuthus primigenius and died about 41 800 years ago. It is considered most likely that she suffocated in mud during a river crossing and the lactic acid produced by bacteria in her stomach partially pickled her. So well preserved is she that there is identifiable milk in her stomach. It was not certain at the time of the discovery that the calf would be preserved at all, as while the original discoverers went to report the find to their local museum, the corpse was lifted and removed for display outside a shop. It was retrieved, with a little damage due to local dogs, and has been widely exhibited since. The permanent display is at the Shimanovsky Museum and Exhibition Centre in Salekhard, Russia. Salekhard is reputedly the only town where the Arctic Circle actually runs through the town.

Completely submerging an organism in what is in effect a preservative is another way in which plant remains or animal corpses can survive for very long periods. One of the ways this can happen is found in tar pits. These are rare sites, but can be quite extensive, for example, Pitch Lake in Trinidad is the largest natural deposit of asphalt in the world, covering about 44 ha. Other such deposits include Binagadi asphalt lake in urban Baka, Azerbaijan, and the second largest lake, Lake Guanoco in Venezuela.

Probably the most investigated and well known of all the asphalt deposits are the Tar Pits of La Brea in Los Angeles. This is the most well studied of the rare tar pits, and it is also unique in having been an asphalt mine during the nineteenth and early‐twentieth century. The position of the tar pits, in what is now an extensive suburban area of Los Angeles, has added to the development of interest in this particular site. The current pits are mostly man‐made, a leftover from asphalt mining during 1913 and 1915, and partially created by deliberate digging for bones. It had been realised for a long time prior to the twentieth century that there were bones in considerable numbers present in the tar pits, but they had been assumed to be those of stray cattle that had wandered in to the tar and become stuck. It is true that animals wandered onto the unsupportive and sticky surface, making the mistake of thinking it was a watering hole, what had not been realised was that these wandering animals were not generally domesticated cattle. It would have been an easy mistake to make for wildlife to imagine the surface was solid as it would accumulate twigs and leaves and form pools on the surface when it rains. This would also attract night flying aquatic insects, such as water beetles that would similarly become trapped in the sticky tar. This is a well‐known phenomenon, where newly made roads with an apparently wet surface will attract night flying aquatic insects when there is a bright moon.

Once in the tar, escape is extremely difficult for the trapped animal, even if they were to escape, the mammalian behaviour of licking fur would render the animal sick due to the toxic compounds in the tar. It is similarly thought that the large numbers of carnivores which are present are due to them being attracted by the plight of the struggling trapped animal. The two predominating carnivorous species that were trapped in La Brea tar were the Sabre Toothed Cat, Smilodon fatalis (Figure 1.1), which is the second commonest skeletal remains of any sort recovered from the tar pits, and the Dire Wolf, Canis dirus. There are more than 400 skulls of Canis dirus on display at La Brea which have been recovered from the tar.

The lack of preserved soft parts in the tar pits has allowed speculation regarding the coat colour of species of Smilodon. They have been represented as plain‐coated or spotted, either of which would be possible. The coat colour of modern felids seems to be broadly dependent on the preferred terrain in which they live, but since there are exceptions to this, it becomes impossible to be sure of the coat in these species.

The formation of these tar deposits starts with a natural seepage of oil from underground reservoirs, as it reaches the surface, the lighter fractions evaporate or are used as an energy supply by some of the microorganisms present, leaving the heavy tar behind. Long after it was known that tar pits contained animal remains, it was not understood why only the skeletons remain. Part of the answer is quite prosaic, it seems that it takes a long time for the corpse to sink, quite long enough for decay to take a considerable hold on the soft parts. Besides this, the tar has residual solvents in it which disrupt the lipids in the body. Lipids are a group of organic molecules less related to each other by their chemical structure as by them being soluble in non‐polar solvents such as benzene and ether. The other major cellular components, proteins, are not soluble in non‐polar solvents, and it is these that would be decayed by fungi and bacteria or scavenged by small insect such as flies.

Figure 1.1 Smilodon fatalis (californicus) skull from La Brea Asphalt, Upper Pleistocene Rancho La Brea tar pits, Los Angeles, California, USA. Staining due to the tar renders the bone permanently discoloured.

Source: Photo. James St John, Creative Commons, generic.

Although preservation of ancient material, plant or animal, by encapsulation can result in very high resolution remains, the most usual way of thinking about preserved remains is as inclusions within rock. This process requires considerable changes in chemical structure and composition, a process described as taphonomy. The final outcome of taphonomy in the most frequently considered situations of fossilisation may appear to be the same, that is leaving a permanent record set in stone, but it takes little time investigating various fossils to see that the mineral nature of fossils can be radically different. This is most notably so when comparing fossils from different areas, as the colours vary quite widely. These colour variations reflect different mineral compositions within the final product of fossilisation, which of course is a reflection of the mineral composition of the rock in which the fossil was formed.

In broad terms and very simple terms, fossilisation resulting in a stone product requires rapid sedimentation of material which will eventually bind in a cement‐like fashion to become rock. The details can, of course, vary enormously from site to site, but in broad terms it always starts with sedimentation. This is one of the reasons that it is generally considered that fossilisation only takes place in shallow seas, lakes or shallow slow rivers and very wet swamp land.

Slow rivers and swamp land are often associated with floodplains, which also accumulate remains washed down stream and silt to cover them. The converse conditions are not so conducive, that is, fossilisation would not normally take place in dry, arid, conditions. This inevitably has some implication for the types of fossils which are most frequently found. Aquatic species will naturally form the bulk of fossilised material, but all species need water to drink and watering holes that attract grazing livestock also attract carnivores, both to drink and as an easy way to gain access to prey species.

A large part of the reason that fossils are not ‘dry found’ is that although mummification through desiccation is an excellent way of preserving mortal remains, we have to consider the length of time they can survive. The longevity of a mineralised fossil plays very well when considered against survival of mummified remains that are simply desiccated. In dry conditions, scavengers and recycling organisms will be active in breaking down bodies that contain nutrients and valuable mineral resources. Even when desiccated, there are a number of invertebrates that can use the material as a source of food. Assuming that burial takes place and the situation is one in which decay and scavenging do not occur, there is still a major long‐term problem of physical stability. In dry conditions, there will generally only be loose compaction of the overlying material, and this implies that there is insufficient mechanical stability of the substrate to guarantee survival of mummified remains. Shifting substrates can be a problem with standard models of fossilisation, unless the fossil is rapidly compacted and incorporated as part of the rock. By contrast, dry mummified remains in loose material, although dry and preserved, will be shifted about and abraded very quickly to dust by the surrounding material. This is very much associated with the time scales which have to be considered when thinking about the age of fossils and the aeons over which they have survived. Taking time to try and comprehend these immense time scales when compared to the age of mankind and associated civilisations is worth the effort, even though it is extremely difficult to appreciate the length of time involved.

It has been suggested that by comparing modern ecosystems with the fossil record, it may be possible to determine the biodiversity and species numbers in extinct ecosystems. By making a range of assumptions, based on ecosystem complexity, it is also possible to estimate the rate at which organisms leave visible traces. Modern studies would indicate that comparable ecosystems, such as rainforests in South America and West Africa, have the same broad biomass divided up into the same numbers of species and individual organisms. By implication, it would seem reasonable that comparable extinct ecosystems would have comparable numbers of species and comparable sizes of populations to modern ecosystems. Needless to say, the species would be radically different, but there would still be primary producers and an energy pyramid leading to the apex predators. Using these broad assumptions we can estimate that, depending upon conditions, anything from 0 to 70% of an ecology can become fossilised, with an average of 30% of biota leaving a trace of some kind. It is a very wide range, which is a reflection of the uncertainty of these sorts of estimates. From these numbers it should be self‐evident that most organisms don't leave any trace at all. If there was little or no recycling of organic remains, in our modern forests and woodlands we would, for example, be wading through the annually discarded antlers of deer.

One of the reasons for this low fossilisation rate, besides the unsuitable terrain for the process to take place, is the recycling of biological material. For the soft parts, the organs and muscles, recycling primarily takes the form of being food for other animals, followed by bacterial and fungal decomposition. With skeletons it is a little different, it is the mineral content which is of value to other animals, rather than the calorific food value. We may assume it is the calcium that is the prime object of recycling, but is not the calcium that is the primarily used part of a recycled skeleton, it is the phosphate and some of the organic material.

For many years, it was assumed that there could be little experimental work possible to investigate the formation of fossils. This changed as interest in fossil fuel formation developed with the increasing demand that became apparent throughout the twentieth century. It has been possible to demonstrate that compression in fine sediments, or in the form of fine clay (Saitta et al. 2019) followed by heating, can produce a result that is very similar to fossilisation. The fine sediment leaves enough space for labile hydrocarbon molecules to escape, which has implications for the fossil fuel industry. At the ultrastructural level, artificial encapsulation of organic material has the same appearance as high‐quality fossils. Long‐term survival of organic residues through aeons of time in the geological record depends primarily on their chemical structure. Hydrophobic water‐insoluble organic molecules can last very well (Bills 1926), while some molecules such as proteins and DNA have a relatively short survival time. Proteins and DNA probably have a short survival time in water, or damp conditions, due to thermodynamically unstable phosphodiester and peptide bonds as well as the instability of some amino acids. Deeply embedded short sequences of nucleic acid, as might be found in the teeth, can be extracted from some preserved material. If the material has undergone high temperatures, or prolonged heating, nucleic acids will break up and will only be present as very small fragments and residues.

Although the general perception of a fossil is of an image, almost an engraved image on stone, or sometimes a three‐dimensional construction, the process of creating a fossil is not a uniform one. This is perhaps self‐evident, since it can hardly be expected that preservation of a heavily calcified mollusc shell would follow the same process as a vertebrate skeleton. There are some parts of the process which are well documented and occur quite commonly. Even the common routes quickly diverge down different paths so that the result is a wide range of fossils being preserved in a variety of different ways.

By far the commonest fossilised forms to be found are those animals that start with a mineralised structure and have high population numbers (Donnovan 1991). These are most obviously molluscs, corals and echinoderms. As a structural element, the most frequently encountered element forming hard parts in the animal kingdom is calcium (Ca). This appears in many forms but is most commonly found among invertebrates in a simple chemical structure, either calcite or aragonite. These are of the same chemical composition, but different crystal structures. In vertebrate skeletons, calcium is conjugated in a different way and forms part of modified hydroxyapatite. This makes up more than 50% of the bone. Hydroxyapatite is a slightly more complicated molecule having the empirical formula which is normally written as Ca₁₀(PO₄)₆·2OH. As the primary structural calcium salt of vertebrates, both in skeletons and in the teeth, it is this molecule which can have the hydroxyl group replaced by fluorine in the teeth, giving it greater resistance to decay.

In whichever form the calcium is found, it is these calcium deposits which form the basis of the most commonly found fossils, coming as they do from shells and skeletons. This is not to say that they remain unaltered, but only that these hard minerals are the starting point for what can be quite complicated chemical changes that are found in diagenesis.

Both calcite and aragonite have the same empirical formula of CaCO₃, but they are differentiated by virtue of the crystal structure that they take up. On close, very close, X‐ray diffraction examination, calcite is a hexagonal (trigonal) system and aragonite is rhombic. Structurally the difference is quite small, but aragonite tends to be less physically stable than calcite. It is probably for this reason that calcite is the most commonly found crystalline calcium salt and is therefore the major component of the large deposits of marble and limestone which are so easily seen on some exposed cliffs. When it is found with magnesium carbonate, MgCO₃, the mixed marble‐like material is described as dolomite.

The distribution of these two forms of calcium carbonate, aragonite and calcite, in biological systems varies depending on the taxa. Although not necessarily exclusive, we would generally expect to find calcite in such wide‐ranging groups as brachiopods, ostracods, foraminifera and some sponges. The alternative structure, aragonite, is more often present as the structural material in molluscs and some sponges. That both forms are found in sponges is indicative of there often being a mixed use of calcium as a structural element in the same taxonomic group.

The process of forming what we would most readily recognise as a mineralised fossil involves the process known as diagenesis. In broad terms, diagenesis describes the changes that sedimentary deposits undergo, both chemically and physically as well as changes due to biological activity, before the process of lithification takes over to form solid rock. The first stage of this is permineralisation, which involves the percolation and deposition of crystalline material from solution into areas where only water‐based solutes can reach. Because it is a crystallisation process, the fine internal detail can be very well preserved. The level of external detail which is preserved also depends on the type of material in which death has taken place. As would be expected, the finer the sediment, the greater the final detail.

Calcium permineralisation is a common early occurrence in fossil formation, as calcium salts quickly saturate ground water being of relatively low solubility. Aragonite is not often preserved unaltered in the geological record and where aragonite fossils do occur they are usually associated with mudstones and marls. Under suitable conditions, it is possible to find ammonites that still have their aragonite shells, the same is also true for bivalve molluscs, but only if conditions are right for preservation without structural modification. The structure of aragonite is less stable than calcite, so where aragonite is present in shell and skeletal structures, it tends to be replaced by calcite. Generally rhombic aragonite is often found to have been replaced by hexagonal calcite, and this usually takes place in one of two ways. It should be noted that although both of these molecules have the same formula, it would seem that when aragonite is replaced by calcite, the calcium carbonate of the structure is not necessarily reused, the calcite deposits coming from extraneous calcium carbonate dissolved in the surrounding water.

The first process for substituting aragonite for calcite involves complete dissolution of the aragonite followed by deposition of calcite in the void that was left. There can be a considerable time lag between dissolution and complete deposition, this is a purely chemical process, unlike the original formation which was under biological control. If calcite deposition is a slow process, it can result in large crystals of calcite being laid down with the loss of a great deal of the original detail. If there is a gap between the two processes, it is reasonable to assume that this is why the original calcium carbonate is not wholly being reused, if at all. During the time between dissolution of the aragonite and deposition of calcite, the shape of the organism is held in place by a cement layer in the host sediment. This can be either as a complete block, in which case infiltration by depositing solutions will tend to take longer, or as coating laid down on the remains of the organism and fused into a cement case within a looser sediment, under these conditions calcite deposition is faster.

The second method by which aragonite can be replaced by calcite takes place across a thin film, with the aragonite being dissolved on one side of the film and calcite being deposited on the other. This process is often referred to as calcitisation and is a process which can retain considerable skeletal detail in the final fossil. It also tends to reuse more of the original calcium carbonate, although this is not necessary if ambient conditions are correct, for example, if the water is a saturated calcium carbonate solution. With the replacement of one form of calcium carbonate with another, there is an almost inevitable reduction in fidelity since the original structure would have had other biomolecules present, such as scaffold proteins and the crystal structure would be very small indeed, controlled by the cells of the living organism. So laying down a calcite copy by simple chemical crystallisation will follow the original mould but will have larger crystals.

Even though we refer to calcite as a uniform substance, the nature of calcite is such that it is described as having two forms itself. These are Low Magnesium Calcite (LMC) and High Magnesium Calcite (HMC). The difference between these two is quite small. LMC has between 1 and 4 mol% MgCO_3, while HMC contains 11–19 mol% MgCO_3. This small difference between the two is enough to make HMC less stable than LMC. Consequently, it is possible for skeletal material that was originally laid down by the organism as LMC to be retained through geological time and consequently retaining structural details down to the micro level. This situation is found in the brachiopods, which start with an LMC structural composition and consequently are sometimes little changed during fossilisation. With the original material remaining in situ it can be used to give a good basis for analysis of carbon and oxygen isotope values from their LMC calcite shells. Similarly, trilobites are also well represented in the fossil record, not just because they were a common part of the ecology, but also because their shells were mostly calcite with some phosphates present.

Structural crystalline minerals other than calcium carbonate can occasionally be found to have replaced calcite in some fossils, although this generally only happens in very specific circumstances. It is not always known in taphonomy what these specific circumstances are, but the fact that the substitutions do not take place commonly would imply specific chemical needs for the process to go to completion. It may also require unusual pressure and temperature to complete the conversion. Silification, that is, substituting silicate minerals in place of the calcium based calcite, is something which happens occasionally. In certain circumstances, silification may have spectacular results, one such is when fossils become converted to opal.

Hydrated amorphous silica (SiO₂ nH₂O) in the form of opal can form fossils which are so decorative that they are often found being broken up for use in jewellery. If the silicates enter a body cavity and precipitate from solution, the external structure can be very well preserved, but not the internal details. As an alternative, should it infiltrate the organic material before decomposition, then good internal details can be preserved. Because deposition of silicon depends on the ground water and basal rocks, it is a very rare combination of events which results in opalised fossils. Some of the very best sites for these opal fossils are found in Australia, usually in active opal mining areas, such as Lightning Ridge in New South Wales and Coober Pedy in South Australia. Although there are other sites in the world where opalised fossils can be found, Australia is the most renowned area. It was in 1987 at Coober Pedy in South Australia that what has become known as Eric the Pliosaur was discovered. This reptile, Umoonasaurus demoscyllus, is one of the most complete opalised vertebrate skeletons known. Not only is it of interest and value as a fossil, it is also of considerable financial value for its opal content alone. The history of the fossil after being discovered is quite convoluted. It was originally sold to a dealer who went bankrupt, at which point it was very nearly sold to a jewellery consortium which had planned to turn it into jewellery at which point it would have been opal with the enhanced value of being a fossil. With help from Akubra Hats and money raised by school children, the fossil was bought for the Australian Museum, where it is now on show.

The formation of opal fossils is a rare event, just as the formation of opals themselves is. Although opalised fossils must have a quite complicated process of formation, we can gain some insight from the way that gem‐stone opals are formed. Opal formation starts as silicon oxide spheres in a silica‐rich solution. These spheres settle under gravity and build up to form the gemstone opal. This process is very slow and to produce the precious colours of opal, the spheres need to be uniform in shape and between 150 and 400 nm in diameter. Although opal is generally made from even‐shaped and sized spheres, there is no long‐range or short‐range order in the stacking of the spheres. There are broadly two forms of opal, opal‐A (sometimes AG or AN) and opal‐C (sometimes CT). Opal‐A can transform into opal‐C under high pressure from overlying sedimentary deposits. This form of opal can sometimes contain as much as 10% by weight of H₂O.

Another mineralisation process which replaces calcium in fossilisation is pyritisation in which iron salts are laid down instead of calcite. This takes place when the water in which the organism died is high in iron sulphides. When surrounding water is high in iron, deposition of FeS₂ is affected by sulphides originating from decaying organic matter, of which there will be considerable amounts in a corpse. Pyritic fossils can be difficult to store because under humid or damp conditions, the iron pyrites of which they are made up, that is FeS₂, will chemically decay into iron oxide and sulphides, sometimes sulphur itself. This is a process which can be hastened by some types of bacteria. If this does start to happen to a pyritic fossil, it is possible to find sulphur deposits on the surface and the fossil may expand as iron oxide is a much larger molecule. This is exactly the same process which causes rust on ferrous metal to flake as the iron oxide pushes itself apart. Iron pyrite is not the only iron containing mineral which can create a fossil impression, but it is by far the commonest. Pyritisation has been shown to be a process capable of recording considerable morphological detail of soft‐bodied organisms of the Ediacaran (Smith 2019).

It is possible for rapid precipitation to occur around a subject being fossilised which will then form a nodule retaining considerable detail in the organism at the centre of the nodule. A mineral that is often found involved in this process of nodule formation is siderite, that is iron (II) carbonate, FeCO₃. The unusual thing about siderite is that it is approximately 48% iron and consequently a valuable iron ore for commercial production of steel. Siderite is a diagenetic mineral in shales where it creates authigenic moulds of fossils as nodules, which have to be split to discover their content. One of the most famous sources of these fossil nodules is the Mazon Creek fossil beds in Illinois, USA, where it is even possible to find fossil sharks. These are rare fossils because although shark teeth are common, their skeletons are cartilaginous and therefore do not normally fossilise before decaying.

Interestingly there is a single species of living gastropod, which is the only animal known to use an iron‐based mineral for its shell. Chrysomallon squamiferum is a gastropod from deep sea thermal vents at depths of 2500 m and greater, where the outer shell contains iron sulphides including the mineral greigite, Fe₃S₄.

While there is a general procedure of one mineral replacing another, or sediment developing in a systematic way over the top of a form which will become fossilised, this is, perhaps, an oversimplification of a dynamic process. It has been shown that the linear progression model may not be so common as a simultaneous process, depending upon tissue type. It is quite likely that local chemistry within the cadaver will alter ion concentrations and influence specific precipitation events. These chemical changes will also be influenced by the state of decay, and it has been suggested that rapid biodegradation can enhance the detail which is eventually left behind (Jauvion et al. 2020). The rapid deposition of some preservative minerals will then be replaced at a slower pace by more robust minerals such as calcite.

As can be appreciated, the formation of fossilised material is a rare chance event which unless interpreted carefully gives a distorted snapshot view of the past. The image which can emerge is one where ecosystems were dominated by the hard shelled or skeletoned. This may broadly be correct, but with the difficulty that soft parts do not generally leave a trace before decay, we may never know the extent or range of some of the species and whole taxons which were on the planet before vertebrates appeared.