Investigating Fossils. Wilson J. Wall

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