|Fossil Preservation :|
When most people see creatures preserved in rock, one of the most baffling questions that immediately spring to mind is how it came to be that way. It’s a rock, after all – what’s it doing with someone’s shell embedded in it? Welcome to taphonomy, and if there’s a stranger or more wondrous aspect of palaeontology, I’m not sure what it is. The question actually has several parts: what was it to begin with? What’s it made of now? And how did it get from one to the other? The number of potential answers to each part is large, but we can constrain things quite neatly by starting at the beginning.
Organisms can be made of a variety of ‘stuff’. There’s the squidgy stuff that makes up most of an organism – organs, fluids, muscles and the like. Then there are crunchy and bendy bits, like the cuticle of arthropods, or our fingernails. These are both what we call ‘soft tissue,’ not because of any measure of their hardness, but because they are organic. Because they are organic, bacteria take great delight in extracting the goodness once the owner no longer cares. As a result, these bits, with a few exceptions, are generally not preserved. I say generally, because there are those spectacular fossil deposits that we call ‘Lagerstätten’ in which even the most fine detail of soft tissues can become fossilized – and it is here that the most bizarre of preservation mechanisms come into play. The huge, vast, overwhelming majority of fossils, however, are of what we call ‘hard parts’ – skeletal elements that were originally made of a biologically secreted mineral. In other words, these parts of organisms, such as the shells of clams, are not organic. They do not decay. Although chemistry and physics have a lot to say in the matter, the chances of these tissues being preserved are boosted from the beginning.
The processes that go into making a fossil start off simple, and get more and more complicated the closer you look into it. We’ll start with the easy bits, and leave the bizarre until you’re more comfortable.
Fossils form when organic remains (whether the shell, the body, or just traces of its passing) become part of the sedimentary sequence. Basically, it needs to be buried. Sediment can accumulate slowly, or be deposited rapidly, depending on the circumstances. For example, parts of the north Pacific today are so far from land that hardly any sediment from the continents reaches them, and so deep that the calcite skeletons of plankton dissolve on the way down (see below). The sediment that is deposited there is mostly windblown dust, volcanic ash, and micrometeorites. As a result, a sedimentary rate of 1 mm in a thousand years is quite normal. In contrast, when major landslips occur around volcanic islands or along fault zones, several metres of sediment (including house-sized boulders) can be deposited in a few seconds. Neither of these cases is much good for fossils. In the first case, the remains are left for years exposed on the surface, and in the second are likely to be destroyed by the violence of the deposition.
Instead, we want some of the vast range of more gentle, but persistent sedimentary environments. For example, in an estuary setting, the river brings a huge amount of sediment into the sea, where the current suddenly stops. Without the force of the flow, the sedimentary particles are swiftly deposited, building up into sandbanks, mudflats and bars. Shells washed around in these conditions can be buried rapidly; while most are eventually broken down by erosion, some become embedded in the sediment. Most of these will be exhumed as the currents change course, or storms wash them away, but a small proportion will make it into final burial, from which their next release might be with a hammer.
Other environments are more reliable. The steady deposition of sediment on the continental shelf, or in lakes, for example, often ensures a constant supply of entombed victims. Unfortunately, the preservation can also be quite poor in many examples of this, because the rate of sedimentation can be rather low. Scavengers prowl over the surface, microorganisms bore into the remains and destroy them, burrowers do the same even after they’re buried, and in general there’s little left. However, all is not lost, for most environments have their little disturbances. Storms sweep sediment from one place to another, small slumps have the same effect, and floods on land can bring in much greater sediment loads than normal; any of these is quite capable of forming the potential for rather good fossils. In coral reefs, which are built largely of the skeletons of the creatures that make them, the entire structure is full of fossils, each burying the ones below.
Of course, this is just scratching the surface. Fossils can form on land as well as under water, but this is less common because the land is generally a place of erosion: even if something is buried in the short term, it is likely to be exhumed and weathered away a short time later, as the sediment that once entombed it is swept out to sea to bury someone else. But it can happen – cave-fills and fissures can be rich hunting grounds, deserts can be gradually built up by the erosion of nearby mountains, and whole regions can sink as a result of tectonic subsidence, when plates themselves are stretched. Alternatively, sap dripping from trees can entrap the unwary; after this, it is largely irrelevant where it may travel, for the potential amber is remarkably resilient. Even volcanic ash can smother an ecosystem, such as at Pompeii, where fossil casts of hundreds humans have been excavated.
But all these factors are just the beginning. Burial of an organism’s remains is a necessary first step, but it is not sufficient to produce a fossil. All sorts of things can affect the eventual outcome, which varies from no trace at all, to perfectly preserved, complete animals, with even the mitochondria preserved within their cells. Here’s where it gets complicated, and this is where we have to tackle things one bit at a time.
Mineral skeletons come in all sorts of varieties. Our bones (assuming that you’re human, or at least vertebrate) are made of a combination of calcium phosphate (a mineral called apatite – easy to remember, because teeth are made of the same stuff…) and organic matter. Purer calcium phosphate is found in a variety of other organisms, especially early in the record of animals. Palaeoscolecid worms, for example, secreted thousands of minute phosphatic plates. Conodonts, being the tiny teeth of early vertebrates, are also phosphatic. Phosphate also forms a major constituent of the ‘organo-phosphatic’ shells of inarticulate brachiopods.
But aside from vertebrates, phosphate is not that common as a biomineral. In contrast, most skeletons are made of one (or more) of the several forms of calcium carbonate. Chalk, as your school should have taught you, is made from the skeletons of uncountable billions of microscopic coccolithophores, a type of marine planktonic algae. In this case, the precise mineral is calcite. Calcite is or was also used for the shells of trilobites and ostracodes, and in the incredibly successful ‘articulate’ brachiopods, which ruled the Palaeozoic but are now a shadow of their past success, clinging onto rocks in a few scattered places. The Palaeozoic corals (rugosans and tabulates) were calcitic as well. Strictly, though, we should be saying low-magnesium calcite, because there is a slightly different mineral (high-Mg calcite) that is used in echinoderms, and in most fossil bryozoans, as well as one group of sponges. Molluscs, on the other hand (except for bivalves that do not burrow, such as oysters and mussels, and a few other exceptions) have a shell made from another form, aragonite. The modern corals (scleractinians), some bryozoans, and a variety of other reef-building creatures also use aragonite. We’ll get onto why this is important shortly.
It doesn’t stop there, though. Most sponges and a variety of single-celled protists secrete silica, as do gastropods in their scraping tool, or radula. This is the same composition as quartz, but disordered rather than crystalline; effectively it’s glass, but is also known as opal. Some sponges are even more bizarre, secreting granules of an unusual iron oxide called lepidocrosite. Some sea-cucumbers join in the fun with granules of iron phosphate, while everything from bacteria to pigeons seems to have made some use of haematite. There’s a snail near deep-ocean hydrothermal vents that secretes scales of iron sulphide (pyrite). Beyond these, you’re into the realm of the truly bizarre, and they’re not likely ever to turn up as fossils.
Minerals in general are decidedly not forever. Under conditions of changing temperature and pressure when buried in the crust, they can become unstable, and revert to something else. More severely, though, they can dissolve. The first challenge is simply to be buried in the first place, as we have seen – but it is more of a challenge than I have suggested, for in addition to erosion, abrasion, and scavengers, chemistry has to be overcome too. Minerals that are perfectly stable in the warm, shallow surface waters can be entirely unstable in the cold, pressurised deep oceans. The most extreme example of this is calcium carbonate (both calcite and aragonite, the latter more severely). For a coccolith that has sadly expired after its stint of photosynthesising in the balmy open ocean, the shell eventually sinks. As it progresses past a couple of kilometres (the exact depth varies between oceans, depending on the chemical composition – particularly the saturation state – of the water), it starts to dissolve. After another kilometre, if there is any left at all, then its fate is sealed. By the time it becomes buried in sediment, there will be nothing left to bury.
But opal has the opposite problem: a sponge dying on a tropical reef is likely to donate all its spicules to the sea water in a matter of a couple of weeks, but those that live in the deep sea can scatter their spicules for eternity, and they will not dissolve. Slowly, the sediment falls through the deeps – or a cataclysmic submarine mudslide covers everything in its path – or something in between – and eventually the skeleton is buried. Here is where the tribulations really begin. Pore spaces within the sediment are full of water, but as you go beneath the surface, it soon turns nasty. Decay of organic matter leads to all the oxygen being used up, and anoxic conditions change the chemistry completely. But the bacteria don’t mind – sulphate reducers and methanogens take over, pumping a variety of noxious compounds into the mud. This can lead to severely acidic conditions, or alkaline ones instead. Concentrations of dissolved ions can go haywire, leading to almost anything being severely undersaturated. An undersaturated solution will take ions from anywhere, and regrettably often it’s from a fossil.
As the sediment on top builds up, the chemistry becomes more constant. The flow of water through the sediment grinds to a halt, and the bacteria are more interested in getting their nutrients from clay minerals. Unfortunately, this is where pressure starts to have an effect. As an extreme case, coal is organic-rich sediment that has been compacted approximately ten to twenty times. Flattened tree trunks are a remarkable sight, and give a feel for what compaction can do. In most mudstones, the compaction factor is about five; in sandstones it is usually a lot less, and in limestones can be effectively zero. Eventually, if your rock is really unlucky, it can be metamorphosed. This involves truly excruciating temperature and pressure, so that the crystals of the rock itself are completely reformed. In high-grade metamorphic rocks, little trace of the original rock is left, the original clays and sand being transformed to garnet, kyanite or biotite. Pretty, but destructive. Even the hardiest fossils generally fail to survive that.
It’s amazing anything ever gets fossilized at all. In fact, if one calculates roughly how many shells have been produced in the Earth’s history, you find a ridiculously small proportion exist as fossils. This is a very good thing. If it were not the case, all the Earth’s store of carbon would have been buried underground long, long ago. There wouldn’t be enough left to make, for example, you. But a scattering of shells have survived (in some form) the dissolution, burial, and everything that geology can throw at them, and it is to these we now turn.
There are several possibilities for the form of a preserved shell. It can be the original, unchanged mineralogy, as is quite often the case with phosphate or low-magnesium calcite. More likely, though, it will have been recrystallised, forming a more stable form of a similar mineral; aragonite to calcite, opal to quartz. Such recrystallisation often yields traces of the original structure preserved as ghosts in the crystals. Many of the less robust minerals, particularly aragonite, will eventually be dissolved. If this happens before the rock is cemented, then the chances are the void will simply vanish under compression. If it occurs later, though, the result is a mould where the skeleton once was. This can then be infilled with anything from calcite to quartz to rhodocrosite, sometimes leading to spectacular fossils. Porous structures such as bone or wood are often permineralized – the holes are filled with some cement, and it is this that preserves the structure, whatever is left of the original.
Sometimes, the gaps remain empty, and all that is left is a mould. In many cases this is what we find when collecting fossils, but it can be an illusion. The last battle of a fossil is with weathering when it reaches the surface once again. Anything calcareous tends to dissolve in the mildly acidic rainwater, leaching out of a rock to leave only cavities behind. Behind the façade, even a few inches into the outcrop of bedrock, the fresh material can yield the replacement, the recystallization, or the original structure of the shells.
Of course, all are equally good fossils. In some cases, we prefer to have the moulds, because we can extract more useful information from the surfaces. This is particularly true of echinoderms such as crinoids, where the internal structure is largely useless. In contrast, bryozoans can only easily be studied through specifically oriented sections through the skeleton; with them, the internal structure is critical. With most fossils, you can get something useful out of any type of preservation, though – albeit perhaps not what you wanted.
The preservation of anything other than mineral skeletons is (with a few exceptions described below) an extremely rare event. Anything that can decay is likely to be lost. Fur, skin, internal organs – all are immensely vulnerable to the destruction by bacteria, fungi and scavengers that assure their recycling back into the ecosystem. But it is not impossible to preserve them, and such things have happened a surprising number of times. These windows into those parts of past life that are normally rather shy are rare and wonderful, and the stories behind their formation are remarkable in themselves.
Firstly, there are some purely organic structures that defy logic by being almost indestructible. We don’t understand how, but basically it takes a very persistent microbe indeed to do anything more than make a couple of holes in them. I’ll mention briefly the main groups. Firstly, spores and pollen. These are made from a substance called sporopollenin, which appears to be immortal. We extract fossils like this by dissolving rocks in concentrated hydrofluoric acid – perhaps the nastiest chemical in existence, from the point of view of a vertebrate. Purely organic matter, however, and particularly such things as spores, are immune. They barely notice the immersion, except perhaps to enjoy their newfound freedom. Chitinozoans are similar. These are believed to be the eggs of some animal, perhaps related to an obscure group of worms called the gastrotrichs. Chitinozoans occur in their billions through the Ordovician to Carboniferous, and have been successfully extracted from significantly metamorphic rocks. For whatever reason, those eggs were meant to last. Slightly less robust, but still often abundantly preserved, are the jaws of polychaete worms, that we call scolecodonts. Again, something in their composition made them rather resilient to decay processes.
The most peculiar and apparently inexplicable example, however, are the graptolites. These are a very important group of fossils with respect to dating rocks, since they are extremely abundant, very widespread, and evolved rapidly in a recognisable sequence. In general the colonial skeletons, made of a series of tubes, are a few centimetres long (although some reached a meter or so), and they are usually preserved as organic carbon – either a shiny film or a thick black deposit. Sometimes they are found preserved in three dimensions, and can be extracted by dissolving limestones in weak acids. In the best examples, the skeletons can be sectioned and viewed with an electron microscope to yield details of how they were put together. And what do we find? Collagen. Although the huge molecules themselves are no longer pristine, they have a characteristic shape that is recognisable. Collagen was a surprising result. Skin is mostly collagen, and needless to say, it doesn’t generally fossilize. There are however different types of collagen, with different levels of robustness. Based on the shape of the preserved molecules in the graptolite skeleton, there seems to be a distinct similarity to rats’ heart tissue. Rats’ hearts don’t tend to fossilize either. Why did graptolites? We simply don’t know, and it’s quite a niggle.
But enough of the ordinary, no matter how puzzling. What we’re really interested in now is how to preserve the extraordinary. This involves the really soft tissue – the muscles and tentacles and rubbery bits that you don’t look at too closely in your seafood. When you think about it, it really is quite extraordinary that such things can ever be fossilized, and yet it has happened repeatedly, in a myriad different ways. Basically, these processes fall into two groups: incomplete decay, and early mineralization.
For incomplete decay, we simply need all the bacteria to pass up a good chance at a meal, and leave a good bit of the carcass behind for as long as it takes to turn it into effectively pure carbon in a solid rock. However… bacteria live everywhere, from superheated water at the bottom of the ocean, to within and upon every animal, living or dead. Some like oxygen, others are poisoned by it. Every possible niche for creatures to be living and dying in has bacteria, on and in the sediment. It seems impossible that incomplete decay could ever be an issue, because there seems to be no way of stopping the decay. Even in amber, where creatures are completely encased in an impervious shield as they die, the body decays through the action of bacteria within the animal itself. All that is left is a perfect surface impression, an insect-shape hole.
So it is quite surprising that some of the most famous fossil deposits, the Cambrian Burgess Shale and the Jurassic Posidonienschiefer of Holzmaden, appear to have been preserved through at least a component of incomplete decay. The Burgess Shale is complicated; we still aren’t sure what most of the fossils are preserved as, let alone how they came to be that way. There is some evidence for clay minerals (aluminosilicates), but another component is definitely a film of organic carbon. But even this may not be a straightforward interpretation, since it occurs in the most unlikely places, coating the long opal spines of the sponge Pirania. These spicules should not have been surrounded by soft tissue, and there is a minimal carbon component in the spicules of living species. Were the spicules of Pirania constructed differently? If not, then we might have to consider whether the carbon film preservation of the Burgess Shale was actually inorganic – mineralization by graphite, perhaps?
So, if we leave the Burgess Shale to one side, all we are left with is Holzmaden, and the equivalent Oxford Clay. Here there are icthyosaurs and sharks with impressions of their skin remaining. The shells of ammonites are entirely dissolved, but their organic coating remains. It’s rather odd. Among the interpretations for Holzmaden’s preservation is a suggestion that the sea floor was almost anoxic, with a oxygenation boundary fluctuating near the sediment surface. Is it also possible that this could account for the inhibition of bacteria; that perhaps they could cope with each set of conditions, but not the switch between them? That might work until the fossils are buried. After that, the bacteria would be in an entirely anoxic zone, and should soon set to work. Again, it’s baffling. My hunch is that when these fossils are examined chemically, they will prove not be entirely carbon at all, but perhaps preserved as films of siderite, or other reduced iron chemicals. But for now, it is a mystery. Overall, the evidence for any processes resulting in unfinished decay of soft tissue seems rather shaky.
The only real way to do it is to transform the fossil to a more stable mineral, before the decay has taken place. This can be either the fossil itself, or a mould of the fossil in the sediment, but it must occur quickly. Anyone who has played with ‘crystal garden’ kits as children will remember the patience required to wait for your crystals to grow, and these were fast-growing minerals under conditions that could hardly fail to yield something pretty. The water was so full of ions that crystals would form with no provocation whatsoever. That is not a natural situation. For fossils, we have to rely initially on seawater, or something similarly refined. We also can’t assume it’s been pumped full of copper sulphate, and although halite (sea salt) and gypsum can form readily enough, they have never yet been found to preserve exceptional fossils.
The minerals that do help to preserve soft tissue are varied, but include calcite, phosphate, aluminosilicates, quartz, siderite (iron carbonate) and pyrite (iron sulphide). In each case, the primary requirement is a rich source of the dissolved components of the mineral (such as iron and sulphide ions). After this, we need a way of templating the fossil with the mineral. And that’s about it, although in each case we must avoid the various things that could stop it happening.
For example, preservation in pyrite is quite well understood. It occurs when organisms are rapidly buried in anoxic sediment, rich in sulphates and dissolved iron. The first stages of decay involve bacteria that specialize in reducing sulphate in order to oxidise their food. If you’ve ever waded through an estuary, and been assailed by black mud smelling of rotten eggs, you’re already familiar with them. As soon as the bacteria begin to attack the carcass, they release copious quantities of sulphide ions (the ‘rotten eggs’ is hydrogen sulphide). These immediately come into contact with the dissolved iron, and promptly precipitate out as FeS. Of course, this is concentrated on the surfaces where the bacteria are active, and quite soon there is a coating of pyrite all over the animal. The process seems so simple that it should happen frequently; in fact, though, there are only three confirmed examples of such deposits (one in the Builth Inlier!), and rumours of a fourth. Others may yet appear, but still, they appear to be rare. Why? Perhaps it is simply a result of the rarity of finding somewhere with sufficient dissolved iron (plus an apparent requirement that there be little organic matter within the sediment), but it is likely there are factors involved that we have not yet considered.
It is worth mentioning in passing another attribute of pyritised fossils – the stunning images that we can get from x-raying the rock. The fossils on the surface are appealing, but difficult to see details on; the pyrite is reflective, mostly embedded, and very fragile. An x-ray, however, reveals extraordinary detail that is otherwise completely obscure, and in many cases reveals fossils that we would not even know were there. Apparently barren slabs of black slate can yield remarkable wonders when examined with this technique.
In another major style of soft-tissue mineralization, the tissues are actually replaced by a mineral, rather than coated with one. The best example of this is phosphate. Firstly, there is a distinctive set of deposits in the Cambrian and Late Precambrian, particularly from China (Doushantuo) and the Baltic area (Ørsten), but with several others scattered over the globe. In these cases, the fossils are small, up to only a few millimetres in size. What they lack in stature, however, they more than make up in detail. Tiny arthropods have been known for many years, with all their appendages intact, even down to the hairs. Even more spectacular is the recent discovery that these faunas include larvae and embryos, in all stages of early cell division. Individual cells are clearly visible in many cases. The other main type of phosphatised soft tissue is characterised by Mesozoic and Cretaceous deposits such as the Santana Formation of Brazil. This place is famous for huge quantities of fish, fossilized in nodules. You can find them for sale in a distressing number of places. What you probably don’t realise, though, is that within the fossils, there is often preservation of muscle fibres, cells, and even subcellular components such as mitochondria, all faithfully replaced by calcium phosphate. In some cases, even large molecules such as collagen fibrils have been preserved. That really takes some beating. A final variation on this theme is a series of ‘shrimp beds’ in the Carboniferous of Scotland and Ireland, in which numerous centimetre-scale organisms have been phosphatised after being washed into a hypersaline lagoon.
And how does phosphatisation take place? We wish we knew. A rich source of phosphate is an obvious point, and that’s not easy to find. In the Santana, it probably came from the decaying remains of shoals of fish killed by overturning of poisonous waters in a restricted lagoon. In the Ørsten, the small size of the fossils suggests that the phosphate source was limited – there simply wasn’t enough to preserve larger organisms. But is that satisfying? Not really. Recent attempts to simulate the preservation with lobster eggs in a lab have yielded some success, but not enough. If it was that easy, why are these deposits so excruciatingly rare above the Cambrian?
Another way of preserving soft tissue is to do nothing to the fossil itself, but instead turn the surrounding sediment into rock, very quickly indeed. This may seem implausible – and indeed it is, on a large scale. But on a small scale, it can happen. Nodules are small growths of a mineral within a sediment deposit. They are usually flattened spheres, most frequently formed from calcite cement crystallizes between the sediment grains. The initial growth of crystals nucleates for some reason (sometimes around a calcitic shell, for example), and then grows outwards through well-understood chemical principles (at least, given a source of more calcite). Usually, nodules form quite late in the burial of a rock, long after soft tissues have decayed. Occasionally, though, something strange about the chemistry can change things a little. In rare cases, the nodules can form within days, or even hours. Conveniently for us, these cases are often the result of a sudden sediment influx, which induced the chemical changes that caused the nodules to form. As a by-product, they also include lots of creatures buried in that same sediment.
Sometimes, as in the Carboniferous-age iron carbonate nodules of the Mazon Creek (Illinois, USA) and Cosely (Staffordshire, England), the nodules form around decaying carcasses, which actually induce the nodule formation the first place. Under these circumstances, an entire ecosystem can be preserved, including jellyfish and obscure groups of worms such as echiurans and chaetognaths, which otherwise have almost no fossil record. The only drawback is that the preservation often lacks detail, with the fossils of soft organisms represented only by vague colour patterns and irregular relief. In other nodule faunas, such as the extraordinary, recently discovered Herefordshire Lagerstätte, calcite nodules formed in a slumped volcanic ash deposit, and just happened to envelop a wide range of small creatures entirely by accident. After burial, and after the astonishingly early nodule formation, the fossils decayed completely, to leave a void – not unlike the way that amber preserves things – that has since been infilled with more calcite. In the Herefordshire fauna, there is a similar level of detail, with fine hairs and spines preserved on animals only a few millimetres across.
The problem with the Herefordshire fossils is that they’re a nightmare to work with. When you split the nodules, all you get is a cross-section through the 3D fossil, which is effectively useless. You can’t dissolve the nodule, because the fossil and the nodule dissolve at about the same rate. You can’t even use x-ray or CT, because although there is a visual contrast between the two, there is no contrast in ‘electron density’ – a result of the calcareous composition. The group studying this fauna have instead come up with a laborious but very effective approach that involves serial grinding – effectively, grinding down the surface about 30 microns at a time, and taking a digital photograph at each stage. They then wrote some software that allowed them to edit the images, and combine them into a fully manipulable, 3D digital reconstruction, in which all the details are astoundingly clear. The only drawback is that the original fossil is now dust on the floor of their lab.
Silica (quartz) can also preserve fossils by encapsulating organisms before they decay, but this also requires unusual conditions. The most famous (and important) example is from the Devonian of Scotland, where the Rhynie and Windyfield Cherts have preserved the flora and fauna of a hot spring ecosystem. These fossils represent the best record we have of early land plants, preserved with wonderful cellular detail of their tissues, and have aided enormously in helping us to understand how the transition to land took place. More rarely, the microscope sections that reveal these structures (the rock is transparent when even a millimetre or so thick) contain the remains of early arthropods – water fleas, centipede-like creatures and the spider-like trigonotarbids. It is an unusual example of exceptional preservation on land, and depends on the chemical solubility of silica in hot water. Basically, hot water can hold more dissolved silica. Hot springs involve very hot water bubbling up through the rocks, and dissolving lots of silica on the way. Out of the ground it comes, sloshes around a bit, and cools down. As the water cools, it can hold less silica, which precipitates around the edges. If these edges happen to contain plants and animals, then they’re likely to become famous, four hundred million years or so down the line. Just look at Rhynia, after all.
Silicification is usually associated with plant communities – it is a very common form of fossilisation of wood, for example – because the plant tissues take a bit longer than animal tissues to break down, and because encouraging chemical bonds form between lignin and silicic acid. To preserve animals as well is indeed unusual, but it does happen, and not always at hot springs. The Llandegely Rocks biota (Builth Inlier again) is mostly sponges, but again is preserved by silicification. In this case there were probably no hot springs – just a shallow bay with a huge input of volcanic glass. Again, the water became oversaturated with silica (hence, perhaps, the huge numbers of sponges living there), and the result when things were buried was for the silica to go berserk. Some sponges are completely dissolved, with the edges held in place by siliceous cement before the sponge decayed. Others had silica nucleating on the spicules to form nodules of quartz in which the sponges were embedded. In a few cases, the middle parts of the sponges dissolved, and the outside was silicified – and here something very odd happened, in that the soft tissue of the sponge has been preserved through replacement by quartz. It’s a minor Lagerstätte, but an interesting one – and so far unique.
There are a variety of other really unique and peculiar types of preservation to be found in the world. One of the most bizarre is the Jurassic fauna at La Voulte-sur-Rhone, in France. Here there are a wide range of animals, including worms, crustaceans and an octopus (!), preserved in marine mudstones. To look at a cross-section of these creatures is to see an anatomical section. Rather than being satisfied with one mineral, La Voulte employed a succession of four – phosphate, calcite, pyrite and (of all things) sphalerite – each preserving different organs, tissues and so on. There’s nothing else like it anywhere. Another example is the extraordinary Eocene oil shale at Messel, where the fossils are actually revealed, under an electron microscope, to be composed of bacteria. The microbes are preserved by siderite (as in the Carboniferous nodules), and form an outline of the soft tissues of a wide range of vertebrates and invertebrates, from bats to beetles.
Alternatively, there’s the Upper Ordovician Soom Shale of South Africa. Here is preserved a mudstone laid down in a glacial lagoon. The fauna is equally odd. Fossils are rare – one has to dig for a week before finding something of any use – but once they appear, they’re spectacular. There are eurypterids, soft-bodied trilobiteoids, and a strange thing that is probably an anomalocarid (leftover monsters from the Burgess Shale). It is most famous, however, for conodonts. These are the minute teeth of tiny vertebrates, widely used in the dating of Palaeozoic rocks. They are very well studied, by a large number of people, and their record is thought to be quite complete. Conodonts occur in the Soom Shale as well, in place at the front end of the complete, eel-like animal with big eyes. The only odd thing (apart from this being one of only two known examples of conodont animal soft tissue) is that fact that they’re ten times the normal size. Other fossils have so far defied description, let alone interpretation. It’s a strange fauna, and an equally strange mode of preservation, for the fossils are made of clay minerals. All the original skeletal minerals have vanished, but the soft tissue has been replaced by clays such as kaolinite. This process appears to have been aided by poisonous bottom-water full of sulphuric acid.
For a more recent, and more easily understandable example, there are the frozen mammoths of the Siberian permafrost. We’re not quite sure precisely how this happened, but it was probably related to falling into freezing mud. Some of these beasts are apparently still edible once they thawed – after only twenty thousand years in the freezer. Personally, I think you’d have to be pretty desperate.
Perhaps the only generalisation we can make about the preservation of fossils, is that it’s difficult to make any generalisations at all. Every occurrence is slightly different, and when we come to ‘exceptional preservation,’ many are radically so. Given the rate of new discoveries in recent decades, it seems inevitable that many more wonders are yet to be uncovered, and anyone may stumble across them. So keep your eyes and minds open - after all, it’s not just the creatures themselves that are interesting.
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