Encyclopedia of Geology. Editor: Richard C Selley, Robin M Cocks, Ian R Plimer. Volume 3, Elsevier, 2005.
This article will describe in detail Fossil Lagerstätten, with a special emphasis on conservation Lagerstätten—deposits containing exceptionally preserved fossils. Concentration Lagerstätten are deposits where fossils are especially abundant and the processes that operate to produce these types of fossil horizons will be described. Fully articulated skeletons and the preservation of the soft parts of organisms, which are usually rapidly lost to decay after death, both constitute examples of exceptional preservation. In order to create these extraordinary fossil deposits, scavengers must be prohibited from disarticulating and consuming the carcass and bacterial putrefaction of soft tissues must be circumvented. Authigenic mineralization of soft tissues produces fossils with the highest degree of fidelity known, where even subcellular features can be recognised. Mineralization processes, including replacement by apatite, pyrite, clays, and silica are documented using examples from conservation Lagerstätten. Even exceptionally preserved fossils show some degree of decay-induced morphological change, so that decay experiments have proved crucial in their accurate interpretation. In addition, decay experiments have determined some of the physical and chemical factors, such as degree of oxygenation, pH, temperature, and soft tissue composition which all affect preservation. Exceptionally preserved faunas not only produce some of the world’s most spectacular fossils, but they are also critical in the reconstruction and understanding of ancient life and evolutionary processes. Such deposits may be temporally constrained because some geological periods are replete in examples compared with others; the putative explanations for such temporal trends are explored.
Lagerstätten, a German word, was originally used to describe mineral and ore deposits of economic worth. Adolf Seilacher first coined the term Fossil Lagerstätten (singular Lagerstatte) to describe a body of rock that is unusually rich in palaeontological information because, either the fossils are so well preserved and/or the fossils are so abundant that they warrant exploitation and scientific attention. The distinction between exceptionally preserved fossils and exceptionally numerous fossils has led to the terms conservation Lagerstätten and concentration Lagerstätten being applied, respectively. It is the conservation Lagerstätten that have received the most attention because they contain fossils that are extraordinary in the quality of their preservation. They may contain completely articulated skeletons or soft-bodied animals composed of nonmi-neralized, so-called soft-parts and soft-tissues. Fossil conservation Lagerstätten include some of the world’s most celebrated deposits, such as the Cambrian Burgess Shale of Canada and the Jurassic Solnhofen Lithographic Limestone of Germany. Exceptionally preserved faunas are confined to particular environmental settings, where a number of processes must occur in order to fossilise the nonmineralised tissues. In recent years, some of these processes have become well documented and will be described below; however, there are still many unanswered questions relating to exceptional preservation.
It should be appreciated that there is no absolute Criterion to discriminate a Lagerstatte from a fossil-iferous horizon. However, this is consistent with the notion that Lagerstätten are not distinct rock units, but are end-members of a range of sedimentary facies in which fossils are preserved.
In these deposits it is the sheer abundance of fossils, although not often very well preserved, that is important and it is vitally important to reveal how such profusion occurred. Concentration may be a real reflection of community ecology or behaviour, or concentration may be a sedimentological artefact produced by time-averaging and physical processes, such as winnowing, which act to distillate skelet al.remains. Two types of concentration deposits are considered in more detail below.
Stratiform Concentration Deposits
Where shelly fossils are locally abundant and form dense concentrations they may be referred to as ‘shell beds’ or ‘coquinas’. They are formed in a variety of ways, reflecting a range of processes and time-scales. For example, they may form from a sudden, rapid influx of shells from a mass mortality, or from the slow accumulation of shells over many years during times of low deposition rates. An example to illustrate concentration by multiple, physical processes is the famous ammonite coquina of the Middle Jurassic (Normandy Coast, France) where beds contain different ammonites representing about 2 million years. The ammonites clearly do not reflect a single mass mortality event, rather sedimentation to preserve the shells (so that they were not corroded) and probably several mixing events during storms that concentrated and yet did not destroy the shells.
Bone bed coquinas comprise an unusually rich concentration of phosphatic vertebrate remains, including bones, teeth, scales, and coprolites. Remains are not articulated and commonly include terrestrial and marine components. There are several models proposed to explain bone bed genesis. Winnowing involves episodes of erosion of the seafloor to rework and concentrate phosphatic material, which was previously more widely dispersed. Condensation involves extremely low sedimentation rates so that the input of phosphatic material is not diluted by sediment, but builds up on the seafloor over long time periods. Another model suggests that other biogenic material, composed of carbonate, may be dissolved, resulting in the apparent concentration of phosphatic material. Finally, in the transgressive lag model, a transgressive sea picks up phosphatic clasts from previously formed sediments and concentrates them into a basal bone bed. Examples of bone beds include the Westbury Formation of the Rhaetian Penarth Group, Wales and the Silurian of Ludlow, England.
In terrestrial and marine environments, fossils may be concentrated in protected environments such as cavities. Terrestrial cave and fissures constitute the most spectacular of these deposits where animals are concentrated either because they used the cavity as a dwelling, or because the cavity formed a death trap, successively killing and preserving animals over time. The Early Holocene Shield Trap Cave (Montana, USA) records many disarticulated bison bones and was formed in a bell-shaped limestone cave after roof collapse.
The Importance of Exceptional Preservation
Entirely soft-bodied organisms or the soft parts of organisms usually rapidly disappear after death, due to decay processes and, consequently, most fossils comprise the remaining hard, mineralized parts of animals. This has two consequences: firstly, entirely soft-bodied animals are usually not preserved (except in conservation Lagerstätten); and secondly, if the animal has hard parts, these become disarticulated as the soft tissues no longer support them. Disarticulation may pose significant problems when trying to understand ancient life. Plants in particular undergo disarticulation when transported and because their component parts behave differently hydrodynamically their leaves, pollen, seed, fruit, bark, stems, roots, etc. often become widely separated, making it difficult to reconstruct the whole plant. Animals also suffer disarticulation with some, such as echinoderms, being more susceptible than others.
Conservation Lagerstätten (exceptionally preserved faunas) provide palaeontologists with a unique and unrivalled window through which to view the biology, ecology, and evolution of life. Up to two-thirds of modern shallow water marine communities are composed of entirely soft-bodied animals (see Palaeoecology). So conservation Lagerstätten are vital because they preserve a more complete picture of community diversity. However, it is important to determine how much of the original biota is represented in a fossil assemblage; in other words, is a particular taxon absent because it was not part of the community, or was it just not preserved? Understanding the taphonomy (processes of fossilization) of different conservation Lagerstatte, individual fossil specimens, and a range of different tissues types, is crucial in gauging preservational bias.
The preservation of soft tissues is also extremely helpful when trying to interpret animal affinities. Many animals have hard parts that are uninformative as to the underlying soft part anatomy. For example, conodonts (microscopic, phosphatic, tooth-like elements) were known for over 100 years before their affinities were resolved. This is because, although conodont elements are made of apatite and therefore have a good fossil record, the rest of the animal, to which these conodonts belonged, had never been found. Fortunately, in 1982, a set of conodont elements was reported with associated soft tissues from the Carboniferous Granton Shrimp Bed near Edinburgh. The analyses of these fossils led to the interpretation that conodonts were in fact vertebrates with an eel-like form, a notochord, V-shaped muscle blocks, a caudal fin, and that the conodont elements were located in the animal’s oral cavity and functioned as a feeding apparatus.
Finally, the preservation of soft-bodied organisms provides palaeontologists with the evidence to explore biological phenomenon such as the Cambrian explosion, the nature of early metazoans, extinctions, and radiations.
Death, Decay, and Destruction
After death, several processes usually occur which would normally inhibit the potential of an animal or plant from becoming a fossil, and this is particularly the case for nonmineralised tissues. Necrolysis is the organic modification to carcasses, and occurs through autolysis, which is the breakdown of cells by their own enzymes, scavenging which is the consumption of carcasses by macroscavengers, decomposition which is degradation by fungi, and putrefaction which is the destruction by bacteria. Biostratinomy describes the physical alterations that affect a carcass prior to burial, and after burial, diagenesis includes the chemical and mechanical changes that take place to fossils. Taphonomy is the study of all of these processes of fossilization, from death of the animal to its discovery as a fossil. In order for exceptional preservation to occur, the taphonomy must be exceptional to bypass decay and ultimately destruction—the normal fate of nonmineralized tissues.
Decay experiments have proved to be an extremely valuable tool to document and understand decay-induced morphological changes and the processes of fossilisation. Decay affects morphology and it is useful to compare the morphological features of a fossil with similar morphological features that have undergone decay, rather than the same features from an extant, living animal. Decay experiments have been undertaken on a range of taxa including vertebrates, from entirely soft-bodied hagfish through to deer carcasses, invertebrates such as cnidarians, various worms, various echinoderms, cephalopods, crustaceans, insects, cephalochordates, and prokaryotes. In addition to assessing decay-induced morphological changes, these experiments have investigated the role of different factors (pH, oxygen concentration, temperature, salinity, and even microbe type) in the process of decay and fossilization. Decay experiments have provided important insights in to why some organic tissues survive decay and how other tissues become replaced very rapidly by authigenic minerals. In the future, understanding decay and fossilization processes may be used to predict the environmental conditions, and thus the type and location of sediments that may favour exceptional preservation.
There are two broad groups of conservation deposits: conservation traps and stratiform conservation deposits.
Conservation traps are deposits that are generally very geographically localised and are usually temporally restricted. They occur where animals and plants are trapped in a setting where macroscavengers are unable to access the remains, and the destruction by bacteria and enzymes is inhibited. Often the processes responsible for preservation in conservation traps are akin to those we employ to preserve our food, such as freezing, drying, and pickling. Examples include deep frozen mammoths from the Pleistocene found in clefts in the glacial permafrost in Siberia, mummified, desiccated ground sloths in caves in South America, and insects trapped in amber resin. Perhaps the most famous conservation trap is the Pleistocene Rancho La Brea tar pits of California, USA, which have preserved three million fossils including mammals, birds, fish, reptiles, invertebrates, and plants. In this respect, Rancho La Brea tar pits are a concentration, conservation trap. Conservation traps are temporally restricted because if conditions change, for example, if a carcass is re-hydrated or thawed, bacteria will resume their relentless destruction of the organic remains.
Stratiform Conservation Deposits (Table 1)
Stratiform conservation Lagerstätten are deposits where incomplete necrolysis occurs throughout the sediment layer(s) rather than being constrained to special, localized settings, as is the case for conservation traps.
|Table 1, a small selection of conservation Lagerstätten with age, location, and brief comments on the environmental conditions and preservation of soft parts (sp.)|
|Doushantuo Formation||Latest Proterozoic||South China||One deposit in carbonaceous shales, another in phosphorites phosphatisation of sp.|
|Chengjiang||Early Cambrian||Yunnan Province, China||Burgess Shale-type fauna, microturbidites various mineral replacement of sp. and possible organic remains|
|Burgess Shale||Middle Cambrian||British Columbia, Canada||Burgess Shale-type fauna, obrution and anoxia, kerogenised organic films and clay mineral replacement of sp.|
|Orsten||Late Cambrian||Southern Sweden, Germany and Poland||Anthraconite limestone nodules in bituminous shale, apatite (phosphatic) coating and replacement of sp.|
|Soom Shale||Late Ordovician||Cape Province, South Africa||Euxinic and anoxic bottom and porewaters, clay mineral replacement of sp.|
|Herefordshire||Silurian||Herefordshire, UK||Calcite nodules in volcanic ash deposit after decay three-dimensional animal void infilled by calcite|
|Hunsruck Slate||Early Devonian||Western Germany||Obrution by microturbidites pyrite replacement of sp.|
|Rhynie Chert||Early Devonian||Eastern Scotland||Plants and arthropods engulfed in Si-rich hotspring water silicification of sp.|
|Mazon Creek||Pennsylvanian Carboniferous||Illinois, USA||Rapid burial in oxygen depleted sediment and siderite nodules sp. represented as highly compressed, light-on-dark impressions|
|Monte San Giorgio||Middle Triassic||Southern Switzerland||Bottom waters anoxic, but surface waters normal marine rare phosphatised sp., mostly articulated skeletons|
|Posidonia Shale||Lower Jurassic||Holzmaden region, Germany||Anoxic bottom waters, soupy sediment and occasional sediment blanketing, sp. of belemnites, cephalopods|
|Solnhofen Limestone||Upper Jurassic||Southern Germany||Hypersaline, oxygen-depleted bottom waters, soupy sediment obrution and microbial mats. Soft parts mostly impressions rare|
|Santana Formation||Lower Cretaceous||NE Brazil||phosphatisation of sp. and organic residues Hypersaline waters, soupy sediment, and mass mortalities many carbonate concretions and sp. phosphatised|
|Grube Messel||Mid Eocene||West Germany||Quiet, anoxic lacustrine waters silhouettes of sp. made of autolithified, sideritic bacterial films|
|Baltic Amber||Late Eocene-Early Oligocene||Baltic, NW Europe||Amber is fossilized resin from trees, which traps arthropods; resin stops bacterial and fungal decay and acts as desiccant and antibiotic|
Scavenging by macro-organisms is a ubiquitous event in most environments and leads to disarticulation, and sometimes wide dispersal of the animal hard parts, and the removal of soft parts. Terrestrial and aquatic environments abound with macroscavengers and they may devour soft tissues in surprisingly little time. For example, a fish carcass introduced to a cold (4°C), dysaerobic seabed off the coast of California was stripped of soft parts and disarticulated in 2-3 days by brittle stars. Clearly, one of the most important prerequisites for exceptional preservation is the precludement of macroscavengers from carcasses and plants. This may be accomplished in a variety of ways, some of which are illustrated below, using examples from fossil conservation Lagerstätten.
Obrution. Catastrophic, rapid burial, by turbidites or tempestites, may help to preserve nonmineralised tissues because it protects the animal (which may be buried dead or alive) from macroscavengers and bioturbators, and it tends to induce anoxia (see below). However, whilst there are fewer bacteria with depth, and anaerobic bacteria are less efficient at breaking down organic carbon, a rapidly buried carcass still requires other processes to occur to inhibit or circumvent the decay of organic remains. Sediment smothering often produces an ecological bias because it will affect bottom-living organisms more than nektonic ones. In the Phyllopod Bed of the Middle Cambrian Burgess Shale (British Columbia, Canada) there is evidence to suggest that much of the biota was catastrophically buried, such as graded beds and variable orientations of specimens relative to bedding. Furthermore, Phyllopod Bed sediments are parallel laminated (i.e., undisturbed) suggesting that, owing to anoxia, conditions in the sediment porewaters were inimical to bioturbators and scavengers.
Soupy substrates. Burial owing to high rates of sedimentation is not the only way of rapidly covering a carcass and thus prohibiting many macroscavengers. If the sediment is soft, or even soupy, carcasses can sink into the sediment and thus be buried. Articulated vertebrate skeletons and the preservation of the soft parts of ichthyosaurs of the Jurassic Oxford Clay (UK) are suggested to have occurred because carcasses sunk into a soupy seafloor. This example is fascinating because areas of vertebrate carcasses that protruded above the soupy sediment were often encrusted and bored, suggesting that vertebrate skeletons provided ‘benthic islands’ of hard substrate, amid the otherwise inhospitable, soupy seafloor.
Stagnation, anoxia. Anoxia is the absence of dissolved oxygen and results if the rate of carbon deposition exceeds that of oxygen supply. Euxinic conditions describe waters that are anoxic and sulphidic. Benthic organisms are prohibited from living in such environments as are bioturbators, resulting in sediments with parallel (i.e., undisturbed) laminations. Carcasses falling into such bottom water conditions have an enhanced potential for exceptional preservation for several reasons. Firstly, macroscavengers would not be able to devour and disarticulate the carcass. Furthermore, anaerobic bacteria putrefy soft tissues more slowly and so increase the time for rapid mineralization (see below) to replace the tissues. In addition, anaerobic bacteria may not be able to utilise very recalcitrant organic matter, which then has the potential to survive into the fossil record as an organic residue. A classic example of a deposit where anoxia and stagnation led to exceptional preservation is the Jurassic Posidonia Shales (Holzmaden, Germany). Here black, bituminous shales containing extremely rare benthonic organisms were overlain by an upper water body rich in oxygen and life. The fauna includes plants and many invertebrates, but the shale is most famous for its complete vertebrate skeletons such as sharks, ichthyosaurs, crocodiles, and pterosaurs. However, anoxia cannot account for the occurrence of the articulated crinoids, because if they lay unprotected on the seafloor, albeit an anoxic one, they would disarticulate in a matter of days. It has been suggested, therefore, that rapid sediment smothering also occurred in the Posidonia Shales.
It should be appreciated that exceptional preservation in several conservation Lagerstätten can be attributable to more than one environmental circumstance. The Jurassic Solnhofen Limestone (Bavaria, Germany) was deposited in a basin where the stratified bottom waters were inhospitable to life owing to high salinity and low oxygen content. Periodic storm events washed animals into the basin, which were then rapidly buried by carbonate ooze. So rapid burial, hypersalinity, and anoxic stratified waters may all have contributed to exceptional preservation. Moreover, on the basis of the anatomy of the commonly preserved Saccocoma, a crinoid, it has been suggested that periodically the bottom waters were not saline, but that the sediment was soupy, allowing carcasses to sink into the limey muds.
Even where organisms are protected from macroscavengers, their soft parts are still vunerable, owing to the action of autolytic enzymes and bacteria. In the presence of oxygen, bacteria decompose soft tissues rapidly. Once oxygen has been used up, decay proceeds anaerobically. In freshwater environments, anaerobic decay is dominated by nitrate reduction and methanogenesis, whereas in the marine environment sulphate reduction and methanogenesis dominate. These reactions all lead to the decomposition of soft tissues, although there is evidence to suggest that anaerobic decay proceeds more slowly and is less efficient than aerobic decay. Decay may be inhibited by microbial mats because these mats effectively limit the diffusion of toxic bacterial metabolic by-products away from the site of decomposition, and the influx of oxygen and sulphate (marine environments) needed to fuel the bacteria. Mats may also aid soft part preservation by trapping organisms on the seafloor and creating a ‘closed’ microenvironment that may well favour rapid mineralization processes (see below). Under certain conditions, bacterial decomposition may also be retarded in fine-grained mudrocks by the prevalence of clay minerals. It has been suggested that the digestive enzymes used by bacteria in decomposition may become adsorbed onto specific clay minerals and thus rendered inert. Such a model has been proposed to explain the preservation of organic films in the Burgess Shale. However, some of the highest fidelity of soft-part preservation comes about when the soft tissues are mineralized before they are decomposed.
Soft Tissue Mineralization
Nonmineralized tissues display a spectrum of resistance to decay depending on their physical and chemical makeup. Decay-resistant (sometimes called recalcitrant) tissues, such as chitin of arthropod cuticles and woody plant tissues, may retain their organic composition as fossils. Decay-susceptible (sometimes called labile) tissues cannot survive unless authigenic minerals rapidly replace them, a process sometimes referred to as permineralisation. Mineralization can be extremely rapid, occurring within days, weeks, and months of death. Authigenic mineralization is controlled by a number of factors, including the organic makeup of the soft tissues, the geochemistry of the sediment, the pH and Eh of the porewaters and the concentrations of mineral forming ions. In addition, bacteria may mediate mineralization by breaking bonds, speeding up reactions, and concentrating authigenic mineral ions on to the tissue surfaces—the exact nature of their participation is not yet understood. In some deposits it is the bacteria themselves that autolithify, and they can pseudomorph details of the underlying animal morphology. For example, in the Eocene oil shale of Grube Messel (Germany) exquisite details, such as single hairs or the barbules of feathers, have been preserved as layers of lithified bacteria.
Only a few minerals are known to replace soft tissues, the most important of which are apatite, pyrite, clay minerals, and silica.
Apatite is the mineral most often involved in replacing soft tissues and it does so with the highest degree of fidelity. Details, down to the subcellular level, such as Cretaceous fish cell nuclei sitting on muscle fibres, have been recorded and such fine detail is possible because the apatite crystallites can be very small (commonly <30nm). Apatite mineralization requires a sufficient concentration of phosphate, which may be released from the decaying animal, or concentrated in the surrounding sediment. Decay experiments have also shown that slightly acidic pH favours apatite authigenesis over carbonate authigenesis, which does not have the same potential to replace soft tissues. Experimental phosphatisation (the replacement of soft tissues by apatite) and the analyses of experimentally produced and fossil apatite textures has greatly improved our understanding of apatite authigenesis. Some of the more spectacular examples of soft tissues replaced by apatite include: delicately preserved tiny arthropods in nodules from the Upper Cambrian ‘Orsten’ (northern and central Europe) and fish muscles, gill filaments, and eggs from the Cretaceous Santana Formation (Brazil).
Pyrite is commonly formed in the sediment of anoxic black shales but it is relatively rarely involved in mineralizing animal soft tissues. Although it is quite commonly associated with the preservation of plant remains where it may preserve cellular details (e.g., Eocene London Clay of England). Pyrite crystals are generally larger than apatite and clay minerals, and this constrains the fidelity of pyritised soft tissues. Pyrite commonly fills voids or coats the gross morphology or outline of tissues and so delicate, rapidly decayed tissues, such as muscle are not known to be pyritised. Replacing animal tissues by pyrite requires that pyrite must grow on the carcass rather than throughout the sediment. In order to do this the sediment must have high concentrations of iron in porewater solutions and low total organic matter. This allows sulphide generated from seawater sulphate by the decay of soft tissues to be trapped instantly by formation of iron sulphide minerals within, or very close to the carcass. An example of soft part pyritisation in seen in the Early Devonian Hunsriick Slate (Germany) where several taxa (including echinoderms, molluscs, arthropods, vertebrates, and cnidarians) are beautifully preserved with some relief owing to early pyritisation. In addition, the soft parts of several taxa from the Early Cambrian Chengjiang biota of China were preserved by pyrite, which later oxidised to create the characteristic pink and orange fossils.
Clay minerals are capable of replacing soft tissues with fidelity slightly less than that of apatite but considerably better than that of pyrite. Clays have an affinity for organic matter when certain geochemical conditions are met. It has been demonstrated that a particularly low pH may be important in this respect, and may explain why, although clay minerals are ubiquitous in fine-grained sediments, clay mineral replacement of soft tissues is not. It is possible that detrital, colloidal clays which are extremely small (1 nm to 1 μm across) may become attracted towards, and template on to decaying soft tissues. Or clay authigenesis may be responsible for soft tissue preservation. In either case, it has been suggested that bacteria play an important role in somehow controlling and mediating the process. Several taxa from the Upper Ordovician Soom Shale of South Africa show soft tissue preservation by clay minerals, including the muscle fibres and fibrils of a conodont animal. Whilst it has been shown that the Middle Cambrian Burgess Shale fossils of Canada are composed of kerogen (or another graphite-like structure), clay minerals are also involved in their preservation and are seen to replace and coat organic surfaces.
Silica and calcium carbonate are also involved in mineralising soft tissues. The Earth’s oldest Lagerstatte, the Apex Cherts (Warrawoona Group, Western Australia) contain silicified microfossils 3450-3470 Ma in age. The Early Devonian Rhynie Chert of Scotland preserves terrestrial arthropods and plants in superb detail. Here chert beds contain beautiful, three-dimensional plant material with preserved cell structures which became silicified when plants were engulfed in Si-rich hotspring waters.
The Role of Nodules
In a number of conservation Lagerstätten, early diagnetic concretions (nodules) formed around the fossil and protected it from later compaction. The Silurian Herefordshire fauna (UK) has produced spectacular three-dimensionally preserved fossils, owing to early stiffening of decaying animals by volcanic ash, followed by calcite infilling of the void left by the decayed tissues, and precipitation of calcite nodules around the fossil. A novel technique was developed to extract the maximum amount of information from these three-dimensional fossils. The nodules containing the fossils are serially ground at 30 μm intervals, and digitally photographed so that a set of ‘slice images’ of the whole fossil is created. These images are then used to create impressive 3D computerised reconstructions of the fossils. Other deposits where 3D preservation occurs owing to concretions include a diverse biota from the Carboniferous Mazon Creek (Illinois, USA) in siderite concretions, and vertebrates and invertebrates from the Lower Cretaceous Santana Formation (NE Brazil) in carbonate concretions.
Temporal Trends in Exceptional Preservation?
It has been suggested that the conditions required to create fossil Lagerstätten have not been uniformly present through the time of metazoans on Earth. In fact, taphonomic windows have appeared, leading to unique exceptionally preserved faunas, and then been ‘closed’. The preservation potential of Cambrian, Ordovician, and Silurian organisms is not greatly different, and yet the Cambrian is relatively replete in fossil Lagerstätten compared with the other two periods. This may reflect a temporal difference in the processes of preservation. For example, it has been suggested that during the Cambrian, deep bioturbators had yet to invade deeper water settings, but after the Cambrian they radiated from onshore to offshore, decreasing the likelihood of organic preservation after the Cambrian. There are several other examples where exceptional preservation appears to be favoured during particular time periods.