Peter A. Allison

The role of anoxia in the decay and mineralization of proteinaceous macro-fossils.

Actualistic experiments have quantified rate of anaerobic decay and associated mineralization around proteinaceous macro-organisms. Carcasses of the polychaete worm Nereis and the eumalacostracans Nephrops and Palaemon were buried in airtight glass jars filled with sediment and water from marine, brackish, and lacustrine environments. Over a period of 25 weeks the contents were examined to determine the state of decay and were chemically analyzed to monitor early diagenetic mineralization (two methods for such analysis are reviewed). Decay processes were active in the experimental conditions despite anoxia and had virtually destroyed the carcasses within 25 weeks. However, decay-rate in the sulfate-reducing marine system was greater than in the methanogenic freshwater environments. Petrological and geochemical analyses of the organic remains identified discrete layers of authigenic iron monosulfide (a pyrite precursor) on the surface of the decaying Nephrops cuticle within weeks of initiating the experiment. Chemical analysis of decomposing flesh showed a marked increase in pore-water calcium content with time. The results clearly show that anoxia is ineffective as a long-term conservation medium in the preservation of soft-bodied fossils. However, decay-induced mineralization can be very rapid so that even a slight reduction in decay rate can lead to improved levels of fossil preservation. Traditionally, stagnation and rapid burial are considered to be the main prerequisites for the preservation of soft-bodied fossils and the formation of Konservat-Lagerstatten . Clearly these factors are only important in that they promote early diagenetic mineralization. This is the only way to halt information loss through decay.

Konservat-Lagerstaetten; cause and classification

A review of the processes required for exceptional preservation of soft-bodied fossils demonstrates that anoxia does not significantly inhibit decay and emphasizes the importance of early diagenetic mineralization. Early diagenesis is the principal factor amongst the complex processes leading to soft-part preservation. The development of a particular preservational mineral is controlled by rate of burial, amount of organic detritus, and salinity. A new causative classification of soft-bodied fossil biotas is presented based upon fossil mineralogy and mineral paragenesis.

Exceptional fossil record: Distribution of soft-tissue preservation through the Phanerozoic

Preservation of soft-bodied fossil biotas (Konservat-Lagerstaten) that preserve traces of volatile nonmineralized tissues (readily degraded by bacteria) are not evenly spaced through geologic time. When compared to outcrop area, exceptional faunas appear to be over-represented in the Cambrian and Jurassic. These concentrations in time correspond to particular environments, indicating that controls on the distribution of exceptional faunas may have operated on a global scale. The reduction in the number of exceptional faunas after the Cambrian may reflect the evolution and diversification of deep bio- turbators. Specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks.

Marine Palaeoenvironmental Analysis from Fossils

The approach of the book is analytical rather than taxonomic, concentrating on a range of techniques. The common thread, however, is that it is palaeontological material that is being considered, whether it be identifiable body fossils, trace fossils, distinctive fossil associates, diagenetically unaltered material or organic compounds. Using a number of methods, and comparing their results, allows di f ferent environmental controls to be isolated and provides more information on the record of past environmental parameters.

Deep-water taphonomy of vertebrate carcasses: a whale skeleton in the bathyal Santa Catalina Basin

Taphonomic processes in deep-water environments differ markedly from those in shallow waters. These differences are illustrated by the preservational style of a large cetacean skeleton lying at the bottom of the Santa Catalina Basin in 1,240 m of water. The degree of skeletal articulation contrasts with that documented in the shallow North Sea where gas-filled, buoyant carcasses dis- articulated during flotation. Increased hydrostatic pressure at greater depth is presumed to have prevented the whale carcass from floating and promoted increased levels of preservation. We present a model that relates gas evolution during decay to carcass buoyancy with depth. Application of this model may ultimately allow the degree of skeletal articulation to be used as a rough index of paleobathymetry.

Paleolatitudinal sampling bias, Phanerozoic species diversity, and the end-Permian extinction

Species diversity varies with latitude. Almost all marine sedimentary rock in the Paleozoic of Europe and North America was deposited in the tropics, whereas only 24% was deposited in these latitudes during Mesozoic and Tertiary time. A new plot of species diversity through the Phanerozoic takes into account the paleolatitudinal distribution of marine sedimentary rock. This plot indicates that the Permian witnessed the acme of species diversity prior to the Cretaceous, and that the Triassic recovery following the end-Permian extinction was much less pronounced than suggested by previous studies.

In situ benthos and paleo-oxygenation in the Middle Cambrian Burgess Shale, British Columbia, Canada

The Middle Cambrian Burgess Shale was deposited under variable levels of oxygenation. During periods of oxygenation, low-diversity shell beds were formed and the muds were colonized by infauna. Under these circumstances pyrite was restricted to anoxic microenvironments and formed small (1 mm) discrete aggregated masses. Soft-bodied organisms were preserved under low-oxygen conditions typified by evenly dispersed pyrite, and an absence of trace fossils and shell beds.

Palaeo-oxygenation: effects and recognition

Abstract Oxygen is a bio-limiting element for metazoa and one of the most important factors influencing species diversity and abundance in the marine realm. Equally, the absence of oxygen is generally considered to be essential for the inhibition of microbial decay and the formation of organic-rich sediments. As such, the determination of depositional palaeo-oxygenation values of ancient rocks has become a fundamental quest for the palaeoecologist and mudrock sedimentologist. In this paper the development of the tripartite anaerobic-dysaerobic-aerobic terminology for oxygen-related biofacies and the recent additions of the exaerobic and poikiloaerobic biofacies are reviewed. The new, non-genetic, oxygen-restricted biofacies (ORB) scheme is also presented. Chemosymbiotic life strategies have been suggested to be important in many extinct black shale taxa but a glance at modern chemosymbionts casts doubt on the significance of this mode of life in ORB. A review of lowest dysaerobic benthic forms throughout the Phanerozoic reveals the repeated occurrence of a few morphologies almost exclusively derived from the brachiopods and bivalves. Geochemical indices of palaeo-oxygen levels are also reviewed. The recent renaissance in the development of criteria for the determination of palaeo-oxygen values has led to the recognition of a considerably greater variety of dysaerobic biofacies in the geological record. This is illustrated with two case studies, the first, from the celebrated Burgess Shale of British Columbia, shows a depositional environment dominated by a fluctuating oxycline. The second case study, from widely separated earliest Triassic marine sections, illustrates the possibility of a widespread (global) dysoxic event at this time.

Variation in rates of decay and disarticulation of echinodermata : implications for the application of actualistic data

Actualistic laboratory experiments have documented the preservation potential of selected echinoderms and have calibrated rate of decay with weight-loss of chitin in litter bags. The results are integrated with field experiments which quantified decay rate in flowing seawater, intertidal sediments and freshwater mud using identical litter bags. Decay rate of chitin was generally slower in freshwater sediments than in marine muddy environments and open seawater. Sediment-induced variation in decay rate is within an order of magnitude. The duration of decay required to promote fragmentation varied little between the two regular echinoids but differed by an order of magnitude between the holothurians

A review of marine palaeoenvironmental analysis from fossils

The papers in this volume critically review the use of fossils, including their inorganic skeletal tissue or their soluble organic remains, for the analysis of palaeoenvironments. The contributions are not limited to traditional palaeontological techniques but are multi-disciplinary, drawing on a host of geochemical, palaeoecological and palaeontological methods. This holistic approach is essential if the potential pitfalls of a strictly uniformitarian approach are to be avoided. If a range of methods are used, and the results compared, then different environmental controls can be isolated. This methodology is of importance to sedimentologists, stratigraphers and palaeontologists who need to maximize their palaeoenvironmental interpretations from palaeontological data. The implications of this work are fundamental to correct interpretations of depositional environments, facies models, sequence stratigraphy and palaeoclimates. The approach taken in the volume is analytical rather than taxonomic. As such, the techniques used to analyse the effects of different environmental parameters are focused on, rather than what can be learnt from the study of particular fossil groups. This approach is therefore different to that found in many texts (e.g. Dodd & Stanton 1981; Clarkson 1986), where the emphasis is on the palaeoecological value of different taxonomic groups and is more similar to the short reviews of ‘Fossils as environmental indicators’ in Briggs & Crowther (1990). This analytical approach leads to a more thorough analysis of palaeoenvironments. By using a range of techniques, from the traditional taxonomic uniformitarianism to the more recently developed geochemical and isotopic analyses of mineralized skeletons and soluble organic tissue from plants