Susan M. Kidwell

Depletion, degradation, and recovery potential of estuaries and coastal seas.

Estuarine and coastal transformation is as old as civilization yet has dramatically accelerated over the past 150 to 300 years. Reconstructed time lines, causes, and consequences of change in 12 once diverse and productive estuaries and coastal seas worldwide show similar patterns: Human impacts have depleted >90% of formerly important species, destroyed >65% of seagrass and wetland habitat, degraded water quality, and accelerated species invasions. Twentieth-century conservation efforts achieved partial recovery of upper trophic levels but have so far failed to restore former ecosystem structure and function. Our results provide detailed historical baselines and quantitative targets for ecosystem-based management and marine conservation.

Taphonomy and paleobiology

Abstract Taphonomy plays diverse roles in paleobiology. These include assessing sample quality relevant to ecologic, biogeographic, and evolutionary questions, diagnosing the roles of various taphonomic agents, processes and circumstances in generating the sedimentary and fossil records, and reconstructing the dynamics of organic recycling over time as a part of Earth history. Major advances over the past 15 years have occurred in understanding (1) the controls on preservation, especially the ecology and biogeochemistry of soft-tissue preservation, and the dominance of biological versus physical agents in the destruction of remains from all major taxonomic groups (plants, invertebrates, vertebrates); (2) scales of spatial and temporal resolution, particularly the relatively minor role of out-of-habitat transport contrasted with the major effects of time-averaging; (3) quantitative compositional fidelity; that is, the degree to which different types of assemblages reflect the species composition and abundance of source faunas and floras; and (4) large-scale variations through time in preservational regimes (megabiases), caused by the evolution of new bodyplans and behavioral capabilities, and by broad-scale changes in climate, tectonics, and geochemistry of Earth surface systems. Paleobiological questions regarding major trends in biodiversity, major extinctions and recoveries, timing of cladogenesis and rates of evolution, and the role of environmental forcing in evolution all entail issues appropriate for taphonomic analysis, and a wide range of strategies are being developed to minimize the impact of sample incompleteness and bias. These include taphonomically robust metrics of paleontologic patterns, gap analysis, equalizing samples via rarefaction, inferences about preservation probability, isotaphonomic comparisons, taphonomic control taxa, and modeling of artificial fossil assemblages based on modern analogues. All of this work is yielding a more quantitative assessment of both the positive and negative aspects of paleobiological samples. Comparisons and syntheses of patterns across major groups and over a wider range of temporal and spatial scales present a challenging and exciting agenda for taphonomy in the coming decades.

Conceptual framework for the analysis and classification of fossil concentrations

Densely fossiliferous deposits are receiving increasing attention for their yield of paleobiologic data and their usefulness in sedimentology and stratigraphy. This trend has created a pressing need for standardized descriptive terminology and a genetic classification based on a coherent conceptual framework. The descriptive procedure outlined here for skeletal concentrations stresses four features -taxonomic composition, bioclastic fabric, geometry, and internal structure-that can be described readily in the field by nonspecialists. The genetic classification scheme is based on three end members, representing biologic, sedimentologic, and diagenetic factors in skeletal concentration. Concentrations created through the simultaneous or sequential action of two or more factors are classified as mixed types. As a conceptual framework for comparative biostratinomic analysis, the broad categories of this ternary classification scheme should facilitate recognition of large-scale temporal and spatial patterns in skeletal accumulation. The usefulness of this approach is suggested by the good agreement between biostratinomic patterns observed in ancient onshore-offshore facies tracts and those predicted across paleobathymetric transects based on modern processes of skeletal concentration.

Models for Fossil Concentrations: Paleobiologic Implications

Four basic types of skeletal concentrations are modeled in terms of changes in sedimentation rate alone. The model categorizes fossil concentrations on the relatively objective basis of their bed contacts, and uses this criterion to infer directional shifts in net sedimentation. This radical simplification of accumulation histories, in which hardpart input is held constant, yields a surprisingly powerful model capable of predicting a broad spectrum of taphonomic and paleobiologic phenomena. Type I concentrations grade from less fossiliferous sediments and terminate in omission surfaces; if hardpart supply is held constant, they record a slowdown from positive to zero net sedimentation. Type II concentrations are the same as Type I but terminate in erosion surfaces (slowdown to negative net sedimentation), and Type III and IV concentrations are characterized by basal erosion or omission surfaces, respectively, grade upward into less fossiliferous sediments, and record increases in net sedimentation from negative or zero rates to positive rates. According to the model, samples collected from successive horizons within any of these shell beds will differ in the degree and type of post-mortem bias owing to differing histories of hardpart exposure at the depositional interface. Moreover, because rates of sediment accumulation govern the abundance of hardparts at the depositional interface and thus many of the physical characteristics of the benthic habitat, the dynamics of fossil accumulation have direct consequences for the structure of benthic communities (taphonomic feedback) and for ecologically controlled species morphometry. The model is highly robust to fluctuations in hardpart input, as judged by its ability to correctly infer modes of formation of concentrations in synthetic stratigraphic sections. In addition, field examples of Type I-IV concentrations show independent evidence of formation during intervals of reduced net sedi- mentation, and many exhibit trends in taphonomic and paleobiologic features expected from the postulated changes in net sedimentation. The model thus provides a testable working hypothesis for the accumulation of fossil material in a wide range of environments, and should be applicable to concentrations of any taxonomic composition, state of preservation, or geologic age. The power and robustness of this heuristic model in fact argue that fossil-rich and fossil-poor strata provide fundamentally different records of past conditions, and that sedimentation rather than hardpart input is the primary control on the nature of the

Taphonomy's Contributions To Paleobiology

Taphonomy established itself in paleontology primarily as a subdiscipline of paleoecology, but it has evolved into a much broader study of the ways in which preservation affects the fossil record. The past decade has seen a change in emphasis from descriptive taphonomic studies of fossil assemblages to more experimental, process-oriented investigations of necrolysis, stratification, and diagenesis of organic remains in modern environments. These actualistic studies are increasing the sophistication of taphonomic analysis in the fossil record by sharpening the diagnosis of bias in paleontological data and by providing a baseline for quantitative modeling of preservational patterns. The analysis of bias is also expanding into the evaluation of temporal resolution in the fossil record (sample acuity, stratigraphic completeness), and taphonomic research is thus contributing to broad-scale problems in evolution, biogeography, and bio- stratigraphy. In addition, taphonomic studies are providing new insights into paleoenvironmental recon- struction and into the direct paleobiological significance of post mortem processes such as the behavior of scavengers and the role of dead hardparts in structuring benthic communities. One of taphonomys most promising new frontiers is comparative analysis applied to different taxonomic groups within assemblages and across environments, tectonic settings, and dimatic regimes. All of this currently active research is contributing to a better understanding of the fossil record as the result of a dynamic, evolving, integrated system of biological and sedimentological processes that have both limited and enhanced knowledge of Earth history.

Stratigraphic Condensation of Marine Transgressive Records: Origin of Major Shell Deposits in the Miocene of Maryland

Cyclic stratigraphic sequences in shallow marine records are commonly characterized by a condensed transgressive lag at the base of thicker, shallowing-upward facies. The standard actualistic model for these thin fossiliferous lags, by which most of the shelf is starved owing to coastal trapping of sediment and fossils are suspected of being reworked because of the association with an erosional ravinement, is contradicted by detailed stratigraphic and taphonomic analysis of Miocene examples in the Maryland coastal plain. The four major shell deposits in the Miocene record are condensed (i.e., demonstrably thin relative to coeval strata), transgressive records of intertidal to subtidal environments (to storm wavebase) and are composed of shells produced locally as transgression proceeded. The complex internal stratigraphies of the shell deposits and the mixture of soft- and shell-bottom faunas indicate condensation under a regime of dynamic bypassing rather than complete sediment starvation; bypassed fine sediments accumulated in deeper water environments below storm wavebase. Deeper, even more basinward parts of the shelf were starved of all sediment size fractions and accumulated shell-poor, bone-rich condensed deposits that lie mid-cycle (bracketing the time of maximum water depth). The base-of-cycle shell deposits and mid-cycle bone bed differ not only in composition and in environment and dynamics of condensation, but also in chronostratigraphic value: the onlapping shell deposits must be diachronous to some degree, whereas the mid-cycle bone bed approximates an isochronous marker for correlation. Thus, in some settings at least, transgressive shelves present a spatial mosaic of condensational and depositional regimes. Regardless of origin, all condensed intervals can time-average assemblages and telescope biostratigraphic datums. They otherwise differ widely, however, in paleontologic attributes and are characterized by highly variable and complex stratigraphic anatomies.

Bivalve taphonomy in tropical mixed siliciclastic-carbonate settings. II. Effect of bivalve life habits and shell types

Abstract Bivalve death assemblages from subtidal environments within the tropical Bocas del Toro embayment of Caribbean Panama permit a test of the extent to which levels of damage are determined by the intrinsic nature of shell supply (proportion of epifaunal species, thick shells, calcitic shells, low-organic microstructures), as opposed to the extrinsic postmortem environment that shells experience. Only damage to interior surfaces of shells was used, to ensure that damage was unambiguously postmortem in origin. We find that facies-level differences in patterns of damage (the rank order importance of postmortem encrustation, boring, edge-rounding, fine-scale surface degradation) are overwhelmingly controlled by environmental conditions: in each environment, all subsets of the death assemblage present the same damage profile. The composition of shell supply affects only the intensity of the taphonomic signature (i.e., percentage of shells affected), and only in environments containing hard substrata (patch reefs, Halimeda gravelly sand, mud among patch reefs). In these environments, epifauna, whether aragonitic or calcitic and whether thin or thick, exhibit significantly higher damage than co-occurring infauna, probably due to the initial period of seafloor exposure they typically experience after death. Thick shells (>0.5 mm), regardless of life habit or mineralogy, are damaged more frequently than thin shells, probably because of selective colonization by fouling organisms. Calcitic shells show no consistently greater frequency of damage than aragonitic shells, and high-organic microstructures yield mixed patterns. Taphofacies surveys in such depositional systems could thus be confidently based on any subset of the fauna, including diagenetically residual assemblages of calcitic shells and thick-shelled molds. Further tests are needed to determine whether the higher levels of damage observed on some subsets of shells are a consequence of greater time-averaging (thus lower temporal resolution), greater exposure time, preferential attack (potential bias in relative abundance), or some combination of these. Paleobiologically, however, the implication is that ecological subsets of bivalve assemblages are not isotaphonomic, either in tangible damage or in probable bias, within hard-substrate environments, although they may be within soft-sediment environments. In actualistic studies, targeting broad classes of taxa for comparison across environments maximizes our ability to extrapolate taphonomic guidelines into the fossil record, where life habits, skeletal types, and shallow subtidal habitats have dramatically different patterns of abundance and deployment.

Experimental disintegration of regular echinoids: Roles of temperature, oxygen, and decay thresholds

Laboratory experiments on regular echinoids indicate that low water temperatures retard organic decomposition far more effectively than anoxia, and that the primary role of anoxia in the preservation of articulated multi-element calcareous skeletons may be in excluding scavenging organisms. When tumbled at 20 rpm, specimens that were first allowed to decay for two days in warm seawater (30°C) disintegrated more than six times faster than specimens treated at room temperature (23°C) and more than an order of magnitude faster than specimens treated in cool water (11°C). In contrast, the effects of aerobic versus anerobic decay on disintegration rates were insignificant. The longer the period that specimens were allowed to decay before tumbling, the greater the rate at which specimens disintegrated, until a threshold time that appears to mark the decomposition of collagenous ligaments. This required a few days at 30°C, about two weeks at 23°C, and more than 4 weeks at 11°C for Strongylocentrotus . Up until this threshold, coronas disintegrate by a combination of cross-plate fractures and separation along plate sutures; cross-plate fractures thus can be taphonomic in origin and are not necessarily related to predation. Specimens decayed for longer-than-threshold periods of time disintegrate virtually instantaneously upon tumbling by sutural separation only. Undisturbed coronas can remain intact for months, sufficient time for epibiont occupation. Rates of disintegration were documented semi-quantitatively by recognizing seven stages of test disarticulation, and quantitatively by tensometer measures of test strength and toughness. The effects of temperature and oxygen on decay and the existence of a decay threshold in disintegration should apply at least in a qualitative sense to many other animals whose skeletons consist of multiple, collagen-bound elements. Regular echinoids should still be perceived as taphonomically fragile organisms, but our results suggest the potential for latitudinal as well as bathymetric gradients in the preservation of fossil echinoid faunas. Echinoid preservation under any given set of conditions should also be a function of taxonomic differences in test construction (particularly stereom interlocking along plate sutures) as suggested by previous workers, although our experiments indicate that these effects should only be significant among post-threshold specimens. A survey of regular echinoids from Upper Cretaceous white chalk facies of Britain substantiates the basic experimental patterns, yielding examples of all disarticulation stages and significant taxonomic differences in quality of preservation. A diverse array of borers and encrusters on fossil coronas also corroborates the post-mortem persistence of some tests on mid-latitude seafloors.

Taphonomic Feedback Ecological Consequences of Shell Accumulation

Sequential changes in benthic community composition have frequently been attributed by marine ecologists and paleontologists to autogenic ecologic succession in the classical sense: a biotically driven process leading to the establishment of a stable climax or mature community (Margalef, 1968; Odum, 1969). In recent years, however, the concepts of deterministic autogenic succession have been modified and supplemented by a greater recognition of the roles of stochastic colonization and of biogenic and physical disturbance in structuring ecological communities in time and space (Colinvaux, 1973; Drury and Nisbet, 1973; Sutherland, 1974, 1981; Horn, 1974, 1976; Connell and Slayter, 1977; Connell, 1978; Lubchenco, 1978; Sousa, 1979a, 1980; White, 1979; Paine and Levin, 1981). Paleontologists have also begun to adopt a more critical approach, recognizing the many processes, both biotic and abiotic, that serve as driving mechanisms for sequential faunal changes.

Bivalve taphonomy in tropical mixed siliciclastic-carbonate settings. I. Environmental variation in shell condition

Abstract Contrary to the geological stereotype of pure-carbonate reef platforms, approximately 50% of shallow shelf area in the Tropics is accumulating siliciclastic and mixed siliciclastic-carbonate sediments. Taphonomic characterization of these settings is thus essential for assessing variation among major facies types within the Tropics, as well as for eventual comparison with higher-latitude settings. Our grab samples and dredge samples of bivalve death assemblages from nine stations in five subtidal habitats in a large marine embayment of Caribbean Panama (Bocas del Toro) provide the first actualistic information on the taphonomic condition of shells in Recent tropical siliciclastic sediments. Focusing on unambiguous damage to bivalve shell interiors, we found that the quality of shell preservation in fine-grained siliciclastics is superb: commonly <10% of specimens are affected by encrustation, boring, edge-rounding, and fine-scale surface alteration via dissolution, microbioerosion, and maceration. Pure-carbonate and mixed siliciclastic-carbonate environments containing hard substrata (patch reefs, Halimeda gravelly sand, mud among patch reefs) contain higher numbers of more severely damaged shells (generally >25%) and also higher diversities of fossilizable encrusters and borers. Disarticulation and fragmentation are pervasive across all environments and are probably related to predation rather than to postmortem processes. As in other shallow subtidal study areas, the taxonomic compositions of death assemblages have not been homogenized by postmortem transport but show high spatial fidelity to the distribution of living species. Assemblages from the five sedimentary environments have distinct taphonomic signatures, but the strongest differences are between the two fine-grained, exclusively soft-sediment siliciclastic environments on the one hand and the three environments containing hard substrata on the other. Experimental tests for rates and agents of damage, still in progress, indicate that the most critical environmental variables are exhumation cycles and burial rate. Bivalve death assemblages from Bocas del Toro demonstrate that damage levels in tropical fine-grained siliciclastic environments are much lower than in closely associated reefs and algal sands, and suggest a less filtered record of biological information.