Taphonomic Comparison Research Articles

Biological and Physical Contributions to the Accumulation of Strombid Gastropods in a Pliocene Shell Bed

Before the evolutionary and ecological information contained in shell beds can be interpreted, the conditions of shell bed formation must be understood. Here the authors investigate the mode and time scale of accumulation of a dense layer of Strombus floridanus in the Pliocene Pinecrest Beds of Florida. They utilize a variety of comparative taphonomic data, including the extent of encrustation and boring on strombid shells of different ontogenetic ages, and on accompanying pelecypods of different ecological types. The taphonomic comparisons enable them to reconstruct more accurately the events of shell bed formation. The formation of the strombid shell layer involved both biological and physical components. The characteristically gregarious behavior of Strombus is reflected in the large number of individuals preserved in this layer. Based on average densities of individuals in strombid colonies today, the authors estimate that a time period of tens to hundreds of years was required to accumulate these fossils. Repeated sediment winnowing by storms, followed by rapid reburial in a regime of at least episodically high sedimentation rates, is the most likely mechanism of accumulation, and accounts for the observed patterns of encrustation and boring on the shells of Strombus and various associated pelecypods. The origin ofmore » Florida's Plio-Pleistocene shelly sands is poorly understood; analysis of this bed may provide a working model for future taphonomic studies.« less

Taphofacies models for epeiric sea environments: Middle Paleozoic examples

Skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur. Aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and, therefore, are useful in discriminating taphonomic facies. For purposes of taphonomic comparisons, skeletonized marine taxa are reduced to five generally defined types: massive, arborescent, univalved bivalved, and multielement. Different skeletal types display different attributes which reflect specific responses to physical, chemical, and/or biological processes. The taphonomic properties of disarticulation, reorientation and sorting, fragmentation, corrosion and abrasion, skeletal dissolution, and early diagenesis display patterns which are predictable with respect to a variety of inter-related physico-chemical parameters. These can be quantified through indices which evaluate relative frequencies of occurrence. Sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event-related properties, thereby generating composite taphofacies. Such events produce taphonomic signature which in themselves correspond to specific environmental conditions; i.e., water depth and event intensity. Indeed, background and event-related processes interact to generate complex, but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments. The comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties. Deductively-derived taphofacies models, corroborated by empirical data from Paleozoic strata, illustrate the distribution of taphonomic properties with respect to environmental energy (turbulence), background sedimentation rate, and sediment oxygenation. These parameters also indirectly influence other, less obvious factors including bioturbation and sediment chemistry. Trends in the distribution of taphonomic properties (taphonomic gradients) permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore-offshore transect within a generalized epeiric sea setting. Taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena. Moreover, taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies.

Taphonomic comparison of passive and active continental margins: Neogene shell beds of the Atlantic coastal plain and Northern Gulf of California

To test for differences in the nature of the fossil record as a function of terrigenous sedimentation, macroinvertebrate concentrations in a recently rifted continental margin (Pliocene Imperial Formation, southeastern California) are compared with a mature passive margin (Miocene Calvert and Choptank Formations, Maryland coastal plain). Previous work suggested that shell beds are primarily a product of absolute low net sedimentation rather than high skeletal input; the duration and environment of low sedimentation of (erosion, omission) determine the quality of paleontologic data through time-averaging and selective preservation of assemblages, and the frequency of low sedimentation episodes determine the stratigraphic density of richly fossiliferous horizons for sampling. Settings of rapid subsidence — and potentially high average sedimentation rates — such as active continental margins are thus expected to have fewer and taphonomically less complex skeletal concentrations than slowly subsiding passive margins, and where steep bathymetric gradients co-occur with high subsidence, skeletal concentrations should be diachronous, patchy in development, and highly variable laterally. These predictions are largely borne out by the shallow marine Imperial Formation, whose average sedimentation rate is several orders of magnitude higher than the Maryland Miocene section. In both tectonic settings, major accumulations record relatively prolonged episodes of low terrigenous sedimentation, attesting to the predictive and explanatory power of a sedimentologic model for fossil accumulation. Major complex shell beds in the Imperial Formation (Latrania member) are exceptional deposits, formed under conditions of sediment starvation along the seaward edges of prograding coastal alluvial fans and fandelta complexes and on bedrock highs; whereas major shell beds in passive margin deposits are stratigraphically condensed records of repeated marine transgressions. Although actualistic experiments suggest that rapid, deep burial is essential to the preservation of skeletal fossils, the stratigraphic record demonstrates that periods of sediment starvation and bypassing — and thus intermittent or retarded shallow burial of shells — are fundamental to the formation of many major skeletal concentrations.