Experimental taphonomy of a decapod crustacean: Long-term data and their implications

Despite being a common element in many modern marine habitats, decapod crustaceans have a patchy yet long-ranging fossil record. While a strong susceptibility to decay early in the post-mortem history is generally acknowledged, specific controls on the preservation potential of this group are poorly understood. To aid in the full understanding of the details of the taphonomic process for this group we report here the results of long-term observations of large numbers of in situ decay experiments. The unique time scale of these experiments (2–13 years) is intended to bridge the gap between short-term decay experiments and the observation of fossil specimens. Complete carcasses of the portunoid decapod, Callinectes sapidus, were placed in mesh bags at the sediment–water interface in 1993in a wide range of depositional environments. In the Bahamas, experiments were deployed in shallow reef settings and down-slope to 280 m. In the Gulf of Mexico experiments were placed in several terrigenous and carbonate depositional environments as well as a few unusual environments such as brine and hydrocarbon seep localities. At least one deployment consisting of two crab carcasses was collected from each locality after 2 years, 8 years, and 13 years. A total of 191 individuals were deployed, collected, and scored for a suite of taphonomic variables during the course of this study. For most individuals, complete loss of soft tissue, disarticulation, and degradation and/or loss of most cuticle occurred after two years of exposure. The remaining elements, including the claws and associated limb parts often survived over the entire 13-year duration of the study. This suggests a greater fossilization potential (at least for claws) than might be indicated by the fossil record. Nonmetric Multidimensional Scaling was employed to reduce the dimensionality of the dataset and detect subtle patterns among the variables. The subsidiary variables depth and bottom type account for a portion of the variation among ordinated samples while time since deployment and burial state at collection do not. Null models hypothesizing equal probability of each taphonomic state allowed consideration of the possibility that the observed patterns are not distinguishable from those resulting from purely stochastic processes. However, ordinations of null model datasets did not compare favorably with the ordination of the actual data suggesting that, however subtle the pattern, there is a taphonomically meaningful structure in the data. Thus, while easily-scored taphonomic variables can help to distinguish some subtle temporal and sediment-related taphonomic patterns among samples, there are likely several important factors that could not be accounted for in this study. Our findings show that crab cuticle is quite durable at the sediment/water interface, but the determining factors for good preservation may not be easily linked to extrinsic environmental factors alone. Decapod preservation may be determined by a complex interaction between environment of deposition (e.g., energy, sediment type, scavenging, microbial activity) and intrinsic factors such as molt stage, sex, and age of the crab.

Field of study: Environmental science, Geography