Comparative Taphonomy of Mollusk Assemblages in Quaternary Freshwater Sequences from the Salado River Basin, Buenos Aires

From the analysis of the taphonomic attributes of the specimens of Heleobia parchappii found in fluvial and paleolacustrine sequences of the lower basin of the Salado River, the surface alteration of their shells was comparatively studied in order to assess the differences between both environments. The main results that were obtained by means of Non-Metric Multidimensional Scaling (NMDS) and analysis of similarity (ANOSIM) tests (R=0.31, p<0.01) enabled the recognition and statistical differentiation of both groups of samples. The taphonomic characteristics presented by those that were recovered from the paleolacustrine environment are more similar to each other, whereas those from the fluvial environment showed greater dispersion. Through the similarity percentage (SIMPER) analysis we could determine that the variables of discoloration and dissolution were the most important features to differentiate both groups, and that the values of the indexes (Total Taphonomic Grades) were always higher in the paleolacustrine assemblages, thus showing greater decay. The differences of preservation could be explained by the residence time of the remains near the water-sediment interface as well as by the differences in the sedimentation rate in both environments, which control the different exposure cycles of the shells. These differences would be related to changes in the hydrological regime (fluctuations in surface and groundwater levels) that, although affecting the entire study area, are more intense and frequent in shallow lakes, therefore favoring the decay of the shells accumulated in this environment.