Taphonomy of recent freshwater molluscan death assemblages, Touro Passo stream, southern Brazil

Abstract

Qualitative and quantitative taphonomic data for the freshwater mollusk shells from death assemblages found in straight and meandering fluvial channels of the Touro Passo Stream, southern Brazil, are here presented. A total of 245 gastropod and 485 bivalve shells were collected and analyzed. Complete valves, showing small chipped margins, dominated the studied shell material. The periostracum was present in the great majority of the mollusk shells, and the ligament was preserved in more than 50% of the studied bivalves. Gastropod shells were intensively (50% to 80%) encrusted by periphyton and Potamolithus egg capsules. Corrasion is one of the main taphonomic signatures exhibited by the mollusk shells, and are represented by small to large pits and holes found in the internal and external shell surfaces. Some evidences indicate that these were acquired soon after death or even during life. Clearly, the presence/absence and the degree of taphonomic alteration of the shells were mediated by intrinsic factors. Some life habits make difficult the determination of post-mortem nature of some taphonomic signatures. In addition, taphonomic signatures do not vary predictably in shells from thanatocoenoses found in the straight or meandering channels. In part, this is due to the reworking of the Pleistocene mollusk shells from fossil-rich layers found in the sandstones of point bar deposits of the meandering channel. Some qualitative and quantitative data are very similar to those found in shells from muddy, organic rich marine bottoms, where acid conditions prevail. In fact, dissolution seems to be the main taphonomic process affecting the shells from the Touro Passo Stream assemblages. However, abrasion is also important in shell damage and roundness, especially in the shells found in assemblages of the straight channel. The spatial mixing and reworking of shells from the studied fluvial subenvironments, the mixture of recently dead shell with fossil remains, and the complex origin of some taphonomic signatures indicate that paleoenvironmental reconstructions, based on freshwater mollusks shells, must be carefully achieved and complemented by sedimentologic and stratigraphic data, even when conducted in typical potamic areas.