Actualistic taphonomy of freshwater mollusks in shallow lakes from the Southeastern Pampas

TIETZE, ELEONOR; DE FRANCESCO, CLAUDIO GERMÁN

Mendoza

Congreso; 4th International Paleontological Congress (IPC4); 2014

The main purpose of paleoecological studies is to perform accurate reconstructions of past environments. Following this objective, it is fundamental to understand and estimate the importance of constructive and destructive processes that influence the development of death and fossil assemblages. The first step is to evaluate which taphonomic processes act during the residence time of remains residing around the sediment-water interface and to what extent do they reflect their original living communities. Southeastern Pampean lakes are very shallow (1-3 m in depth). Depositional processes dominate over erosive ones, allowing shell accumulation accessible to be used in environmental reconstruction. Mollusks are abundant organisms in these lakes, commonly recorded alive, dead and fossil. With the aim of understanding the taphonomic processes that affect freshwater mollusk remains, compositional fidelity and preservation was assessed in four lakes applying qualitative and quantitative (uni- and multivariate) techniques. A total of 15 species were collected. Two of them were allochthonous species for freshwater habitats, and were thus, excluded from the analyses. Despite the homogeneity of the area, differences in diversity and taphonomy were recorded in living and death assemblages. Life and death assemblages varied greatly among lakes both in terms of abundance (n= 67?3102) and richness (2?9). Some death assemblages presented higher abundances than life assemblages, while other death assemblages were less abundant than their molluscan community. These differences in abundance affected the fidelity in richness but not in Shannon-Weiner and Simpson indexes. However, all death assemblages were represented by within-habitat time-averaged samples without several effects of premortem and postmortem processes. The taphonomic attributes that mainly affected shells were loss of proteinaceous parts, fragmentation and fine-scale surface alteration. Life/death agreement and taphonomic differences were related both to environmental conditions (extrinsic factors) and faunistic composition (intrinsic factors). In these environments, which are highly dynamic and productive, one of the extrinsic factors affecting death remains is carbonate saturation, which may vary among lakes, seasons and years. Besides, productive systems favor microbioerosion by promoting carbonate dissolution facilitated by boring microorganisms. In the case of intrinsic factors differential preservation and input rates of thin-/thick-shelled species were especially important, despite the high dominance of one species, Heleobia parchappii. Although the degree of taphonomic alteration across lakes and environments was variable, all mollusk assemblages still preserved their biological signature from the precursor communities.