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

TIETZE, ELEONOR; DE FRANCESCO, CLAUDIO GERMÁN

Mendoza

Congreso; 4th International Paleontological Congress; 2014

International Paleontological Association

The main purpose of paleoecological studies is toperform accurate reconstructions of past environments. Following thisobjective, it is fundamental to understand and estimate the importance ofconstructive and destructive processes that influence the development of deathand fossil assemblages. The first step is to evaluate which taphonomicprocesses act during the residence time of remains residing around thesediment-water interface and to what extent do they reflect their originalliving communities. Southeastern Pampean lakes are very shallow (1-3 m indepth). Depositional processes dominate over erosive ones, allowing shellaccumulation accessible to be used in environmental reconstruction. Mollusksare abundant organisms in these lakes, commonly recorded alive, dead andfossil. With the aim of understanding the taphonomic processes that affectfreshwater mollusk remains, compositional fidelity and preservation wasassessed in four lakes applying qualitative and quantitative (uni- andmultivariate) techniques. A total of 15 species were collected. Two of themwere allochthonous species for freshwater habitats, and were thus, excludedfrom the analyses. Despite the homogeneity of the area, differences indiversity and taphonomy were recorded in living and death assemblages. Life anddeath assemblages varied greatly among lakes both in terms of abundance (n=67?3102) and richness (2?9). Some death assemblages presented higher abundancesthan life assemblages, while other death assemblages were less abundant thantheir molluscan community. These differences in abundance affected the fidelityin richness but not in Shannon-Weiner and Simpson indexes. However, all deathassemblages were represented by within-habitat time-averaged samples withoutseveral effects of premortem and postmortem processes. The taphonomicattributes that mainly affected shells were loss of proteinaceous parts,fragmentation and fine-scale surface alteration. Life/death agreement andtaphonomic differences were related both to environmental conditions (extrinsicfactors) and faunistic composition (intrinsic factors). In these environments,which are highly dynamic and productive, one of the extrinsic factors affectingdeath remains is carbonate saturation, which may vary among lakes, seasons andyears. Besides, productive systems favor microbioerosion by promoting carbonatedissolution facilitated by boring microorganisms. In the case of intrinsicfactors differential preservation and input rates of thin-/thick-shelledspecies were especially important, despite the high dominance of one species, Heleobiaparchappii. Although the degree of taphonomic alteration across lakes andenvironments was variable, all mollusk assemblages still preserved theirbiological signature from the precursor communities.