Differential preservation of the snail Heleobia parchappii (d?Orbigny) in freshwater and saline shallow lakes of the Argentine Pampas

DE FRANCESCO, CLAUDIO GERMÁN; HASSAN, GABRIELA S.

Montevideo

Workshop; Workshop Actualistic Taphonomy in South America; 2017

It has been observed that in Holocene lacustrine successions of the Pampa Plain(Argentina), mollusk assemblages deposited under inferred brackish-saline lakeconditions exhibit higher abundance and better preservation than those deposited in freshwater, suggesting that preservation may be strongly related to salinity in these environments. To explore this hypothesis, we conducted an actualistic field-based study of live/dead fidelity and preservation of the euryhaline mud snail Heleobia parchappii (target species) in six shallow lakes representing a gradient of salinity (between 0.5 and 42 ppt). Live/dead fidelity included the comparison of the abundance and size of shells occurring alive (LA) with those occurring dead (DA). Preservation was evaluated through the analysis of shell fragmentation and surface alteration in DAs. Live/dead fidelity in abundance was poor. Dead shells were more abundant in higher saline lakes whereas live snails showed their lowest abundance here. This pattern was more evident in DAs deposited over the littoral shore of lakes (older assemblages) suggesting a taphonomic origin for such concentrations. Shell size was similar between LAs and DAs, suggesting a good live/dead fidelity. Fragmentation was lower in the most saline lakes, suggesting less destructive conditions for postmortem preservation. Shell surface alteration did not show significant differences among lakes. These preliminary results highlight the importance of lake salinity as a possible taphonomic agent that promote mollusk preservation. It is possible that the higher taphonomic alteration of shells in freshwater may be related to changes produced by microbial maceration of organic matrix, as these lakes are far more productive than saline ones. While experimental studies addressing the causes of shell alteration in freshwater lakes are needed, the present findings provide evidence in that direction, with implications for explaining the high concentration of well-preserved shells usually found in Holocene sedimentary successions.