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

DE FRANCESCO, CLAUDIO G.; HASSAN, GABRIELA S.

Montevideo

Workshop; First Workshop Actualistic Taphonomy in South America; 2017

Universidad de la República, Facultad de Ciencias

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 infreshwater, suggesting that preservation may be strongly related to salinity in theseenvironments. To explore this hypothesis, we conducted an actualistic field-basedstudy of live/dead fidelity and preservation of the euryhaline mud snail Heleobiaparchappii (target species) in six shallow lakes representing a gradient of salinity(between 0.5 and 42 ppt). Live/dead fidelity included the comparison of theabundance and size of shells occurring alive (LA) with those occurring dead (DA).Preservation was evaluated through the analysis of shell fragmentation and surfacealteration in DAs. Live/dead fidelity in abundance was poor. Dead shells were moreabundant in higher saline lakes whereas live snails showed their lowest abundancehere. This pattern was more evident in DAs deposited over the littoral shore of lakes(older assemblages) suggesting a taphonomic origin for such concentrations. Shellsize was similar between LAs and DAs, suggesting a good live/dead fidelity.Fragmentation was lower in the most saline lakes, suggesting less destructiveconditions for postmortem preservation. Shell surface alteration did not showsignificant differences among lakes. These preliminary results highlight theimportance of lake salinity as a possible taphonomic agent that promote molluskpreservation. It is possible that the higher taphonomic alteration of shells infreshwater may be related to changes produced by microbial maceration of organicmatrix, as these lakes are far more productive than saline ones. While experimentalstudies addressing the causes of shell alteration in freshwater lakes are needed, thepresent findings provide evidence in that direction, with implications for explainingthe high concentration of well-preserved shells usually found in Holocenesedimentary successions.