SALINITY-RELATED PRESERVATION OF MOLLUSKS IN SHALLOW LAKES: IMPLICATIONS FOR THE UNDERSTANDING OF THE LACUSTRINE PAMPEAN FOSSIL RECORD

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

PALAIOS

SEPM-SOC SEDIMENTARY GEOLOGY

Lugar: Lawrence; Año: 2023 vol. 38 p. 111 - 124

0883-1351

Premortem and postmortem processes significantly influence the formation of themolluscan fossil record in freshwater environments. Despite their importance forpaleoenvironmental studies, they remain poorly understood. In Pampean shallowlakes, Holocene shell deposits of the euryhaline snail Heleobia parchappii show arelation with salinity, as preservation seems to be favored by brackish-saline waterconditions. To explore if this pattern may respond to ecological (i.e., differential survivaland reproduction) or taphonomic processes acting differently in freshwater andbrackish-saline environments, we conducted a field-based study comparing premortem(abundance, length, width/length ratio and crushing resistance in living and deadshells) and postmortem (fragmentation, fine-scale surface alteration and loss ofperiostracum of dead shells) attributes along a modern lacustrine salinity gradient (0.5-40 ppt) in the Pampa plain of Argentina. Snails from saline lakes were smaller andmore rotund than those from freshwater lakes, exhibiting higher abundances andresistances in death assemblages. They showed the highest fidelity in shell length andthe best states of preservation, which were similar to values recorded in fossil shells.We concluded that shells deposited in saline lakes are better preserved than thosedeposited in freshwater lakes, giving rise to highly abundant shell concentrations,analogues to those shell-rich fossil levels recorded in Pampean lakes. Suchabundance does not reflect the natural abundances of living snails, but is the result ofthe combined influence that less destructive environments and better shell intrinsicproperties have on preservation.