First record of cyanobacteria microboring activity in Pampean shallow lakes of Argentina

Bioerosion led by cyanobacteria occurs in terrestrial, freshwater and marine environments and depends on several ecological and climatic conditions. Therefore, the traces left by these boring organisms can give additional information about the paleoenvironment. In the case of freshwater environments of Argentina, the dissolution of mollusk shells has mainly been attributed to chemical and biological processes. The effect of microbioerosion processes in taphonomic lost is still lacking. With the aim of identifying microbioerosion processes, mollusk shells were studied. The specimens collected were specially picked when a biofilm covered the shell or dissolution marks were present. The shell surface of some specimens were scrapped removing the biofilm or the shells were dissolved with HCl, and the biological remains were observed under light microscope. Other specimens were treated with 2.5% sodium hypochlorite to remove organic matter, and then coated with gold and analyzed using a scanning electronic microscope. Surface pitting and circular to ovoid penetrations were observed. The endolithic cyanobacteria found could be identified as belonging to the family Pseudanabaenaceae. This is the first record of microbioerosion borings in shells of Heleobia parchappii in a pampean shallow lake, and the presence of the trace produced producing cyanobacteria. The record of this endolithic bioerosion traces enhances the discussion of taphonomic processes affecting mollusk shells in freshwater settings, and may allow adjusting the composition of fossil assemblages and in consequence paleoecological interpretations.