Bioerosion on spinicaudata shells from a Triassic freshwater paleolake, Mendoza, Argentina

Abstract

Bioerosion by microorganisms is termed microbioerosion. In aquatic systems, microbioerosion affects various substrates, most commonly relatively soluble ones such as sulfates, phosphates, and carbonates. However, the record of microbioerosion in fossil calcareous organisms in lake environments is limited. In northern Mendoza Province, at the locality Paramillos de Uspallata, Triassic sedimentary rocks outcrop, including the Agua de la Zorra Formation, which was deposited in a deltaic and lacustrine system. This study assesses possible bioerosion by microorganisms on spinicaudata shells from the lacustrine sequence and discusses their preservation potential in lacustrine environments. Spinicaudatans from the Agua de la Zorra Formation were analyzed under a binocular microscope and via scanning electron microscopy. Traces, the product of microbioerosion, preserved as molds, were assigned as Type 1 and 2 and were then compared with those generated by cyanobacteria, bacteria, chlorophyte, and fungi that inhabit fresh and brackish water environments. Microbioerosion occurs post-mortem and enhances dissolution of the spinicaudata shells. This, together with bias in extraction methodologies and sampling, is considered the reason for the rarity of spinicaudatans with traces of bioerosion in the lacustrine record.