REDESCRIPTION AND PALEOENVIRONMENTAL SIGNIFICANCE OF HELEOBIA AMEGHINI (DOERING, 1884) (GASTROPODA: RISSOOIDEA) FROM THE LATE PLEISTOCENE OF BUENOS AIRES PROVINCE, ARGENTINA

REDESCRIPTION AND PALEOENVIRONMENTAL SIGNIFICANCE OF HELEOBIA AMEGHINI (DOERING, 1884) (GASTROPODA: RISSOOIDEA) FROM THE LATE PLEISTOCENE OF BUENOS AIRES PROVINCE, ARGENTINA. New remains of Heleobia ameghini (Doering, 1884) are analysed. This is an extinct late Pleistocene gastropod species recovered from alluvial sections along the middle course of the Lujan River. Detailed observations allowed establishing the main diagnostic characteristics of the species; these are compared with the data provided in the original description of this taxon. Its paleoenvironmental significance is also discussed in context with the stratigraphic setting and the accompanying fauna. Heleobia ameghini is characterized by its small size, which varies between 0.75 and 6.75 mm in length. Shells are globose, with the last whorl highly developed, in a similar way to the modern Heleobia piscium (d'Orbigny, 1835), from which it differs by their relative higher size and the lower convexity of the whorls. The poor preservation of the specimens and the bell-shaped size-frequency distribution suggest an important taphonomic alteration, which may be consequence of the dissolution processes that occurred during the biostratinomic or diagenetic stage. It is concluded that Heleobia ameghini tolerated significant salinity fluctuations, inhabiting fluvial environments subjected to recurrent episodes of storm-flooding alternated with moments of water-flow decrease. The latter were responsible for the development of ponds and eventually dry ponds during drought periods and thus higher salinity values.