Systematics, distributional and palaeoenvironmental aspects of Littoridinaaustralis (D'Orbigny, 1835) (Mesogastropoda) in the Marine Quaternary of Argentina (South America)

Littoridina SOULEYET, 1852 (Mesogastropoda, Hydrobiidae) is the best repres ented genus among the molluscan taxa recorded in the marine late Quaternary from Argentina. It is distinguished by its ubiquity and high abundance alo ng the coastal area from Rio de La Plata down to southern Buenos Aires Prov ince, both in beach ridge and coastal lagoon and estuarine facies and tidal flats. It is practically absent in Holocene littoral depositis from Patago nia and a very scarce component of the Pleistocene deposits from the whole Argentine coast. At present it is not recorded along the Argentine Sea or o ccurs only rarely in scarce quantities and restricted distribution, always associated to marginal marine environments. Most specimens collected from t he marine Quaternary are assignable to Littoridina australis (D'ORBIGNY, 18 35), a dominant species (50--> 80%) in the Holocene from La Plata down to B ahia San Bias (Buenos Aires Province). Due to the similarity of the origina l illustrations and of the type material of L. australis crassa, L. isabell eana, L. australis nana and L. conexa with L. australis, these taxa are con sidered synonyms. L. australis is proposed as a polytipic species with thre e main morphs (ecomorphs) which represent adaptations to different environm ental conditions, mostly of salinty:'A' (wide and short, very scarce, simil ar to L. charruana and L. conexa),'B' (identical to the lectotypo de L. aus tralis, similar to L. australis crassa; predominant) and 'C' (strongly elon gated, similar to L. isabelleana and L. australis nana, with variations acc ording to size; scarce). At present'B' is dominant in mixohaline conditions ,'C' in polyhaline environments and 'A' in oligo-mixohalin sites. The morph ological variations observed within the abundant material (fossil and moder n), geographically and through time, would be a consequence of the influenc e over the littoral benthonic communities of environmental and climatic cha nges occurred during the late Quaternary. A trend was observed towards a ma jor abundance of the morph 'C' (polyhaline) in the northern sector of the d istribution, Samborombon Bay, and before ca. 5 ka A.P., whilst the morph 'B ' (mixohaline) predominates in the central and southern sectors of the dist ribution, Mar Chiquita and Bahia Blanca, and is dominant after ca. 4.5 ka. This in turn would be a response to a progressive decrease of salinity alon g the littoral zone following the Climatic Optimum (Holocene 'Hypsithermal' , >7-5 ka A.P.), in coincidence with the sea-level land temperature) decrea sing trend until the present.