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

Littoridina SOULEYET, 1852 (Mesogastropoda, Hydrobiidae) is the best represented genus among the molluscan taxa recorded in the marine late Quaternary from Argentina. It is distinguished by its ubiquity and high abundance along the coastal area from Rio de La Plata down to southern Buenos Aires Province, both in beach ridge and coastal lagoon and estuarine facies and tidal flats. It is practically absent in Holocene littoral depositis from Patagonia 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 occurs only rarely in scarce quantities and restricted distribution, always associated to marginal marine environments. Most specimens collected from the marine Quaternary are assignable to Littoridina australis (D'ORBIGNY, 1835), a dominant species (50--> 80%) in the Holocene from La Plata down to Bahia San Blas (Buenos Aires Province). Due to the similarity of the original illustrations and of the type material of L. australis crassa, L. isabelleana, L. australis nana and L. conexa with L. australis, these taxa are considered synonyms. L. australis is proposed as a polytipic species with three main morphs (ecomorphs) which represent adaptations to different environmental conditions, mostly of salinty:'A' (wide and short, very scarce, similar to L. charruana and L. conexa),'B' (identical to the lectotypo de L. australis, similar to L. australis crassa; predominant) and 'C' (strongly elongated, similar to L. isabelleana and L. australis nana, with variations according to size; scarce). At present'B' is dominant in mixohaline conditions,'C' in polyhaline environments and 'A' in oligo-mixohalin sites. The morphological variations observed within the abundant material (fossil and modern), geographically and through time, would be a consequence of the influence over the littoral benthonic communities of environmental and climatic changes occurred during the late Quaternary. A trend was observed towards a major abundance of the morph 'C' (polyhaline) in the northern sector of the distribution, Samborombon Bay, and before ca. 5 ka A.P., whilst the morph 'B' (mixohaline) predominates in the central and southern sectors of the distribution, 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 along the littoral zone following the Climatic Optimum (Holocene 'Hypsithermal', >7-5 ka A.P.), in coincidence with the sea-level (and temperature) decreasing trend until the present.