Biogeographic distribution of the Holocene molluscs from the Argentine-Uruguayan littoral

The biogeographic distribution of the Holocene marine molluscs from the Arg entine-Uruguayan littoral was analyzed. Following the present-day denominat ion, two biogeographic regions were defined: the Argentina Province and the Magellan Province. The 197 species formed several faunal groups: species w idely distributed within the region; species with intermediate range; and s pecies restricted to specific areas. With few exceptions, the Holocene faun a consists of species living in the adjacent waters. The geographic persist ence of the species indicates, on the whole, similar climatic conditions du ring the Holocene. Nevertheless, minor temperature fluctuations produced sl ight displacements of the distribution range of some taxa. From an ecologic al viewpoint, the major differences between the two provinces are the incre ase of the epifauna in the Magellan Province and a higher proportion of bur rowing species in the Argentina Province, in accordance with the predominan ce of rocky or sandy substrata, respectively.