Late Miocene molluscs from the southwestern Atlantic Ocean (Argentina and Uruguay): a palaeobiogeographic analysis

The analysis of the structure and evolutionary history of the Late Miocene marine molluscs along the southwestern Atlantic littoral indicates that these faunas did not give rise to the present assemblages of the Argentinean Province of the same region, the latter being characterised by a typically transitional association (in the sense of Scarabino [(1977) Com. Soc. Malac. Urug. 4, 177–285]) that developed after Miocene times once the cold Malvinas (Falkland) Current was completely operating. The Late Miocene Valdesian and Paranaian Provinces, mainly composed of paratropical warm-water faunas, are defined in this paper. The former includes assemblages recovered from the Puerto Madryn Formation. Its southern limit is placed at 42°S, while in the north its boundary with the Paranaian Province is situated approximately between 37 and 39°S. The Paranaian Province includes assemblages belonging to the Paraná and Camacho Formations. It extends northwards along the Uruguayan and southern Brazilian littoral, but its northern boundary still remains unknown. An incipient thermal gradient is recognised, as the temperature values for the Paranaian Province are slightly higher than those for the Valdesian Province. Taking into account that in the Valdesian Province water temperature was in fact warm, it is concluded that the cold Malvinas (Falkland) Current still had a very low activity in the area during the Late Miocene. The presence of upwelling phenomena is also suggested by the presence of widely distributed turritellid beds in the northern Patagonian region.