Paleobiogeography of the Danian molluscan assemblages of Patagonia (Argentina)

A detailed quantitative analysis of bivalves and gastropods reported from Danian marine rocks of Patagonia (Argentina) allowed the identification of three molluscan biogeographic units that, from north to south, are identified as the Rocaguelian, Salamancan, and Dorotean Bioprovinces. Molluscan assemblages comprise Cosmopolitan, Paleoaustral, Gulf Coastal Plain and Endemic genera, which collectively give this fauna a distinctive signature, preventing them to be considered as related to any other assemblages recorded in the high latitudes of the Southern Hemisphere. Results obtained through the present research support the idea that the Weddellian Province did not extend to Patagonia during Danian times, proving that the geographic isolation of this region during that interval was enough to allow the development of separate biogeographic units from other austral regions. Moreover, it is demonstrated that, although significant even in northern Patagonia, Paleoaustral taxa were not dominant elements of Danian faunas. Composition of the molluscan faunas reflects the presence of warm-temperate waters in the region, and records a latitudinal temperature trend with slightly higher values in northern Patagonia than in the south. In addition, a discussion of the age of the Paleocene faunas has been incorporated. The assemblages contained in the Dorotea formation (Austral Basin), Lefipán and Roca formations (Cañadón Asfalto Basin), and Salamanca formation (Cañadón Asfalto and Golfo San Jorge basins) have been placed in the early Danian, based on the correlation with the accurately dated molluscan faunas of the Roca formation exposed in the Neuquina–Colorado Basin, and that of the Sobral and López de Bertodano formations (Antarctica).