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A reappraisal of available information concerning fossil species of the genus Bouchardia Davidson 1850 reveals occurrences in various stratigraphic units ranging from about the uppermost Cretaceous-Palaeocene boundary to the Mio-Pliocene. Whereas the genus has not been reported from certain late Tertiary marine units of southern Patagonia, it does survive to the present in Brazilian seas. The geographical distribution of confirmed records is displayed in palaeocontinental reconstruction maps. Thus a conspicuous latitudinal displacement in a northwards direction along the eastern (Atlantic) margin of South America becomes apparent. Furthermore, typical environmental conditions and autecological features known for the only living species, B. rosea (Mawe), include an apically counterweighted shell well suited for free lying on a clean, coarse-grained sedimentary bottom in platform and coastal areas, with maximum abundance achieved in euhaline, nutrient-rich waters, 18 to 26 m deep , ideally between 20°-22°C. On the assumption that the above mentioned ecological requirements have remained approximately the same throughout the Cenozoic for congeneric species, which do show similar skeletal adaptations and taphonomic evidences, the following conclusions can be drawn: (I) During the Palaeogene the shallow waters along the coasts of southern Patagonia and the Antarctic Peninsula should have reached significantly warmer temperatures than those presently recorded in the same region ; (2) In Neogene times, the southern limit for such relatively high temperature levels in the shallow waters of the western South Atlantic shores seems to have been progressively displaced towards more northerly zones as reflected by the brachiopod migration; (3) In the context of known geotectonic events and their inferred palaeoceanographic consequences, the opening of the Drake Passage and the resulting establishment of the circum-polar Antarctic current system, is suggested as the most likely triggering factor. This caused a major rearrangement of the oceanic circulation pattern and a widespread cooling in the area as well, eventually leading to the withdrawal of representatives of Bouchardia to the more restricted present day distribution held by extant propulations. As a complement to the study, selected illustrative specimens are figured.

revista brasileira de geociências

DISTRIBUTION AND PALAEOENVIRONMENTAL SIGNIFICANCE OF THE GENUS BOUCHARDIA (BRACHIOPODA, TEREBRATELLIDINA): ITS BEARING ON THE CENOZOIC EVOLUTION OF THE SOUTH ATLANTIC