Les genres Costa et Paleocosta (Ostracodes) du paléogène aquitain: Les structures de populations,témoins des paléoenvironnements. Description de Paleocosta aquitanica nov. sp.

Costa tricostata (Reuss) and Paleocosta aquitanicanov. sp. are polymorph species of the Paleogene Aquitaine. In this study, their constituent forms are both described and illustrated. These are particularly ornamented and show Mesogene affinites; their geographical distribution area is limited within the Aquitaine Basin and Bay of Biscaye.In bathyal environments, the spatial distribution of the Costa, reveals three domains characterized by an increase of faunal density and polymorphism and a morpho- functional adaptation through the strengthening of the shell architecture. We attribute these changes to the effects of an adaptative strategy of the populations within an environment of increasing dynamic activity either near drainage channels and/or accumulation zones of terrigenous material continentally-derived, or within highly intensive upwelling zones.In less shallow environments and/or more close tooceanic influences, the Costa and Paleocosta populations flourish but lose their polymorphism. They announce the ecologic limit of the species.