Les crises du Dévonien supérieur par l'étude des faunes d'ostracodes dans leur cadre paléogéographique
Abstract Ostracod evolution, studied at a global scale, can contribute to an evaluation of the palaeobilogical crises during the Late Devonian. The mostly pelagic Entomozoidae are not affected. Data on benthic neritic ostracods come from the Western Canadian platform, Europe, the Russian platform, North Africa and the Middle East. Species extinction rates vary according to provinces and areas. Extinction occur at the Givetian/Frasnian, Lower/Upper Frasnian and Frasnian/Famennian boundaries. Only five families seem to disappear during the Late Devonian. The appearance of new assemblages since the earlisst Famennian is worldwide on platforms. The Thuringian ecotype ostracod assemblages underwent a major change since the Early Famennian. It is characterized by many species with spinous carapaces and is known in the Paleotethys during Famennian-Dinantian times. It is related to a glacial event in the North of the Gondwana which generated cold deep water currents invading the paleotethysian basins. The Thuringian ecotype allows to propose a paleobathymetric model. Its species composition shows a double origin: by evolution of Early Devonian roots and with lineages proceeding from circalittoral species. Causes of these Late Devonian events can be understood in a particular paleogeographic reconstruction where the Paleotethys has no western faunal communication since the Givetian/Frasnian boundary. Warming water, associated with an oxygen deficiency, eliminated several species at the Givetian/Frasnian and Lower/Upper Frasnian boundaries. A first cooling at the Frasnian/Famennian time was sufficient to decimate species strongly adapted to the Frasnian conditions. The generalized regression at this time probably increased this last phenomenon.