Similarity analysis of Ostracoda faunas in the Western Tethys during the Late Pliensbachian-Early Toarcian (Early Jurassic)

The present study is focused on the biogeographic dynamic quantification tested on ostracods of Upper Pliensbachian-Lower Toarcian. The calculation of similarity and distance indices made it possible to compare the contents of nine regions belonging to the palaeobiogeographic province of Western Tethys. The generic comparison realized for each chronological interval (Emaciatum Zone of the Late Pliensbachian and Polymorphum and Levisoni zones of the Early Toarcian) shows the presence of two subprovinces of ostracods during this time interval, the most stable in the westernmost part of the Tethys composed by the Traras (Algeria) and Lusitanian (Portugal) basins. This was probably facilitated by the presence of communication seaways. The other subprovince for ostracod is located in the eastern regions mainly composed by the Aquitanian Basin (France) and southern Germany. In regions such as the Iberian Cordillera (eastern Spain), the composition of the ostracod assemblages changed with more affinity to the eastern subprovince during the Late Pliensbachian and to the western subprovince during the Early Toarcian. Other regions such as the Central High Atlas (Morocco) and Umbria Marche Basin (Italy) show a remoteness of the ostracod assemblages during the studied interval. Otherwise, the Pielou criterion calculation confirms the absence of a geographical or chronological gradient among the studied regions probably related to oceanographic changes (the sea-level rise, the global warming and the development of oxygen depleted conditions in north central Europe).