Une nouvelle faune de bryozoairesde l'Hauterivien inférieur du Jura

Abstract

In the Jura, a new bryozoan fauna was found in the upper part of the Pierre jaune and the lower part of the Urgonien jaune (lower Hauterivian, Nodosoplicatum and Cruasense Zones). Almost the half of the species appear here for the first time. Four species are new: Leiosoecia bognensis, Lobomesenteripora musiegensis, Perfodiastopora vuachensis, Petalopora flavapetrensis. The last is the oldest species of Petalopora, a genus previously known only from the Upper Cretaceous. Therefore, including this new fauna, a succession of five faunas appears in the Jura during the Valanginian and the lower Hauterivian. This rapid turnover, and more generally the variations in the geographical position and composition of the species present in the region, are related to modifications of the marine palaeogeography and climate. Fauna 1 exists in lower Valanginian and in the base of the upper Valanginian (Verrucosum Zone), in relation to the carbonate platform of the “Calcaire roux”. Fauna 2 is in connexion with a regression and a climatic cooling in the upper Valanginian (Trinodosum Zone). Next, in the top of the upper Valanginian (Callidiscus Zone) and in the basal Hauterivian (Radiatus Zone) a transgression and a concomittant increase in temperature allow the extension of Fauna 3 (known since the Provence till NW Germany). At last, Faunas 4 and 5 are in connexion with the settlement and the progradation of a new carbonate platform in the upper part of Lower Hauterivian (Nodosoplicatum and Cruasense Zones).