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The ammonites of the semiforme and fallauxi zones from the tithonian of the ardeche southeast france stratigraphy paleontology paleobiogeography


, : The ammonites of the semiforme and fallauxi zones from the tithonian of the ardeche southeast france stratigraphy paleontology paleobiogeography. Palaeontographica Abteilung A Palaeozoologie-Stratigraphie 219(1-3): 1-87

In the present paper the ammonite faunas of the Early Tithonian semiforme and fallauxi Zones of Ardeche are revised. In spite of their paleobiogeographic and biostratigraphic importance, these faunas had not be re-studied since TOUCAs' monography in 1890. This revision is based on a new bed-by-bed sampling carried out in ten sections, seven of which are located in the type-region of Le Pouzin. This allowed us to elucidate the vertical range of the species as well as important lithological changes over the study area. It has been possible to recognize a distinctive facies change from north to south of the Mio-Pliocene basaltic flows constituting the "plateau des Coirons". The "Calcaires noduleux", which are characteristic in the northern area (Le Pouzin area), are replaced by grey, massive limestomes in the South (Vogue, Saint-Laurent-sous-Coirons). Also, the occurrences of breccias, which appear to be related to turbidity currents, become increasingly rarer toward the south. As regards biostratigraphy, it has been possible to trace the boundary between the semiforme and fallauxi Zones by means of Richterella assemblages. The fallauxi Zone is in fact well recognizable only in its lower part, that is the richteri Subzone. Above this unit, a stratigraphic gap has been detected in the Le Pouzin area, while in the Southern sector the transitional beds to the "Calcaires blancs" formation are either barren or they contain uncharacteristic faunas. Our paleontological study resulted in the determination of 23 species in the considered rock interval. Two of these, "Glochiceras" blaschkei and Parapallasiceras toucasi, are new. Sexual dimorphism has been detected in most of the studied groups, as well as the intra-specific variability. The faunas of the semiforme and fallauxi zones in the mediterranean province are fairly homogeneus. The following faunas can be recognized: a) a mediterranean fauna, can be recognized: a) a mediterranean fauna, characterized by Simoceratids; b) a submediterranean fauna. This has been further subdivided into a european type (including the faunas of Ardeche) and a eurocaucasian type, with greatly reduced diversity. The fauna of Ardeche is characterized, as a whole, by pulses in the occurrence of distinctive mediterranean forms. The only submediterranean forms in Ardeche are perisphinctids with the exception of Richterella, which occurs mostly in the north-tethyran areas in spite of its mediterranean origin.

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