Ammonite faunas, OAE and the Pliensbachian-Toarcian boundary (Early Jurassic) in the Apennines

The critical Jurassic event known as Early Toarcian Oceanic Anoxic Event (OAE) has been the subject of several studies, but its palaeontological characterization is still problematic, and its dating is therefore debated. The rich ammonite faunas of the Mediterranean Tethys (Italy, Greece, Albania, North Africa, southern Spain, etc.) demonstrate that the OAE separates two very different assemblages, resulting from a strong post-OAE biological renewal. Faunas before the anoxic event include many taxa already present in late Pliensbachian (evolute Phylloceratida; Reynesocoeloceratinae, Protogrammoceratinae and Arieticeratinae), whereas after the anoxic event new forms radiated (Nodicoeloceratinae, Harpoceratinae and Hildoceratinae; afterwards also groups as Mercaticeratinae, Phymatoceratidae and Hammatoceratidae). As these assemblages show remarkable differences from their Northwest European equivalents, in the Apennine it is necessary to use a Mediterranean zonation, here formally established. The examined data suggest accurate boundaries for the OAE, which separates our first two Toarcian Zones, corresponding to Tenuicostatum and Serpentinum standard chronozones. Within this context, the Apennine assemblage placed immediately below the anoxic event is an endemic fauna with more than 15 ammonite genera, two of which (Secchianoceras and Petranoceras) are exclusive of the interval in question. Most abundant are Phylloceratidae (Phylloceras, Lavizzaroceras, Calaiceras and Harpophylloceras), Juraphyllitidae (Meneghiniceras) and Lytoceratidae (Lytoceras); Dactylioceratidae (Dactylioceras, Eodactylites and Secchianoceras) and Hildoceratidae are comparatively less numerous, but these latter show the largest taxonomic diversity (Fontanelliceras, Trinacrioceras, Protogrammoceras, Petranoceras, etc.). The description of this fauna improves the knowledge of a poorly understood interval immediately preceding the OAE, thus allowing better correlations in the Mediterranean area.