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. h Ammonites, chronostratigraphy, Early Jurassic, OAE, Tethys.Massimiliano Bilotta [dotpal@unipg.it],
Comparing the Early Jurassic ammonoids found in three Italian Apennine sections (Bosso, Pallareto and Furlo) with those reported in the literature for some South American sections (Argentina and Chile), we noted that these two areas share the same late Sinemurian–early Pliensbachian faunal succession: Paltechioceras in the late Sinemurian (Raricostatum Zone), Catriceras in the earliest Pliensbachian (Jamesoni Zone, Taylori Subzone), Miltoceras in the middle part of the early Pliensbachian (Jamesoni Zone, Polymorphus Subzone). An overall similarity to Apennine and South American ammonite faunas of the same age can be found for some sections of Morocco and partially also for southern Spain. One of the Italian taxa (an unpublished early Pliensbachian Apennine genus, ascribable to the family Polymorphitidae) is reported in the South American Pacific coast by the middle to late part of the early Pliensbachian (Ibex Zone). Evolving towards more and more discoidal morphologies, it might have given rise to an endemic American ammonite group, which is represented by the genera Eoamaltheus Hillebrandt and Fanninoceras McLearn. This phylogenetic reconstruction is based on several lines of evidence (shell morphology, ontogenetic development, aspect of the suture lines, biostratigraphy), and can be safely traced back to the basal Pliensbachian strata of the Tethyan Palaeoprovince. As far as we know, the examined ammonoid taxa are exclusively found in the western Tethys (e.g. Italian Apennines) and American Pacific coast (e.g. Argentina), but they have never been recorded elsewhere, and they show no evidence of pantropical distribution. Our biostratigraphic and phylogenetic data suggest the existence of a late Sinemurian–early Pliensbachian marine connection between the western Tethys (Italian Apennines) and the South American Pacific coast (Argentina). This is the so-called Hispanic Corridor, the existence of which has already been supported by many authors using several geological and palaeontological pieces of evidence. According to our interpretation, as well as the conclusions of other workers, it was a temporary and intermittent seaway, which allowed a partial faunal exchange for some taxa (including certain shallow-water ammonoids) during Sinemurian and Pliensbachian times. This palaeobiogeographic model consistently relates the biostratigraphy and evolution of some western Tethys and South American ammonoids to the major palaeogeographic changes affecting this vast area. In any case, our working hypothesis does not have any claim of universal applicability; we deem the proto-Atlantic seaway model to be the most suitable (until proof of the contrary) for the ammonites we examined, but it may not be necessarily valid for all the currently known taxa.
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