A small collection of ammonoids from the Upper Triassic Scillato Formation at Pizzo Mondello (Agrigento, Sicily) is studied. These specimens were collected within the framework of a project intended to provide integrated high-resolution bio-chronostratigraphic support for the Upper Carnian-Norian magnetostratigraphic scale defined at this site, which is located in an historical area from which G.G. Gemmellaro collected and monographed Upper Triassic ammonoids at the beginning of the 20th century. The specimens, which were collected utilizing the bed-by-bed sampling method, represent the first collection of Upper Triassic ammonoids described from western Sicily since Gemmellaro's time. Several levels of the Pizzo Mondello section yielded ammonoids, but very few beds have provided more than one specimen. This scarcity of specimens has resulted in a complex taxonomie analysis because Gemmellaro, who described 166 new species, did not explain the variability of many of his taxa. Sixteen taxa belonging to eleven genera are described. They include Placites sp. ind., Discotropites plinii (Mojsisovics), Anatropites sp. ind., Microtropites cf. paronai, Metathisbites cf. affinis, Hyattites cf. praefloridus, Projuvavites boehmi (Gemmellaro), Projuvavites inflatus (Gemmellaro), Gonionotites cf. italicus, Gonionotites aff. recuperoi, Pregriesbachites n. gen., P. bukowskii (Gemmellaro), Dimorphites noricus n. sp., Dimorphites selectus Mojsisovics, Dimorphites sp. and Discophyllites insignis. Among the new taxa, Dimorphites noricus n. sp. formalizes the nomen nudum "Dimorphites n. sp. 1", which has been quoted in the literature for several years as the index ammonoid for the lowest subzone of the Norian stage. The small collection documents the Discotropites plinii and Gonionotites italicus subzones of the uppermost Carnian Spinosus Zone as well as the Dimorphites noricus and D. selectus subzones of the Jandianus Zone, the first zone of the Lower Norian. This chronostratigraphic classification not only confidently ties the Pizzo Mondello succession to the Tethyan chronostratigraphic scale, but it is also crucial for the calibration of the Halobia and conodont bioevents identified in the section. Also discussed is the chronostratigraphic ammonoid-based correlation of the Pizzo Mondello section with Feuerkogel (Austria), Jomsom (Nepal), West Union Canyon (Nevada, USA), Black Bear Ridge (British Columbia, Canada) and the Yana Okhotskaya River (Siberia, Russia). These are the most complete ammonoid-bearing sections in the world that span the Carnian/Norian boundary, and they exhibit a discrete distribution of ammonoid-bearing beds across the boundary. Ammonoids will not provide the primary marker event for the definition of the GSSP of the Norian, but they are crucial for the selection of the most significant events based on other groups.
|Titolo:||Late Carnian-Early Norian ammonoids from the GSSP candidate section Pizzo Mondello (Sicani Moutains, Sicily)|
|Parole Chiave:||Upper Triassic; Carnian/Norian boundary; Ammonoids; Biostratigraphy; Sicily; Sicanian Domain; Scillato Formation|
|Settore Scientifico Disciplinare:||Settore GEO/01 - Paleontologia e Paleoecologia|
Settore GEO/02 - Geologia Stratigrafica e Sedimentologica
|Data di pubblicazione:||mar-2012|
|Appare nelle tipologie:||01 - Articolo su periodico|