Generic turnovers of Carnian/Norian conodonts : climatic control or competition?

Pizzo Mondello section (western Sicily, Italy) is a 430 m thick continuous succession of marine cherty limestones, named Calcari con Selce, ranging from Late Carnian to Rhaetian (Nicora et al., 2007), characterized by almost uniform facies and rich in fossils (e.g. conodonts, radiolarians, ammonoids, bivalves) and, thus, suitable for detailed phylogenetic and taxonomic studies on conodont populations. Conodont biostratigraphy around the Carnian/Norian boundary is still problematic due to an inferred provincialism that affects most of the conodont species and to the occurrence of a great number of transitional forms, linked to the fast recovery of conodonts after the Julian/Tuvalian crisis (Carnian, Late Triassic) (Rigo et al., 2007). The lower 144 metres of the succession, Tuvalian to Lacian in age (Carnian-Norian), were sampled in detail for a biostratigraphic and ecological study of five Upper Triassic conodont genera: Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella, and Norigondolella. After a taxonomic revision of the studied genera, meant to define the most significant morphological features for their classification, a statistical approach was applied to the study of conodont populations. Quantitative curves of the absolute abundances for each genus show potential ecological competition first between Paragondolella-Carnepigondolella and later between Metapolygnathus-Epigondolella (and partially between Epigondolella-Norigondolella). Recognition of morphoclines among species, integrated by a similar ecological behaviour, support the phylogenetic derivation of Norigondolella and Metapolygnathus from Paragondolella and that of Epigondolella from Carnepigondolella. Furthermore, cross checks of the quantitative curves demonstrate the presence of three major assemblage changes: at metre 64.76 (named event T1) Carnepigondolella is replaced by its descendant Epigondolella in a evolutionary step; at metre 80 (event T2) Epigondolella is substituted by the mass occurrence of Metapolygnathus and at metre 95 (event T3) Metapolygnathus is succeeded by advanced Epigondolellae species and by Norigondolella. In looking for environmental explanations to these biological events, the quantitative curves of the conodont assemblages are compared to coeval δ18O and δ13C isotopic curves, based on new data from Pizzo Mondello section. The comparison shows a correspondence between higher 13C/12C ratios and event T2 (and partially event T3), but not with T1. This is in accordance with the interpretation of event T1 as an evolutionary turnover, supported by the occurrence of the transitional forms between the genera involved in the turnover (i.e,, Carnepigondolella and Epigondolella), and it demonstrates that conodont evolution is not affected by possible environmental changes Instead, a possible influence of environmental conditions on the absolute abundances of all the studied genera is observable: while Epigondolella proliferate when seawater δ13C ranges between 2.1‰ and 2.5‰, Carnepigondolella proliferate in the range between 1.6‰ and 2.1‰; Metapolygnathus instead appears to be limited to environmental conditions related to higher δ13C in seawater when δ13C is higher than 2.5‰. We explain this behaviour interpreting the genus Metapolygnathus as an opportunist taxon, exactly as its forerunner, the genus Paragondolella and thus, supporting their phyologenetic relationship