Possible influence of the δ13C variations on the Carnian/Norian conodonts diffusion.

Quantitative analyses of the absolute abundances of the species belonging to the five most widespread Upper Carnian/Lower Norian conodont genera (Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella, and Norigondolella) from the Pizzo Mondello section (Monti Sicani, Western Sicily, Italy), GSSP candidate for the Norian stage (Balini et al., 2008), show potential ecological competition between these genera. Cross checks of the quantitative curves evidence the presence of three major assemblage changes, named T events: at T1 Carnepigondolella is replaced by its descendant Epigondolella in an evolutionary step; at T2 Epigondolella is substituted by the mass occurrence of Metapolygnathus and at T3 Metapolygnathus is succeeded by advanced Epigondolellae and by Norigondolella. A comparison between these quantitative curves and coeval δ18O and δ13C isotopic curves, obtained from the carbonate bulk of the section, shows a correspondence between higher13C/12C ratios and event T2 (and partially event T3), but not with T1. Thus, while the shifts in the C isotopic composition of seawater do not affect conodont evolution, they have an influence on the diffusion of the studied genera, which react differently to the δ13C variations. Epigondolella and Carnepigondolella proliferate in fact when seawater δ13C is lower than 2.5‰, Metapolygnathus is instead limited to environmental conditions related to δ13C values higher than 2.5‰ (Mazza et al., 2009). The rapid expansion of photosynthetically active organisms in the Upper Carnian may explain these perturbations in the carbon cycle and have influenced the trophic chain of the different conodont animal genera, producing thus mutual local migrations.