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

Abstract

Pizzo Mondello section (western Sicily, Italy) yields a very rich conodont record, ranging from Late Carnian to Rhaetian in age, shedding new light into the evolution of conodonts across the Carnian/Norian boundary. 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 Middle Carnian crisis. The lower 144 m of the succession, Tuvalian to Lacian in age, 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 genera studied, 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 between Paragondolella– Carnepigondolella and later between Metapolygnathus– Epigondolella (and partially between Epigondolella– Norigondolella). Recognition of morphoclines among species, integrated by a similar ecological behaviour, supports the phylogenetic derivation of Norigondolella and Metapolygnathus from Paragondolella and that of Epigondolella from Carnepigondolella. Furthermore, cross checks of the quantitative curves evidenced the presence of three major assemblage changes: at metre 64.76 (named event T1) Carnepigondolella is replaced by its descendant Epigondolella in an 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 conodont assemblages are compared to coeval δ 18O and δ 13C isotopic curves, based on new data from Pizzo Mondello. From the comparison, a correspondence appears between higher 13C/ 12C ratios and the interval between events T2–T3, but not with event T1. This is in accordance with the interpretation of event T1 as an evolutionary turnover. In more detail, we observe the possible influence of environmental conditions on the absolute abundances of all the studied genera: while Epigondolella proliferate when seawater δ 13C 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 δ 13C values in the seawater. We explain this behaviour by interpreting the genus Metapolygnathus as an opportunist taxon, exactly as its forerunner, the genus Paragondolella.