Unravelling conodont (Conodonta) ontogenetic processes in the Late Triassic through growth series reconstructions and X-ray microtomography

The ontogenetic series of ten Carnian-Norian (Late Triassic) platform conodont species, belonging to the genera Paragondolella, Carnepigondolella, Epigondolella, and Metapolygnathus are reconstructed in this paper. The growth series are based on rich populations derived from almost monospecific samples from the upper Carnian-Rhaetian succession of Pizzo Mondello (Sicani Mountains, Western Sicily, Italy), a GSSP candidate for the Norian stage. Six to twelve ontogenetic stages have been recognized for each species, based on examination of more than 350 SEM photos, integrated with X-ray synchrotron microtomographic images of their internal structure. These findings have given rise to a revaluation of the systematics and the phylogenesis of the Late Triassic conodonts from an ontogenetic perspective, leading to important refinements of their biostratigraphy and to a reinterpretation of their evolutionary processes. Herein it is observed that conodonts develop a morphological identity already in their most juvenile stages, thus, for the first time, they now have a specific taxonomic identity. Changes to the stratigraphic ranges of important Epigondolella species such as E. rigoi, E. triangularis, and E. uniformis are noted, and their First Appearing Data (FADs) now result younger in age, calling into question previous phylogenetic models. More articulated phylogenetic relationships with the carnepigondolellids have emerged, questioning the hypothesis that Epigondolella is a monophyletic group and suggesting the existence of dfferent polyphyletic lineages deriving anagenetically from several Carnian Carnepigondolella species. Subsequently in the middle Norian the epigondolellids evolve into the genera Mockina, Parvigondolella, and Misikella, which show reduced dimensions and neotenic characters. The present study provides a tool for the interpretation of this evolutionary process. We observed that the morphologies of these three genera are analogous to progressive juvenile stages of the epigondolellids: the mockinae are comparable to early adult, late juvenile, and juvenile stages of the epigondolellids, while the parvigondolellids and the misikellae to their early juvenile stages. Consequently, it is deduced that the final evolutionary trend of conodonts in the Late Triassic is thus the result of a gradual process of progenesis that started already in the middle Norian.