Improved taxonomic definition based on the ontogenetic series of Griesbachian-Dienerian conodonts from the Early Triassic of northwestern Pakistan

As one of the most important intervals in Earth life evolution, the latest Permian to earliest Triassic witnessed the greatest mass extinction and most profound recovery of marine biotas. In this period, gondolellid conodonts went through a major faunal turnover and morphologic change marked by platform reduction during the late Griesbachian to Dienerian. However, the details of this morphologic change process remain poorly known. The resultant Dienerian segminate conodonts also display remarkable morphologic variations related to growth, which has caused a great deal of confusion in taxonomy. Ontogenetic and phylogenetic studies are believed to be essential in understanding the developmental process and evolution of organisms and thus will be helpful for improving taxonomic definitions and revealing temporal changes of morphology. Here, based on abundant and well-preserved specimens from the Salt Range and Surghar Range of northwestern Pakistan, we described in detail the morphologic variants and reconstructed the ontogenetic series for seven stratigraphically important Griesbachian-Dienerian conodonts belonging to Clarkina, Neoclarkina, Neospathodus, and Sweetospathodus. A further Dienerian key species Eurygnathodus costatus is also included in discussion based on the work of Lyu et al. (2020). The phylogenetic relationships among these taxa were reconstructed by means of cladistic approach. Some important evolutionary trends were recognized. Morphometric data including element length, platform width and denticle number were collected based on global occurrences and were plotted to illustrate the ontogenetic trajectory. Comparisons of ontogenetic trajectories revealed complex heterochronic patterns for platform growth. The reduction of platform may result from progenesis and neoteny in late Griesbachian Neoclarkina species and further from postdisplacement of the flange in Dienerian segminate conodonts. The Dienerian Neospathodus lineage may represent a paedomorphosis of the flange, whereas the lineage of Sw. kummeli-Eu. costatus was probably characterized by a peramorphosis of the platform through acceleration.

Automated summary

This study focused on the morphological changes and ontogenetic series of important Griesbachian-Dienerian conodonts based on well-preserved specimens from northwestern Pakistan. By reconstructing their phylogenetic relationships and analyzing morphometric data, the study identified complex evolutionary trends and heterochronic patterns in platform growth. The reduction of platform in late Griesbachian species was attributed to progenesis and neoteny, while the Dienerian segminate conodonts exhibited postdisplacement of the flange. Additionally, the study suggested that the Dienerian Neospathodus lineage represented a paedomorphosis of the flange, while the Sw. kummeli-Eu. costatus lineage may have undergone peramorphosis of the platform through acceleration.