A Geometric Morphometric Study of Outline Morphological Variation for the Late Permian Conodont Clarkina

The Clarkina is one of the most representative conodont genus of the Late Permian, and plays a key role in chronostratigraphic subdivision and global stratigraphic correlation for the Lopingian and the Permian–Triassic boundary (PTB). Traditional taxonomic systems are mostly based on qualitative descriptions of characters such as platform, carina, and denticles, which lack support from systematic quantitative analyses. In this study, a total of 72 well-preserved Clarkina P1 elements were selected for geometric morphometric analysis. Landmarking and data acquisition were performed using TPSDig software: six fixed landmarks were set at key positions of the platform of conodont elements, and 50 sliding semi-landmarks were evenly distributed along both lateral margins of the platform to characterize platform outline. TPS.Util software was then used to configure sliding curves for the TPS-formatted data. The obtained data were imported into PAST software for Generalized procrustes analysis (GPA) to eliminate differences in size, position, and orientation among individuals. Principal component analysis (PCA) was subsequently conducted in PAST to identify the major directions of morphological variation among samples. Finally, thin-plate spline deformation grids were used to visualize the principal patterns of morphological change. PCA results show that Clarkina exhibit a continuous distribution in morphospace. The first principal component (PC1), as the major axis of morphological variation, reflects gradual changes in platform width, anteroposterior extension, and overall proportionality. The second principal component (PC2) accounts for a smaller proportion of morphological variation, mainly corresponding to differences in posterior platform outline, local morphological details, and secondary proportions. Individuals do not form distinct species clusters in the PC1–PC2 morphospace; instead, morphospaces of different species overlap extensively, displaying a continuous morphological gradient along PC1. These results indicate that the platform outline ofClarkina P1 element is characterized by prominent continuous variation, and interspecific morphological boundaries are not distinct in geometric morphospace. Thin-plate spline deformation grids further demonstrate that platform widening and posterior platform elongation are the dominant directions of platform morphological change in Clarkina.