Morphological Contraction in Conodonts around the Mid-Carboniferous Event

The mid-Carboniferous event, also referred to as the Serpukhovian biodiversity crisis, coeval with the onset of the Late Paleozoic Ice Age (LPIA), instigating a profound reorganization of marine ecosystems even comparable in magnitude to the “Big Five” mass extinctions. Among pelagic nekton, this event caused that conodont fauna dominated by genera Gnathodus and Lochriea gave way to that by Declinognathodus, Idiognathoides, and others. While benthic comunities, notably fusuline foraminifera, underwent rapid post-crisis diversification, pelagic conodonts experienced a sustained decline in taxonomic diversity. However, the biostratigraphic resolution of conodont zonations significantly increases from the Mississippian into the Pennsylvanian benifiting from rapid variations in key diagnostic characters. This juxtaposition challenges the morphological reconstruction hypothesis wherein increased morphological disparity typically precedes taxonomic diversity. Further, it remains ambiguous whether the observed Pennsylvanian variability reflect genuine macroevolutionary expansion or taxonomic over-splitting of localized platform ornamentation features. Here, we compiled a comprehensive dataset of 1007 SEM images spanning the late Visean to early Moscovian from South China. We employed two quantitative approaches: (1) Geometric Morphometrics (GMM) utilizing semi-landmarks to capture P1 element contour morphology, and (2) Deep Learning-based feature extraction to quantify latent space representations of oral view platform ornamentation patterns. Both two independent methods converge on a singular consistnet conlusion that morphological disparity converged significantly following the mid-Carboniferous event. GMM results reveal a pronounced contraction of the occupied morphospace in the Pennsylvanian, accompanied by a directional shift in the morphospace centroid towards narrower platform, indicating selective extinction or directional selection. Two key disparity metrics, Sum of Variances (SOV) and Sum of Ranges (SOR), derived from both GMM result and deep-learning latent features exhibit a significant decline from the Mississippian to the Pennsylvanian. We interpret this pattern as selective pruning of morphospace periphery. The extinction of Gnathodus and Lochriea eliminated specialized, morphologically disparate members, the survived Pennsylvanian clades retained the capacity for rapid morphological variation, while their phenotypic repertoire was constrained within a narrowed morphospace. Under the global cooling trend of the LPIA, selection likely favored convergence toward an “optimal feeding morphology” adapted for severe environmental pressure. Consequently, the frequent morphological variations observed in Pennsylvanian conodonts represent iterative perturbations within a restricted morphospace rather than genuine morphological innovations facilitating niche expansion, explaining the observed contradiction between high biostratigraphic resolution and long-term macroevolutionary decline.