Paleobiogeographic Evolution of Graptolites from the Late Katian (late Ordovician) to the Aeronian (llandovery, Silurian): Quantitative Analysis Based on a Global Occurrence Dataset

A series of significant paleoenvironmental changes and biological evolutionary events occurred from the late Katian of the Late Ordovician to the Aeronian of the Llandovery (Silurian). These include the expansion and retreat of the Gondwanan ice sheet, rapid global sea-level fluctuations, drastic perturbations in marine environments, the End Ordovician Mass Extinction (EOME), and the subsequent biotic recovery and radiation. As the most important macroplanktonic group of the Early Paleozoic, graptolites exhibited a global distribution, rapid evolutionary turnover, and high sensitivity to changes in global climate and oceanic conditions. Consequently, the study of their paleobiogeographic provincialism and evolution is key to understanding the coordinated development of biotas and environments during this critical interval. The impact of mass extinction events on paleobiogeographic patterns has long been a focal point of paleobiology research. With the continuous accumulation of fossil occurrence data and the rapid advancement of quantitative paleobiogeographic methods, the field is progressively shifting from qualitative description to quantitative analysis. Recent studies suggest that the rate of species diversity change across the EOME interval may have been significantly higher than traditionally recognized. However, the concurrent changes in the global paleobiogeographic distribution of graptolite faunas during this period still lack systematic investigation. In this study, we systematically compiled graptolite species, locality, and age data from published literature and database resources. Species taxonomic assignments were reviewed and standardized according to a unified systematic framework, and precise correlation of fossiliferous horizons across localities was achieved using a high-resolution biostratigraphic scheme. Based on this, we established a comprehensive global dataset of graptolite occurrences spanning the late Katian to the Aeronian. Subsequently, cluster analysis and Non-metric Multidimensional Scaling (NMDS) were employed to delineate graptolite biogeographic realms and provinces, and to elucidate their interrelationships and patterns of temporal change. Furthermore, network analysis was applied to reveal the connectivity strength, structural characteristics, and evolutionary dynamics of biotas across different regions in both time and space. Integrating these results with global paleogeographic, paleoclimatic, and paleoenvironmental reconstructions, we explore the relationships between graptolite distribution patterns and factors such as paleolatitude, paleogeographic configurations (oceanic connectivity), and paleobathymetry. Preliminary results indicate that graptolite biogeography underwent a rapid collapse and subsequent reorganization of provincialism across the extinction interval, a process that appears temporally correlated with changes in diversity dynamics and faunal turnover.