The Morphological Evolution of Cephalopods from the Late Cambrian to Ordovician in South China 1,2

The Great Ordovician Biodiversification Event established the fundamental framework of the Palaeozoic Evolutionary Fauna (PEF), with cephalopods as a key component. Nautiloid cephalopods thrived in the Early Paleozoic marine, characterized by long stratigraphic ranges, rapid evolution, and abundant fossil records. As top predators of the food web, their morphological evolution is crucial for understanding broader evolutionary patterns in PEF. Morphological evolution is a study hot spot of the evolutionary palaeobiology. Several key hypo-theses emphasize the roles of directional trends and environmental factors, including Cope's Rule (directional size increase), the Lilliput Effect (size reduction in mass extinction survivors), and Bergmann's Rule (latitudinal size gradients linked to temperature). During the Great Ordovician Biodiversification Event (GOBE), cephalopod shells morphology varied significantly, shifting from the generally small and simple forms of the late Cambrian to the more diverse and giant forms of Ordovician. However, this understanding is currently based on qualitative interpretations from previous studies, without systematic quantitative research has yet been conducted. The research methods include palaeontological morphometric analysis and evolutionary palaeoe-cological analysis. The materials are from 67 sections in South China, comprising more than 1000 specimens and 227 genera, covering 71 publications. Cephalopod shells were reconstructed and measured to calculate total shell length, apical end and volume. Overall, this study explores the patterns of cephalopod morphological evolution and the driving mechanisms underlying these changes. It clarified the significance of cephalopod faunas in the GOBE, and provides important fossil evidence for understanding the GOBE.