AI-Powered Analysis of Global Trilobite Diversity and Morphology During the Late Ordovician Mass Extinction

During the Ordovician-Silurian boundary interval, the Hirnantian Glaciation and the first major biodiversity crisis of the Phanerozoic, the Late Ordovician Mass Extinction (LOME), occurred. As one of the Phanerozoic “Big Five” extinction events, LOME is widely regarded as being closely linked to environmental changes such as anoxia induced by the Hirnantian Glaciation. However, due to the lack of high temporal resolution data in most previous studies, evolutionary patterns of different clades remain unclear. Trilobites, one of the most diverse clades during the Paleozoic, suffered catastrophic losses during this event, never recovering to the same level of dominance in the marine ecosystem and ultimately disappearing during the end-Permian Mass Extinction. Although previous studies (stage- or biozone-level, generally ~1-3 Myr) based on individual or limited number of stratigraphic sections suggested marine anoxia as the driver of benthic extinctions or the main cause of biodiversity decline, the vast morphological and occurrence data of trilobites have not been fully utilized to depict the morphological evolution of marine life due to technical constraints, hindering our understanding of the evolutionary history of life during this critical interval.

Here we compiled global trilobite fossil records and morphological descriptions spanning LOME from literature. Using the newly developed quantitative stratigraphic method, HORSE, we analyzed tens of thousands of fossil records to generate a global high-resolution trilobite diversity curve (~25 kyr as imputed temporal resolution) which has never been achieved before. The manual, labor-intensive annotation hindered the development of image-based large-scale annotated fossil datasets, thereby limiting large-scale morphological data analysis. However, high-dimensional embeddings extracted from morphological descriptions with large language models (LLMs) quantified global trilobite morphological similarities and allowed the generation of a high-resolution morphological disparity curve. Comparison between these two curves revealed that, while severe biodiversity losses are a defining feature of mass extinction events, its impacts on morphological disparity are more complicated. Although greater morphological disparity typically indicates higher ecological or functional diversity, the coupled diversity and disparity dynamics during the glaciation could be explained by either the intensity of extinctions or strong internal constraints. This study aims to reveal in significant detail the connections between marine biodiversity changes and morphological evolution during the Hirnantian Glaciation and LOME, as well as the relationships between these biotic changes and abiotic factors, thereby enhancing our understanding of the patterns and underlying mechanisms of the Late Ordovician Mass Extinction.