Morphological disparity in Cambrian–Ordovician trilobite cephala and pygidia suggests functional canalization is taxonomically controlled

Abstract

The evolutionary trajectories of morphological and ecological diversity in trilobites from the Cambrian to the Ordovician reveal a dynamic interplay of extinction, diversification and functional constraints. By integrating analyses of cephalic and pygidial morphological disparity, this study uncovers a taxonomically controlled functional canalization that shaped trilobite evolution. During Cambrian Series 2, cephalic disparity reflected niche partitioning, while pygidial disparity suggested diverse niche occupation, supported by a low ecological disparity and minimal interspecies competition. The Redlichiid-Olenellid Extinction Carbon Isotope Excursion (ROECE) gave rise to environmental stresses that constrained morphospace occupation while promoting ecological diversification. Subsequent transitions from the Drumian to the Ordovician displayed increasing ecological diversity, with cephala adapting to broader ecological niches and pygidia demonstrating mechanical specialization through morphological canalization. These trends culminated in the Great Ordovician Biodiversification Event (GOBE), characterized by peak taxonomic richness and pronounced ecological disparity. Our analyses suggest that disparity patterns differed when examined at different taxonomic levels: class level did not match those observed at the order level. This reflects lineage-specific constraints on morphospace occupation. This study pioneers an integrative approach to morphological, ecological and taxonomic diversity, revealing how functional specialization, taxonomic constraints and environmental perturbations collectively shaped trilobite disparity over 77 million years.