Trilobite cranidial and pygidial disparity patterns: two sides of the story

Estimates of morphological disparity provide a powerful tool for assessing macroevolutionary and macroecological dynamics in palaeobiology. Trilobites, an iconic group of Palaeozoic marine arthropods, serve as a model group for investigating aspects of early animal evolution. Most studies focus on cephalon or cranidial morphology as it reflects key ecological traits. However, recent studies indicate discrepancies in disparity patterns across different anatomical structures. Here, we explore the interplay between the cranidium and pygidium of trilobites to further understand how these structures contribute to macroevolutionary interpretations. Our results show discordant patterns of disparity for these structures, suggesting differential constraints. Further, a Procrustes superimposition of both morphospaces indicates that main differences are caused by taxa with extreme morphologies for both structures. In general, pygidial morphology does not accurately reflect ecological diversity; rather, the morphological disparity of pygidia and taxonomic diversity behave similarly, even at key moments of community restructuring where cranidial disparity clearly decouples from diversity. In contrast, cranidial morphospace clearly reflects ecological roles or strategies in these trilobite communities, providing a promising approach to a more complete perspective on biodiversity and ecosystem functioning.