Hierarchical importance of factors impacting trilobite cephalic disparity

1Trilobite cephala show high disparity owing to their supporting of key functional ecological structures, such as sensory and feeding structures, and those enabling important behaviours like enrollment and exoskeleton moulting. Previous studies analysed associations between cephalic morphometry, trends through time and with diversity, and these behaviours. However, no studies have yet quantitatively compared the importance of these different varied factors on cephalic morphometry. Here, we explore the association between trilobite cephalic morphometry and taxonomic variables, moulting behaviour, and facial suture type in Principal Components Analysis morphospace, and assess whether these variables can predict cephalic morphometry using Linear Discriminant Analyses. We employed Linear Mixed-Effects Models (LMMs) to quantify the hierarchical importance of these variables for determining cephalic disparity. The dataset used comprises cephalon outlines for 762 species of trilobites from across the globe and through the entire Palaeozoic. We found that variation in cephalic morphometry is not clearly associated with moulting behaviour, nor is moulting behaviour linked to facial suture type. Trilobite moulting behaviour is evidently too phenotypically plastic, and/or preservationally impacted, to produce clear signals across broad datasets. Results demonstrate the utility of LMMs for palaeontological datasets; we found that family-level assignment overall well explains cephalic morphometry, more so than order-level assignment which shows significant overlap in morphospace. Geological Period, facial suture type, and moulting behaviour had little impact on cephalic morphometry. We thereby demonstrate that trilobite cephalic outline morphometry is generally in accordance with trilobite family-level systematics and highlight the use of LMMs for interrogating broad-scale fossil morphological data.