Multiple origins of dorsal ecdysial sutures in trilobites and their relatives

Abstract

Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites – a hyperdiverse group of tens of thousands of species -moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans mean that it is unclear whether these sutures were evolved deep within Artiopoda, or evolved multiple times within the group. Here we describe new well-preserved material ofAcanthomeridion, a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features betweenAcanthomeridionand trilobites. We used two coding strategies treating structures as homologous or independently derived across multiple phylogenetic analysis techniques (parsimony or Bayesian inference), and showed that regardless of these variables, the sutures crucial for the success and growth of the hyperdiverse trilobites evolved multiple times within Artiopoda.

Distinct causes underlie double-peaked trilobite morphological disparity

“…The variation in functional morphology of the cephalon is intrinsically linked to the varied life modes of trilobites; different cephalic shapes and structures have been suggested to be adaptations to different feeding modes (e.g., Fatka and Szabad, 2011; Fortey and Gutiérrez-Marco, 2022; Fortey and Owens, 1999; Hegna, 2010; Hughes, 2000; Pearson, 2017), life modes (e.g., Bault et al, 2023b; Cherns et al, 2006; Esteve et al, 2021; Fortey, 2014), or specific behaviours (e.g., Drage, 2019; Henningsmoen, 1975; Suárez and Esteve, 2021). However, the evolution and extent of disparity of the trilobite cephalon remains unclear, with uncertainty around the unstable high-level trilobite taxonomy (Adrain, 2013, 2011; Paterson, 2019), the potential homology of cephalic structures (Du et al, 2023; Hughes, 2003; Park and Kihm, 2017), and the adaptation of cephalic shape to hypothetical life mode.…”