A reappraisal of the early Cambrian artiopod <i>Acanthomeridion serratum</i> reveals cephalic appendages specialized for durophagy

The Artiopoda, exemplified by trilobites, represents a diverse group of euarthropods that thrived during the early Paleozoic. Acanthomeridion serratum , an early diverging artiopodan with a detailed exoskeleton record in the lower Cambrian (Series 2, Stage 3) Chengjiang Lagerstätte, presents challenges in understanding its soft tissues, particularly its post-antennal cephalic appendages. In this study, new micro-computed tomography (micro-CT) analysis reveals that these appendages feature a transversely elongated protopod with teeth-like gnathobasic spines, a delicate endopod without a prominent walking claw and a multisegmented exopod with widely spaced lamellae. Smaller sub-gnathobasic spines alternate with the larger teeth-like gnathobasic spines. The robust protopod, characterized by medially opposed gnathal edges, suggests a feeding function, indicative of a durophagous lifestyle. Phylogenetic analyses place A. serratum as a sister taxon to Zhiwenia , occupying a basal position in the Artiopoda. These findings provide direct evidence of durophagous predation in the Cambrian, likely driving the evolution of biomineralized shells or hard exoskeletons in various metazoan groups.