Rethinking artiopod evolution: insights from the anatomy of <i>Acanthomeridion</i> from the early Cambrian of China

Abstract Recent Euarthropoda phylogenies incorporating Cambrian taxa provide a robust framework for testing evolutionary scenarios. Despite considerable progress, the interrelationships of Artiopoda continue to be ambiguous. Here we reassess Acanthomeridion serratum from the Chengjiang Lagerstätte based on new material and updated phylogenetic analyses. Acanthomeridion is an unusual artiopodan characterized by lateral stalked eyes, a trilobite-like dorsal suture system and a postero-ventral plate resembling the postventral plate of aglaspidids, covering the anal region. Updated phylogenetic analyses consistently place Acanthomeridion within Vicissicaudata, closely related to the iconic Sidneyia from the Burgess Shale. Morphological evidence, including stalked lateral eyes, optic notch, gnathobasic cephalic appendages and a postero-ventral plate, also supports this relationship. Importantly, we refine the terminology of appendicular derivatives and propose two homologous categories derived from different trunk appendages: pre-terminal appendages and ventral sclerotized plates. In parallel, we identify potential evolutionary trends within Artiopoda, including the migration of eyes from lateral to dorsal positions and from mobile to sessile forms, as well as the emergence of complex suture systems. Collectively, these findings highlight Acanthomeridion as a key taxon for understanding the evolution of artiopodan vision, ecdysial systems and appendicular derivatives, and provide new evidence for resolving the internal phylogeny of Artiopoda.