Cambrian Artiopoda Reveals a Constraint in Euarthropod Brain Evolution

Fossilized traces of neuropils, nerves and ganglia have demonstrated that cerebral organization in Cambrian arthropods conforms to a ground pattern defining one of today's two existing euarthropod clades, Mandibulata and Chelicerata^1-8. Artiopoda - a third clade including trilobites and soft-bodied relatives - persisted until the late Carboniferous^9,10, but its cerebral organization has remained unknown. Here we identify and reconstruct fossilized neural traces of the artiopodan Xandarella spectaculum ¹⁰, which reveal an expanded prosocerebrum associated with paired ocelli, a truncated protocerebrum supplied by substantial lateral eyes, and salient deutocerebral antennular lobes. This arrangement predicts reliance on chemosensory-guided foraging, with visual processing largely limited to dorsal orientational cues and simple local motion signals. The artiopodan brain thus reveals clade-specific modifications of homologous domains of the euarthropod cerebral ground pattern^4,6-8 established in the early Cambrian.