A pancrustacean brain and its visual systems define a Cambrian great appendage arthropod.

Early Cambrian fossils from the Chengjiang biota demonstrate that over half a billion years ago early stem euarthropods existed coevally with representatives of already recognizable crown groups. Prominent stem taxa were Fuxianhuia protensa and Alalcomenaeus whose cerebral and ganglionic traits identify them as, respectively, stem mandibulates and stem chelicerates. Here we report features of the visual systems and brain of the enigmatic lower Cambrian stem euarthropod Jianfengia multisegmentalis, which despite the absence of trunk tagmatization reveals neural traits that justify its inclusion within stem Pancrustacea also known as Tetraconata. The sutured eyestalks of Jianfengia, typifying crown Malacostraca, terminate as compound eyes populated by ommatidia that reveal tetradic structures indicative of cone-building Semper cells. Preserved neuropils and tracts in the eyestalks of Jianfengia resolve nested optic neuropils as occur in eucrustaceans. The nauplius-like eyes of Jianfengia and their associated nerves supply a discrete forebrain region, as they do in pancrustaceans. These attributes distinguish Pancrustacea from stem Chelicerata and extant Myriapoda as does the organization of the jianfengiid deutocerebrum and tritocerebrum. In the absence of external traits that differentiate identities amongst stem euarthropods, neuroanatomical traits provide a powerful tool for discerning characteristic distinctions and correspondences for elucidating euarthropod relationships. Here, we demonstrate that cerebral arrangements in Jianfengia correspond to those of pancrustaceans existing today.