Brain anatomy of the Cambrian fossil Jianfengia multisegmentalis informs euarthropod phylogeny.

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 on the visual systems and brain of the enigmatic lower Cambrian euarthropod Jianfengia multisegmentalis, which reveals neural traits suggestive of Pancrustacea despite its possession of 'great appendages'. As occur in pancrustaceans, three nested optic neuropils are resolved in the eyestalks of Jianfengia, together with rostral ocelli and their associated nerves supplying a discrete forebrain region. Sutured eyestalks typifying crown Malacostraca provide compound eyes populated by ommatidia revealing structures suggesting cone-building cells. These and other neuroanatomical traits provide a powerful tool for resolving euarthropod relationships. Phylogenetic analyses deploying neural traits of Jianfengia, other Cambrian taxa, and extant Euarthropoda elucidate the status of Jianfengia as sister to total Mandibulata and reveal the short-bodied 'great appendage' Leanchoiliidae as sister to total Chelicerata. Together these data provide independent evidence for a 23 year-old proposition that 'great appendage' morphology defines the early stem from which derived the two branches of the euarthropod tree of life.