Cerebral Organization in the trilobite Yunnanocephalus yunnanensis

The clade Trilobita is recognized for its extraordinary morphological diversifications amidst a highly constrained body plan. This raises the question of whether trilobite nervous systems were conservative or comparably diverse. However, despite the identification of thousands of trilobite species and increasing evidence for neural preservation in other Paleozoic euarthropods, the recognition of a fossilized trilobite brain has remained elusive. Here we describe the identification of a trilobite brain and its spatially constrained position within its cranidium. An unusually preserved specimen of the lower Cambrian trilobite Yunnanocephalus yunnanensis reveals a bilaterally organized organ connected to its lateral eyes. Comparisons to the brain of a trilobitomorph xandarellid support the interpretation of this organ as a modest cerebrum comprising prosocerebral, protocerebral, and deutocerebral domains, an organization typifying the euarthropod cerebral ground pattern. These findings suggest that the cerebrum of Yunnanocephalus could have supported the multimodal sensory processing and behavioral repertoires of trilobites, while retaining the deep evolutionary conservation of brain organization across euarthropod lineages