Marine Origin of the Arachnid Brain Reveals Early Divergence of Chelicerata

Fossils from the lower Cambrian provide crucial insights into the origin of arthropod lineages: Mandibulata, exemplified by centipedes, insects, and crustaceans; and Chelicerata, whose descendants include sea spiders and horseshoe crabs and terrestrial spiders, scorpions, and ticks. One fossil genus claimed as an early Cambrian chelicerate is Mollisonia , defined by its clustered limbs beneath a rostral carapace, segmented trunk and pygidium, approximating a prosoma-opisthosoma-like organization. Here, we demonstrate singularly preserved trace neuronal tissues that identify a cerebral organization of M. symmetrica uniquely distinct from brains attributed to upper stem euarthopods. The fossilized neural traces reveal a copious asegmental cerebrum lying above contiguous ganglia associated with their clustered appendages. This arrangement exclusively characterizes the central nervous systems of extant arachnids. Comparison with the brains of megacheiran chelicerates illuminates the marine origin of Arachnida at the base of the chelicerate tree of life leading to today’s most versatile arthropod predators.