NEUROANATOMY IN A MIDDLE CAMBRIAN MOLLISONIID AND THE ANCESTRAL NERVOUS SYSTEM ORGANIZATION OF CHELICERATES (Invited Presentation)

Recent years have witnessed a steady increase in reports of fossilized nervous tissues among Cambrian total-group euarthropods, which allow reconstructing the early evolutionary history of these animals. Here, we describe the central nervous system of the stem-group chelicerate Mollisonia symmetrica from the mid-Cambrian Burgess Shale. The fossilized neurological anatomy of M. symmetrica includes optic nerves connected to a pair of lateral eyes, a putative condensed cephalic synganglion, and a metameric ventral nerve cord. Each trunk tergite is associated with a condensed ganglion bearing lateral segmental nerves, and linked by longitudinal connectives. The nervous system is preserved as reflective carbonaceous films underneath the phosphatized digestive tract. M. symmetrica demonstrates that the neuroanatomical organization observed in extant Euarthropoda dates back to the mid-Cambrian. Our results suggest that M. symmetrica illustrates the ancestral organization of stem-group Chelicerata before to the evolution of the derived neuroanatomical characters observed in Cambrian megacheirans and extant representatives.