NEW CAMBRIAN FOSSILS FROM UTAH ILLUMINATE THE EARLY EVOLUTION OF NERVOUS AND SENSORY SYSTEMS IN CTENOPHORES

Ctenophores, or comb jellies, are a group of predatory, mainly holopelagic macroinvertebrates, whose controversial phylogenetic position has prompted several competing evolutionary scenarios regarding the origin of animal body plans. The recently proposed placement of ctenophores as a sister-group of all other animals in particular has led to the hypothesis that ctenophores evolved their nervous and muscular systems convergently with other eumetazoans. Molecular clock estimates predict that the living diversity of ctenophores has a relatively late origin, probably in the late Paleozoic, but their gelatinous bodies typically exclude them from the conventional fossil record. Here, we describe two new ctenophores from the Cambrian Marjum Formation of Utah, which illuminate the early evolution of nervous and sensory features in the phylum. Thalassostaphylos elegans gen. et sp. nov. is reconstructed as having 16 comb rows, a well-developed oral skirt, and an apical organ with ciliated polar fields. Ctenorhabdotus campanelliformis sp. nov. features ca. 24 comb rows, a well-developed oral skirt, and a prominent apical organ enclosed in by a skeletonized capsule. C. campanelliformis preserves neurological tissues as carbonaceous films concentrated around the apical organ and ciliated furrows, which connect to a transverse circumoral nerve ring via longitudinal giant axons underlying the comb rows. The presence of interconnected aboral and oral nerve centers deviates from the neurological organization of living ctenophores, but resembles certain cnidarians. Our findings demonstrate substantial complexity in the nervous system of Cambrian ctenophores, and refine our understanding of the stepwise evolution of the body plan of the ctenophore crown group.