Re-evaluating the palaeobiology and affinities of the Ctenocystoidea (Echinodermata)

Ctenocystoids are a poorly understood group of fossil deuterostomes, with little known about their morphology, palaeobiology or affinities. A three-dimensional specimen of the ctenocystoid Ctenocystis utahensis from the Middle Cambrian Spence Shale (western USA) was imaged using X-ray microtomography and digitally reconstructed. This allowed the species to be described in greater detail than previously possible, providing a number of morphofunctional insights. Two openings are identified: (i) an anterior opening controlled by moveable plates in the distinctive ctenoid apparatus and (ii) a wide posterior periproct. A putative third body opening (hydropore and/or gonopore) is not present, contrary to previous reconstructions. Ctenocystis utahensis was most probably epibenthic, facing into water currents during life. Suspended food particles entered paired lateral grooves within the ctenoid apparatus through gaps between neighbouring ctenoid plates; when the ctenoid apparatus was open, material could pass from these lateral grooves to an internal chamber, which led into the main body cavity. Our reconstruction demonstrates that the plates of the ctenoid apparatus and the lateral feeding grooves are not homologous with paired brachioles, refuting suggestions that ctenocystoids are derived blastozoans. Instead, it is probable that ctenocystoids are basal stem-group echinoderms, sharing several plesiomorphies (a bilateral body plan, pharyngeal gill slits and a paired water vascular system) with the latest common ancestor of echinoderms plus hemichordates. Reconstructing the anatomy of ctenocystoids in three-dimensions is critical to a full understanding of their palaeobiology and phylogenetic position.