Phylogenetic affinities and evolution of the Early Cambrian hexangulaconulariids

The abrupt appearance of abundant small shelly fossils (SSFs) near the base of the Cambrian System signals a key event in the evolutionary history of the Kingdom Metazoa. Among the probable cnidarian taxa, the SSF family Hexangulaconulariidae (Terreneuvian Series) is characterized by a fan-shaped, biradially symmetrical periderm having distinct apical and abapical portions, the latter of which bears multiple faces. Documented herein are three new hexangulaconulariid specimens from Cambrian Stage 2 on the Yangtze Platform (South China). The new specimens exhibit nine or 11 faces on each of the two sides of the abapical portion. Results of a Bayesian analysis of the phylogenetic relationships among hexangulaconulariids, carinachitids, conulariids, olivooids, and extant cnidarian genera indicate that hexangulaconulariids constitute a well-defined clade within a polytomic, stem-group Medusozoa that also includes carinachitids, conulariids and olivooids. The evolution of hexangulaconulariids featured multiple instances of increase in the total number of faces, modification of the morphology of the interfacial boundaries, and increase in the total size of the periderm. Increase in the number of faces appears to have been achieved in part by the transformation of a single ancestral face into three faces, through the development of a longitudinal furrow or ridge along each of the two adapically converging loci of inflection points (angular bends) of the prominent transverse ribs of the ancestral face.