Living medusozoans and theirMiddle Cambrian ancestors are characterized fundamentally by four-fold symmetry.<br />The symmetric pattern of their earlier antecedents during the Ediacaran–Cambrian transition, traditionally<br />expected to be tetramerous, needs to be reconsidered in the light of Cambrian pentamerous fossil medusozoans.<br />Here we present a microanatomic analysis of three tiny tetramerous specimens from the Lower Cambrian<br />Kuanchuanpu Formation in southern China; they display diagnostic characteristics of Cubomedusae, including<br />paired but well-separated interradial tentacles deep in the subumbrellar cavity, interradial septa connected by<br />claustra, coronalmuscles at the subumbrella, and an annular velariumsuspended by twelve frenula. These fossils<br />likely represent three new taxa of a monophyletic stem group of cubomedusans, which is further confirmed by<br />our phylogenetic analysis based on the data matrix with 25 taxa and 107 characteristics. Among these fossil<br />cubozoans and co-occurring pentamerous forms, the specimen ELISN31-31 is mostly close to the crown-group<br />Cubomedusaewith regard to its narrow vascular cavity and the endodermic perradial fusion. Diverse symmetrical<br />patterns among different lineages of early Cambrian medusozoan candidates might have been independently<br />evolved in different classes of medusozoans.

Early Cambrian; Kuanchuanpu Formation; Tetra-radial symmetry; Cubozoan; Embryos

Northwest University; Kyoto University; Nanjing Institute of Geology and paleontology, CAS; China University of Geosciences (Beijing); Chang'an University; Tohoku University; The University of Tokyo