Divergent evolution of medusozoan symmetric patterns: Evidence from the microanatomy of Cambrian tetramerous cubozoans from South China

Abstract

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