Abstract

Cnidarians are phylogenetically located at nearly the base of the “Tree of Animals”, whose early evolution has a profound impact on the rise of bilaterians. However, the early diversity and phylogeny of this “lowly” metazoan clade has been hitherto enigmatic. Fortunately, cnidarian fossils from the early Cambrian could provide key insights into their evolutionary history. Here, based on a scrutiny of the purported hyolith Burithes yunnanensis Hou et al. from the early Cambrian Chengjiang biota in South China, we reveal that this species shows characters distinct from those typical of hyoliths, not least a funnel-shaped gastrovascular system with a single opening, a whorl of tentacles surrounding the mouth, and the lack of operculum. These characters suggest a great deviation from the original definition of the genus Burithes, but rather a close affinity with cnidarians, here reassigned to Palaeoconotuba gen. nov. Bayesian inference of phylogeny based on new anatomical traits identifies a new clade, including Palaeoconotuba and Cambrorhytium, as a stem group of sessile medusozoan cnidarians that are united by the synapomorphies of developing an organic conical theca and a funnel-like gastrovascular system. This study unveils a stem lineage of medusozoans once evolving a lifelong conical theca from the early Cambrian.