A macroscopic jellyfish from the early Cambrian of south China

Medusozoan cnidarians possess a free-swimming medusa (jellyfish) life phase that is developed from a benthic polyp phase. While medusozoan polyps have a well-known fossil record since the late Ediacaran, fossil medusae are surprisingly sparse, with no convincing macroscopic forms until the middle Cambrian. The morphology and palaeoecology of early medusae thus remain largely unknown due to this cryptic evolutionary history. Here, we describe Mycozoon insolitum gen. et sp. nov. from the early Cambrian of south China. Mycozoon shares several morphological aspects with medusae, including a bell-shaped umbrella with a manubrium and a well-preserved gastrovascular system. Our phylogenetic analyses recover Mycozoon as the oldest known macroscopic jellyfish, with a close affinity to hydrozoans. The presence of more than four unbranched radial canals, alongside a large manubrium that exceeds the height of the subumbrella, provides crucial evidence for the evolution of feeding anatomy in early medusae. Our findings suggest that early medusozoans, together with other diploblasts, had reached a relatively high taxonomic diversity by the early Cambrian. A new fossil cnidarian, Mycozoon insolitum, from the early Cambrian Chengjiang biota, south China, is interpreted as a macroscopic jellyfish bearing a tough umbrella and a well-defined digestive system.