A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale.

Cnidarians are regarded as one of the earliest-diverging animal phyla. One of the hallmarks of the cnidarian body plan is the evolution of a free-swimming medusa in some medusozoan classes, but the origin of this innovation remains poorly constrained by the fossil record and molecular data. Previously described macrofossils, putatively representing medusa stages of crown-group medusozoans from the Cambrian of Utah and South China, are here reinterpreted as ctenophore-grade organisms. Other putative Ediacaran to Cambrian medusozoan fossils consist mainly of microfossils and tubular forms. Here we describe Burgessomedusa phasmiformis gen. et sp. nov., the oldest unequivocal macroscopic free-swimming medusa in the fossil record. Our study is based on 182 exceptionally preserved body fossils from the middle Cambrian Burgess Shale (Raymond Quarry, British Columbia, Canada). Burgessomedusa possesses a cuboidal umbrella up to 20 cm high and over 90 short, finger-like tentacles. Phylogenetic analysis supports a medusozoan affinity, most likely as a stem group to Cubozoa or Acraspeda (a group including Staurozoa, Cubozoa and Scyphozoa). Burgessomedusa demonstrates an ancient origin for the free-swimming medusa life stage and supports a growing number of studies showing an early evolutionary diversification of Medusozoa, including of the crown group, during the late Precambrian-Cambrian transition.