The earliest pelagic jellyfish with rhopalia from Cambrian Chengjiang Lagerstätte

• Earliest-known crown-group medusa from the Cambrian Stage 3.
• First report of Chengjiang medusa with rhopalia.
• The medusae have been a predatory member of the pelagic food web at the Cambrian Stage 3.

Modern cnidarian medusae generally show a triphasic life cycle with the succession of a larva, a sessile polyp and a pelagic medusa stage. The debate around the metagenesis of sessile polyps into pelagic medusae has lasted for more than 100 years. When pelagic forms originated is not clear. Hitherto, the earliest crown-group medusae have been found at Cambrian Stage 5 (traditional Middle Cambrian, 509 Ma) in Utah, while diverse stem-group medusozoans were found in the basal Cambrian Fortunian Stage. No reliable medusae have been found from Cambrian Series 2 Stage 3 (ca. 521 Ma), although the marine benthic community teemed with many phyla of bilaterians, sponges and ctenophores. Here, we reinterpret Yunnanoascus haikouensis Hu et al. (2007), originally described as a ctenophore, as a pelagic, predatory, crown-group medusozoan, based on the presence of rhopalia, possible radial canals and marginal tentacles. The medusae were a predatory member of the pelagic food web at the middle level of the ocean at Cambrian Stage 3.

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