The oldest diverse jellyfish fauna reinterpreted as sessile polypoid dinomischids (stem-group Ctenophora)

The Drumian (<i>c.</i> 500 myr) Marjum biota of Utah is the most diverse (<i>c.</i> 150 species) exceptionally preserved Cambrian fossil assemblage in the United States. Compared to other Burgess Shale-type biotas in the country, it is also notable for the prominence of pelagic organisms, including panarthropods, cambroernids, ctenophores and vertebrates. The Marjum strata have yielded the oldest diverse jellyfish assemblage (Cnidaria Medusozoa), with representatives of the classes Cubozoa, Hydrozoa and Scyphozoa. Though previously known from 10 specimens only, this remarkable fauna suggested early morphological diversification of medusozoans by the middle Cambrian. Prompted by the discovery of over 50 new specimens, our restudy of these putative early medusozoans reveals that their bodies were protected by a multi-element sclerotized integument and composed of three regions: (1) a variably shaped basal stalk; (2) a calyx forming the main body cavity and housing most internal organs; and (3) a terminal filter feeding apparatus of partially skeletonized tentacles (likely 18) surrounding a central cavity. Each tentacle bears two rows of transverse dark stains, from which possible remains of pinnules occasionally project. These features are diagnostic of dinomischids, an enigmatic clade of epibenthic, sessile organisms controversially linked to the origin of ctenophores. We describe three new dinomischid taxa – <i>Calathites macrocalyx</i> sp. nov., <i>Theiokylixia cartwrightae</i> gen. et sp. nov., and <i>Dinomischus nudus</i> sp. nov. – alongside a single specimen of a second, indeterminate <i>Dinomischus</i> species. Although likely transported before burial, the Marjum fossils frequently preserve, sometimes exquisitely, internal anatomical details. Their reinterpretation reduces the known diversity of Cambrian medusae but significantly expands the temporal and geographical range of dinomischids to the Drumian of NW Laurentia, while refining our understanding of their morphological and taphonomic variation. https://zoobank.org/urn:lsid:zoobank.org:pub:CB9FB334-F901-4665-9476-BC733EEFD357