Stranded jellyfish in the lowermost Cambrian (Corduban) of Spain

Ninety discoid structures of big size occurring on a bedding plane of Nemakit-Daldynian to Tommotian sandstones (i.e. Corduban in the Spanish scale of Cambrian stages) from south-western Spain are described. Cross-cutting relationships between discoid structures and associated trace fossils, as well as evidence for penecontemporaneous deformation of sediment laminae below the discoids, permit to interprete these structures as impressions of ancient, soft-bodied marine organisms. Taphonomic, biometric, and morphological studies suggest that they are outer moulds of both sides, subumbrellar and exumbrellar, of ancient jellyfish of hydrozoan coelenterates, whose canals resemble the modern genus Aequorea. With diameters up to 88 cm, Cordubia was a giant by hydromedusae standards. Palaeoichnological, lithological and sedimentological features suggest an extremely shallow, marine environment, where these organisms stranded during a single event. Cordubia gigantea gen. et sp. nov. is the oldest record of unquestionable jellyfish. The palaeontological site described here is outstanding, also because its anomalous record of hydrozoans in coarse siliciclastic rocks and the high number of specimens on a single top bedding plane of ca. 120 m. Since soft-bodied biotas of NemakitDaldynian to Tommotian ages are very scarce, this site may provide a better understanding of the important taphonomic and biological events across the Neoproterozoic/Phanerozoic transition, particularly the evolution of hydrozoans.