Precambrian–Cambrian Fossil Record of Polypoid Scyphozoans  

Recent molecular studies indicate that the phylum Cnidaria originated during the Proterozoic Eon.  However, the fossil record of Precambrian cnidarians is problematical.  Various Ediacaran fossils that were originally interpreted as cnidarian medusae or polyps have been removed from this group, and the cnidarian affinity of various polyp-like fossils from the lower–middle Cambrian has also been called into question. 

The scyphozoan cnidarians are characterized today by a prominent medusoid stage and a relatively inconspicuous polyp. Ediacaran and Cambrian scyphozoan have large polyps, which  in some cases are encased in a phosphatic periderm.  The Ediacaran Vendoconularia triradiata, known from a single Ediacaran specimen, is most similar to conulariid scyphozoans (Late Cambrian to Late Triassic) – although unlike these fossils it may have lacked a mineralized periderm. Certain more-or-less tubular genera from the Lower to Middle Cambrian have also been interpreted as scyphopolyps.  Hyolithellus and Byronia are most similar to the peridermal tubule of extant coronates, and were probably coronate polyps themselves.  Cambrorhytium and Cambrovitus were probably polypoid scyphozoans or hydrozoans.  Sphenothallus, whose range continued to the Permian, was either a scyphozoan or hydrozoan polyp.  It occurs both in shallow shelf carbonates and in dark shales deposited in deep slope settings.  The Lower Cambrian small shelly fossils (SSFs) Arthochites, Carinachites, Emeiconularia, and Hexaconularia have been interpreted as conulariid scyphozoans.  Together these four genera exhibit certain gross morphological characteristics that appear to be shared only with Late Cambrian and younger conulariids.  However, the laminar microstructure and internal skeletal structures (septae, carinae, and transverse walls) of these SSFs is not conulariid-like. Further, Hexaconularia exhibits a distinct apical region, probably an embryonic shell, which is absent in conulariids.  The apparent absence of conulariids or conulariid-like fossils in middle Cambrian strata presents another problem for the hypothesis of a conulariid/scyphozoan affinity for conulariid-like SSFs. 

Like modern scyphozoan polyps, Byronia, Cambrorhytium, Cambrovitus, and Sphenothallus were sessile organisms that were attached by an apical holdfast to biological substrates such as brachiopod shells or other tubular fossils.  With the possible exception of Sphenothallus, all of these taxa appear to have been solitary.  None of the conulariid-like SSFs exhibit a basal holdfast or other intrinsic evidence (e.g. close association with possible substrates) of a sessile mode of life.