Article: Ediacaran-like fossils in Cambrian Burgess Shale-type faunas of North America
Simon Conway Morris
A number of fossils from the Stephen Formation (Burgess Shale, Middle Cambrian, British Columbia) and Parker Slate (Lower Cambrian, Vermont) resemble Ediacaran taxa. Thaumaptilon walcotti gen. et sp. nov., known from three specimens, consists of a broad frond bearing a central rachis from which arise branches bearing possible zooids. The holdfast is relatively elongate and slightly swollen. T. walcotti approaches closely a number of Ediacaran frond-like fossils, especially Charniodiscus, Vaizitsinia and Khatyspytia. All these taxa appear to be pennatulacean anthozoans (Cnidaria). Mackenzia costalis Walcott, known from about seventy specimens, is an elongate bag-like organism. The exterior was probably thrown into longitudinal folds, and the interior may have borne septa. Attachment to the sea-bed is indicated by holdfasts of eocrinoid stems and brachiopod/sponge spicule aggregations. M. costalis may be compared to several Ediacaran taxa including Inaria, Protechiurus and possibly Platypholinia. The affinities of M. costalis are uncertain, but a place within the actinarian anthozoans seems possible and this animal probably had a cnidarian-grade of organization. Emmonsaspis cambrensis (Walcott), long interpreted as a possible chordate, is shown to be a frond-like fossil with angled branches arising from the mid-line. No trace of a holdfast exists in any of the three specimens. E. cambrensis resembles a number of Ediacaran frond-like fossils, but similarities to taxa such as Pteridinium may be superficial. The two remaining taxa are known from only single specimens. Gelenoptron tantaculatum gen. et sp. nov., originally described by Walcott as Redoubtia polypodia (pars), is tentatively interpreted as a chondrophorine, with evidence for a float and tentacular margin. A unique specimen consisting of a disc with annuli and tentacles is regarded as another type of chondrophorine. It occurs in association with M. costalis. The description of these animals as hold-overs from the Ediacaran assemblages casts some doubt on the general validity of Seilacher's concept of Ediacaran taxa representing a distinctive body-plan, known as the Vendobionta, separate from the metazoans.