Cnidarian taphonomy and affinities of the Ediacara biota

Plaster impressions and sand casts of extant medusae, a chondrophoran, and a pennatulid share basic structural characteristics with fossils in the Upper Proterozoic Ediacara assemblage. Impressions of extant medusae and Proterozoic circular impressions show general similarities in arrangement and position of radial and concentric structures and a central raised boss. However, annular rings and radial grooves are more numerous in the Proterozoic fossils and strongly folded or deformed fossils are rare as compared with impressions of modem medusae. Recent pennatulids yield impressions that are more deformed and irregular than the Proterozoic genus Charniodiscus. The greater frequency of deformation of most simulated fossil medusoids relative to Precambrian circular impressions implies that Proterozoic medu-soids were substantially stiffer than many modern taxa of comparable sizes. Many fossils with abundant circular rings have no constructional counterparts among the extant forms studied here and their medusoid affinities should remain in doubt. The structural simplicity of impressions of Ediacara organisms and extant cnidarians suggests that their mutual similarities may be due to convergence. However, there is no compelling morphological reason to reject the claim that some Proterozoic fossils may share affinities with living cnidarians. □ Taphonomy. Ediacara biota, cnidarians, phylogenetic relationships.