Discoidal fossils of the Ediacaran biota: A review of current understanding

Discoidal fossils, despite being the oldest, youngest, and most common elements of the Ediacaran biota, have not received their fair share of attention. Taxonomy of discoidal fossils is currently dubious, and some forms have not been properly re-examined since the initial incorrect descriptions as medusae. Attachment discs of benthic stalked forms, which adhere to microbial mats at the sediment-water interface, are unequivocally present without stalks or other upper parts in most discoidal Ediacaran assemblages. However, many discoidal assemblages are likely to have represented a heterogeneous mixture of benthic discoidal organisms, including bacterial colonies, fungi, actinian-grade cnidarians, and perhaps poriferans. Such organisms probably account for the vast majority of fossils in the Fermeuse Formation of Newfoundland and similar assemblages from Norway, England, and Wales. Discs in the underlying complex Mistaken Point assemblages, however, likely mostly represent holdfasts. Other complex assemblages, such as those of South Australia and the White Sea of Russia, unequivocally contain more than one biological construction responsible for the discoidal structures, but holdfasts likely represent a significant proportion. The disc-dominated Ferneuse assemblages and the nearby rangeomorph-dominated Mistaken Point assemblages are unlikely to merely represent different taphonomic windows on identical communities, as previously suggested, but rather reflect environmental control on both biotic composition and taphonomy.