Discoidal fossils of the Ediacaran biota: a review of current understanding B. A. MACGABHANN Department of Earth and Ocean Sciences, National University of Ireland Galway, University Road, Galway, Ireland (e-mail: b.macgabhann1@nuigalway.ie) Abstract: 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 organ- isms, 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 under- lying complex Mistaken Point assemblages, however, likely mostly represent holdfasts. Other complex assemblages, such as those of South Australia and the White Sea of Russia, unequivo- cally contain more than one biological construction responsible for the discoidal structures, but holdfasts likely represent a significant proportion. The disc-dominated Fermeuse assemblages and the nearby rangeomorph-dominated Mistaken Point assemblages are unlikely to merely rep- resent different taphonomic windows on identical communities, as previously suggested, but rather reflect environmental control on both biotic composition and taphonomy. Discoidal fossils were the first described elements of the Ediacaran biota (Billings 1872) and are by far the most abundant component. In terms of age, they are the oldest (Hofmann et al. 1990) and, indeed, the youngest (Crimes & McIlroy 1999; Hagadorn et al. 2000) Ediacaran-type fossils known. For this reason they represent, potentially, the most important constituent of the biota; however, knowledge of both disc palaeoecology and affinities does not reflect this. In particular, current understanding of disc taxonomy is rather convoluted. Indeed, one of the legacies of initial descriptions of Ediacaran discs as medusae has been a plethora of generic and specific names in the literature. Many of these were described or are known only from single (often unrepresentative, damaged or incomplete) specimens, with genera and species often distinguished by only minor differences, which may, in fact, be taphonomic arte- facts or morphologically insignificant from a taxo- nomic perspective. It did not help that most original descriptions did not separate interpretation from observation, leaving them rather bloated with inappropriate and wildly incorrect interpretative terminology. The situation was perhaps best sum- marised by Runnegar and Fedonkin (1992), who noted, with reference to the genus Ediacaria, that ‘Many of the fossils that have been referred to E. flindersi do not resemble the holotype, and speci- mens identified as E. flindersi by different authors may be totally dissimilar. This is a common situ- ation in the literature and the result is a jumble of useless and misapplied names’ (Runnegar & Fedonkin 1992, p. 379). In light of this, this contribution sets out to sum- marize our current knowledge of the Ediacaran discs, highlighting several major issues and incon- sistencies, and outlining potential directions for future research. History of research Serious study of Ediacaran discoidal fossils began after Sprigg (Sprigg 1947, 1948, 1949) described the first-found elements in the type area in South Australia as ‘jellyfish’, and Glaessner and Wade (Glaessner 1959, 1984; Glaessner & Wade 1966; Wade 1969, 1972) more or less followed this interpretation, considering them stranded pelagic medusae. Seilacher disagreed, describing the discs as a ‘heterogeneous group of trace fossils and the remains of unidentified benthic organisms’ (Seilacher 1984, p. 163), but description of discs as ‘medusae’ continued (e.g. Sun 1986a, b). In the meantime, others had suggested that at least some discs actu- ally represented the basal parts of frond-like taxa (e.g. Charniodiscus concentricus Ford, 1958), and this has been borne out by later work (e.g. Jenkins & Gehling 1978; Laflamme et al. 2004). From:VICKERS-RICH, P. & KOMAROWER, P. (eds) The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications, 286, 297–313. DOI: 10.1144/SP286.21 0305-8719/07/$15.00 # The Geological Society of London 2007.