MICROBIAL ORIGIN OF SOME OF THE EDIACARAN FOSSILS

Peculiar discoidal structures do not only constitute the major bulk of the late Neoproterozoic Ediacaran fossil assemblage but also predate in the fossil record its first appearance. Being originally assigned to fossilized jellyfishes, the Ediacaran discoidal fossils have subsequently been re-interpreted as moulds of unidentified benthic organisms and holdfasts of frondose objects. However, morphology and preservation of some of the discoidal structures, including Ediacaria and Cyclomedusa, allows their alternative interpretation as being impressions of bacterial and fungal colonies. Apart from similar discoidal shape, such interpretation is also supported by a common array of observed organizational patterns (concentric rings, radial structures, central dome or crater), which in microbial colonies are produced under conditions of nutrient deprivation. Some of the discoidal fossils could be compared with so-called “fairy rings”, the concentric ring-shaped surface structures formed in microbial mats as a reaction to diurnal chemical cycles. The discoidal fossils are preserved as an integral part of impression of a bacterial-algal film and when deformed, they behave as thin membranous sheets. Discoidal structures of microbial origin are expected to occur in older as well as younger sedimentary sequences. Therefore, careful analysis and distinguishment between (1) microbial structures, (2) holdfasts of frondose objects, and (3) other sedentary organisms is necessary.