Evidence of organic structures in Ediacara-type fossils and associated microbial mats

Abstract

Ediacara-type fossils represent a group of soft-bodied organisms, mainly known from imprints in Proterozoic coarse-grained siliciclastic sedimentary rocks. Circular compressions of Beltanelliformis brunsae and remains related to Ediacara-type fossils, such as Cucullus fraudulentus, and Mucuplagum primitivum are reported here in an organic mode of preservation from the Neoproterozoic Doushantuo and Liulaobei Formations of China. They can be interpreted as prokaryotic colonies. A charnid fossil with circular attachment disc and stalk, but torn-off frond, is documented in a three-dimensional and partly organic mode of preservation from the Neoproterozoic Ust-Pinega Formation (White Sea coast, Russia). According to their morphology and structure, the Charniidae are not regarded by us as permatulaceans. Modern Myxobacteria illustrate that macroscopic size, complexity, and even compartmentalization can also be developed by prokaryotic colonies. Part of the Ediacara-type fossils may therefore represent prokaryotic colonies or symbiotic organisms involving prokaryotes. Finally, direct evidence indicates that biofilms with associated prokaryotic sheaths, preserved in both organic and pyritic fashion, form the wrinkled surfaces ("elephant skin") that were preferentially colonized by Ediacara-type fossils. This finding supports previous interpretations, based on comparative morphological and sedimentological approaches, that ancient wrinkle structures were microbial mats.