Anatomical information content in the Ediacaran fossils and their possible zoological affinities

Various modes of preservation of Ediacaran fossils in different sediments, quartz sand at Zimnie Gory in northern Russia and lime mud at Khorbusuonka in northern Yakutia, show that the sediment was liquid long after formation of the imprints and that its mineralogy did not matter. A laminated 2 mm thick microbial mat is preserved intact at Zimnie Gory. It stabilized the sediment surface allowing formation of imprints on it. The soft body impressions on the under surface of the sand bed and within it developed owing to formation of a less than 1 mm thin death mask by precipitation of iron sulfide in the sediment. Fossils of the same species or even parts of the same organism may be preserved differently. Internal organs either collapsed, their cavities being filled with sediment from above, or resisted compression more effectively than the rest of the body. This allows restoration of the original internal anatomy of Ediacaran organisms. At Zimnie Gory numerous series of imprints of Yorgia on the clay bottom surface with the collapsed body at their end represent death tracks. The environment of formation of the Ediacaran fossils was thus inhospitable to most organisms. Those adapted to it, namely the radially organized frondose Petalonamae (of possible ctenophoran affinities), anchored in the mat with their basal bulbs. They evolved towards sessile life possibly in symbiosis with photo- or chemoautotrophic microorganisms. Vagile Ediacaran organisms belong mostly to the Dipleurozoa (somewhat resembling chordates and nemerteans), characterized by a segmented dorsal hydraulic skeleton, intestine with metameric caeca, and serial gonads. Only a fraction of the actual Precambrian faunal diversity is represented in the Ediacaran biota.