DEATH, DECAY AND DESTRUCTION OF DICKINSONIA

Dickinsonia is the iconic Ediacaran fossil, a characteristic late Precambrian megafossil of South Australia and the Russian White Sea. Dickinsonia has been compared with annelid worms, jellyfish, scleractinian corals, or extinct Vendobionta, and the unusual preservation of such a soft-bodied creature in quartz sandstones attributed to a late Precambrian regime of decay different from that of today. Measurements of width and length of specimens of Dickinsonia costata in collections of Adelaide University and the South Australian Museum show indeterminate isometric growth, more like a colonial or thalloid organism than a free-living worm. Dickinsonia shows great variation in thickness independent of width or length, but traces of ribbing on the poorest of impressions. I interpret this as evidence of decay comparable with wilting and skeletonization of a fossil leaf or lichen, unlike the clotting and distortion during decay found in known fossil worms and jellyfish. Even effaced remains of Dickinsonia are raised compressions, sometimes arrayed in arcs that have been interpreted as slime trails or tumble tracks. I interpret these elevated platforms as rhizinous bases of decayed crustose lichens arranged in fairy rings. They were sessile creatures, as no overlapping specimens of Dickinsonia have ever been found. One specimen shows embayment beside another, comparable with competitive interactions of encrusting bryozoans. Slightly bent or shrunken specimens of Dickinsonia reveal limited flexibility, but there is no known brittle deformation to support theories of pyritic or sideritic “death masks”. Dickinsonia was thin and ground hugging, as it did not disrupt overlying cross-bedding. It was also resistant to compaction by overburden, like fossil lichens such as Spongiophyton and Thucomyces, and more compaction-resistant than fossil logs, jellyfish or worms. The growth and decay of Dickinsonia was more like that of a plant or lichen than a worm, jellyfish or coral. Aerobic decay was very effective during the late Precambrian, as also shown by paleosols with isotopically-organic carbon yet low organic carbon content, and by fossil actinobacteria (Primoflagella).