Fossils, pseudofossils and problematica from the Ura Formation: Ediacaran of the Patom Basin, Siberia

The pseudofossils, problematica and actual macrofossils from the Middle Ediacaran Ura Formation of the Patom Basin of Siberia are critically reviewed. Both the pseudofossils and problematica are flat-clasts of ooid grainstones that are interpreted as a product of submarine gravitational or storm-induced erosional episodic events. Some extremely flat batten-shaped ooid and cementstones 2D-clasts closely resemble shreds of ribbon-like macrophyte phytoleims randomly scattered on a bedding plane. They can potentially be considered as a cement-incrustation along the algae thalli buried within ooid sands or shales, however, there is no certainty in their taxonomic identification. At the same time, other abiotic or microbially induced formation mechanisms of these fragments cannot be excluded. The Ura macrofossils, similar in size to the 2D-clasts, are described from the same outcrop, approximately 60 m downsection. They are represented mainly by the ribbon-shaped carbonaceous compressions of Liulingjitaenia alloplecta that demonstrate the stratigraphically lowest appearance of the Miaohe-type fossils in mid-Ediacaran (pre-Shuram, 580-570 Ma) strata of Ura Formation in close association with Doushantuo-type acanthomorphic acritarchs. These fossils are present as a detrital component or as in situ buried large (decimeter scale) colonial clusters. The colonies covered significant surface areas of the soft muddy sea-floor beneath wave-base. L. alloplecta themselves, commonly attributed to green macroalgae, could also be considered as composite colonies of filamentous microalgae or cyanobacteria.