Ediacaran biota in the aftermath of the Kotlinian Crisis: Asha Group of the South Urals

The Asha Group of the South Urals is dominated by laminated shales and thin siltstone–sandstone alter-nations interpreted as a prograding low-energy inner shelf depositional system. The succession includesseveral thick sharp-based sandstone bodies comprising interstratified fine- to medium-grained fine-laminated, planar-laminated, hummocky-, convolute- and wave-bedded, planar and trough cross-beddedsandstones, occasionally pebble- to boulder conglomerates, regarded as tidal, deltaic and shoreface depo-sitional systems. The sandstone shoreface bodies are interpreted as forced-regressive deposits strandedin offshore positions during subsequent transgressions. Although palaeoecological and taphonomic con-text of the Asha Group is favourable for the Ediacara-type biofacies, the associated fossil assemblagesnevertheless are depauperate and consist of frondomorph holdfast structures, palaeopascichnids, microbial colonies, arumberiamorph structures, as well as lithified microbial substrates (shagreen texture,biolaminites). In terms of fidelity and fossil completeness, preservation of Ediacaran fossils in the AshaGroup is by no means inferior to that seen in other Ediacaran macrofossil localities. The low biodiversityof Ediacaran macrofossils can be attributed to a relatively young Ediacaran age of the Asha Group, which isconstrained by a U–Pb zircon date of 547.6 ± 3.8 Ma from an ash bed in the lower part of the sedimentarysuccession. The uppermost part of the Asha Group has yielded bilobed burrows with a backfill struc-ture suggesting an affinity with the ichnogenus Didymaulichnus from the Ediacaran–Cambrian boundarystrata. The Asha Group therefore offers an important glimpse into the history of the Edacaran biota inthe aftermath of the Kotlinian Crisis that caused extinction of dickinsoniomorphs, tribrachiomorphs andbilateralomorphs in wave- and current-agitated shoreface depositional systems ∼550 million years ago,but did not significantly affect frondomorphs and palaeopascichnids.