Did Ediacaran NeoProterozoic Kotlinian Mass Extinction Pave way for the Cambrian Explosion?

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

Authors:

Kolesnikov et al

Abstract:

The Asha Group of the South Urals is dominated by laminated shales and thin siltstone–sandstone alternations interpreted as a prograding low-energy inner shelf depositional system. The succession includes several thick sharp-based sandstone bodies comprising interstratified fine- to medium-grained fine-laminated, planar-laminated, hummocky-, convolute- and wave-bedded, planar and trough cross-bedded sandstones, occasionally pebble- to boulder conglomerates, regarded as tidal, deltaic and shoreface depositional systems. The sandstone shoreface bodies are interpreted as forced-regressive deposits stranded in offshore positions during subsequent transgressions. Although palaeoecological and taphonomic context of the Asha Group is favourable for the Ediacara-type biofacies, the associated fossil assemblages nevertheless 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 Asha Group is by no means inferior to that seen in other Ediacaran macrofossil localities. The low biodiversity of Ediacaran macrofossils can be attributed to a relatively young Ediacaran age of the Asha Group, which is constrained by a U–Pb zircon date of 547.6 ± 3.8 Ma from an ash bed in the lower part of the sedimentary succession. The uppermost part of the Asha Group has yielded bilobed burrows with a backfill structure suggesting an affinity with the ichnogenus Didymaulichnus from the Ediacaran–Cambrian boundary strata. The Asha Group therefore offers an important glimpse into the history of the Edacaran biota in the aftermath of the Kotlinian Crisis that caused extinction of dickinsoniomorphs, tribrachiomorphs and bilateralomorphs in wave- and current-agitated shoreface depositional systems ∼550 million years ago, but did not significantly affect frondomorphs and palaeopascichnids.