The occurrence of Dickinsonia in non-marine facies ; La aparición de Dickinsonia en facies no marinas

Dickinsonia in the central Urals occurs in a succession transitional from marginal marine to non-marine. Even within this sequence Dickinsonia appears to be restricted to the least marine facies interpreted as a coastal lagoon surrounded by tidal flats. Another puzzling aspect is the complete absence of other Ediacaran taxa that are usually found together with Dickinsonia in other fossil localities. In fact, the Sinii Kamen Member of the Cherny Kamen Formation yielded an assemblage of Ediacaran macrofossils including aspidellamorph and mawsonitomorph holdfasts, frondomorphs, palaeopascichnids, and concentric ring structures of microbial origin; however, fossils of Dickinsonia have never been found in this association.


Introduction
Dickinsonia is an iconic late Ediacaran taxon representing benthic macroscopic soft-bodied organisms of uncertain phylogenetic affinity. It occurs in the Flinders Ranges of South Australia, the Dniester Valley in Podolia and the Southeast White Sea area, in lithofacies that have conventionally been regarded as shallow marine. Dickinsonia has also been described from the central Urals; however, the depositional context of the fossiliferous strata remains elusive.

Results
The fossils are confined to a 144 m-thick fining upward succession referred to as the Konovalovka Member, which is part of the ca. 1300 m-thick Cherny Kamen Formation cropping out in the lower reaches of Sylvitsa River, the right tributary of the Chusovaya River (58°01'04" N, 58°40'15" E). The Cherny Kamen Formation in general records a gradual transition from basinal mudstone and marine prodelta to lower/upper delta plain in a foreland basin setting (Grazhdankin et al., 2009). The Konovalovka Member consists of four facies associations: flaser-bedded sandstone (F1), hummockybedded sandstone (F2), biolaminated sanstone (F3), and wave-rippled sandstone (F4).
The biolaminated sandstone facies association ( Fig. 1; F3) comprises intervals (0.2 m thick) of biolaminated sandstone, siltstone and mudstone interstratified with packages (0.03-3.4 m) of waverippled sandstone with siltstone and mudstone interbeds and intervals (0.02-4.9 m) of alternating siltstone and mudstone. Less common are waverippled, hummocky-and parallel-bedded sandstones, as well as occasional sandstone units with climbing ripple cross-lamination (0.05-0.2 m). The intervals of alternating siltstone and mudstone are characterised by desiccation cracks and salt crystal pseudomorphs. Microbial mats played a very important role in sediment stabilisation, which could explain the scarcity of clusters of flat mudstone clasts in this facies association. The F3 facies association is interpreted as a tidal flat depositional environment subject to episodic flooding and subaerial exposure.
The wave-rippled sandstone facies association ( Fig. 1; F4) is sandwiched between two thick intervals of biolaminated sandstone. The facies association comprises intervals (0.2-0.9 m) of finely alternating wave-bedded sandstone, mudstone and siltstone interbeds interstratified with packages (0.05-1.4 m) of wave-rippled sandstones with finely laminated siltstone and mudstone interbeds. Occasionally, the facies also includes desiccation cracks and salt crystal pseudomorphs. The main difference from the biolaminated sandstone facies association is the ubiquitous wave-ripple lamination, which is interpreted as the result of slight increase in sediment input/reworking that suppressed microbial mat formation. The facies association F4 is here tentatively interpreted as a lagoon within a tidal flat depositional environment, although the overall depositional setting of the biolaminated and wave-rippled sandstones facies is very similar. It is the waverippled sandstone facies association that yielded a numerically abundant assemblage of Dickinsonia preserved in situ (Sozonov et al., 2019) (Fig. 2).
The biolaminated sandstones of the Konovalovka Member are succeeded by the cross-bedded sandstone facies association of the Krutikha Member ( Fig. 1; F5). The Krutikha Member of the Cherny Kamen Formation reaches 400 m in thickness and comprises multistoried cross-bedded sandstones regularly interstratified with intervals of fine alternating sandstone, siltstone and mudstone. The lithofacies is also characterised by broad channels up to 3.2-m thick filled with medium-grained sandstones. The lowermost part of the channels exhibits coarse parallel and wavy bedding; the middle part consists of sandstones with multistoried unidirectional tabular cross-bedding; the uppermost part is trough crossbedded. The thickness of cross bedding varies between 1 and 7 cm. The channels contain abundant clusters of flat mud pebbles in the lower part and along the cross-beds. Another distinctive feature of the facies is the abundance of diverse oscillation ripple marks, occasional salt crystal pseudomorphs, and casts of desiccation cracks. This facies association is interpreted as a result of the gradual progradation of a delta plain under high sediment supply regime (Grazhdankin et al., 2009).

Conclusions
In the central Urals, Dickinsonia occurs in a succession transitional from marginal marine to non-marine. Dickinsonia is restricted to coastal lagoon environments surrounded by tidal flats that have yielded no other Ediacaran taxa: e.g., the Sinii Kamen Member (Cherny Kamen Formation) has yielded an assemblage of Ediacaran macrofossils including aspidellamorph and mawsonitomorph holdfasts, frondomorphs, palaeopascichnids, and concentric ring structures of microbial origin; however, fossils of Dickinsonia have never been found in this association.