Konstantin E. Nagovitsin

Carbonate-hosted Avalon-type fossils in arctic Siberia

Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleo-ecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionally with taphonomic phantoms of Avalon-type taxa (Miaohe-type taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be responsible for the morphological distinctiveness of Ediacaran macrofossils and the profound disparity in the taxonomic composition between fossil assemblages. On the other hand, the exclusion of Avalon-type fossils from carbonaceous compressions (Miaohe-type preservational window) is a real taphonomic signal that provides an important constraint on the properties of certain Ediacaran tissues.

The oldest evidence of bioturbation on Earth

Detailed study of the Khatyspyt Formation in arctic Siberia has shown that, contrary to common expectations, the earliest ichnofabric is of late Ediacaran age, records a food-seeking behavior, and is found in a distal carbonate ramp setting where it co-occurs with fossils of Avalon-type Ediacaran soft-bodied organisms. Preserved depth of bioturbation reaches 5 cm. The ichnofabric solely comprises Nenoxites , the oldest known meniscate trace fossil, which is interpreted as burrows with terminal backfill structure formed as a result of active displacement of sediment to form a tunnel within the substrate, followed by emplacement of the material behind the animal as it progressed through the sediment. In addition to being the most reliable paleontological evidence for the existence of bilaterians at ca. 555 Ma, the late Ediacaran bioturbation is regarded as a key step in the escalatory “engineering” of Phanerozoic ecospace leading to sudden diversification of macroscopic organisms and macrocommunities.

Late Vendian Miaohe-type ecological assemblage of the east European platform

Upper Vendian rocks of the East European Platform (EEP) are characterized by the presence of the White Sea fossil biota, which colonized the region from the southeastern White Sea area to the Central Urals [1]. The White Sea biota includes ecological assemblages of the Avalon (Newfoundland), Ediacara (South Australia), and Nama (Namibia) types, each related to certain environmental conditions [2]. In 2006, we found a previously unknown and morphologically diverse assemblage of carbonaceous macroscopic fossils in the fine-grained aluminosiliciclastic rocks of the Perevalok Formation (Sylvitsa Group) in the Central Urals. Together with the organic-walled macrofossils from the rocks of the Lyamtsa Formation (Valdai Group) in the southeastern White Sea area, the carbonaceous fossils from the Perevalok Formation represent a new (fourth) ecological assemblage of the White Sea fossil biota. The ecological assemblage is older than 557‐558 Ma [3, 4] and includes macroscopic microbial colonies, multicellular and coenocytic eukaryotic macroalgae. In the Late Vendian history of the EEP, this assemblage predated the appearance of the world’s most diverse soft-bodied assemblage, which was found in the overlying rocks of the White Sea area (Verkhovka and Erga formations) and Central Urals (Chernokamen Formation) [1, 2, 5]. The carbonaceous fossils are confined to a thick (200‐400 m) transgressive sequence at the base of the Upper Vendian succession in the southeastern White Sea area and Central Urals (Fig. 1). The lower part of the sequence (laminated mudstones with layers of volcanic tuffs) is gradually replaced upsection by thinly interbedded siltstones and mudstones with rare layers of wave-bedded sandstones. The sequence was formed by the advance and periodic retreat of storm-dominated coastal depositional setting into subaqueous muddy planes with relatively quiet sedimentation in the course of oscillating wane in transgression. Fossiliferous intervals contain thin laminae of phosphorites and organic matter and mark the peak of shallow-water transgression over the platform. In the southeastern White Sea area, this sequence correlates with the Lyamtsa Formation and lower part of the Verkhovka Formation; in the Central Urals, with the upper part of the Staropechny and Perevalok formations [6, 7] (Fig. 1).

Ediacaria in the Siberian hypostratotype of the Riphean

The study of stratigraphy and paleontology of the Riphean/Vendian boundary strata is fundamental to decoding the transitional (Neoproterozoic) stage in the evolution of the biosphere. During this stage, the Proterozoic-style biota with limited morphological diversity, small size of individual organisms, lack of biogeographic zonation, and low rates of evolutionary turnover was replaced by the diverse Phanerozoic-style biota, with bioprovinciality and high dynamics of macroevolutionary processes [1]. The study of the transitional period invokes the analysis of the most representative Upper Proterozoic sections along the periphery of the Siberian Craton. Here, the uppermost Riphean was separated into the Baikalian Complex [2], which was proposed as a Regional Stage [3] or, later, as an Erathem of the General Stratigraphic Chart for the Precambrian of Northern Eurasia and equivalent to the Cryogenian of the International Stratigraphic Chart for the Precambrian [4]. Ediacaran fossils recently discovered in the Ui Group suggest that the biosphere shift commenced during the Baikalian. In 2005, large isolated outcrops of light- and yellowish gray quartzitic and arkosic sandstones, light gray siltstones and greenish gray mudstones were studied in the middle reaches of the Maya River on a segment between the mouths of the Malyi Kandyk and UlakhanKrestyakh Creeks (eastern slope of the Aldan Shield) (Fig. 1). The sections comprise the stratotype of the Kandyk Formation of the Ui Group, Upper Riphean [5, 6]. The total thickness of the succession exceeds 300 m (Fig. 1). The upper part of the sequence can be traced in large-blocky Felsenmeere, screes, and isolated outcrops at the mouth of the Yudoma River, where it is overlain by gray flaggy dolostones and oncolitic limestones of the Yudoma Group of Vendian age [2, 6, 7]. The studied sedimentary succession of the Kandyk Formation consists of two large depositional systems. The first depositional system, up to 100 m in thickness (Fig. 1, Member 1), has a distinctive complexly rhythmic stratification pattern composed of alternating sheets of wavy-bedded sandstones (0.1‐0.4 m), intervals of siltstone‐mudstone couplets, and sheets of wavy-laminated limestones (up to 0.7 m). The second depositional system, over 200 m thick (Fig. 1, members 2‐4), has a relatively simple structure: thick (up to 70 m) packages of thin- and wavy-laminated and thick-bedded and cross-bedded sandstones are interstratified with thick (up to 65 m) intervals of siltstone‐mudstone couplets.