The Ediacaran-Cambrian transitional strata record one of the most important evolutionary turnovers in marine ecosystems, associated with the origin and diversification of modern phyla (the Cambrian Explosion; e.g. Erwin et al., 2011; Zhuravlev & Wood, 2018; Wood et al., 2019). One of the major innovations that first appeared in the terminal Ediacaran benthic communities was mineralized exoskeleton. Small skeletal fossils (SSF; a diverse polyphyletic group of mm-sized biomineralized fossils) first appeared in the terminal Ediacaran and flourished in the Terreneuvian. They commonly reveal considerable morphological similarity with typical Cambrian macroscopic mineralized eumetazoans (molluscs, brachiopods, hyoliths etc.), but their phylogenetic affinity as well as the exact time of emergence of macroscopic mineralized benthos in the Terreneuvian remain debatable. Herein we report morphologically-diverse complex of macroscopic fossils from the basal Cambrian Stage 2 strata of the Olenek Uplift (northeastern Siberian Platform), which closely resemble certain small skeletal taxa, typical for the Terreneuvian.

Within the Olenek Uplift, the Ediacaran-Cambrian transitional strata comprise a mixed carbonate- siliciclastic succession that is defined as the Kessyusa Group, including three formations (in stratigraphical order): Syhargalakh, Mattaia and Chuskuna (Nagovitsin et al., 2015). The studied material originates from the upper part of the Mattaia Formation, representing a coarsening-upward succession (92 m thick) from offshore siltstones and mudstones to the lower shoreface cross- and wave-bedded coarse-grained sandstones, oolitic grainstones and calcimicrobe framestones (Marusin, 2016).

The studied fossils are localized in the package of the lower shoreface calcimicrobe framestones and intraclastic limestones that have been early referred to as the Suordakh Member and interpreted as a microbial-dominated, isolated carbonate platform (16.6 m thick) (Nagovitsin et al., 2015; Marusin, 2016). This informal unit also hosts diverse small skeletal assemblages (association of Nochoroicyathus sunnaginicus Siberian Assemblage Zone), including proposed index-taxa of the Cambrian Stage 2 Watsonella crosbyi and Aldanella attleborensis (Khomentovsky & Karlova, 1993; Nagovitsin et al., 2015). Strongly positive δ¹³C_carb values (up to +4.4‰) in carbonates of the Suordakh Member and U-Pb zircon age 529.7 ± 0.3 Ma of tuffs in the middle of the interval (Kaufman et al., 2012; Marusin, 2016) further confirm the Cambrian Age 2 of the fossil-bearing upper Mattaia Formation (Fig. 1). Similar facies in the overlying Chuskuna Formation also bear macroscopic forms, herein studied.

The fossils comprise macroscopic (3.5-92 mm) steinkerns (internal moulds of shells), exposed on the bedding planes and weathered and shear surfaces of the limestones of the Suordakh Member. The fossils are filled with intraclastic and micritic limestones, similar to the host rock (Fig. 2) and generally lack any relics of mineralized walls. It considerably limits standard preparation technique, applied for extraction of small skeletal fossils (slow rock-dissolution in 2% or 5-10% acetic acid), because of high risk of the fossil being dissolved.

Far exceeding typical size of the small skeletal fossils, the studied material reveals striking morphological similarity with specific Fortunian and Cambrian Age 2 small skeletal taxa, which were early documented in the upper Mattaia Formation (Khomentovsky & Karlova, 1993; Nagovitsin et al., 2015). The macroscopic assemblage of the upper Mattaia Formation includes helcionellid, stenothecid and kharkhaniid molluscs, anabaritids and circothecid hyoliths (Fig. 1).

In the study section, hecionellid molluscs include flat cap-shaped forms with circular cross-section and sub-central or slightly posteriorly-displaced apex Asiapatella sinuata (up to 7 mm in diameter; Fig. 2A) and Asiapatella undulata (up to 9 mm; Fig. 2B); laterally-compressed flat cap-shaped moulds with rounded apex inclined posteriorly, convex anterior field and short concave posterior Bemella parula (4-7 mm; Fig. 2D) and Bemella simplex (3.5-7 mm; Fig. 2C); moderately-high cap-shaped, laterally-compressed specimens with the apex, going far off the posterior margin Igorella emeiensis (10–21 mm; Fig. 2E) and I. maidipingensis (5.5-10 mm long; Fig. 2F).

Stenothecid molluscs are herein preserved as laterally-compressed bilateral cap-shaped steinkerns, semicircular in lateral view and ornamented with thin concentrical ribs and folds, with distinct (Anabarella cf. plana; up to 4.5 mm; Fig. 2G) or small rounded apex (Watsonella crosbyi; up to 8 mm; Fig. 2H), reaching or even overhanging the short concave posterior margin. The only species Barskovia hemisymmetrica of kharkhaniid molluscs is identified in the Suordakh Member. It comprises internal mould (5 mm in diameter) of low turbospiral sinistral shell with no evidence of internal sculpture (Fig. 2I).

The identified anabaritids are preserved as straight triradial tubular steinkerns (up to 35 mm long and up to 5 mm in diameter), with smooth surface and rounded triangular cross-section (Anabarites licis; Fig. 2J, L) and with helicoidal bands (Anabarites volutus; Fig. 2K).

Circothecids comprise a diverse morphological group of extinct Cambrian organisms, putatively assigned to orthothecid hyoliths, with conical mineralized shell, simple morphology and no dorso-ventral differentiation (Missarzhevsky, 1989). These fossils are abundant in the Suordakh Member and represent gently-curved conical steinkerns with circular cross-section and thin transverse ornamentation (Conotheca mammilata; 4-15 mm long; Fig. 2M), horn-shaped smooth conical moulds (Kotuitheca cf. curta; 12 mm long; Fig. 2N) and straight or gently-curved forms with oval to egg-shaped cross-section and thin transverse ornamentation (Turcutheca annae; up to 30 mm long; Fig. 2O). The basal interval of the Suordakh Member also embeds large-sized fragments (up to 92 mm long; up to 24.5 mm in diameter) of conical steinkerns, circular to ovoidal in the cross-section (Fig. 2Q). The fossils exhibit no relics of calcite wall, but covered with thin transverse-annulated crust. Although fragmental preservation limits precise phylogenetic affinity, the primitive morphology of these forms putatively assigns them to circothecids (Marusin & Grazhdankin, 2018).

The basal beds of the Suordakh Member also contain numerous macroscopic fossils, comprising triangular pyramids (4-20.5 mm long), filled with the sediment identical to the host. The surface of the pyramids is covered with various symmetrical sculptural elements (isolated bumps on the median line of the faces, series of chevron-like ridges tapering towards the aperture) (Fig. 2P). The lateral faces of the pyramids commonly display plastic deformations. Phylogenetic affinity of the fossils remains debatable, since their general morphology meets analogues among conulariids, orthothecid hyoliths or anabaritids.

Among all the macroscopic forms documented in the Kessyusa Group, only anabaritids and circothecids Turcutheca occasionally reveal relics of calcite wall, which can be noted both macroscopically on the specimens (Fig. 2K, O) and in thin sections (Fig. 2L).

There are two major hypotheses that alternatively explain the morphological similarity and stratigraphic discrepancy of the earliest skeletal macro- and microfossils: (i) the late Ediacaran and earliest Cambrian small skeletal fauna is a phylogenetic ancestor of the Cambrian macroscopic organisms, exhibiting similar morphology (e.g. Rozanov et al., 1969; Runnegar & Jell, 1976); and (ii) small skeletal fossils comprise taphonomical artifacts. The second hypothesis assumes that all SSF represent larval or juvenile ontogenetic stages of macroscopic skeletal organisms. In contrast, preservation potential of thin calcareous shells of adult forms was crucially limited by preservation conditions (sediment reworking and compaction) and techniques applied for extraction of the mineralized forms from the rock (Dzik, 1991; Martí Mus et al., 2008). Our study reveals that typical Terreneuvian small skeletal fossils (anabaritids, molluscs, circothecid hyoliths) co-occur with their macroscopic analogues, and hence challenges the first hypothesis.

The relics of calcite wall, fragmentally preserved and exclusively associated with anabarithids and certain circothecid hyoliths (Fig. 2K, L, O) on the studied material, demonstrate that at least these Cambrian problematic conical and tubular benthic organisms had already reached macroscopic sizes (up to 35 mm long) and were covered with calcite shells at the very beginning of the Cambrian Age 2. In contrast, abundant plastic deformations documented on the largest fossils (Fig. 2P, Q) implicates that ~530 Ma these benthic organisms were most likely covered by semi-rigid, possibly non-mineralized exoskeleton.