2.a. General setting

The Digermulen Peninsula is located in Finnmark in the northeastern part of Norway, outlined by Tanafjorden to the SE and Langfjorden to the NW (Fig. 1a). It is c. 35 km long and 18 km wide at the base of the fjords, ending to the NE in the pointed Digermulen headland. Its highest point is 639 m above sea level, but the mountain plateau forms a denudated expanse, with steep transitions to the coast and several incised valleys. The strata are well exposed and dip slightly to the west with the strike sub-parallel to the coastline. They form part of the Lower Allochthon Gaissa Thrust Belt of the Finnmark Caledonides (Rice, Reference Rice, Corfu, Gasser and Chew2014). Siliciclastic rocks dominate, belonging to the Smalfjorden, Nyborg, Mortensnes, Stáhpogieddi and Breidvika formations, forming the Cryogenian to basal Cambrian Vestertana Group (Fig. 1b). Both the Smalfjorden and Mortensnes formations contain diamictites related to a glacial regime, with the Nyborg Formation representing an inter-glacial deposit. The Stáhpogieddi and Breidvika formations are post-glacial marine sequences. Above this follow the Duolbagáisá, Kistedalen and Bearalgáisá formations, constituting the younger marine lower Cambrian (Stage 3) to Lower Ordovician (Tremadocian) Digermulen Group (Reading, Reference Reading1965) (Fig. 1b). The rocks have undergone tectonometamorphic deformation related to the Scandian Orogeny (Meinhold et al. Reference Meinhold, Jensen, Høyberget, Arslan, Ebbestad, Högström, Palacios, Agić and Taylor2019 a,b, Reference Meinhold, Roberts, Arslan, Jensen, Ebbestad, Högström, Høyberget, Agić, Palacios and Taylor2020), resulting in a postmature overprint of 200–250 °C with very localized, low epizonal metamorphic conditions of ∼300 °C (Meinhold et al. Reference Meinhold, Wemmer, Högström, Ebbestad, Jensen, Palacios, Høyberget, Agić and Taylor2019 b).

Fig. 1. (a) Geology of the Digermulen Peninsula (based on Siedlecka et al. Reference Siedlecka, Reading, Williams and Roberts2006), with the study area marked by a rectangle. (b) Stratigraphy of the Vestertana and Digermulen groups with estimated thickness of strata (Banks et al. Reference Banks, Edwards, Geddes, Hobday and Reading1971; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017, fig. 1d). Stage divisions are based on Nikolaisen & Henningsmoen (Reference Nikolaisen and Henningsmoen1985, Reference Nikolaisen and Henningsmoen1990), Högström et al. (Reference Högström, Jensen, Palacios and Ebbestad2013), McIlroy & Brasier (Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017) and Palacios et al. (Reference Palacios, Högström, Ebbestad, Agić, Høyberget, Jensen, Meinhold and Taylor2020). The horizontal grey bar shows the interval studied herein. The black triangles reflect upwards-coarsening parasequences (based on McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017). Ord. – Ordovician; Trem. – Tremadocian; G – glaciogenic diamictites. (c) Detailed map of the study area with the studied sections marked by red bars.

The Cryogenian–Ordovician succession on the Digermulen Peninsula is c. 3000 m thick and is the only fossiliferous site in Scandinavia with sedimentation across the Ediacaran–Cambrian transition, seemingly without any significant breaks (Högström et al. Reference Högström, Jensen, Palacios and Ebbestad2013). Age constraints on the Vestertana Group are based on glacial unconformities and the presence of trace and body fossils (Banks et al. Reference Banks, Edwards, Geddes, Hobday and Reading1971; Högström et al. Reference Högström, Jensen, Palacios and Ebbestad2013; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017; Jensen et al. Reference Jensen, Högström, Høyberget, Meinhold, McIlroy, Ebbestad, Taylor, Agić and Palacios2018 b).

2.b. The Ediacaran–Cambrian boundary beds on the Digermulen Peninsula

The post-glacial shallow marine succession of the Vestertana Group starts with the Stáhpogieddi Formation, which is divided into the Lillevannet, Indreelva and Manndrapselva members (Fig. 1b). In the latter unit, Reading (Reference Reading1965) recognized three sets of resistant quartzite bands in his ‘red quartzitic sandstone member’, and although the modern name was used already by Banks (Reference Banks, Crimes and Harper1970), the member was formally named the Manndraperelva Member by Banks et al. (Reference Banks, Edwards, Geddes, Hobday and Reading1971), now referred to as the Manndrapselva Member in accordance with the modern spelling (Siedlecka et al. Reference Siedlecka, Reading, Williams and Roberts2006). It is estimated to be c. 190 m thick, with three parasequences (McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017), that probably represent fourth order units. They are referred to as cycles herein to conform to previous literature.

The basal cycle shows a gradual transition from the underlying Indreelva Member into reddish sandstone (c. 33 m thick), followed by two coarsening-upward regressive cycles c. 60 m and c. 36 m thick, respectively (Banks et al. Reference Banks, Edwards, Geddes, Hobday and Reading1971; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017; Meinhold et al. Reference Meinhold, Jensen, Høyberget, Arslan, Ebbestad, Högström, Palacios, Agić and Taylor2019 a; this study). Each of the parasequences consists of mudstone and fine sandstone beds of various thickness and lateral extension, with the sandstone beds becoming thicker towards the top of each parasequence and ending in a set of massive sandstones (see Banks et al. Reference Banks, Edwards, Geddes, Hobday and Reading1971; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017, fig. 2 for sedimentological details).

In the field (NE side of the peninsula), the massive sandstone units in each parasequence form weathering resistant ridges that have a reddish lower part and a whitish upper part. The uppermost sandstone package of the third cycle of the Manndrapselva Member is c. 9 m thick (red and white quartzite), but the boundary to the lower member of the succeeding Breidvika Formation is somewhat arbitrarily placed at the top of the last red sandstone (Reading, Reference Reading1965), which is at the transition to the whitish sandstone in the middle of this set (see field photos and logs for further details). The name Breidvika Formation was first used by Føyn (Reference Føyn and Dons1960) but described in detail by Reading (Reference Reading1965), who recognized two members, 240 m and 340 m thick, respectively (McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017); these are treated as informal (Högström et al. Reference Högström, Jensen, Palacios and Ebbestad2013). The lowermost part of the lower member of the Breidvika Formation appears similar to the upper parasequence of the underlying Manndrapselva Member, forming a c. 30 m thick parasequence ending in a c. 5 m thick sandstone set with a red to white transition within.

Whereas the depositional environment of the Manndrapselva Member broadly represents the offshore to wave-dominated lower shoreface, the lower member of the Breidvika represents a near-shore shelf and offshore facies (McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017). The rocks of the lower member consist of alternating mudstone and thin sandstone beds interpreted as tempestites (Banks et al. Reference Banks, Edwards, Geddes, Hobday and Reading1971; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017).

The Ediacaran–Cambrian boundary is indicated by palaeopascichnids in the second cycle, and trace fossils and organic-walled microfossils close to the base of the third cycle of the Manndrapselva Member; the latter, together with the succeeding lower member of the Breidvika Formation, encompasses the Fortunian Stage of the Terreneuvian Series (Högström et al. Reference Högström, Jensen, Palacios and Ebbestad2013; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017; Jensen et al. Reference Jensen, Högström, Høyberget, Meinhold, Palacios, Taylor, Ebbestad, Agić and McIlroy2017, Reference Jensen, Högström, Almond, Taylor, Meinhold, Høyberget, Ebbestad, Agić and Palacios2018 a,b). McIlroy & Logan (Reference McIlroy and Logan1999) and McIlroy & Brasier (Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017) reported the conical skeletal fossil Ladatheca cylindrica (Grabau, Reference Grabau1900), in association with the tubular foraminiferan Platysolenites antiquissimus (Eichwald, Reference Eichwald1860), from the boundary interval between the lower and upper members of the Breidvika Formation. In the Avalonian successions in Newfoundland, the Ladatheca cylindrica Zone lies in the upper part of the Fortunian Stage, although the taxon extends through the overlying Watsonella crosbyi Zone in Newfoundland, where it co-occurs with Platysolenites antiquissimus. The top of the Fortunian (and the base of the succeeding Stage 2) has been proposed to lie within the Watsonella crosbyi Zone (Landing et al. Reference Landing, Geyer, Brasier and Bowring2013).

2.c. The Ediacaran–Cambrian boundary beds in Baltica

The traditional Baltic and East European stratigraphical terminology encompasses horizons, series and suites, which are broadly used as regional stages, groups and formations, respectively (Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990). The base of the Cambrian has been placed at the base of the Lontovan Regional Stage (Sokolov, Reference Sokolov1984, Reference Sokolov1997), but is more commonly placed at the base of the Rovnian Stage (Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990; Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011; Meidla, Reference Meidla2017), which has been inferred to approximate with the Global Stratotype Section and Point (GSSP) for the base of the Cambrian in Newfoundland, Canada (Landing, Reference Landing1994). The basal two stages, the Rovnian and Lontovan, are of importance in relation to the succession on the Digermulen Peninsula (Fig. 2).

Fig. 2. Traditional trace fossil and acritarch zonation of the Rovnian and Lontovan (Terreneuvian, Cambrian) in Baltica compared to the Terreneuvian biozones in the GSSP section in Newfoundland (eastern Avalonia). The acritarch zonation for Baltica is based on Moczydłowska (Reference Moczydłowska1991), while the other zones follow Nielsen & Schovsbo (Reference Nielsen and Schovsbo2011). The biozones for Newfoundland are based on Palacios et al. (Reference Palacios, Jensen, Barr, White and Myrow2018). Domi. – Dominopolian.

The Rovno layers were named by Kir’yanov (Reference Kir’yanov, Ishchenko, Kiriakov and Krandievsky1968) for strata in Ukraine that are characterized by abundant sabelliditids and the predominance of simple, smooth acritarchs. The lower boundary was drawn at the appearance of sabelliditids. Subsequently, the layers were taken to comprise the Rovno Horizon in the East European Platform characterized by the widespread occurrence of Sabellidites cambriensis and the absence of Platysolenites (Kir’yanov, Reference Kir’yanov1969), encompassing the Sabellidites cambriensis Zone (Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990). However, in the traditional usage (Areń & Lendzion, Reference Areń and Lendzion1978; Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990), Platysolenites co-occurs with Sabellidites in the uppermost part of the Sabellidites cambriensis Zone. The Sabellidites cambriensis Zone is rarely referred to in more recent studies in Baltica (but see Meidla, Reference Meidla2017), while it is used as a biozone in the GSSP section in Newfoundland (Landing et al. Reference Landing, Myrow, Benus and Narbonne1989) (Fig. 2).

Definitions of the Lontova beds and the Platysolenites antiquissimus Zone were developed in the 1920s and 1930s by the Estonian geologist Armin Öpik (Mens & Pirrus, Reference Mens and Pirrus1977, Reference Mens, Pirrus, Raukas and Teedumäe1997; Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990) for the ‘Blue clay’ of Estonia. The stratotype for the formation and stage is in the Kunda quarry in NE Estonia. The base of the Platysolenites Zone was drawn at the appearance of the acritarchs Granomarginata, Tasmanites tenellus and leiosphaerids and not by the first occurrence of Platysolenites (Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990). Moczydłowska (Reference Moczydłowska1991) refined the acritarch stratigraphy, proposing the Asteridium tornatum–Comasphaeridium velvetum Zone to encompass the Lontovan Stage, and a Platysolenites antiquissimus Interval-Zone with the lower boundary at the first occurrence of Platysolenites. Thus, it corresponds to the Asteridium tornatum–Comasphaeridium velvetum Zone and includes the upper part both of the traditional Sabellidites cambriensis Zone and the uppermost part of the Rovnian Stage (Moczydłowska, Reference Moczydłowska1991, fig. 11). The upper boundary corresponds to the base of the Skiagia ornata–Fimbriaglomerella membranacea Zone (Moczydłowska, Reference Moczydłowska1991) (Fig. 2).

On the Digermulen Peninsula most of the Rovnian Stage encompasses the third cycle of the Manndrapselva Member, while the upper part of the Rovnian and all of the Lontovan Stage are encompassed by the Breidvika Formation (Fig. 2) (see also McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017, fig. 2).

6.a. Results

About 70 m of the Manndrapselva section were logged (Fig. 8a). The lower parts of the lower member of the Breidvika Formation are partly covered by vegetation, which prevented sampling. The flat landscape and low dip of the strata preferentially exposed upper surfaces of the beds, while lower surfaces with trace fossils are more difficult to find. In total, 46 samples with Sabellidites cambriensis were recovered. In the lower part of the section (Manndrapselva Member) specimens are rare and non-gregarious. The trace fossil Treptichnus pedum was registered at c. 9 m and 16 m below the reference level (= 0 m) in the section (Fig. 7d, e), together with Sabellidites cambriensis. The 9 m level below the reference level correlates with the position of Treptichnus pedum recorded by Högström et al. (Reference Högström, Jensen, Palacios and Ebbestad2013) from the Bárdeluovttjohka section. Specimens of Sabellidites are more common already in the lower part of the lower member of the Breidvika Formation, often with abundant specimens on the bedding surfaces. The stratigraphically youngest samples were found just above the upper boundary of the first parasequence in the Breidvika Formation (Fig. 8).

The Avžejohka section offered the best exposures of the fissile mudstone (Fig. 8b), with 18 samples collected. Some of the surfaces were covered with Sabellidites specimens. Sabellidites is common within the first 5 m above the thick red/white quartzite and with a few specimens occurring more than 25 m above the quartzite.

Farmer et al. (Reference Farmer, Vidal, Moczydłowska, Strauss, Ahlberg and Siedlecka1992) recorded Sabellidites at 9 m and 16 m above the base of the lower member of the Breidvika Formation in the Bárdeluovttjohka section, while Högström et al. (Reference Högström, Jensen, Palacios and Ebbestad2013) extended the stratigraphic range in the first parasequence. For the present study, nearly 70 m of the section was logged. A total of 26 samples were collected (including 12 samples from the 2011 expedition). Specimens are found stratigraphically lower in the Bárdeluovttjohka section, and Treptichnus pedum is also recorded at a lower level than in the Manndrapselva section (Jensen et al. Reference Jensen, Högström, Almond, Taylor, Meinhold, Høyberget, Ebbestad, Agić and Palacios2018 a, fig. 7c; but see above). The acritarch Granomarginata prima is found at c. 12 m below the reference level and Asteridium tornatum at c. 23 m above the reference level (data from Högström et al. Reference Högström, Jensen, Palacios and Ebbestad2013; Palacios et al. Reference Palacios, Jensen, Barr, White and Myrow2018).

The sections correlate well across the c. 5 km distance between the southernmost and northernmost outcrop, with a similar distribution and abundance of Sabellidites. Specimens are rare in the third cycle of the Manndrapselva Member, but they are consistently present from c. 6.5 m above the lowest level of Treptichnus pedum and upwards (Fig. 8). A single specimen was registered by Jensen et al. (Reference Jensen, Högström, Almond, Taylor, Meinhold, Høyberget, Ebbestad, Agić and Palacios2018 a) c. 2 m below the lowest Treptichnus pedum specimen in the outcrop to the south of the Bárdeluovttjohka section (Fig. 8c). Sabellidites is common and abundant in the first 5 m above the base of the first parasequence of the Breidvika Formation with two specimens recognized further 15 m upsection (Figs 6h, 8b). The observed upper range of Sabellidites is thus at c. 55 m into the lower member of the Breidvika Formation.

7.a. General remarks

Nielsen & Schovsbo (Reference Nielsen and Schovsbo2011, p. 223) adopted the Rovnian and Lontovan stages developed in Eastern Europe (Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990) for the lower Cambrian of Scandinavia, with the Kotlinian being the uppermost stage of the Ediacaran. A detailed overview of the distribution of Sabellidites in a number of sections on Baltica and elsewhere is found in the online Supplementary Material.

The Rovnian Stage stratotype is taken at level 100–152.7 m of core 4 at Klevan in the Rivne region of Ukraine, being 29 to 53 m thick in the area (Kir’yanov, Reference Kir’yanov1969) (Fig. 9b, locality 11a), with an additional reference section at the outcrop along the Ternava River (a tributary of the Dniester River) at the village of Kitaygorod, Khmelnytsky region, Ukraine (Fig. 9b, locality 11c) (Konstantinenko & Kir’yanov, Reference Konstantinenko, Kir’yanov and Gozhyk2013). The Rovnian Stage is characterized by a mass-occurrence of Sabellidites occurring without Platysolenites antiquissimus (Kir’yanov, Reference Kir’yanov, Ishchenko, Kiriakov and Krandievsky1968, Reference Kir’yanov1969) but with a low-diversity leiosphaerid acritarch assemblage (Konstantinenko & Kir’yanov, Reference Konstantinenko, Kir’yanov and Gozhyk2013).

Fig. 9. (a) Map of Newfoundland, Canada, with location of the GSSP section at Fortune Head on the Burin Peninsula. (b) Simplified map of northern and eastern Europe with main localities discussed herein and in the online Supplementary Material, surface and subsurface occurrences of lower Palaeozoic rocks and major structural elements. Location of core drilling sites and distribution of Palaeozoic rocks based on Zoricheva (Reference Zoricheva, Zoricheva and Volkov1963), Rozanov (Reference Rozanov, Keller and Rozanov1980, Reference Rozanov, Rozanov and Łydka1987), Kuzmenko & Burzin (Reference Kuzmenko and Burzin1996), Mens & Pirrus (Reference Mens, Pirrus, Raukas and Teedumäe1997), Silaupa et al. (Reference Silaupa, Fokin, Lazauskienè, Stephenson, Gee and Stephenson2005), Maslov et al. (Reference Maslov, Grazhdankin, Podkovyrov, Ronkin and Lepikhina2008), Nielsen & Schovsbo (Reference Nielsen and Schovsbo2011) and Podkovyrov et al. (Reference Podkovyrov, Maslov, Kuznetsov and Ershova2017). Boundaries of the Baltic shield based on Torsvik & Rehnström (Reference Torsvik and Rhenström2003), Gee et al. (Reference Gee, Bogolepova, Lorenz, Gee and Stephenson2006) and Mazur et al. (Reference Mazur, Krzywiec, Malinowski, Lewandowski, Aleksandrowski and Mikołajczak2018).

The stratotype of the Lontovan Stage is in the Kunda quarry in NE Estonia with additional data from the Lontova drill core (interval 14.0 to 88.3 m) (Mens & Pirrus, Reference Mens and Pirrus1977, Reference Mens, Pirrus, Raukas and Teedumäe1997). The Lontovan Stage has been considered equivalent in age to provisional Stage 2 of the Terreneuvian (Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011; Meidla, Reference Meidla2017 and references therein). However, it seems to be older and corresponds mostly to the Fortunian, and to Stage 2 only in part (Moczydłowska & Yin, Reference Moczydłowska, Yin, Zhao, Zhu, Peng, Gaines and Parsley2012; Palacios et al. Reference Palacios, Jensen, Barr, White and Myrow2018, Reference Palacios, Högström, Ebbestad, Agić, Høyberget, Jensen, Meinhold and Taylor2020; Slater et al. Reference Slater, Harvey and Butterfield2018).

Two acritarch assemblages and subzones were originally recognized in the basal Cambrian of the East European Platform and considered typical for the Rovnian and Lontovan stages (Mens & Posti, Reference Mens, Posti, Männil and Mens1984; Mens et al. Reference Mens, Bergström and Lendzion1990; Jankauskas & Lendzion, Reference Jankauskas and Lendzion1992; Raevskaya, Reference Raevskaya2005 and references therein). The low-diversity Asteridium–Comasphaeridium assemblage Zone was developed as the first recognizable zonation for the Polish part of the East European Platform and Baltoscandian successions, and considered equivalent to the latest Rovno and Lontova horizons (Moczydłowska, Reference Moczydłowska1991). The zone is characterized by Asteridium tornatum, Comasphaeridium velvetum, Granomarginata prima and Granomarginata squamacea. Jachowicz-Zdanowska (Reference Jachowicz-Zdanowska2013) recorded microfossils, believed to be older but including Granomarginata, in the Pulvinosphaeridium antiquum–Pseudotasmanites assemblage Zone on the Brunovistulicum Terrane in southern Poland and northeastern Czech Republic (west of the Teisseyre–Tornquist Zone). Szczepanik & Żylińska (Reference Szczepanik and Żylińska2016) placed this zone in the earliest Fortunian.

The post-Lontovan Skiagia–Fimbriaglomerella Zone corresponds to the Dominopolian Stage and the occurrence of the first trilobites in Baltoscandia of the Schmidtiellus mickwitzi Zone (Moczydłowska, Reference Moczydłowska1991; Nielsen & Schovsbo Reference Nielsen and Schovsbo2011; Palacios et al. Reference Palacios, Högström, Ebbestad, Agić, Høyberget, Jensen, Meinhold and Taylor2020). In the review by Moczydłowska & Yin (Reference Moczydłowska, Yin, Zhao, Zhu, Peng, Gaines and Parsley2012), the Asteridium–Comasphaeridium Zone took a wider scope and encompasses the entire Fortunian Stage and extends into the undefined Stage 2, thus including both the Rovnian and Lontovan in terms of Baltic stages.

The acritarchs Granomarginata prima and Granomarginata squamacea, together with rare Asteridium tornatum, are present at the base of a Lontovan Stage on the East European Platform (Volkova et al. Reference Volkova, Kir’yanov, Piscun, Pashkyavichene, ]ankauskas, Keller and Rozanov1979, Reference Volkova, Kir’yanov, Piscun, Pashkyavichene, ]ankauskas, Urbanek and Rozanov1983; Moczydłowska, Reference Moczydłowska1991). The first two occur in the Fortunian type section in Newfoundland, but here the name-bearing taxa of the assemblage zone are missing (Palacios et al. Reference Palacios, Jensen, Barr, White and Myrow2018, Reference Palacios, Högström, Ebbestad, Agić, Høyberget, Jensen, Meinhold and Taylor2020). Nearly the entire Terreneuvian and most of Stage 2 instead encompass the Granomarginata Zone, and identification of the Asteridium–Comasphaeridium assemblage Zone can only be made when the index fossils of these small acanthomorphic acritarchs are present (Palacios et al. Reference Palacios, Jensen, Barr, White and Myrow2018, Reference Palacios, Högström, Ebbestad, Agić, Høyberget, Jensen, Meinhold and Taylor2020). Granomarginata itself may be older (Agić et al., Reference Agić, Högström, Jensen, Ebbestad, Vickers-Rich, Hall, Matthews, Meinhold, Høyberget and Taylor2021) and the Cambrian occurrences may represent the tail end of a more typical Ediacaran assemblage.

The Vendian (Period) has been used in East European literature since the mid-1960s, with the Vendian–Cambrian boundary usually placed at the base of the Lontovan Regional Stage (Sokolov, Reference Sokolov1984, Reference Sokolov1997). Estonian and many Ukrainian workers on the other hand preferred to place the boundary at the base of the Rovnian Stage, especially following the definition of the basal Cambrian GSSP (Mens et al. Reference Mens, Bergström and Lendzion1990; Kir’yanov, Reference Kir’yanov2006; Velikanov, Reference Velikanov2009; Velikanov & Melnychuk, Reference Velikanov, Melnychuk and Gozhyk2013; Meidla, Reference Meidla2017). The Vendian of older Polish literature, with the upper boundary placed at the base of the Lontovan, is often replaced by Ediacaran in more modern studies without adjustments of the Ediacaran–Cambrian boundary level (see discussion pertaining to this in Section 7b and 7d).

7.b. Sabellidites cambriensis as an index fossil for the lowermost Cambrian

Sabellidites cambriensis has been recognized as a lowermost Cambrian zonal fossil since the mid-1960s and is widely used in correlation across Scandinavia and the East European Platform and with Siberia (Sokolov, Reference Sokolov and Sokolov1965, Reference Sokolov1997; Kir’yanov, Reference Kir’yanov1969; Martinsson, Reference Martinsson and Holland1974; Mens, Reference Mens, Keller and Rozanov1980, Reference Mens, Rozanov and Łydka1987; Bergström & Ahlberg, Reference Bergström and Ahlberg1981; Lendzion, Reference Lendzion1983; Bergström & Gee, Reference Bergström, Gee, Gee and Sturt1985; Mens et al. Reference Mens, Bergström and Lendzion1987, Reference Mens, Bergström and Lendzion1990). A general sequence with Sabellidites cambriensis in the oldest strata (Rovno), followed by, or slightly overlapping with, Platysolenites antiquissimus, and the appearance of a more diverse shelly fauna in the succeeding strata (Lontovan) became well established. The lower Cambrian facies on the East European Platform shows a strong lateral continuity (Rozanov & Zhuravlev, Reference Rozanov, Zhuravlev, Lipps and Signor1992), which facilitates a broad correlation across the region.

However, in the 1990s and onwards the stratigraphical value of both Sabellidites cambriensis and Platysolenites became questioned. It was argued that the lack of associated diagnostic organic-walled microfossils (OWM) for the older strata, long ranges of the macroscopic taxa, strong facies dependence and diachronic facies distribution gave a low biostratigraphical value (e.g. Vidal & Moczydłowska, Reference Vidal and Moczydłowska1992, Reference Vidal and Moczydłowska1995; Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011; Pacześna, Reference Pacześna2014; Szczepanik & Żylińska, Reference Szczepanik and Żylińska2016), and the once widespread use of Sabellidites for correlation was disbanded.

Although the arguments against the stratigraphical usefulness to some extent are valid, they do not render Sabellidites cambriensis and Platysolenites biostratigraphically uninteresting. The evolutionary faunal sequence during earliest Cambrian time in Baltica is consistent and was developed in broadly the same type of widespread dominantly siliciclastic facies within a relatively short time interval; the Rovnian and Lontovan succession in Baltica span perhaps 10–15 Ma (Nielsen & Schovsbo Reference Nielsen and Schovsbo2011, p. 287). The flooding of an essentially flat craton (peneplain) in earliest Cambrian time was stepwise and proceeded by a series of rapid transgressive–regressive cycles with initially large clastic supplies to the basins, followed by less clastic supply as the sea level rose (Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011). The peri-cratonic deposits are thick and complete, while epi-cratonic deposits lack the oldest strata and the facies are much reduced in thickness. Sabellidites and Platysolenites are found in the fine-grained facies, representing periods of drowning and sea level highstand. This may pose a problem as the facies may develop differently along the platform and also be diachronous.

The distribution of OWM and small carbonaceous fossils (SCF) is therefore crucial. These are often restricted to certain facies or rocks with certain organic content (Woltz et al. Reference Woltz, Porter, Agić, Dehler, Junium, Riedman, Hodgskiss, Wörndle and Halverson2021), but this is not the same as their occurrence being facies dependent. As pelagic organisms, they may be reliable index fossils and especially useful for correlation, regardless of their distribution being better represented in particular fine-grained facies. As such, it is possible to state that the distribution of these fossils, for the purpose of biostratigraphy, are independent of facies. Thus, the co-occurrence of assemblages with Sabellidites and/or Platysolenites, trace fossils and certain OWM and potentially SCF is therefore important for dating and correlation of the sedimentary rocks in Baltica regardless of facies.

The terminal Ediacaran and very earliest Cambrian has a depauperate microfossil record with mainly leiosphaerids, although new studies have demonstrated a higher diversity (Jachowicz-Zdanowska, Reference Jachowicz-Zdanowska2011, Reference Jachowicz-Zdanowska2013; Szczepanik & Żylińska, Reference Szczepanik and Żylińska2016; Arvestål & Willman, Reference Arvestål and Willman2020; Agić et al., Reference Agić, Högström, Jensen, Ebbestad, Vickers-Rich, Hall, Matthews, Meinhold, Høyberget and Taylor2021). The lowest Cambrian strata are followed by an assemblage with the first small acanthomorphic acritarchs of the Asteridium–Comasphaeridium assemblage Zone, where Asteridium is a good marker for the start of Cambrian-type OWM assemblages. Note that the occurrence of Asteridium does not necessarily indicate the lowermost possible Cambrian.

As not all the stratigraphical components (i.e. Sabellidites, Platysolenites, trace fossils, OWM, SCF, shelly fossils) are present or well documented in all sections, correlation across Baltica is still challenging (see online Supplementary Material). However, Sabellidites is a useful taxon to include, as it is a distinct component of the earliest Cambrian evolutionary fauna, has a wide distribution and is fairly easy to recognize. The supposed Ediacaran range of Sabellidites cambriensis is equivocal for a number of reasons. Sabellidites cambriensis is found just below both the GSSP and Treptichnus pedum on Newfoundland and possibly just below Treptichnus pedum also on the Digermulen Peninsula, and as such is present in the Ediacaran. However, at both sites the co-occurrences of these two taxa are less than 5 m apart, and defining confident intervals for the extremely close stratigraphic proximity in these several hundred metre thick sequences is essentially irrelevant for the distribution. For practical purposes, it is found that they both first occur at the very base of the Cambrian and that the distribution of Sabellidites cambriensis therefore is truly basal Cambrian. It is thus not a diagnostic index fossil of a Terminal Ediacaran Stage as suggested by Muscente et al. (Reference Muscente, Bykova, Boag, Buatois, Mángano, Eleish, Prabhu, Pan, Meyer, Schiffbauer, Fox, Hazen and Knoll2019), but essentially of the basal Cambrian.

Stratigraphically old occurrences of Sabellidites cambriensis on the East European Platform cited in the literature are largely erroneous as pointed out by Sokolov (Reference Sokolov1997), although sabelliditid-like forms such as the saarinids Calyptrina and Saarina are found already in the Redkino Stage (Gnilovskaya, Reference Gnilovskaya1996; Sokolov, Reference Sokolov1997) (see online Supplementary Material).

In Poland, the original lower Cambrian boundary was traditionally placed at the base of the Włodawa Formation, encompassing the old usage Rovno Sabellidites Zone (Areń & Lendzion, Reference Areń and Lendzion1978; Lendzion, Reference Lendzion1983). Subsequent works, however, followed the Russian usage of the Vendian, with the upper boundary near the top of the Włodawa Formation. Thus the Sabellidites Zone, associated with a low-diversity leiosphaerid acritarch assemblage, was placed in the uppermost Vendian of the Russian usage (Moczydłowska & Vidal, Reference Moczydłowska and Vidal1986; Moczydłowska, Reference Moczydłowska1991, Reference Moczydłowska and Ahlberg1998). Therefore, the ensuing Asteridium–Comasphaeridium assemblage Zone encompassed the lower Cambrian, including the top of the Rovnian and the entire Lontovan and the Platysolenites Zone.

Later, however, the Vendian was simply equated with the Ediacaran in Poland (see for instance Moczydłowska, Reference Moczydłowska2008; Pacześna, Reference Pacześna2014), therefore by default assigning an Ediacaran age for the Sabellidites Zone. Furthermore, the Asteridium–Comasphaeridium Zone was taken to encompass the entire Fortunian Stage and extend into the undefined Stage 2 (Moczydłowska & Yin, Reference Moczydłowska, Yin, Zhao, Zhu, Peng, Gaines and Parsley2012), thus encompassing both the Rovnian and Lontovan in terms of the traditional Baltic stages. This view contrasts markedly with that in which a basal Fortunian leiosphaerid assemblage occurs prior to the Asteridium–Comasphaeridium Zone in both the Newfoundland type section and in Baltica (Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011; Szczepanik & Żylińska, Reference Szczepanik and Żylińska2016; Palacios et al. Reference Palacios, Jensen, Barr, White and Myrow2018, Reference Palacios, Högström, Ebbestad, Agić, Høyberget, Jensen, Meinhold and Taylor2020; Slater et al. Reference Slater, Harvey and Butterfield2018). With addition of the stratigraphical data on the distribution of Sabellidites cambriensis in Newfoundland and on the Digermulen Peninsula it seems clear that the taxon has an insignificant Ediacaran range, and a more extensive Ediacaran distribution of this taxon in Poland or elsewhere is most likely not the case (see also online Supplementary Material).

The sections with Sabellidites cambriensis on the Digermulen Peninsula described herein are the only outcrops in Baltica that offer a comprehensive record of the stratigraphical distribution of this taxon around the Ediacaran–Cambrian transition. The succession is directly comparable to that at Fortune Head on Newfoundland, with Sabellidites co-occurring with a diverse trace fossil association and OWM in a section seemingly without significant sedimentary breaks. The lower range of Sabellidites on the Digermulen Peninsula is firmly established, whereas the observed upper range may still prove to overlap with that of Platysolenites antiquissimus, as seen elsewhere in Baltica.

7.c. Platysolenites

The tubular foraminiferan Platysolenites has been regarded as a long-ranging taxon in Scandinavia, believed to be extending from the sub-trilobitic succession (Lontovan Stage) into the trilobite-bearing Holmia kjerulfi assemblage Zone (Vergalian–Rausvian stages) (Skjeseth, Reference Skjeseth1963; Bergström, Reference Bergström and Taylor1981; Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011). This assumption is partly based on two minute and doubtful fragments found in the Redalen Member of the Ringstrand Formation in the Mjøsa area of Norway reported by Vogt (Reference Vogt1924). A much-cited correlation of this finding within the younger Brennsætersaga Member of the Ringstrand Formation by Skjeseth (Reference Skjeseth1963) is probably erroneous and its occurrence is likely older, i.e. in the Skiagia–Fimbriaglomerella assemblage Zone (see discussion in Høyberget et al. Reference Høyberget, Ebbestad and Funke2019).

Rare specimens attributed to Platysolenites antiquissimus have been reported from the Sõru Formation (Dominopolian Stage, Rusophycus parallelum Zone) and basal Lükati Formation of western Estonia (Dominopolian Stage, Skiagia–Fimbriaglomerella assemblage Zone) (Mens & Pirrus, Reference Mens and Pirrus1977, Reference Mens, Pirrus, Raukas and Teedumäe1997; Mens, Reference Mens2003), which then represent the highest stratigraphical occurrence in Baltica.

In Newfoundland, McIlroy et al. (Reference McIlroy, Green and Brasier2001) reported one specimen of Platysolenites in the West Centre Cove Member of the Bonavista Formation, comprising the upper Aldanella attleborensis interval of the Sunnaginia imbricata Zone (Fletcher, Reference Fletcher2006). This is close to the upper part of the Terreneuvian (Palacios et al. Reference Palacios, Jensen, White and Barr2011) and only slightly older than the Lükati Formation and Mjøsa occurrences. Kouchinsky et al. (Reference Kouchinsky, Bengtson, Landing, Steiner, Vendrasco and Ziegler2017) found Platysolenites antiquissimus in carbonate facies in the Anabar Uplift on the Siberian Platform, where it ranges from low in the Anabarites trisulcatus Zone of the Nemakit–Daldyn Formation through the overlying Emyaksin/Medvezhya formations, its upper range possibly being within the Stage 2 – Stage 3 transition beds, which is close to the range in the Lükati Formation. Thus, the youngest records of Platysolenites seem to extend at least to the end of the Terreneuvian, being slightly younger on the Siberian Platform than most of the occurrences in Baltica (Lontovan and lowermost Dominopolian stages).

The earliest occurrences of Platysolenites in Newfoundland are within the lowermost Watsonella crosbyi Zone, while its range starts earlier in Siberia in the lower part of the Fortunian (Anabarites trisulcatus Zone) (Kouchinsky et al. Reference Kouchinsky, Bengtson, Landing, Steiner, Vendrasco and Ziegler2017). On the Digermulen Peninsula the oldest occurrence is within the lower part of the upper member of the Breidvika Formation, co-occurring with specimens of Ladatheca (McIlroy & Logan, Reference McIlroy and Logan1999; McIlroy & Brasier, Reference McIlroy, Brasier, Brasier, McIlroy and McLoughlin2017), which places the occurrence temporally very near to that in Newfoundland. In other parts of Finnmark, Platysolenites is found in beds attributed to the upper part of the lower member of the Breidvika Formation (see Högström et al. Reference Högström, Jensen, Palacios and Ebbestad2013).

Platysolenites is also found in Cambrian strata of California, Avalonian England and Wales, and southwestern Spain (Firby-Durham, Reference Firby-Durham1977; Vidal et al. Reference Vidal, Palacios, Moczydłowska and Gubanov1999; McIlroy et al. Reference McIlroy, Green and Brasier2001) generally associated with helcionelloid molluscs (Gubanov, Reference Gubanov2002).

Moczydłowska (Reference Moczydłowska1991) proposed a Platysolenites antiquissimus Interval-Zone, ranging from its first occurrence to the Schmidtiellus mickwitzi Zone (Skiagia–Fimbriaglomerella assemblage Zone). Based on other occurrences discussed herein (see online Supplementary Material) and in McIlroy et al. (Reference McIlroy, Green and Brasier2001), the distribution of Platysolenites is largely restricted to the Lontovan Stage of Baltica and it is a useful marker across this palaeocontinent.

7.d. Correlation of Sabellidites cambriensis in Baltica

The sections illustrated in Figure 10 are used to draw broad inferences on the correlation of Sabellidites cambriensis in Baltica (for discussion of sections see online Supplementary Material). It should be emphasized that the range and occurrences of macrofossils in the cores is partly a matter of chance and the OWM record is therefore of great importance. Additionally, diagnostic OWM zones are unknown in several successions, and these are marked as ‘arbitrary’ in Figure 10. Clearly, more detailed studies are needed to assess the biostratigraphical potential of Sabellidites, evaluated in the context of other macrofossils, OWM and the SCF record within each region. The correlation of Sabellidites cambriensis outside Baltica is not discussed further here, but it would be helpful to understand more about its distribution in the type sequences on Newfoundland.

Fig. 10. Tentative correlation of a selection of basal lower Cambrian sections in Baltica compared to the GSSP section at Fortune Head on the Burin Peninsula, Newfoundland in Canada. Note that this section is half the vertical scale in order to fit within the figure. The section names and locality numbers correspond to those found on the map (Fig. 9) and are referred to in the text and in the online Supplementary Material. Metres are indicated for the drill cores, to facilitate comparison. The distribution and ranges of the various taxa are discussed in the text and in the online Supplementary Material. Stratigraphical column, left-hand side: C. t. – Cruziana tenella; A. – Asteridium tornatum–Comasphaeridium velvetum assemblage Zone. Fortune Head: Ran. – Random Formation. Luobákti section: 1 – Lower sandstone member; 2 – Lower siltstone member; 3 – Red and green siltstone member; 4 – Upper sandstone member; 5 – Upper siltstone member; AS – Alum Shale Formation. Bornholm: H – Hadeborg Member. Estonia, Kunda section: V. Fm. – Voronka Formation; S – Sämi Member; K – Kestla Member; L – Lükati Formation.

Sabellidites cambriensis co-occurs in its oldest range with simple leiosphaerids and Granomarginata in Newfoundland, the Digermulen Peninsula, Poland and Ukraine (Granomarginata/leiosphaerids in Fig. 10). These two OWM components are reported with Sabellidites in other areas of Baltica, but usually also include Asteridium, although precise ranges are often unknown.

Platysolenites and Sabellidites show partly overlapping ranges in Estonia, the Moscow Basin, Ukraine and the East European Platform of Poland, whereas they do not overlap in Newfoundland, Scandinavia, Polish territories west of the Teisseyre–Tornquist Zone and Belarus. The situation in the White Sea area is unresolved. A single Sabellidites specimen in ‘member 5’ on Newfoundland occurs well above the earliest record of Platysolenites, although the main distribution is within ‘member 2’ and directly comparable with the range observed on the Digermulen Peninsula. On the other hand, Platysolenites also occurs stratigraphically above the highest Sabellidites record, in the upper Terreneuvian Bonavista Formation, giving it a range close to that seen elsewhere in Baltica, i.e. in the Lontovan (see the Fortune Head column in Fig. 10). The main difference is that the thicker succession in Newfoundland would give a larger overlap of the ranges than in the more condensed successions elsewhere, but with only two single occurrences this assumption is at the moment conjectural.

Aldanella co-occurs with Platysolenites in Newfoundland, at one locality in Northern Norway, in Estonia and in Poland. In Poland the uppermost range of Sabellidites may overlap with the occurrences of Aldanella, while in Estonia the recorded occurrences are c. 40 m apart. The presence of OWM is noted in all areas, although the precise range of OWM taxa relative to Sabellidites and Platysolenites is not always clear and more studies are needed.

The northernmost occurrences in Scandinavia, outside of the Digermulen Peninsula, are in the Luobákti section (see online Supplementary Material). Jensen & Grant (Reference Jensen and Grant1998) suggested an early Cambrian age for the entire succession, contrasting the earlier notion that it was late Precambrian (Vidal, Reference Vidal1981); Nielsen & Schovsbo (Reference Nielsen and Schovsbo2011) placed it entirely within the Dominopolian (Skiagia–Fimbriaglomerella Zone). Stodt et al. (Reference Stodt, Rice, Björklund, Bax, Halverson, Pharaoh, Arnaud, Halverson and Shields-Zhou2011) suggested that the oldest beds, the lower c. 10 m, correlate with the Ediacaran Lillevannet and Manndrapselva members in Finnmark, which poses challenges. The much discussed Vakkejokk breccia overlying the Treptichnus pedum level in the area may represent a proximal impact ejecta layer (Ormö et al. Reference Ormö, Nielsen and Alwmark2017), and if this is correct it is only of use in the local correlation. Sabellidites occurs in the lower siltstone member (see online Supplementary Material), which suggests a position in the basal Lontovan following the arguments presented here (unit 2 in Fig. 10), with Platysolenites in the lower part of the red and green mudstone (unit 3 in Fig. 10). The top part of the upper siltstone member of the Torneträsk Formation (unit 5 in Fig. 10, called the Grammajukku Formation by Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011) contains Holmia kjerulfi and stratigraphically higher trilobites (Vergalian–Rausvian), which then gives the upper age constraint at this section. Without a diagnostic OMW record (Vidal & Moczydłowska, Reference Vidal and Moczydłowska1996) a firm correlation of this section is difficult, especially for the lowermost part of the succession (Stodt et al. Reference Stodt, Rice, Björklund, Bax, Halverson, Pharaoh, Arnaud, Halverson and Shields-Zhou2011).

The Bornholm succession contains abundant Sabellidites in the Hadeborg Member but no other fossils. The boundary with the underlying Nexø Formation is sharp and conformable but may represent a sequence boundary (Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011). A Lontovan or older age for this occurrence is inferred (Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011), and the older option (Rovnian) is tentatively preferred here owing to the lack of Platysolenites.

The wide spatial distribution of the Sabellidites-bearing level in the White Sea area is even more extensive than on the Digermulen Peninsula, but the widespread occurrence of Sabellidites in a narrow stratigraphical interval is comparable. The presence of Asteridium and Granomarginata in the upper range of Sabellidites is confirmed in drill cores, but the ranges of the OWM relative to the ranges of Sabellidites and Platysolenites and the position relative to the Padun Group is unknown. At the moment the Padun Group succession is attributed to the lower Cambrian (Lontovan) (Kuznetsov et al. Reference Kuznetsov, Belousova, Alekseev and Romanyuk2014), although the lower part of the Nyugus and Zolotitsa formations may very well prove to be older.

The Estonian section serves as a model for the St Petersburg area and central Moscow Basin. The upper range of Sabellidites overlaps with Platysolenites, which is found throughout the Lontova Formation coinciding with the appearance of acritarchs of the Asteridium–Comasphaeridium Zone. The Lomonosov Formation of the St Petersburg area contains Platysolenites and Granomarginata, but seemingly not Asteridium (Podkovyrov et al. Reference Podkovyrov, Maslov, Kuznetsov and Ershova2017). A comparable distribution is seen in, for instance, the Toropets core in the Moscow Basin where Sabellidites in the Danilov beds (approximately equivalent with the Nekrasovo Formation) co-occurs with Asteridium in its lowest range and with Platysolenites in its upper range (Kirsanov, Reference Kirsanov, Zuravleva and Rozanov1974). Both Asteridium and Platysolenites range into the overlying Rusanov beds (= Lezha Formation), where other sabelliditids are also present.

The Polish sections are re-interpreted here, placing the Ediacaran–Cambrian boundary at the lowest occurrence of Sabellidites in the Włodawa Formation with reference to the Łopiennik core (see online Supplementary Material). The distribution of fossils is quite similar to that seen in the comparably thick sequences of Newfoundland and the Digermulen Peninsula, starting with Sabellidites and simple leiosphaerids before the appearance of Granomarginata. In the Mazowsze Formation, Sabellidites generally co-occurs with acritarchs of the Asteridium–Comasphaeridum Zone, resembling the situation seen in the Estonian successions. Other OWM and SCF, presumed to be of late Ediacaran age, occur just 6 m below the lowest occurrence of Sabellidites in the Włodawa Formation of the Łopiennik IG 1 core (Moczydłowska, Reference Moczydłowska2008; Moczydłowska et al. Reference Moczydłowska, Budd and Agić2015). They represent depauperate assemblages dominated by mainly Proterozoic prokaryotes, including some long-ranging taxa like Ceratophyton, known from the upper Ediacaran and lower Cambrian. Two specimens identified as the Tonian (c. 800 Ma) taxon Valkyria borealis (Moczydłowska, Reference Moczydłowska2008, fig. 8a, b) also resemble the early Cambrian problematicum Baltinema rana (Slater et al. Reference Slater, Harvey, Guilbaud and Butterfield2017, fig. 11a–m). The Włodawa specimens have shorter lateral protrusions and lack a longitudinal stripe within the main body characteristic of Valkyria (Butterfield et al. Reference Butterfield, Knoll and Swett1994) and are truncated, so more specimens need to be examined to confidently distinguish between Valkyria and Baltinema. Palaeopascichnus delicatus has been reported from the underlying Lublin Formation, but the figured specimen (Pacześna Reference Pacześna1986, pl. 1, fig. 2) is unusual in having a marginal border. Pacześna (Reference Pacześna1989) listed Harlaniella from the Lublin Formation, but this material has not been figured or described.

Although not exhibited in the section of Ukraine in Figure 10, where only the Bolshoi Obzyr drill core section is shown, the biostratigraphy appears similar to that of Poland, with Sabellidites occurring with leiosphaerids, Granomarginata and simple trace fossils followed by co-occurrence with Platysolenites. In the Ukrainian succession, the overlap of the ranges of sabelliditids and Platysolenites is short.

7.e. Rovnian and Lontovan

The Asteridium–Comasphaeridium Zone has since the 1990s been the first recognized OWM zone in the lower Cambrian of Baltica and characteristic of the Lontovan, originally including the occurrence of Granomarginata in the East European Platform of Poland (Moczydłowska, Reference Moczydłowska1991). Granomarginata is widespread and common in the Terreneuvian across the globe but also found infrequently in the upper Ediacaran (Agić et al., Reference Agić, Högström, Jensen, Ebbestad, Vickers-Rich, Hall, Matthews, Meinhold, Høyberget and Taylor2021). In Newfoundland, the oldest occurrence is well below the first occurrence of the Asteridium tornatum and the Asteridium–Comasphaeridium Zone as interpreted by Palacios et al. (Reference Palacios, Jensen, Barr, White and Myrow2018), who established a Granomarginata Zone.

However, this distribution of a Granomarginata Zone in Baltica is not clear-cut (Fig. 10). On the Digermulen Peninsula, Granomarginata is found with Sabellidites close to the Ediacaran–Cambrian boundary, while in Newfoundland Granomarginata appears just above Sabellidites and leiosphaerids. Furthermore, Asteridium appears in the upper range of Sabellidites on the Digermulen Peninsula, but both occurrences are well below that of Platysolenites. The distribution of Sabellidites and OWM in the Ukraine is comparable to that in Newfoundland, with a simple leiosphaerid assemblage followed by an assemblage with Granomarginata, whereas Asteridium seems to first appear in the Dominopolian (Talsy) Stage (Volkova et al. Reference Volkova, Kir’yanov, Piscun, Pashkyavichene, ]ankauskas, Keller and Rozanov1979, Reference Volkova, Kir’yanov, Piscun, Pashkyavichene, ]ankauskas, Urbanek and Rozanov1983; Konstantinenko & Kir’yanov, Reference Konstantinenko, Kir’yanov and Gozhyk2013); Sabellidites co-occurs with the first two assemblages, and Platysolenites with the Granomarginata assemblage (correlated with the Lontovan). In the East European part of Poland, it is not clear that the range of Granomarginata is lower than the Lontovan, while it seems to range into the Rovnian in the Polish territories west of the Teisseyre–Tornquist Zone where it occurs with simple leiosphaerids (Jankauskas & Lendzion, Reference Jankauskas and Lendzion1992; Jachowicz-Zdanowska, Reference Jachowicz-Zdanowska2013; Szczepanik & Żylińska, Reference Szczepanik and Żylińska2016).

The base of the Lontovan is not well constrained or defined, but it may be possible to reach a better biostratigraphical resolution for the stage. A Granomarginata Zone may either be distinguished as a lower zone, apart from the Asteridium–Comasphaeridium Zone, or included as a lowermost subzone of the latter zone. The base of the stage would also encompass the lower range of Platysolenites (the Platysolenites Interval-Zone sensu Moczydłowska, Reference Moczydłowska1991) and the upper range of Sabellidites, giving an overlapping range zone of the two. Additional sabelliditids like Paleolina and Sokoloviina, a shelly fauna including Aldanella and well-developed OWM of the Asteridium–Comasphaeridium Zone further distinguish the stage.

The assemblage zone itself is inadequate to establish the lower boundary of the Lontovan, for which OWM first appearance datum and range zones are needed (Szczepanik & Żylińska, Reference Szczepanik and Żylińska2016; Palacios et al. Reference Palacios, Jensen, Barr, White and Myrow2018). The tentative correlation of the local stages in Figure 10 is based on the assumption that the base of the Rovnian coincides with the base of the Treptichnus pedum Zone, and that the base of the Lontovan essentially corresponds to the first occurrence of Platysolenites following the Platysolenites Interval-Zone of Moczydłowska (Reference Moczydłowska1991), and close to the base of the Asteridium–Comasphaeridium Zone.

However, both on the Digermulen Peninsula and Newfoundland, the occurrence of Platysolenites and Asteridium (as an index species for the Asteridium–Comasphaeridium Zone) does not follow this pattern, and the base of the Lontovan would be high up in the Platysolenites level in those sections if the first occurrence of Platysolenites is used to define the base of this zone. This conundrum is difficult to resolve as long as the first appearance of Asteridium or Comasphaeridium is not well known in several areas, or the distribution differs as it does in Newfoundland. Thus, because of the lack of adequate fossil data from many sections, the placement of the Rovnian–Lontovan boundary in Baltica still remains somewhat arbitrary.