Organic-walled microfossils from the lower Cambrian of North Greenland: a reappraisal of diversity

The lower Cambrian Buen Formation of North Greenland is best known for its preservation of soft-bodied metazoans within the Sirius Passet Lagerstätte (Conway Morris et al. 1987; Conway Morris 1998; Peel and Ineson 2011; Harper et al. 2019). This formation has also yielded a diverse shelly fauna (Peel and Willman 2018) and fragmentary remains of non-mineralised Cambrian animals preserved in the form of small carbonaceous fossils (SCFs; Slater et al. 2018a; Wallet et al. 2021). Shales and mudstones from the Buen Formation also produce abundant organic-walled microfossils (principally acritarchs and filaments), yet these non-metazoan components of the biota have received relatively little focussed attention (the only analysis to date being that of Vidal and Peel 1993).

Acritarchs constitute some of the most abundant, spatially widespread and temporally continuous fossil remains from the Proterozoic to the Palaeozoic (e.g. Huntley et al. 2006; Knoll et al. 2006; Agić and Cohen 2021). In Cambrian deposits, these vesicular remains are reasonably interpreted as various cysts, envelopes and vegetative cells produced during the life cycle of unicellular phytoplankton (Tappan 1980; Colbath and Grenfell 1995; Talyzina and Moczydłowska 2000; Butterfield 2007; Moczydłowska 2010, 2016). As such, the Cambrian acritarch record represents a crucial window into primary productivity from a time when tiered ecosystems were first evolving.

Nevertheless, knowledge of acritarch biology and phylogenetic affinity has been drastically limited by their relatively simple morphology, and limited array of characters. For this reason, acritarchs are not assigned to any particular clade or group, but instead are considered incertae sedis and classified using an informal taxonomy based solely on morphology. Despite drawbacks, this classification scheme has great practical value and has enabled extensive use of acritarchs for biostratigraphical purposes (e.g. Volkova et al. 1979, 1983; Vidal 1981a; Moczydłowska and Vidal 1986, 1992; Eklund 1990; Moczydłowska 1991, 1998; Moczydłowska and Zang 2006; Zang et al. 2007; Ahn and Zhu 2017; Palacios et al. 2018, 2020), not least among Cambrian successions where alternative stratigraphical constraints are scarce (e.g. Palacios and Vidal 1992; Young et al. 1994; Vanguestaine et al. 2002; Palacios et al. 2012; White et al. 2012). The continued search for biostratigraphically significant acritarch morphologies has resulted in the creation of numerous form taxa, particularly from the late 1950s to early 1990s (Servais and Paris 2000). Nevertheless, the high variability seen among acritarch morphologies has rarely been acknowledged explicitly in species diagnoses. Difficulties in recognising ontogenetic, phenotypic and taphonomic sources of variation within and between established form species has resulted in a noticeable degree of taxonomic confusion (Stricanne and Servais 2002; Servais et al. 2004; Mullins and Servais 2008). A further problem that has afflicted organic-walled microfossils in particular, is the issue of taxonomic inflation, whereby multiple acritarch taxa previously described in isolation have been found to be subcomponents of multicellular organisms (e.g. Butterfield 2004, 2005; Smith et al. 2015; Shan et al. 2023).

Exactly how to accommodate this complex disparity into discrete and consistent taxonomic units has been debated extensively (Downie and Sarjeant 1963; Staplin et al. 1965; Sarjeant and Stancliffe 1994; Servais 1996; Fatka and Brocke 2008 and references therein). Disparity analyses of large acritarch populations have helped to resolve some of these taxonomic issues by laying bare the extent of morphological overlaps between established form taxa, particularly among Ordovician assemblages (Servais et al. 2004; Fatka and Brocke 2008 and references therein; Wang et al. 2017; Yan et al. 2017; Kroeck et al. 2020, 2021). However, Cambrian taxa have remained largely exempt from such population analyses (Kroeck et al. 2020; Wallet et al. 2022), meaning that currently used diagnoses reveal little about the hidden variation surrounding taxonomic boundaries.

Here we use organic-walled microfossils from the Buen Formation to inspect the disparity of a selection of well-known early Cambrian acritarch taxa. Palynological assemblages that formed the basis of original taxonomic descriptions of acritarchs from the Buen Formation (Vidal and Peel 1993) were re-examined and complemented with new material extracted using a gentle acid-maceration procedure (identical to the method used for SCF extraction; Butterfield and Harvey 2012). These combined records were scrutinised from a population-based perspective (Le Hérissé 1989; Fatka and Brocke 2008; Stricanne and Servais 2002; Servais et al. 2004; Wang et al. 2017; Yan et al. 2017; Kroeck et al. 2020, 2021; Wallet et al. 2022), in which the frequency distribution of observed morphologies is used to delimit broad groupings and evaluate taxonomic boundaries. This approach shows potential to uncover cases where taphonomy, ontogeny, or other factors are complicating taxonomic identification and blurring estimations of biological diversity.

A detailed account of the geological context of early Cambrian deposits in North Greenland is given by Higgins et al. (1991), Vidal and Peel (1993), Ineson and Peel (1997, 2011), and Peel and Willman (2018). Sedimentological and stratigraphical information are briefly summarised here.

The lower Cambrian Buen Formation is exposed in a number of localities across North Greenland, the majority cropping out along a west-east trending belt that terminates along the southern margins of Peary Land (Figure 1A). Successions from the Buen Formation record an episode of siliciclastic deposition informally subdivided into two members (Figure 1B). A sandstone-dominated lower member consists of multiple shallowing-upwards cycles with cross-beds and signs of bioturbation, and has been interpreted as representing deposition in shallow subtidal environments (Bryant and Pickerill 1990; Ineson and Peel 1997). A mudstone-dominated upper member consists of dark intercalations of mudstones and siltstones, with occasional sandstone interbeds, likely recording deposition in deeper, outer shelf settings (Vidal and Peel 1993). Sandstone interbeds become increasingly common towards the top of the upper member, evidencing a transition to inner-shelf conditions.

The mudstone-dominated basal portion of the upper member of the Buen Formation is well exposed at the Brillesø site (Figure 1A). In terms of palaeobiological context, this section has previously yielded acritarchs (Vidal and Peel 1993) and a relatively diverse metazoan biota recorded as macrofossils (Peel and Willman 2018) and SCFs (Slater et al. 2018a; Wallet et al. 2021). Biostratigraphical records of acritarchs from the Heliosphaeridium dissimilare–Skiagia ciliosa Zone (Vidal and Peel 1993) and trilobites from the Nevadia addyensis and Olenellus zones (Hollingsworth 2011; Peel and Willman 2018) constrain the age of the section to Cambrian Stages 3–4 (Montezuman–Dyeran; Figure 1C), possibly encompassing or lying just above the Stage 3/Stage 4 boundary.

This study is based on a reinvestigation of nine palynological slides from a previous study (Vidal and Peel 1993) complemented by three new samples collected from the same levels in the Brillesø site (Figure 1). GGU samples 184002 were collected from Brillesø locality 1 (B1; Figure 2B); while GGU samples 184003 and 184004 were collected from Brillesø locality 2 (B2, Figure 2B). A thickness of 9 m separates samples from B1 and B2. Detailed sampling information is given in Peel and Willman (2018) and Wallet et al. (2021, 2022).

The palynological material described by Vidal and Peel (1993) consists of taxa recovered from a larger range of samples covering various localities in North Greenland. These taxa include Archaeodiscina umbonulata Volkova 1968, ?Celtiberium geminum Fombella 1977, Dictyotidium perforatum Vidal 1993, Dictyotidium priscum Kiryanov & Volkova 1979 in Volkova et al. 1979, Fimbriaglomerella membranacea (Kirjanov 1974) Moczydłowska and Vidal (1988), Fimbriaglomerella minuta (Jankauskas 1979) Moczydłowska and Vidal (1988), Globosphaeridium cerinum (Volkova 1968) Moczydłowska 1991, Goniosphaeridium primarium (Jankauskas 1979) Downie 1982, Goniosphaeridium sp. A, Goniosphaeridium sp., Heliosphaeridium notatum (Volkova 1969) Moczydłowska 1991, Tasmanites volkovae Kirjanov 1974 and Trachysphaeridium timofeevi Vidal 1976. These taxa previously described from beyond the Brillesø locality will not be discussed in further detail in the present paper.

The palynological slides originally described in Vidal and Peel (1993) were prepared following a standard processing technique (Vidal 1988) and yielded a majority of small (< 50 µm) acritarchs. Newly processed samples were handpicked following gentle acid-maceration of rock samples in order to prospect for additional elements of the biota that may not have been detected previously. This procedure has been widely used to recover acritarch biotas in the Proterozoic (e.g. Butterfield et al. 1994; Sergeev et al. 2011; Tang et al. 2017) and SCFs in the Cambrian (e.g. Harvey et al. 2011; Butterfield and Harvey 2012; Slater et al. 2017; Slater and Bohlin 2022). Many acritarchs are too small to be handpicked using a stereomicroscope, and a larger mesh size is used to sieve preparations for hand picking. For these reasons, hand-picked palynomorphs tend to capture the larger-sized component of biodiversity that is often diluted (or fragmented) in conventional palynological preparations (Butterfield et al. 1994; Harvey and Pedder 2013).

In order to evaluate taxonomic boundaries, handpicked and conventional acritarchs from North Greenland were first classified according to established form taxonomy. Morphological variations within and between conventional form taxa were then assessed to define population-based categories. Within- and between-species variations are described in Section 6, and summarised in Section 3.3. Intermediate morphologies that occupy morphospace between multiple closely similar taxa were assigned to an established taxon upon critical analysis of their character combinations. Intermediate forms between two strongly distinct taxa (e.g. different genera) were more readily classified using open nomenclature. About 200 specimens were counted on each of the nine palynological slides to assess the relative abundance of recovered taxa and morphotypes (Supplementary material).

Specimens with a MGUH prefix are from the palynological preparations of Vidal and Peel (1993), deposited in the Natural History Museum of Denmark, Copenhagen. New specimens recovered in this study have a PMU prefix, and are hosted at the Museum of Evolution, Uppsala, Sweden.

The recovered biota encompasses a broad range of morphotypes attributable to 49 form taxa, 15 of which are described for the first time in North Greenland (Figure 2). The majority of the diversity was captured by palynological preparations investigated by Vidal and Peel (1993), but newly processed and hand-picked samples revealed the presence of several larger taxa that were not detected among previous studies, including Germinosphaera bispinosa Mikhailova 1986 emend. Butterfield et al. 1994, Tortunema wernadskii (Schepeleva 1960) Butterfield et al. (1994), Pterospermopsimorpha sp., an unknown form with similarities to Tappania, Palaeosiphonella sp. and other abundant large filaments. In conventional palynological preparations, small- (< 20 µm) and medium-sized (20–60 µm) acanthomorphs dominate the biota (Figure 3A), although significant differences were detected between assemblages from different samples (Figure 3C). Medium-sized acanthomorphs are dominated by acritarchs from the genus Skiagia, while small-sized acanthomorphs mostly consist of agglomerations of Comasphaeridium species (Figure 3B).

The revised list of form taxa present at Brillesø is presented in Figure 2. From this locality, Vidal and Peel (1993) reported a list of 29 form species, 27 of which were described. Descriptions of newly recovered taxa and amendments to previous taxonomic descriptions are placed in Section 6 (Systematic Palaeontology).

Newly recorded taxa include Alliumella? sp. (⁰⁰⁰¹, figure 1), Asteridium pallidum (Volkova 1968) Moczydłowska 1991 (⁰⁰⁰², figure 16; ⁰⁰⁰³, figures 3b, 4b), Comasphaeridium silesiense (Moczydłowska 1998) Jachowicz-Zdanowska 2013 (⁰⁰⁰², figure 8), Eomicrocystis malgica Golovenok & Belova 1986 (⁰⁰⁰³, figure 13), Fimbriaglomerella? sp. (⁰⁰⁰⁴, figure 9), Germinosphaera bispinosa Mikhailova 1986 emend. Butterfield et al. 1994 (⁰⁰⁰¹, figures 2–5), Granomarginata prima Naumova 1960 (⁰⁰⁰⁴, figure 11), Palaeosiphonella sp. (⁰⁰⁰⁵), Parmasphaeridium implicatum (Fridrichsone 1971) Jachowicz-Zdanowska 2013 (⁰⁰⁰⁶, figure 20), Polytrichoides spp. (⁰⁰⁰⁷, figure 22), Pterospermella solida (Volkova 1969) Volkova 1979 (in Volkova et al. 1979; ⁰⁰⁰⁴, figures 5–8), Pterospermopsimorpha sp. (⁰⁰⁰⁸), Skiagia cf. compressa (⁰⁰⁰⁶, figure 9), Skiagia pura Moczydłowska 1988 emend. (⁰⁰⁰⁶, figures 7–8), Symplassosphaeridium sp. (⁰⁰⁰³, figure 12), and Tortunema wernadskii (Schepeleva 1960) Butterfield et al. (1994) (⁰⁰⁰⁹, figure 3b). In addition, four unknown forms resembling Archaeodiscina (⁰⁰¹⁰, figure 9), Cymatiosphaera (⁰⁰¹⁰, figure 13), Tappania (⁰⁰⁰¹, figure 6) and Tasmanites (⁰⁰¹¹, figures 6–9) are described.

Unambiguous taxa that were previously recorded by Vidal and Peel (1993) and require no additional remarks or emendations are illustrated here but will not be described further (full taxonomic descriptions for these species can be found in Vidal and Peel [1993]). These taxa include Asteridium lanatum (Volkova 1969) Moczydłowska 1991 (⁰⁰⁰², figure 17; ⁰⁰⁰³, figure 4a), Asteridium tornatum (Volkova 1968) Moczydłowska 1991 (⁰⁰⁰², figure 19; ⁰⁰⁰³, figure 4c), Heliosphaeridium dissimilare (Volkova 1969) Moczydłowska 1991 (⁰⁰⁰², figure 24), Heliosphaeridium lubomlense (Kirjanov 1974) Moczydłowska 1991 (⁰⁰⁰², figure 25), Leiovalia tenera Kirjanov 1974 (⁰⁰¹², figure 16), Multiplicisphaeridium dendroideum (Jankaukas 1976) Jankaukas and Kirjanov 1979 (in Volkova et al. 1979; ⁰⁰⁰², figure 27), Skiagia compressa (Volkova 1968) Downie 1982 (⁰⁰⁰⁶, figure 6), Skiagia orbiculare (Volkova 1968) Downie 1982 (⁰⁰⁰⁶, figure 4) and Skiagia scottica Downie 1982 (⁰⁰⁰⁶, figure 1).

Other taxonomic identifications conducted by Vidal and Peel (1993) have been revised on the basis of our analysis of acritarch disparity. In particular, specimens previously attributed to Elektoriskos sp. A and Retisphaeridium dichamerum Staplin et al. (1965) have been placed in Skiagia ornata (Volkova 1968) Downie 1982 and Leiosphaeridia sp. A, respectively (Section 6). Specimens previously referred to as Lophosphaeridium sp. and Pterospermella sp. are here recognised as Lophosphaeridium tentativum Volkova 1968 (⁰⁰¹¹, figures 2–4) and Pterospermella velata Moczydłowska 1988 (⁰⁰⁰⁴, figures 1–3), respectively. On the other hand, the poorly preserved, unfigured specimen of Goniosphaeridium varium (Volkova 1969) Downie 1982 previously described by Vidal and Peel (1993) could not be localised and its presence in the Buen Formation is questionable. Comasphaeridium sp. A Downie 1982 is synonymised with Comasphaeridium mackenzianum Baudet et al. (1989) (⁰⁰⁰², figures 11–12); and Comasphaeridium longispinosum Vidal 1993 (⁰⁰¹³, figures 1–2) is renamed Comasphaeridium? brillesensis nom. nov. The diagnosis of Comasphaeridium? densispinosum Vidal 1993 (⁰⁰¹⁴) is emended and the taxon is placed in Pearisphaeridium gen. nov., becoming Pearisphaeridium densispinosum (Vidal 1993) comb. nov. Specimens previously recognised as A. ordensis (⁰⁰⁰², figure 20) are transferred to Heliosphaeridium? cf. dissimilare. Other minor emendations have been made to the descriptions of the following species: Comasphaeridium agglutinatum Moczydłowska 1988 (⁰⁰⁰², figures 9–10; ⁰⁰⁰³, figure 2a), Comasphaeridium molliculum Moczydłowska and Vidal (1988) (⁰⁰⁰², figures 4, 7), Comasphaeridium strigosum (Jankaukas 1976) Downie 1982 (⁰⁰⁰², figures 5–6), Comasphaeridium cf. strigosum (⁰⁰⁰², figures 1–2), Goniosphaeridium? cf. volkovae (⁰⁰⁰², figure 26), Granomarginata squamacea Volkova 1968 (⁰⁰⁰⁴, figures 14–20), Heliosphaeridium cf. coniferum (Downie 1982) Moczydłowska 1991 (⁰⁰⁰², figures 13–15), Heliosphaeridium obscurum (Volkova 1969) Moczydłowska 1991 (⁰⁰⁰², figure 21); Leiosphaeridia spp. (⁰⁰¹², figures 1–15), Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991 (⁰⁰¹⁰, figure 1), Lophosphaeridium tentativum Volkova 1968 (⁰⁰¹⁰, figures 2–4), Lophosphaeridium truncatum Volkova 1969 (⁰⁰¹⁰, figures 5–8), Skiagia ciliosa (Volkova 1969) Downie 1982 (⁰⁰⁰⁶, figures 2–3, 14–17), Skiagia ornata (Volkova 1968) Downie 1982 (⁰⁰⁰⁶, figure 5), Tasmanites bobrowskae Ważyńska 1967 (⁰⁰¹¹, figure 1) and Tasmanites tenellus Volkova 1968 (⁰⁰¹¹, figures 2–5).

Continuous variations are evident between most of the recorded acritarch taxa (Figure 4). In particular, small-sized acanthomorphs (x¯ = 8.9 µm in diameter) from the Asteridium–Comasphaeridium–Heliosphaeridium plexus exhibit variably coalesced, thread-like to spine-like processes showing wide variations in length, in total spanning the range of seven form species (Comasphaeridium agglutinatum, Comasphaeridium mackenzianum, Heliosphaeridium cf. coniferum, Heliosphaeridium? cf. dissimilare, Asteridium pallidum, Asteridium lanatum, and Asteridium tornatum; ⁰⁰⁰², figures 10–19; ⁰⁰⁰³, figures 1–4). Heliosphaeridium dissimilare, H. lubomlense and H. obscurum appear to lie outside this continuum (by virtue of their larger vesicle size [x¯ = 12.6 µm; Figure 5] and more sparsely distributed processes), but themselves form a continuum anchored around a continuous series of variation in the shape and size of their processes (⁰⁰⁰², figures 20–26). Even individual specimens (e.g. ⁰⁰⁰², figure 23) can simultaneously exhibit processes that are broad, narrow, solid and hollow; and are occasionally separated from the vesicle interior by a dark plug-like structure (⁰⁰⁰², figure 25a).

Transitional morphologies between Comasphaeridium strigosum, C. molliculum and C. silesiense are also common (⁰⁰⁰², figures 3–8), and occasionally grade into smaller forms resembling C. mackenzianum or C. agglutinatum (⁰⁰⁰², figure 9; Figure 4). At the opposite end of this spectrum are Comasphaeridium cf. strigosum vesicles, which have unusually long and broad processes (⁰⁰⁰², figures 1–2) that resemble those of Skiagia ornata or Skiagia cf. compressa (⁰⁰⁰⁶, figure 9), but lack funnel-shaped process tips. The Skiagia genus itself also encompasses a continuous range of forms grading from typical cyst-like vesicles having funnel-shaped process tips, to Baltisphaeridium-like morphologies covered in relatively long processes having apparently sharp or blunt tips (⁰⁰⁰⁶, figures 10–11), and Globosphaeridium-like morphologies having shorter spine-like processes (⁰⁰⁰⁶, figure 13). Some processes in these latter morphotypes terminate in small funnels (⁰⁰⁰⁶, figure 13a) characteristic of Skiagia.

Morphological overlap is also prevalent among species of ornamented sphaeromorphs. These include intermediate forms between all three Lophosphaeridium-type ornaments (⁰⁰⁰⁶, figures 18–19; ⁰⁰¹⁰, figures 4a, 5b), L. truncatum vesicles covered in localised dark patches reminiscent of the protuberances of Archaeodiscina (⁰⁰¹⁰, figure 8), overlapping L. truncatum-type and L. tentativum-type ornaments (⁰⁰¹⁰, figures 7a, b); overlapping L. tentativum-type and Tasmanites tenellus-type ornaments (⁰⁰¹¹, figures 3a, b, 5a), and continuous variations in pore size between all Tasmanites species (⁰⁰¹¹, figures 1–4). These morphotypes often exhibit truncated processes reminiscent of those found in Skiagia- or Comasphaeridium-type acanthomorphs (⁰⁰⁰⁶, figure 18; ⁰⁰¹⁰, figures 2a–c, 4a, 5a, 6a, 7b; ⁰⁰¹¹, figures 4a, 5b). Similarly, the numerous hollow conical processes of Skiagia ciliosa tend to develop an artificial ornament of pores surrounded by a darker rim, a surface sculpture reminiscent of Tasmanites volkovae (⁰⁰¹⁰, figures 10–11).

Rimmed acritarchs from the genus Pterospermella also exhibit continuous variation – in this case the supporting elements of the outer membrane display a continuum of forms resulting in transitional types between Pterospermella velata and Pterospermella solida (⁰⁰⁰⁴, figure 5). Flimsy radiating rods in these acritarchs occasionally resemble radial ‘muri’ connecting the central body to the outer membrane in Fimbriaglomerella (⁰⁰⁰⁴, figure 9a), a taxon that also closely resembles Skiagia scottica (Figure 4AT–AU). At the other end of this morphological sequence, rimmed specimens showing various degrees of opacity in the central body and a somewhat spongy texture may approach the morphological range of Granomarginata (⁰⁰⁰⁴, figure 4). The outer rim of Granomarginata also tends to develop a filamentous appearance that resembles densely distributed Comasphaeridium-like processes (⁰⁰⁰⁴, figures 19–21; Figure 4AM).

Despite their morphological simplicity, filamentous taxa are also grounded in continuous variations. For example, a single branched filament assigned to Palaeosiphonella sp. (⁰⁰⁰⁵, figure 5) shares numerous characteristics with a population of filaments attributable to Siphonophycus (⁰⁰⁰⁵). These shared features include a generally large size (> 15 µm in width), a sinuous morphology with localised constrictions, numerous compression folds, and bulbous or rounded terminations (⁰⁰⁰⁵, figures 5, 20–26). In contrast, narrower Siphonophycus filaments (⁰⁰⁰⁷) tend to exhibit an evenly tubular morphology, and occasionally occur in bundles similar to Polytrichoides (⁰⁰⁰⁷, figure 22), or feature a central lineation reminiscent of series of cellular trichomes seen in Eomicrocoelus (⁰⁰⁰⁷, figures 26–27).

Our new sampling has on the one hand considerably increased the known diversity of acritarchs from the Buen Formation – yet at the same time, our population-based analysis of disparity suggests that many of the recorded form taxa are simply taphomorphs, or just straightforward cases of phenotypic variation. Estimations of taxonomic richness based on form-taxa in such organic-walled microfossil assemblages are therefore problematic. Below we analyse the frequency distribution of observed morphologies to identify broad populations, evaluate their relationship with conventional form taxa, and refine our understanding of acritarch diversity in the Buen Formation.

The majority of form taxa we analysed appear to be embedded within a continuum of morphological variation that spans at least specific, and in some cases generic boundaries (Figure 4). Perhaps most significantly, such continuity between morphotypes is identified among acritarchs that at first appear to possess a different underlying construction, namely pteromorphic (i.e. rimmed), acanthomorphic (i.e. process-bearing), and sphaeromorphic acritarchs (i.e. lacking processes). Although the origin(s) of these overlaps may vary (for example, they may reflect life cycle stages), the impact of taphonomy on acritarch morphology is acknowledged as a significant source of this variation, which artificially generates intermediate morphologies (Grey and Willman 2009).

Taphonomic processes can have a profound impact on important taxonomic criteria even among macrofossil groups (Sansom et al. 2010), but acritarchs are especially susceptible to secondary distortion owing to their relatively simple morphologies. Indeed, acanthomorphic and pteromorphic acritarch morphologies tend to acquire a sphaeromorph-like construction as diagnostic surface features are lost through abrasion. This default trend is illustrated by the diverse ornamentations of apparently sphaeromorphic vesicles recovered here, including L. truncatum-like and L. tentativum-like specimens having subtle processes (⁰⁰¹⁰, figures 2a–2c, 4a, 5a, 6a, 7b), or vesicles covered in both pores and tubercles (⁰⁰¹⁰, figures 10–11; ⁰⁰¹¹, figure 5a). Possibly these represent vesicles adorned with hollow broken processes, where the inner and outer surfaces have coalesced during burial and flattening (Martí Mus 2014; Slater et al. 2018a, 2020). Similarly, various degrees of abrasion among Skiagia vesicles are evident as a distinct morphological gradient from Baltisphaeridium-like acritarchs lacking funnel-shaped process terminations (⁰⁰⁰⁶, figures 10–12), ranging through Globosphaeridium cerinum, Lophosphaeridium dubium and L. truncatum morphologies exclusively preserving the robust, apparently solid proximal portion of processes (⁰⁰⁰⁶, figures 13–15, 18–19). Some of these taphomorphs are strikingly similar to previously figured specimens of L. dubium and G. cerinum (e.g. Jachowicz-Zdanowska 2013, pl. 16, fig.16; Palacios et al. 2020, figures 5b, d, 6a, b). A critical reappraisal of the respective records of these species would be needed to clarify their stratigraphical and morphological range and detect signs of taphonomic overprinting, a prospect that could be facilitated by the use of simple quantitative criteria. For example, sculptural elements in Skiagia-derived forms exhibit larger size variations compared to their more convincing sphaeromorphic counterparts assigned to L. tentativum, Tasmanites tenellus and T. bobrowskae (Figure 6). By contrast, L. truncatum, which was originally defined as having truncated ciliae (Volkova 1969), is indistinguishable from ‘false’ sphaeromorphs derived from Skiagia in the recovered assemblage.

In addition to the taphonomic convergence of non-sphaeromorphs into sphaeromorphs, a number of taxonomically significant features exhibit variations that are likely secondary in origin. These include the diagnostic outer rim of Granomarginata that tends to develop a filamentous texture when abraded (⁰⁰⁰⁴, figures 13–14, 18–21), a process that culminates in rare, poorly preserved transitional forms attributable to either Granomarginata or C. strigosum (⁰⁰⁰⁴, figure 21; Spina et al. 2021, pl. 1, fig. 6–8; Agić et al. 2021). The relative scarcity of transitional forms between C. strigosum and Granomarginata (2 specimens recovered) implies that these taxa can relatively easily be distinguished in the recovered assemblage. In contrast, the general tendency of thin, densely distributed processes to coalesce with one another at their base hinders the recognition of a range of small-sized (< 20 µm) acritarch taxa (⁰⁰⁰², figures 10–15; ⁰⁰⁰³, figures 1–2); namely, C. agglutinatum (⁰⁰⁰², figures 9–10; ⁰⁰⁰³, figures 1–2; Figure 4), a taxon defined by its basally agglutinated processes; C. mackenzianum (⁰⁰⁰², figure 11), a taxon covered in thin filiform processes; and H. cf. coniferum (⁰⁰⁰², figure 14), a taxon characterised by expanded process bases (e.g. ⁰⁰⁰², figures 13–14). Process bases are also particularly prone to taphonomic distortion, as evidenced by the wide range of process morphologies observed on individual Heliosphaeridium vesicles (⁰⁰⁰², figures 13, 23, 25). Substantial variations in basal process width are also known from Skiagia (Moczydłowska 2011, pl. 14, figures 5–6; Wallet et al. 2022), and have been interpreted as possibly resulting from the taphonomic flattening of the surrounding vesicle wall. The morphology of process bases is an important species-level taxonomic criterion in Heliosphaeridium and Skiagia (Volkova 1968, 1969; Downie 1982; Moczydłowska 1991), and has been a persistent source of uncertainty in our taxonomic assessments. The multiple orientations and deformation of processes after mounting also contribute to variations in overall process morphology, while the taphonomic condensation of intracellular contents along processes (Staplin et al. 1965; Wood 2009) can account for variations in process colour, including darkened bases resembling plugs (⁰⁰⁰², figures 13a, 25a). Similar structures are found in a range of Skiagia species (⁰⁰⁰⁶, figures 1a, 3a, 8a, 9a, 12a, 14a; Wallet et al. 2022), but are considered diagnostic of S. ciliosa (Palacios et al. 2020). The unequal processes of clustered acanthomorphs, possessing otherwise even vesicle sizes (⁰⁰⁰³, figures 2–4), also suggests that surface abrasion is a significant source of disparity, particularly among small acanthomorphs from the Asteridium–Comasphaeridium–Heliosphaeridium plexus (⁰⁰⁰², figures 13–16, 20–25; Figure 4).

The issue of continuous variation among established acritarch taxa is increasingly being acknowledged (e.g. Stricanne and Servais 2002; Servais et al. 2004; Kroeck et al. 2021; Wallet et al. 2022). Nevertheless, how best to accommodate such continua of forms into rigid classification schemes relying on discrete morphological categories has yet to be resolved. Originally, this had been addressed by identifying broad groupings among large populations of acritarchs, in order to minimise intermediate forms between them (Downie and Sarjeant 1963). For example, a 20-µm size break has been recognised among large populations of acanthomorphic acritarchs (Downie 1958), a boundary that has been used to distinguish the large-sized (> 20 µm) taxon Baltisphaeridium Eisenack (1958), emend. Downie and Sarjeant (1963) from the small-sized (< 20 µm) taxon Micrhystridium Deflandre (1937), emend. Downie and Sarjeant (1963). These two size classes can clearly be identified in the studied assemblage, with only two intermediate forms between them (Figure 5).

This simple classification scheme was to some extent successful, however, with time Baltisphaeridium and Micrhystridium have grown into taxonomic ‘waste-baskets’ (Loeblich 1970; Moczydłowska 1991) encompassing a menagerie of forms with markedly distinct stratigraphical ranges. With a particular focus on applications to biostratigraphy, subsequent research on early Cambrian acritarchs has established new taxonomic units based on more subtle features, leading to a sharp rise in the number of published acritarch taxa (Servais and Paris 2000). In this context, Baltisphaeridium was dismantled into Skiagia (Downie 1982), Comasphaeridium (Downie 1982), Globosphaeridium (Moczydłowska 1991) and Lophosphaeridium species (Moczydłowska 1991); while Micrhystridium was split into Asteridium, Ammonidium, Heliosphaeridium and Comasphaeridium species (Lister 1970; Moczydłowska 1991; Sargeant and Vavrdová 1997). Although the splitting of ‘waste-basket’ taxa has substantially refined acritarch taxonomy and biostratigraphy, it has brought about a noticeable degree of confusion (Stricanne and Servais 2002; Servais et al. 2004; Mullins and Servais 2008) when applied in practice for two main reasons: (1) the partial overlaps existing between many acritarch species make identification equivocal, and conceal the presence of transitional forms (Wallet et al. 2022; Figure 18); and (2) the submicron-scale diagnostic features used for distinguishing between certain acritarch taxa require a combination of high optical resolution and exquisite state of preservation to be reliably detected. While the former likely obscures our understanding of phytoplankton palaeobiology by artificially splitting ontogenetic or ecophenotypic sequences into numerous form-taxa, the latter implies that imperfectly preserved acritarchs cannot be identified with certainty. In particular, the inner structure of processes (e.g. hollow versus solid, presence of septa or plugs) is an important genus-level taxonomic criterion (Moczydłowska 1991) that has been impractical to implement in many instances (e.g. in acritarchs bearing an apparent mixture of hollow and solid processes; Downie and Sarjeant 1963; Volkova 1968), and is largely unknown among small Micrhystridium-type acritarchs having submicron-wide processes (Sergeev et al. 2020).

Our results suggest that a direct use of species diagnoses in the studied assemblage leads to the taxonomic splitting of taphonomic series and other morphological continua of forms. A careful evaluation of the frequency distributions of recovered morphologies reveals ten abundance peaks (n > 10) corresponding to the following populations (Figure 7):

1. Heliosphaeridium (⁰⁰⁰², figures 20–25), distinguished from other Micrhystridium-type acritarchs in having a comparatively larger size (Figure 5) and more sparsely distributed processes;

2. Asteridium (⁰⁰⁰², figures 16–19), distinguished from other Micrhystridium-type acritarchs in their small dimensions, short processes (Figure 5), and a tendency to form densely packed aggregates (⁰⁰⁰³, figure 4);

3. Small (< 20 µm) Comasphaeridium (⁰⁰⁰², figures 10–15), distinguished from other Micrhystridium-type acritarchs in having small dimensions, long processes (Figure 5) and a tendency to occur in loose clusters (⁰⁰⁰³, figures 1–3);

4. Medium (20–40 µm) Comasphaeridium (⁰⁰⁰², figures 1–8), distinguished from Skiagia in having simple filiform processes;

5. Granomarginata (⁰⁰⁰⁴, figures 11–21), distinguished from Pterospermella in having a central discoid and outer rim of subequal thickness and a spongy and/or filamentous texture;

6. Skiagia (⁰⁰⁰⁶, figures 1–19), distinguished from other acanthomorphs in having funnel-shaped process tips;

7. Tasmanites (⁰⁰¹¹, figures 1–5), distinguished from other sphaeromorphs in having a perforated wall;

8. Synsphaeridium (⁰⁰⁰³, figures 5–11), distinguished from other sphaeromorphs in forming compact, highly organised clusters;

9. Palaeosiphonella (⁰⁰⁰⁵), distinguished from other filaments in having a large size, bulbous or semi-circular terminations, localised swellings/constrictions, and a branching habit;

10. Siphonophycus (⁰⁰⁰⁷, figures 1–21, 23–27), distinguished from other filaments in having a simple narrow, non-branching, tubular morphology.

With the exception of Comaspheridium, these peaks broadly correlate with acritarch genera, even though some populations are not monogeneric (e.g. Heliosphaeridium cf. coniferum here lies within the small Comasphaeridium population; see Supplementary material). In addition to these ten population-based groupings, 20 morphologically distinct singletons and small populations can be detected (Supplementary material), representing a total of 30 morphotypes. This informed measure of acritarch diversity contrasts markedly with the 49 form species recognised in the recovered assemblage, many of which represent arbitrary portions of abundance peaks (e.g. C. mackenzianum/H. cf. coniferum), taphonomic variants (e.g. H. obscurum; C. agglutinatum; A. pallidum), or rare transitional forms (Heliosphaeridium? cf. dissimilare; C. cf. strigosum). This case-study illustrates how conventional taxonomic practice can artificially inflate estimates of species richness in a typical early Cambrian assemblage – a finding that questions currently used measures of phytoplankton diversity (e.g. Nowak et al. 2015; Harper et al. 2021; Kroeck et al. 2022). The population-based approach adopted here offers the prospect of discretising disparity into objective units on the basis of observed frequency distributions; i.e. by defining boundaries so as to minimise the number of intermediate forms between categories. These local acritarch populations are crucial starting points for teasing apart the various factors that impinge on acritarch morphology – a pre-requisite for any meaningful estimation of acritarch diversity. For instance, it seems legitimate to consider the continuous range of Skiagia morphotypes recovered here (⁰⁰⁰⁶; Figures 4 and 5) as various ontogenetic and/or ecophenotypic expressions of one or two species (Wallet et al. 2022). In contrast, rare transitional forms such as S. pura, S. ciliosa (morphotype C) or Comasphaeridium cf. strigosum are unlikely to form part of a regular life cycle, and may instead be regarded as phenotypic abnormalities resulting from environmental stress, natural genetic variation (as seen in modern and fossil phytoplankton; e.g. Le Hérissé 1989, 2002; Ellegaard 2000; Falasco et al. 2009; Delabroye et al. 2012), or hybridisation events between closely related species (Bendif et al. 2019). Additional sampling of acritarch disparity across a variety of Cambrian settings may help to shed further light on the origin(s) of the phenotypic variation seen within individual populations, and refine our understanding of diversity at the base of early Palaeozoic food webs.

The updated list of species present in the assemblage conforms to previous correlations with the Heliosphaeridium dissimilare–Skiagia ciliosa (H–S) Zone of the East European Platform (Cambrian Stages 3–4; Vidal and Peel 1993; Wallet et al. 2022; Figure 2). Nevertheless, this interpretation implies pushing back the earliest record of Comasphaeridium silesiense, and expanding the range of Pterospermella velata, Asteridium pallidum and Comasphaeridium agglutinatum. Some of these species have been defined based on characters that are susceptible to taphonomic alteration, and can easily be mistaken for similar, but longer ranging taxa. For instance, the basally agglutinated processes of C. agglutinatum is observed in a wide range of more or less similar form taxa including C. strigosum (⁰⁰⁰², figures 5–6), C. mackenzianum (⁰⁰⁰², figure 11) and H. cf. coniferum (⁰⁰⁰², figure 13); while A. pallidum (⁰⁰⁰³, figure 3b, 4b) differs from A. lanatum (⁰⁰⁰², figure 17; ⁰⁰⁰³, figure 4a) only by having irregular truncated processes (Volkova 1968; Moczydłowska 1991). Comparable criteria (degree of separation and length of processes) have been used to distinguish between C. strigosum (early Cambrian), C. silesiense (Miaolingian–Furongian) and C. molliculum (Terreneuvian–Cambrian Stages 3–4), all of which show considerable morphological overlap in the recovered assemblage. Among pteromorphic acritarchs, secondary abrasion can result in morphologies grading towards G. prima (⁰⁰⁰⁴, figure 4, 12–13), a taphonomic process that may partly explain the anomalous occurrence of this taxon below and above its typical stratigraphical range in the Asteridium tornatum–Comasphaeridium velvetum (A–C) Zone (Volkova et al. 1983; Jankauskas and Lendzion 1992; Palacios et al. 2011).

Taphonomic sources of disparity compound upon the natural variation seen among different acritarchs, including those from the biostratigraphically significant genus Skiagia. The species-level taxonomy of Skiagia likely represents the complex life cycle history of one or a few biological species (Moczydłowska 2010; Wallet et al. 2022), and the recognition of further taphonomically generated variation in this taxon highlights its succeptibility to taxonomic confusion, particularly with respect to longer-ranging sphaeromorphs (e.g. L. dubium, L. truncatum, T. tenellus; Figure 2). Similarly, H. lubomlense (⁰⁰⁰², figure 25), H. obscurum (⁰⁰⁰², figure 21) and H. dissimilare (⁰⁰⁰², figure 24) are united by continuous variations in process morphology that are interpreted as being at least partly taphonomic in the present setting. Among all three species, Heliosphaeridium dissimilare shows the most extensive stratigraphical record ranging from the Terreneuvian to Cambrian Stage 4 (Palacios et al. 2020 and references therein; Figure 2), but whether the full disparity of the Heliosphaeridium genus is expressed throughout this interval remains to be determined.

The recognition of index taxa within continua of forms does not in itself undermine their biostratigraphical potential, as ontogenetic and/or phenotypic variants may have been added to an existing life cycle over the course of evolution without requiring events of speciation. Further, a high potential for morphological convergence among acritarchs suggests that multiple biological species and their phenotypic, ontogenetic and taphonomic variations could be represented within a single continuum of forms. This implies that portions of morphological continua can have biostratigraphical potential, yet our results suggest that these biostratigraphically significant portions of disparity have not been explicitly defined or distinguished from taphonomic sources of variation. Achieving this prospect would require a reinspection of acritarch disparity within and among successive biozones.

Alongside the background of small cyst-like acritarchs detected in previous palynological investigations of the Buen Formation (Vidal and Peel 1993), our additional sampling revealed more than a hundred large (60–250 µm) and/or asymmetrical acritarchs and abundant filamentous microfossils (^{0001,0005,0008,0009,0012,0013}). In contrast to typical Cambrian acanthomorphic acritarchs (e.g. Skiagia), most of which probably represent the remains of phytoplankton, the substantially larger size of these palynomorphs is suggestive of a benthic habit, particularly when their occasional asymmetry and consequent impact on buoyancy are considered (^{0001,0005,0013}; Butterfield 1997). Similarly, the abundant filaments recovered here show a tendency to preserve as agglomerations (⁰⁰⁰⁹, figure 1–2, 9) and mat-like associations (⁰⁰⁰⁹, figure 3–5) that likely covered the seafloor. These contribute to a small but increasing inventory of probable benthic organisms among Cambrian organic-walled microfossils, including for instance mat-like cyanobacterial colonies from the Miaolingian Kaili Formation (Harvey et al. 2011, fig. 1 A–C), the Cambrian Stage 4 filamentous Baltinema rana (Slater et al. 2017), the ‘giant’, actively growing Terreneuvian acritarch Lontohystrichosphaera grandis (Slater et al. 2018b), and the sheet-like problematicum Retiranus balticus (Slater et al. 2018b). These larger, likely benthic elements are relatively common among Proterozoic organic-walled microfossil assemblages, yet are often missing from descriptions of Cambrian assemblages. In the studied samples, large acritarchs and filaments were preferentially recovered using a low-manipulation protocol involving hand-picking of individual specimens, while conventional palynological preparations yielded a predominance of small acritarchs and amorphous debris, only occasionally preserving the remains of larger forms. The widespread use of conventional acid preparation techniques applied to Cambrian samples, and a general focus on biostratigraphically significant taxa may have contributed to this relative scarcity of information on large benthic acritarchs from Cambrian assemblages.

Traditionally, most Proterozoic and Phanerozoic acritarchs have been placed into separate form genera (Vidal and Nystuen 1990; Moczydłowska et al. 1993; Willman et al. 2006; Moczydłowska 2011). This taxonomic divide has been established on the basis of general patterns of variation in acritarch assemblages across the Ediaracan–Cambrian transition; namely (1), Cambrian acanthomorphic acritarchs are often one degree of magnitude smaller than their Proterozoic counterparts (< 75 µm; Vidal and Nystuen 1990; Butterfield 1997), (2) Cambrian acanthomorphic acritarchs often exhibit compelling evidence for excystment structures (Butterfield 1997; Willman et al. 2006), and (3) Cambrian acanthomorphic acritarchs have a different wall structure compared to their Proterozoic counterparts (Vidal and Nystuen 1990; Arouri et al. 2000; Talyzina and Moczydłowska 2000; Marshall et al. 2005; Willman et al. 2006; Willman 2009). While these general observations are supported by an abundant and widespread Ediacaran–Cambrian acritarch record, emerging evidence for some outliers to this pattern are creating taxonomic puzzles, particularly since morphological differences between Proterozoic and Phanerozoic acritarchs are not clearly captured by genus-level diagnoses. For example, recovered specimens of Comasphaeridium? brillesensis (Vidal 1993) nom. nov. (⁰⁰¹³, figures 1–2) are on average twice as large (> 120 µm in central body diameter) as the largest known Comasphaeridium species in the Cambrian acritarch record (Comasphaeridium maximum Palacios 2015), and lack evidence for excystment features. In this respect, these specimens may better conform to the Ediacaran genus Appendisphaera Moczydłowska et al. (1993) than to any other potential Cambrian taxon. Similar problems surround the classification of the specimen attributed to Unknown Form 3 (⁰⁰⁰¹, figure 6), which exhibits distinctive neck-like protrusions and asymmetrical processes, a morphology that falls within the known disparity of the Palaeoproterozoic–Mesoproterozoic taxon Tappania plana Yin 1997. On the other hand, Pearisphaeridium densispinosum (Vidal 1993) comb. nov. (⁰⁰¹⁴) exhibits a thick wall with unambiguous excystment structures typical of Cambrian acanthomorphs, but differs from the latter in having moderately large dimensions (53–79 µm in vesicle diameter) and biform processes (⁰⁰¹⁴, figures 7a, 7b, 9a, 9b). Apart from their widened terminations, the two process morphologies of Pearisphaeridium densispinosum (Vidal 1993) comb. nov. bear close similarities with those of the Ediacaran taxon Cavaspina basiconica Moczydłowska Vidal and Rudavskaya (1993) (Moczydłowska et al. 1993, fig. 8; Liu and Moczydłowska 2019, fig. 39), being either tubular or swollen/conical at the base, and grading into a more slender, hair-like distal portion.

While a few Ediacaran macrofossil and microfossil taxa can be recognised as parts of true boundary-crossing lineages based on their complex combination of characters (e.g. Cochleatina, Swartpuntia, Thaumaptilon; Hagadorn et al. 2000; Budd and Jensen 2017; Slater et al. 2020; Cuthill 2022), the bulk of the organic-walled microfossil record consists of likely convergent, simple morphologies that do not allow phylogenetic connections to be drawn across the Proterozoic–Cambrian boundary. Despite these limitations, the acritarch record remains one of the few statistically robust, continuous, and widespread fossil records available to palaeontologists (Butterfield 2003), and as such has been used intensively in large-scale studies of species richness and assemblage compositions in deep time (e.g. Knoll 1994; Vidal and Moczydłowska-Vidal 1997; Knoll et al. 2006; Huntley et al. 2006; Cohen and Macdonald 2015; Riedman and Sadler 2018; Zheng et al. 2020). In this context, the recognition of Proterozoic-like morphologies in Cambrian acritarch biotas poses important taxonomic dilemmas. While considering them as bona fide Cambrian form-taxa is at odds with the phenetic concept of form-taxonomy and artificially deepens the divide between Proterozoic and Cambrian acritarch biotas, classifying them into existing Proterozoic form taxa carries the risk of expanding taxonomic ranges over long periods in which these taxa are not recorded. Once again, considering form species in the context of their wider population variations may help to shed some light on these taxonomic issues. In the recovered assemblage, Unknown Form 3 and Comasphaeridium? brillesensis (Vidal 1993) nom. nov. are present as small (n < 2) populations that only capture a minor portion of the disparity seen within their candidate Proterozoic counterparts (i.e. Tappania [Yin 1997; Prasad et al. 2005; Leiming et al. 2005; Nagovitsin et al. 2010; Javaux and Knoll 2017. Singh et al. 2019] and Appendisphaera [Moczydłowska et al. 1993; Liu et al. 2014; Liu and Moczydłowska 2019; Anderson et al. 2019; Xiao et al. 2022]); an observation that casts serious doubts on any potential relationship between them. On the other hand, the distinct process morphology of Pearisphaeridium densispinosum (Vidal 1993) comb. nov. warrants its placement into a new genus, while the superficial similarities between this taxon and Cavaspina basiconica can only be judged as equivocal at present.

Newly recovered organic-walled microfossils from the Buen Formation substantially increase the known diversity of acritarchs and filamentous microfossils from the early Cambrian of North Greenland. A total of 49 form species are reported, of which 15 are described for the first time in the region. Part of this new diversity derives from the application of a low-manipulation acid maceration procedure that yielded large delicate acritarchs and mat-forming filaments – a rare record of presumably benthic organisms in Cambrian successions. On the other hand, our analysis of acritarch disparity has revealed that taphonomy is a significant source of the morphological variation within and among the recorded form taxa. Careful examination of within- and between-species variations allowed us to pare down the diversity to 30 distinct morphotypes, ten of which represent reasonably discrete abundance peaks. This study is an early step towards clarifying the morphological disparity of Cambrian acritarch taxa, understanding its origins, and refining our measures of within-assemblage diversity in Cambrian successions.

The species described here are listed in alphabetical order regardless of whether they represent acritarchs (i.e. vesicular remains of unknown affinity) or filaments (i.e. tubular remains of unknown affinity). The term ‘central body’ is applied following regular usage (Evitt 1963), referring to the central portion of the vesicle from which various elements (e.g. processes or outer membrane) protrude. This differs from the ‘inner body’ (e.g. Moczydłowska 1991), which refers to an internal spheroid occupying the central cavity of the vesicle. New taxonomic names follow the International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code).

The number of specimens given for each taxon is a qualitative estimation of the total number of specimens observed on all studied slides, including hand-picked specimens. A subset of these specimens was randomly counted to obtain relative abundance data from conventional palynological slides (Supplementary material). Measurements were conducted on a selection of well-preserved specimens.

Institutional abbreviations. GGU, Grønlands Geologiske Undersøgelse (Geological Survey of Greenland), Copenhagen, Denmark, now a part of the Geological Survey of Denmark and Greenland (GEUS). MGUH, Geological Museum (formerly MMH, Mineralogical Museum), Copenhagen, Denmark, now a part of the Natural History Museum of Denmark.

INCERTAE SEDIS

Genus Alliumella Vanderflit 1971 (in Umnova and Vanderflit 1971)

Type.Alliumella baltica Vanderflit, 1971 (in Umnova and Vanderflit 1971), early Cambrian of Russia.

Alliumella? sp.

⁰⁰⁰¹, figure 1 

Material. One specimen from GGU sample 184004.

Measurements.N = 1. Diameter of central body 21 µm; process length 7 µm; basal process width 2 µm; apical process width 1 µm.

Description. Smooth-walled vesicle having a single narrow, hollow tubular process freely communicating with the vesicle cavity. The process is slightly expanded at the base and does not appear to taper substantially towards the blunt termination.

Remarks. Recovered specimens differ from Alliumella baltica in having a narrower and more tubular process morphology, being only slightly expanded at the base and apparently not tapering into a pointed tip. Co-occurring specimens of Germinosphaera bispinosa further differ from Alliumella? sp. in their larger dimensions and wider process base.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Genus Asteridium Moczydłowska 1991 

Type.Asteridium lanatum (Volkova 1969) Moczydłowska 1991, lower Cambrian Holmia Zone of Poland.

Asteridium pallidum (Volkova 1968) Moczydłowska 1991 

⁰⁰⁰², figure 16; ⁰⁰⁰³, figures 3b, 4b 

Synonymy. 2013 Asteridium pallidum (Volkova 1968) Moczydłowska 1991; Jachowicz-Zdanowska, pp. 49–50, pl. 4, figure 6 [cum syn.].

Material. About 30 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 28. Diameter of central body 7.8–11.5 µm (x¯ = 9.4 µm; σ = 1.0), maximum process length 1–2.3 µm (x¯ = 1.5 µm; σ = 0.4).

Description. Spheroidal vesicles covered in numerous irregularly distributed processes. Processes have thickened, expanded bases producing an apparent pustulose ornamentation on the vesicle surface and imparting a somewhat irregular outline to the vesicle. Processes vary substantially in length, even on single specimens. Vesicles tend to occur in clusters of several dozen cells of similar size.

Remarks. The species differs from A. lanatum in having more irregular processes with blunt terminations (Volkova 1969; Moczydłowska 1991), and from A. tornatum in having longer processes (Volkova 1968). However, the propensity of processes to break beyond their thickened base can produce either short thorn-like ornaments (as in A. tornatum) or irregular blunt processes (as in A. pallidum) from A. lanatum morphologies. The co-occurrence of A. pallidum, A. lanatum and A. tornatum in compact clusters having an overall even cell size (⁰⁰⁰³, figure 4) strongly supports the conspecific origin of these form-taxa in the studied setting.

Occurrence and stratigraphical range. Cambrian Stage 3 of the East European Platform (Skiagia ornata–Fimbriaglomerella membranacea Zone; Volkova 1968; Volkova et al. 1979, 1983; Moczydłowska 1991; Jachowicz-Zdanowska 2011a), Denmark (Moczydłowska and Vidal 1992), Canada (Baudet et al. 1989) and Ireland (Gardiner and Vanguestaine 1971); early Cambrian–Miaolingian of Belgium (Vanguestaine 1978, 1992) and Cambrian Stages 3–4 of North Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Genus Comasphaeridium Staplin et al. 1965 emend. Sarjeant and Stancliffe (1994)

Type.Micrhystridium (= Comasphaeridium) cometes (Valensi 1948) Staplin et al. (1965), Middle Jurassic of France.

Comasphaeridium agglutinatum Moczydłowska 1988 

⁰⁰⁰², figures 9–10, ⁰⁰⁰³, figure 2a 

Synonymy. 2013 Comasphaeridium agglutinatum Moczydłowska 1988; Jachowicz-Zdanowska, p. 51, pl. 5, figure 1 [cum syn.].

Material. About 20 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 17. Diameter of central body 7.1–11.6 µm (x¯ = 8.5 µm; σ = 1.3), process length 2.3–7.8 µm (x¯ = 4.3 µm; σ = 1.4).

Description. Spheroidal vesicles having densely distributed, thin processes with agglutinated basal portions. When complete, processes are relatively long, reaching 70% of the central body diameter.

Remarks. Coalescence between neighbouring processes has been observed in a range of Comasphaeridium morphotypes having densely distributed processes (⁰⁰⁰², figures 5–6, 8–11), including in clustered acanthomorphs having broadly similar morphologies (⁰⁰⁰³, figures 1–3). This suggests that basal agglutination is a taphonomic feature resulting from the secondary flattening and coalescence of densely distributed processes. The C. agglutinatum specimens recovered here are best interpreted as taphonomic variants of C. mackenzianum or H. cf. coniferum, which together have similar dimensions (diameter of central body 6.5–10.8 µm [x¯ = 8.4 µm; σ = 0.95]; process length 2.4–6.5 µm [x¯ = 4.8 µm; σ = 1.0]; N = 38).

Occurrence and stratigraphical range. Terreneuvian of Poland (Asteridium tornatum–Comasphaeridium velvetum Zone; Moczydłowska and Vidal 1986; Moczydłowska 1988, 1991) and Newfoundland (Asteridium interval Zone; Palacios et al. 2018); Terreneuvian and Cambrian Stage 3 of South China (Zheng et al. 2020; Wang et al. 2022), the Czech Republic and Poland (Ichnosphaera flexuosa–Comasphaeridium molliculum Zone; Vavrdová et al. 2003; Jachowicz-Zdanowska 2011a, 2013; Szczepanik and Żylińska 2021) and Cambrian Stages 3–4 of North Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Comasphaeridium? brillesensis (Vidal 1993) nom. nov.

⁰⁰¹³, figures 1–2 

Synonymy. 1993 Comasphaeridium longispinosum Vidal 1993; Vidal and Peel, pp. 19, 21, figure 6a

Holotype. MGUH 21.516, GGU sample 184004, England finder coordinates H.46, Figure 16B.

Derivation of name. After Brillesø in North Greenland, the type locality of the species.

Material. One specimen from GGU sample 184002 and one specimen from GGU sample 184004.

Measurements.N = 2. Diameter of central body 122–160 µm, process length 42–47 µm.

Diagnosis. Large teardrop-shaped acritarch covered in numerous long filiform processes exceeding 25% of the vesicle diameter. Processes are extremely thin (< 0.5 µm) and tightly entangled.

Description. Large teardrop-shaped vesicle covered in slender, entangled processes representing 26% to 39% of the vesicle diameter. Processes appear to be distributed predominantly on the polar regions of the vesicle.

Remarks. Vidal and Peel (1993) described a new species under the name Comasphaeridium longispinosum Vidal 1993, but this name had already been used by Hagenfeldt (1989a) for a different taxon. The junior homonym Comasphaeridium longispinosum Vidal 1993 is here renamed Comasphaeridium? brillesensis nom. nov. while maintaining the original holotype. Vidal and Peel (1993) provided a brief description for this taxon (but no formal diagnosis). We clarify the diagnosis of Comasphaeridium? brillesensis (Vidal 1993) nom. nov. here based on a restudy of the holotype plus an additional newly recovered specimen (⁰⁰¹³, figure 1). Attribution to the genus Comasphaeridium is supported by the hair-like process morphology of the recovered specimens, yet the large size of these specimens exceeds that of other known Palaeozoic Comasphaeridium species (Cramer and María del Carmen 1977; Moczydłowska 1988, 1991; Yao et al. 2005; Jachowicz-Zdanowska 2013; Palacios 2015). The scarcity of available specimens (n = 2) precludes an accurate determination of whether the bipolar distribution of processes and the teardrop shape of the vesicles are original and consistent biological features. For these reasons, the genus-level taxonomic placement of Comasphaeridium? brillesensis (Vidal 1993) nom. nov. remains uncertain. An additional specimen (⁰⁰¹³, figure 3) with an elongated, oval vesicle covered in dense filiform processes may be included in this taxon, but also shows superficial similarities with co-occurring crustacean remains including mandibles and potential labra (Wallet et al. 2021, fig. 8F, 8N–Q).

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993; this study).

Comasphaeridium mackenzianum Baudet et al. (1989)

⁰⁰⁰², figures 11–12, ⁰⁰⁰³, figure 1 

Synonymy. 1982 Comasphaeridium caesariatum Wood & Clendening, p. 260, pl. 2, figures 1–3.

1982 Comasphaeridium sp. A Downie, p. 260, figure 6m.

1989 Comasphaeridium mackenziana Baudet et al., pp. 138–140, pl. 1, figures 14–17.

1989 Comasphaeridium sp. A Downie, 1982; Baudet et al. pl. 1, figures 18–19

1993 Comasphaeridium sp. A Downie, 1982; Vidal and Peel, p. 17, figure 8j.

1993 Heliosphaeridium coniferum (Downie 1982) Moczydłowska 1991; Vidal and Peel, p. 25, figures 8g, k.

Material. Several hundreds of specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 37. Diameter of central body 6.5–10.8 µm (x¯ = 8.4 µm; σ = 1.0), process length 2.4–6.5 µm (x¯ = 4.8 µm; σ = 1.0).

Description. Small spheroidal to oval vesicles having densely distributed, thin filiform processes that tend to entangle into broader threads. Processes may be as long as the total diameter of the central body.

Remarks. Baudet et al. (1989) reported their specimens on a vesicle diameter (Y) vs. process length (X) plot which was used as a basis for the creation of a new species, C. mackenzianum, distinct from Downie’s Comasphaeridium sp. A (1982). According to their classification, every specimen falling above the line Y/X = 1.33 was attributed to C. mackenzianum, and those falling below to Comasphaeridium sp. A. However, the distribution of these species on the plot does not show clear groupings on either side of the Y/X = 1.33 line; this boundary appears to have been defined arbitrarily. A similar plot using populations from the present study showed no evidence of quantitative separation between C. mackenzianum and Comasphaeridium sp. A (Figure 8), and the average dimensions of Comasphaeridium sp. A reported by Vidal and Peel (1993; vesicle diameter 8.4 µm, process length 5.5 µm) clearly fall outside the assumed range of this species below the Y/X = 1.33 line. For these reasons, we consider the boundary between C. mackenzianum and Comasphaeridium sp. A to be artificial, and synonymise these two species. Bundles of processes in C. mackenzianum also tend to break at various locations, which occasionally results in irregular forms similar to Asteridium pallidum (⁰⁰⁰², figure 16). Nevertheless, vesicles attributed to Asteridium tend to have a thickened, slightly widened base compared to C. mackenzianum, and intermediate forms between these taxa are relatively rare. In contrast, C. mackenzianum is difficult to distinguish from the recovered Heliosphaeridium cf. coniferum population owing to the propensity of C. mackenzianum processes to entangle into broader, thick threads having an apparently conical morphology reminiscent of H. coniferum (⁰⁰⁰², figure 12; ⁰⁰⁰³, figure 1). Specimens having such thick processes were previously assigned to H. coniferum (Vidal and Peel 1993, fig. 8g, k), but are here recognised as taphonomic, ontogenetic and/or ecophenotypic variants of C. mackenzianum on the basis of observed population variations. Other specimens having more clearly defined, wider process bases that are less likely to result from taphonomic alteration of C. mackenzianum are here assigned to H. cf. coniferum (e.g. ⁰⁰⁰², figure 14). Specimens from the type locality of H. coniferum (Downie 1982) differ from either C. mackenzianum and H. cf. coniferum in having much fewer processes with wider bases.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Sweden (Skiagia ornata–Fimbriaglomerella membranacea and Heliosphaeridium dissimilare–Skiagia ciliosa zones; Moczydłowska 2002), Scotland, Norway and Alberta (Downie 1982), Spain (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Palacios et al. 2006), Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993) and Cambrian Stage 3 of Tennessee (Wood and Clendening 1982) and Canada (Baudet et al. 1989).

Comasphaeridium molliculum Moczydłowska and Vidal (1988)

⁰⁰⁰², figures 4, 7 

Synonymy. 2013 Comasphaeridium molliculum Moczydłowska and Vidal (1988); Jachowicz-Zdanowska, p. 52, pl. 16, figure 23 [cum syn.].

Material. About 20 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 14. Diameter of central body 16.5–31.8 µm (x¯ = 23.5 µm; σ = 4.1), process length 4.7–12 µm (x¯ = 8.4 µm; σ = 2.3).

Description. Oval to spheroidal vesicles covered in densely distributed, thin filiform processes having distinct bases. The flexible nature and entangling habit of processes often result in an even filamentous rim surrounding the central body.

Remarks.C. molliculum differs from C. strigosum in having thinner, clearly separated processes (Moczydłowska and Vidal 1988; Moczydłowska 2002). However, both criteria vary continuously in the studied assemblage and the two species were therefore segregated on an arbitrary basis, following a qualitative evaluation of the degree of separation and width of processes in individual vesicles. For example, specimens having relatively thin processes with unclear bases owing to their dense distribution on the vesicle surface (⁰⁰⁰², figure 5) were assigned to C. strigosum; while specimens having relatively thick processes with clearly separated bases (⁰⁰⁰², figure 4) were assigned to C. molliculum.

Occurrence and stratigraphical range. Cambrian Stages 3 of the East European Platform (Skiagia ornata–Fimbriaglomerella membranacea Zone; Volkova et al. 1979), Canada (Palacios et al. 2018), Estonia (Moczydłowska 2011); Poland (Moczydłowska 1991; Jachowicz-Zdanowska 2013) and the Czech Republic (Jachowicz-Zdanowska 2013); Cambrian Stages 3–4 (Skiagia ornata–Fimbriaglomerella membranacea and Heliosphaeridium dissimilare–Skiagia ciliosa zones) of Denmark (Moczydłowska and Vidal 1992), Sweden, Norway (Moczydłowska and Vidal 1986; Hagenfeldt 1989b; Moczydłowska et al. 2001; Moczydłowska 2002) and Greenland (Moczydłowska and Vidal 1986; Vidal and Peel 1993); Terreneuvian of the Czech Republic and Poland (Vavrdová et al. 2003; Szczepanik and Żylińska 2021), early Cambrian of Ireland (Brück and Vanguestaine 2004).

Comasphaeridium silesiense (Moczydłowska 1998) Jachowicz-Zdanowska 2013 

⁰⁰⁰², figure 8 

Synonymy. 2013 Comasphaeridium silesiense (Moczydłowska 1998) Jachowicz-Zdanowska; pp. 52–53, pl. 26, figure 3; pl. 30, figures 1–11 [cum syn.].

Material. One specimen from GGU sample 184004.

Measurements.N = 1. Diameter of central body 17 µm, process length 9 µm.

Description. Spheroidal vesicle having densely distributed, partly fused, thin processes of approximately equal size. Processes are relatively long, representing about half of the vesicle diameter.

Remarks.Comasphaeridium silesiense has previously been distinguished from C. strigosum on the basis of its generally longer processes (ca. 50% of the vesicle diameter) of approximately equal length (Moczydłowska 1988; Jachowicz-Zdanowska 2013) fused at their tips (Palacios et al. 2017). The close similarities between these two taxa have nonetheless remained problematic given the susceptibility of these criteria to taphonomic breakage and coalescence, and the continuous variations they show in the present C. strigosum/C. molliculum/C. silesiense material indeed questions their reliability as diagnostic characters.

Occurrence and stratigraphical range. Miaolingian–Furongian of Poland (Moczydłowska 1998; Jachowicz-Zdanowska 2013), Libya (Albani et al. 1991), Ireland (Moczydłowska and Crimes 1995; Brück and Vanguestaine 2004; Vanguestaine and Brück 2008); Miaolingian of Spain (Fombella 1979, 1986, 1987; Palacios et al. 2006; Palacios 2015), Iran (Ghavidel-Syooki 2019; Spina et al. 2021), Canada (Palacios et al. 2017) and Baltica (Hagenfeldt 1989b; Moczydłowska 1999); Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Comasphaeridium strigosum (Jankaukas 1976) Downie 1982 

⁰⁰⁰², figures 5–6 

Synonymy. 2013 Comasphaeridium strigosum (Jankaukas 1976) Downie 1982; Jachowicz-Zdanowska, p. 51, pl. 5, figure 1 [cum syn.].

non 1982 Comasphaeridium sp. A.; Downie, p. 260, figure 6m.

1993 Comasphaeridium cf. strigosum (Jankaukas 1976) Downie 1982; Vidal & Peel, p. 21, figures 7c, h. non 1993 Comasphaeridium strigosum (Jankaukas 1976) Downie 1982; Vidal & Peel, p. 21, figure 7g.

Material. About 20 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 6. Diameter of central body 20.3–25.5 µm (x¯ = 22 µm; σ = 1.9), process length 6.8–11.7 µm (x¯ = 9.0 µm; σ = 2.0).

Description. Spheroidal to oval vesicles having densely distributed, thin filiform processes. Processes are relatively long, representing 27–55% of the diameter of the central body.

Remarks. The specimens included in this population meet the majority of criteria for inclusion in C. silesiense (e.g. fused process tips [Palacios et al. 2017]; a process length exceeding a third of the vesicle diameter [Jachowicz-Zdanowska 2013]), but differ from the latter in having apparently unequal process lengths. Since this criterion was used as a diagnostic feature of C. silesiense (Jachowicz-Zdanowska 2013), C. strigosum was segregated from C. silesiense on this basis. However, it should be noted that taphonomic breakage can result in unequal process lengths, and that the entangling habit, sinuous morphology and dense distribution of processes in these species do not permit to measure the length of processes accurately. The relatively uniform distribution of vesicle dimensions among medium-sized Comasphaeridium vesicles (Figure 9) suggests that C. strigosum and C. silesiense likely form part of the same population. Jachowicz-Zdanowska (2013) synonymised Comasphaeridium sp. A Downie, 1982 with Comasphaeridium strigosum. Judging from Downie’s figured specimen (1982, fig. 6m) and considering the dimensions of a similar population reported by Baudet et al. (1989; diameter of the central body 5–11 µm) from Belgium, Comasphaeridium sp. A Downie, 1982 appears to be substantially smaller than C. strigosum, and is instead placed in synonymy with C. mackenzianum.

Occurrence and stratigraphical range. Terreneuvian to Miaolingian of the East European Platform (Asteridium tornatum–Comasphaeridium velvetum Zone; Volkova 1969; Volkova et al. 1979; Moczydłowska 1991); Cambrian Series 2 of Kazakhstan (Ogurtsova 1985; Sergeev et al. 2020); Cambrian Stages 3–4 to Miaolingian of Norway (Holmia Zone; Vidal 1981a; Downie 1982; Moczydłowska and Vidal 1986), Scotland (Downie 1982); Sweden (Moczydłowska and Vidal 1986; Hagenfeldt 1988, 1989b; Eklund 1990), Denmark (Moczydłowska and Vidal 1986, 1992), Spain (Palacios and Vidal 1992), Poland and the Czech Republic (Jachowicz-Zdanowska 2013); Greenland (Vidal 1979; Downie 1982; Moczydłowska and Vidal 1986; Vidal and Peel 1988, 1993) and Canada (Palacios et al. 2017); Terreneuvian to Cambrian Stage 4 of the Siberian Platform (Vidal et al. 1995); lower Cambrian of Svalbard (Knoll and Swett 1987).

Comasphaeridium cf. strigosum

⁰⁰⁰², figures 1–2 

Synonymy.non 1993 Comasphaeridium cf. strigosum (Jankauskas) Downie 1982; Vidal & Peel, p. 21, figure 7c.

1993 Comasphaeridium cf. strigosum (Jankauskas) Downie 1982; Vidal & Peel, p. 21, figure 6b.

Material. Two specimens from GGU sample 184003.

Measurements.N = 2. Diameter of central body 25.1–26 µm; process length 16.8–18 µm; process width ∼ 0.6 µm.

Description. Oval to spheroidal vesicles covered with densely distributed, thin, long and entangled processes with a tendency to fuse at their tips. The morphology of process tips appears to be blunt or truncated in most cases. The length of processes exceeds two thirds of the diameter of the central body.

Remarks. These specimens are interpreted as transitional forms between C. strigosum and S. ornata. C. cf. strigosum resembles S. ornata in its overall dimensions, and potentially in having faint funnel-shaped process terminations (⁰⁰⁰², figure 2a). However, substantial uncertainty surrounds the latter observation, not least given the propensity of processes to overlap and fuse. Vesicles attributed to C. cf. strigosum show a predominance of Comasphaeridium features warranting their taxonomic placement in this genus; namely, a dense cover of flexible, filiform processes. C. cf. strigosum differs from C. strigosum in having unusually long processes and a slightly larger central body size.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993; this study).

Genus Eomicrocystis Golovenok and Belova (1984)

Type.Eomicrocystis malgica Golovenok & Belova 1986, late Mesoproterozoic of Siberia.

Eomicrocystis malgica Golovenok and Belova (1986)

⁰⁰⁰³, figure 13 

Synonymy. 2017 Eomicrocystis malgica Golovenok and Belova 1986; Javaux and Knoll, pp. 206–207, figures 2.16–2.18 [cum syn.].

Material. Two specimens from GGU sample 184003.

Measurements.N = 2. Diameter of vesicle 2.4–4.1 µm; maximum length of clusters 31.2–65.3 µm.

Description. Irregular clusters of loosely packed, small, oval to spheroidal vesicles lacking an enclosing envelope.

Remarks. The species differs from E. elegans and Synsphaeridium in the smaller size of vesicles in the cluster, and from Symplassosphaeridium in forming loosely packed aggregations.

Occurrence and stratigraphical range. Mesoproterozoic of Siberia (Golovenok and Belova 1986), Canada (Hofmann and Jackson 1994) and Montana (United States; Adam et al. 2016); Mesoproterozoic and Neoproterozoic of Australia (Cotter 1997; Javaux and Knoll 2017); India (Prasad and Asher 2001; Singh and Babu 2013) and Mauritania (Beghin et al. 2017); Neoproterozoic of the Democratic Republic of Congo (Baludikay et al. 2016) and Estonia (Arvestål and Willman 2020); Cambrian Stage 4 of China (Wang et al. 2022); Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Genus Fimbriaglomerella Loeblich and Drugg (1968)

Type.Fimbriaglomerella divisa Loeblich and Drugg (1968), Early Devonian of the United States.

Fimbriaglomerella? sp.

⁰⁰⁰⁴, figure 9 

Material. One specimen from sample 184004.

Measurements.N = 1. Diameter of the vesicle 31.8 µm, diameter of the inner body 22.1 µm.

Description. Vesicle consisting of a spheroidal, dark brown inner body surrounded by an irregular, thin and translucent outer membrane that appears to be supported by flimsy radial elements resembling muri.

Remarks. The recovered specimen is similar to Fimbriaglomerella membranacea in having a filmy membrane and numerous muri-like elements, but the exact arrangement of these radial structures and the shape of their contact with the outer membrane is obscured by the dark appearance of the inner body. In the absence of evidence for polygonal luminae forming a reticulum on the vesicle surface, the generic attribution of the recovered specimen remains tentative, particularly given its resemblance to Pterospermella solida or P. vitalis (e.g. Moczydłowska 1991, pl. 4, fig. E).

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Genus Germinosphaera Mikhailova 1986 emend. Butterfield et al. (1994)

Type.Germinosphaera bispinosa Mikhailova 1986, Neoproterozoic of Siberia.

Germinosphaera bispinosa Mikhailova 1986 emend. Butterfield et al. (1994)

⁰⁰⁰¹, figures 2–5 

Synonymy. 2019 Germinosphaera bispinosa Mikhailova 1986; Miao et al. pp. 187–188, figure 5d-f [cum syn].

Material. One specimen from GGU sample 184002, two specimens from GGU sample 184003 and two specimens from GGU sample 184004.

Measurements.N = 5. Diameter of vesicle 48.3–98.5 µm (x¯ = 66.6 µm; σ = 22.2); maximum process length 19–42 µm (x¯ = 26.5 µm; σ = 9.4); basal process width 6.8–54.9 µm (x¯ = 24.7 µm; σ = 18); distal process width 4.4–11.9 µm (x¯ = 8.3 µm; σ = 3.1).

Description. Smooth-walled, oval to spheroidal vesicles having one or two bipolar processes arranged in the equatorial plane. Processes widen upon contact with the vesicle cavity, with which they communicate freely. Process terminations are truncated and opened.

Remarks. The form genus Germinosphaera accommodates acritarchs showing considerable morphological variation that has previously been discretised into numerous form-species. Subsequent taxonomic studies (Butterfield et al. 1994; Miao et al. 2019) demonstrated that species previously recognised as G. fibrilla, G. jankauskasii, G. guttaformis, G. rudis and G. tadasii likely represent taphonomic or ontogenetic variants of a single taxon, G. bispinosa. The diagnosis of this species has been emended by Miao et al. (2019) to encompass all acritarchs having a psilate wall and one or several tubular processes distributed irregularly or equatorially on the vesicle surface. These characteristics fully conform to the morphology of the G. bispinosa specimens recovered here, even though the recovered population only shows a small portion of the disparity seen in Germinosphaera from Proterozoic assemblages.

Occurrence and stratigraphical range. Mesoproterozoic–Neoproterozoic of Russia (Timofeev et al. 1976; Mikhailova 1986; Jankauskas et al. 1989; Mendelson and Schopf 1992; Schopf and Klein 1992; Semikhatov et al. 2000; Vorob’eva et al. 2009; Shuvalova et al. 2021), Canada (Loron et al. 2019, 2021) and the Democratic Republic of the Congo (Baludikay et al. 2016); late Paleoproterozoic to Neoproterozoic of North China (Yan 1995; Yin and Guan 1999; Li et al. 2019; Miao et al. 2019, 2021); Neoproterozoic of Sweden (Loron and Moczydłowska 2018; Moczydłowska et al. 2018), Svalbard (Knoll et al. 1991; Butterfield et al. 1994), Australia (Zang 1995), Brazil (Lehn et al. 2019) and India (Prasad et al. 2005); latest Ediacaran to early Cambrian of India (Prasad and Asher 2001); Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Genus Goniosphaeridium Eisenack 1969 emend. Kjellström 1971 

Type.Baltisphaeridium (= Goniosphaeridium) polygonale (Eisenack 1931) Eisenack 1959, Silurian of Baltica.

Goniosphaeridium? cf. volkovae

⁰⁰⁰², figure 26

Synonymy. 1993 Goniosphaeridium volkovae Hagenfeldt 1989b; Vidal & Peel, p. 25, figure 8h.

Material. One specimen from GGU sample 184002.

Measurements.N = 1. Diameter of central body 12.8 µm, process length 6 µm, basal process width 1.5 µm.

Description. Spheroidal vesicle having about 25 long, horn-like, relatively broad processes gradually expanding towards the base. Process tips appear to be sharp.

Remarks. Specimens previously attributed to Goniosphaeridium volkovae Hagenfeldt 1989b by Vidal and Peel (1993) have a central body of 7–8 µm in diameter, and a process length of 1.5–4 µm (N = 20). Of the 20 specimens identified by Vidal and Peel (1993), only one could be located on the studied slides (Vidal and Peel 1993, fig. 8h). This specimen slightly exceeds the size range reported by Vidal and Peel (1993), but is still substantially smaller than G. volkovae populations from elsewhere (e.g. in Sweden: central body diameter 18–28 µm; process length 5–14 µm; Hagenfeldt 1989b; in Poland and the Czech Republic: central body diameter 20–30 µm; process length 10–15 µm; Jachowicz-Zdanowska 2013), and smaller than the lower size limit of 20 µm stated in the diagnosis of Goniosphaeridium (Kjellström 1971). Despite size differences, this specimen is similar to G. volkovae in gross morphology, and was therefore tentatively attributed to the species. Alternatively, the size and morphology of the studied specimens could form part of the transitional range of forms between C. mackenzianum/agglutinatum and H. coniferum, or lie within the disparity of the Heliosphaeridium genus.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993).

Genus Granomarginata Naumova 1960 

Type.Granomarginata prima Naumova 1960, Terreneuvian of Estonia.

Granomarginata prima Naumova 1960 

⁰⁰⁰⁴, figure 11 

Synonymy. 1960 Granomarginata prima Naumova; p. 114, pl. 3, figure 10.

1968 Granomarginata prima Naumova 1960; Volkova, p. 25, pl. 4, figure 10–13; p. 10, figure 6.

1979 Granomarginata prima Naumova 1960; Volkova et al. p. 25, pl. 18, figure 1–4.

1986 Granomarginata prima Naumova 1960; Moczydłowska & Vidal, p. 211, figure 8j.

? 1986 ‘Comasphaeridium-like fossil’; Germs et al. p. 56, figure 5i.

? 1987 Granomarginata prima Naumova 1960; Knoll and Swett, pp. 916–917, figure 8.10, 8.14.

1991 Granomarginata prima Naumova 1960; Moczydłowska, p. 56, pl. 3, figure c.

2002 Granomarginata prima Naumova 1960; Jankauskas, pl. 1, figure 6.

non 2010 Granomarginata prima Naumova 1960; Prasad et al. p. 420, pl. 1, figure 6.

2011b Granomarginata prima Naumova 1960; Jachowicz-Zdanowska; p. 91, figure 4I.

2011 Granomarginata prima Naumova 1960; Palacios et al. figure 3b.

? 2013 Granomarginata prima Naumova 1960; Högström et al. p. 103, figure 6C.

2018 Granomarginata prima Naumova 1960; Palacios et al. p. 1739, figure 6a, b.

? 2021 Granomarginata prima Naumova 1960; Agić et al. pp. 14–15, figure 4e, f.

Material. One specimen from GGU sample 184004.

Measurements.N = 1. Diameter of the discoid 11.9 µm, width of the rim 2.5 µm.

Description. Vesicle consisting of a central discoid surrounded by a narrow equatorial rim. The surface texture of the vesicle is spongy and exhibits little to no signs of folding. The vesicle is translucent, light-brown in colour, and has an apparently even thickness from the centre to the periphery. The central discoid has an oval, clearly defined outline.

Remarks. Whilst the species is represented by a single specimen in the recovered assemblage, it is fully consistent with the type material described by Naumova (1960) in terms of overall size, surface texture and width of the rim. Granomarginata prima differs from G. squamacea in having a comparatively narrow rim, but no quantitative boundary has been formally defined between these two taxa (Naumova 1960; Volkova 1968). A population of G. prima from the Terreneuvian of Newfoundland shows a ratio of r = 0.16–0.25 between the width of the rim and the diameter of the discoid (Palacios et al. 2018). Considering a boundary of r = 0.25 between G. prima and G. squamacea, the recovered specimen falls within the range of G. prima (Figure 10; r = 0.21). A problematic aspect of this criterion is its susceptibility to taphonomic abrasion, a process that may lead G. squamacea morphologies to grade into G. prima. It is possible that this taphonomic process accounts for the relatively continuous range of r values observed in the Granomarginata population (Figure 10); however, close inspection of the recovered G. prima specimen reveals no sign of marginal abrasion which, together with its small overall dimensions, supports its assignment to G. prima. Specimens from India attributed to G. prima have not been described and are figured by a single cluster of vesicles lacking an obvious equatorial fringe and a spongy texture. For these reasons, we reject their attribution to G. prima. Granomarginata prima specimens from Svalbard (Knoll and Swett 1987), Canada (Agić et al. 2021), Namibia (Germs et al. 1986; Agić et al. 2021) and Norway (Högström et al. 2013; Agić et al. 2021) exhibit an opaque discoid surrounded by a thinner translucent equatorial rim. Although these specimens may represent thermally altered Granomarginata, the resulting morphology is equally similar to abraded Pterospermella (e.g. Moczydłowska 1988; Palacios et al. 2018, fig. 5d), Pterospermopsimorpha (e.g. Yin and Guan 1999, fig. 3.11) or Simia (e.g. Samuelsson et al. 1999, fig. 7a, g). Indeed, the poor state of preservation of these specimens does not permit meaningful evaluation of surface textures (e.g. spongy appearance, presence of wrinkles and folds), which are essential criteria for the identification of Granomarginata (Naumova 1960; Volkova 1968). The recovery of rare intermediate forms between Granomarginata prima and Pterospermella velata in the studied assemblage (⁰⁰⁰⁴, figure 4) further highlights a potential for taxonomic confusion between these pteromorphic genera. For these reasons, we regard occurrences of G. prima having an opaque central discoid (Germs et al. 1986; Knoll and Swett 1987; Högström et al. 2013; Agić et al. 2021) as ambiguous.

Occurrence and stratigraphical range. Ediacaran of Poland (Jachowicz-Zdanowska 2011b) and potentially Canada (Agić et al. 2021), Namibia (Germs et al. 1986; Agić et al. 2021) and Norway (Högström et al. 2013; Agić et al. 2021); Terreneuvian of Finland (Slater and Willman 2019); Fortunian–Cambrian Stage 2 of Canada (Granomarginata and Asteridium tornatum–Comasphaeridium velvetum Zones; Palacios et al. 2011, 2018, 2020); Cambrian Stages 2–3 of the East European Platform (Asteridium tornatum–Comasphaeridium velvetum and Skiagia ornata–Fimbriaglomerella membranacea Zones; Volkova 1969; Volkova et al. 1979; Moczydłowska 1991; Jankauskas and Lendzion 1992); Cambrian Stages 3–4 of Sweden, Greenland (Holmia B Zone; Moczydłowska and Vidal 1986; Moczydłowska 1991; this study), Poland and Ukraine (Holmia Zone; Jachowicz-Zdanowska 2011a); lower Cambrian of Kazakhstan (Ogurtsova 1985) and potentially Svalbard (Knoll and Swett 1987); Miaolingian of China (Yin et al. 2009).

Granomarginata squamacea Volkova 1968 

⁰⁰⁰⁴, figures 14–20

Synonymy. ? 1987 Granomarginata squamacea Volkova 1968; Knoll and Swett, p. 917, figure 8.11.

1991 Granomarginata squamacea Volkova 1968; Moczydłowska, p. 57; pl. 3, figure D–F [cum syn.].

? 1995 Granomarginata squamacea Volkova 1968; Vidal & Moczydłowska, p. 504, figure 6.1.

? 2002 Granomarginata squamacea Volkova 1968; Moczydłowska, p. 201, figure 9.4.

? 2002? Granomarginata squamacea Volkova 1968; Vanguestaine et al. p. 57, pl. 3, figure 4.

2004 Granomarginata squamacea Volkova 1968; Brück and Vanguestaine, p. 207, figure 4.1–4.3

2011 Granomarginata squamacea Volkova 1968; Moczydłowska, pp. 127–129, pl. 7, figure 1–4.

2011 Granomarginata squamacea Volkova 1968; Palacios et al. pp. 48–50, figure 3A.

2018 Granomarginata squamacea Volkova 1968; Palacios et al. p. 1735, figures 5c, 6c.

? 2020 ‘Simia-like acritarch’; Arvestål & Willman, p. 26, figure 12D.

? 2021 Granomarginata squamacea Volkova 1968; Agić et al. p. 15, figure 4a–d.

Material. About 50 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 28. Diameter of the discoid 17–54 µm (x¯ = 24.9 µm; σ = 8.8), width of the rim 7.3–23.7 µm (x¯ = 14.5 µm; σ = 3.7).

Description. Vesicles consisting of a central discoid surrounded by a broad equatorial rim. The surface texture of the vesicle is spongy and exhibits little to no signs of folding. Vesicles range in colour from light-brown to darker orange-brown hues. The optical density of the wall appears to be relatively even throughout individual vesicles, only increasing upon contact between the equatorial rim and the central discoid. This contact may be sharp or gradual, resulting in more-or-less clearly defined discoids. The equatorial rim tends to develop a filamentous appearance, particularly along its outer margins.

Remarks. As for G. prima, the opacity of the central discoid is here regarded as an obstacle to identification owing to the general similarity between Granomarginata, Pterospermella (Moczydłowska 1988), Pterospermopsimorpha (e.g. Miao et al. 2019, fig. 7a, b) and Simia (e.g. Samuelsson et al. 1999, fig. 7b, c). In particular, G. squamacea specimens from Spitsbergen (Knoll and Swett 1987, fig. 8.11) and Namibia (Agić et al. 2021, fig. 4a) exhibit a clearly defined, thick-walled discoid that do not show obvious signs of spongy texture and is surrounded by a much thinner, finely wrinkled outer membrane. In these respects, these specimens are nearly identical to Pterospermella velata (e.g. Moczydłowska 1991, pl. 4, fig. A–D), particularly since this taxon is here shown to occasionally develop the spongy or filamentous appearance of Granomarginata (⁰⁰⁰⁴, figure 1a, 4a). For the same reasons, forms having an opaque central discoid (Moczydłowska 2002, fig. 9.4; Agić et al. 2021, fig. 4c–d) or an overall poor state of preservation (Vidal et al. 1995, fig. 6.1; Agić et al. 2021, fig. 4b) are considered as equivocal at present. Future population-based studies may help to clarify the morphological relationships between pteromorphic acritarch taxa, and in turn clarify the origin(s) of these ambiguous morphotypes.

Occurrence and stratigraphical range. Potentially Ediacaran of Norway, Namibia and Estonia (Arvestål and Willman 2020; Agić et al. 2021); Lower Cambrian and Miaolingian of the East European Platform (Volkova 1969; Volkova et al. 1979; Moczydłowska 1981, 1991) and Canada (Downie 1982; Martin and Dean 1983; Palacios et al. 2011, 2018); lower Cambrian of Kazakhstan (Ogurtsova 1985), potentially eastern Siberia (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal et al. 1995), Scotland (Downie 1982), Ireland (Vanguestaine et al. 2002; Brück and Vanguestaine 2004), Estonia (Skiagia ornata–Fimbriaglomerella membranacea Zone; Moczydłowska 2011), Sweden, (Holmia and Schmidtiellus Mickwitzi Zones; Vidal 1981a, 1981b; Moczydłowska and Vidal 1986; Hagenfeldt 1989b; Moczydłowska 2002), Denmark, Norway (Holmia Zone; Vidal 1981a, b; Moczydłowska and Vidal 1986; Hagenfeldt 1989b), Svalbard (Knoll and Swett 1987), Greenland (Downie 1982; Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study) and Finland (Tynni 1978); Miaolingian of Morocco (Vanguestaine and Van Looy 1983).

Genus Heliosphaeridium Moczydłowska 1991 

Type.Micrhystridium (= Heliosphaeridium) dissimilare (Volkova 1969) Moczydłowska 1991, lower Cambrian Protolenus Zone of Poland.

Heliosphaeridium? cf. dissimilare

1993 Asteridium ordensis (Volkova 1968) Vidal and Peel (1993); Vidal & Peel, p. 15; figure 9B.

⁰⁰⁰², figure 20

Material. One specimen in GGU sample 184002.

Measurements.N = 1. Diameter of central body 8.5 µm, process length 2.0 µm, basal process width 0.7–0.9 µm.

Description. Spheroidal vesicle having straight, sparsely distributed, tubular processes of moderate length. Processes appear to be hollow and truncated at their distal ends.

Remarks. Four specimens were initially assigned to A. ordensis by Vidal and Peel (1993), but only one could be located on the studied slides. Vidal and Peel (1993) transferred Downie’s taxon Micrhystridium ordensis to Asteridium. However, Moczydłowska (1991) had placed Micrhystridium ordensis in synonymy with Heliosphaeridium dissimilare. The recovered specimen indeed displays attributes of H. dissimilare, including relatively sparsely distributed processes having a tubular morphology and an apparently hollow construction. However, the specimen differs from Heliosphaeridium in its smaller dimensions (Figure 11), which are more consistent with Asteridium, or acritarchs from the small (< 20 µm) Comasphaeridium plexus. For these reasons, the genus-level attribution of the recovered specimen remains uncertain, although it is tentatively assigned to H. dissimilare following Moczydłowska’s interpretation of Micrhystridium ordensis. This rare morphotype could represent a phenotypic or ontogenetic variant of the Heliosphaeridium population, or perhaps a hybrid form between species of the Heliosphaeridium and Asteridium plexus.

Heliosphaeridium cf. coniferum

⁰⁰⁰², figures 13–15

Material. Six specimens in GGU sample 184004.

Measurements.N = 3. Diameter of central body 8.7–10.8 µm (x¯ = 9.6 µm; σ = 0.9), process length 4.5–6.4 µm (x¯ = 5.2 µm; σ = 0.8).

Description. Spheroidal to oval vesicles having numerous (n > 30) and relatively long processes (40–70% of the diameter of the central body) with expanded bases. Process tips appear sharp, blunt or slightly expanded.

Remarks. On the basis of observed variations in vesicle size and process morphology (Figure 4), recovered Heliosphaeridium cf. coniferum specimens are here interpreted as rare transitional forms between C. mackenzianum/agglutinatum and the usually slightly larger acritarchs from the Heliosphaeridium genus (Figure 5; Sergeev et al. 2020, fig. 3). Heliosphaeridium cf. coniferum indeed differs from C. mackenzianum and C. agglutinatum in having clearly expanded and separated process bases, and from Heliosphaeridium coniferum (Downie 1982) Moczydłowska 1991 in having more densely distributed processes with narrower bases. Some specimens referred to as H. coniferum by Moczydłowska (1991, pl. 8, fig. B–D; 1998, fig. 31A–C) and Sergeev et al. (2020, fig. 7.1–4) might also fall within this transitional category, having rather densely distributed processes with clearly expanded bases.

Occurrence and stratigraphical range. Assuming transitional variants are present in previously described populations of H. coniferum (Downie 1982) Moczydłowska, 1991: Cambrian Stages 3–4 of Poland (Skiagia ornata–Fimbriaglomerella membranacea and Heliosphaeridium dissimilare–Skiagia ciliosa zones; Moczydłowska 1991), Scotland, Alberta (Downie 1982), South Kazakhstan (Sergeev 1992, 2020), Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993), and Miaolingian of Poland (Acadoparadoxides oelandicus Superzone; Moczydłowska 1998).

Heliosphaeridium obscurum (Volkova 1969) Moczydłowska 1991 

⁰⁰⁰², figure 21

Synonymy. 1991 Heliosphaeridium obscurum (Volkova 1969) Moczydłowska, p. 60, pl. 8, figure O [cum syn.].

1998 Heliosphaeridium obscurum (Volkova 1969) Moczydłowska 1991; Moczydłowska, p. 79–80 [cum syn.].

2013 Heliosphaeridium coniferum (Downie 1982) Moczydłowska 1991; Jachowicz-Zdanowska, pl. 16, figure 9.

2020 Heliosphaeridium obscurum (Volkova 1969) Moczydłowska 1991; Sergeev et al. Appendix A, pp. 14–15, figures 4.5–4.8, 4.17–4.20, 6.13–6.14 [cum syn.].

Material. Eight specimens from GGU sample 184002.

Measurements.N = 8. Diameter of central body 9.2–13.1 µm (x¯ = 11.6 µm; σ = 1.3), process length 2.3–4.4 µm (x¯ = 3.3 µm; σ = 0.7), basal process width 0.6–1.0 µm (x¯ = 0.8 µm; σ = 0.2; Figure 9).

Description. Oval to spheroidal vesicles having relatively few, unevenly distributed processes with slightly widened bases. The length of processes is variable, but does not exceed 40% of the vesicle diameter. Process tips appear blunt or truncated.

Remarks. The species only differs from H. dissimilare in having an oval central body with processes of unequal length (Volkova 1969). Given their closely similar morphologies, observed population variations (Figures 4 and 11) and the prominence of H. dissimilare in the synonymy list of H. obscurum, the two species likely originate from a single biological taxon uniting various morphotypes from the Heliosphaeridium genus. The figured but undescribed H. coniferum specimen from the Goczałkowice Formation of Poland (Jachowicz-Zdanowska 2013) does not display the typical attributes of either H. coniferum or H. cf. coniferum, instead consisting of an oval vesicle with few, irregularly distributed, short blunt processes with moderately widened process bases. This combination of features is compatible with the diagnosis of H. obscurum.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Poland, the Czech Republic (Skiagia–Eklundia varia Assemblage Zone; Jachowicz-Zdanowska 2013), Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993) and Alberta (Downie 1982); Cambrian Stage 3–4 and Miaolingian of the East European Platform (Heliosphaeridium dissimilare–Skiagia ciliosa and Volkovia dentifera–Liepaina plana zones; Moczydłowska 1991; Volkova 1969; Volkova et al. 1979, 1983), Sweden (Moczydłowska and Vidal 1986; Hagenfeldt 1988, 1989b) and South Kazakhstan (Ogurtsova 1985; Sergeev et al. 2020); early Cambrian and Miaolingian of Ireland (Gardiner and Vanguestaine 1971; Vanguestaine 1978); Cambrian of Turkey (Erkmen and Bozdoğan 1981).

Genus Leiosphaeridia Eisenack 1958 emend. Downie and Sarjeant (1963)

Type.Leiosphaeridia baltica Eisenack, 1958, Ordovician of Baltica.

Remarks. Morphologically simple sphaeromorphic taxa such as Leiosphaeridia are regarded as highly polyphyletic (Butterfield et al. 1994, Butterfield 1997; Talyzina and Moczydłowska 2000; Willman and Moczydłowska 2007; Javaux and Knoll 2017), encompassing both prokaryotes and disarticulated elements and/or ontogenetic stages of larger eukaryotes (Butterfield 2004, 2005; Slater et al. 2018b). A specific-level subdivision of Leiosphaeridia has been proposed based on vesicle size and wall thickness (Jankauskas et al. 1987, 1989), but the continuous variation of these characters in individual Leiosphaeridia populations (Butterfield et al. 1994; Riedman and Porter 2016; Javaux and Knoll 2017) questions its practical value. Acknowledging the character limitations of simple sphaeromorphic acritarchs, Javaux and Knoll (2017) suggested that recognising distinct populations within individual leiosphaerid assemblages may be easier than subjecting them to established form-taxonomy. Following this view, we discretised the disparity seen in the recovered Leiosphaeridia population into three main categories based on vesicle size and folding patterns. Although the latter criterion is taphonomic in origin, it may be regarded as a coarse expression of the diverse structural characteristics of the wall seen in TEM/SEM and Raman spectroscopic analyses of Leiosphaeridia (Talyzina and Moczydłowska 2000; Javaux et al. 2004; Gong et al. 2010; Javaux and Knoll 2017; Pang et al. 2020; Strother and Wellman 2021).

Leiosphaeridia sp. A

⁰⁰¹², figures 1–5, 15

Synonymy. 1993 Retisphaeridium dichamerum Staplin, Jansonius & Pocock; Vidal and Peel, p. 29, figure 9e–f.

Material. More than a hundred specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 26. Diameter of vesicle 19.3–199.2 µm (x¯ = 100.9 µm; σ = 53.2).

Description. Spheroidal, oval or lenticular vesicles having a smooth surface. Folding patterns, if present, are apparently random or follow the vesicle margin. Excystment, when present, is by median split or circular pylome.

Remarks. Type A leiospheres exhibit a wide range of sizes, colours and apparent wall thicknesses that likely encompass multiple biological taxa. Folds and wrinkles also show various morphologies, being occasionally arranged radially from a densely wrinkled centre (⁰⁰¹², figure 15) in the manner of a pteromorph, or grading into thick bifurcating lines (⁰⁰¹², figure 2) similar to those seen in type B spheroids. Specimens previously identified as Retisphaeridium dichamerum Staplin et al. (1965) in previous investigations of acritarchs from the Buen Formation (Vidal and Peel 1993) do not provide unequivocal evidence for the rupture of the vesicle into plates, a character that has been regarded as a Miaolingian innovation (Palacios et al. 2020). These specimens are more conservatively interpreted as part of the Leiosphaeridia sp. A morphological spectrum, consisting of smooth-walled spheroids with linear folds arranged in somewhat polygonal patterns.

Leiosphaeridia sp. B

⁰⁰¹², figures 6–9 

Material. Twelve specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 12. Diameter of vesicle 23.6–138.6 µm (x¯ = 93.2 µm; σ = 37.1).

Description. Oval to spheroidal vesicles having a moderately thick, densely wrinkled wall. One or two optically dense compression folds are observed on the vesicle surface, often dividing into lower-order, narrower sinuous fold lines and wrinkles paralleling the main folding axis. Excystment not observed.

Remarks. The colour and thickness of the wall appears to be relatively constant in the studied population, but size varies widely among either hand-picked (23.6–110.3 µm) or conventional (59.7–122 µm) palynomorphs.

Leiosphaeridia sp. C

⁰⁰¹², figures 10–14 

Material. One specimen from GGU sample 184002 and six specimens from GGU sample 184004.

Measurements.N = 5. Diameter of vesicle 154.9–240.8 µm (x¯ = 186.8 µm; σ = 32.3).

Description. Large, flimsy vesicles having an irregular outline. Extremely thin wrinkle lines cover the entire vesicle surface, clustering into thick, densely compressed areas. The vesicle margin exhibits two orders of irregular bulbous outgrowths; with first-order outgrowths consisting of agglomerations of second-order outgrowths.

Remarks. Marginal outgrowths may be interpreted as bud-like structures in an asymmetrical, actively growing, presumably benthic eukaryote similar to Gemmuloides doncookii Samuelsson and Butterfield (2001) (also see Adam et al. 2017, fig. 2H). However, these bulbous protrusions are more conservatively interpreted as portions of deformed vesicle wall resulting from secondary compression and folding.

Genus Lophosphaeridium Timofeev 1959 ex Downie 1963 emend. Lister 1970 

Type species.Lophosphaeridium rarum Timofeev ex Downie, 1963 [=Lophosphaeridium rarum Timofeev, 1959], Lower Ordovician of Russia.

Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991 

⁰⁰¹⁰, figure 1 

Synonymy. 1968 Baltisphaeridium dubium Volkova 1968; p. 18, pl. 1 figures 8,9, pl. 11 figure 6

1979 Baltisphaeridium dubium Volkova 1968; Volkova et al. p. 9, pl. 5 figures 3, 4

1982 Lophosphaeridium tentativum Volkova 1968; Downie, p. 279, figure 11n

1986 Baltisphaeridium dubium Volkova 1968; Moczydłowska & Vidal, figure 11E–F

?1991 Lophosphaeridium dubium (Volkova 1968); Moczydłowska, p. 61, pl. 3 figure A

1993 Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991; Vidal & Peel, p. 27, figure 9G

?2002 Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991; Vanguestaine et al. p. 57, pl. 3 figures 12–13

2002 Lophosphaeridium sp.; Vanguestaine et al. p. 57, pl. 3 figure 8

2013 Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991; Jachowicz-Zdanowska, pl. 16 figures 17, 18

?2016 Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991; Szczepanik & Żylińska, pl. 3 figures 8, 10

?2016 Lophosphaeridium cf. dubium (Volkova 1968) Moczydłowska 1991; Szczepanik & Żylińska, pl. 3 figure 9, pl. 4 figures 11, 12

2016 Lophosphaeridium truncatum Volkova 1969; Szczepanik & Żylińska, pl. 4 figure 10

?2020 Lophosphaeridium dubium (Volkova 1968) Moczydłowska 1991; Palacios et al. 2020, figure 5B–D

Material. One specimen from GGU sample 184003 and two specimens from GGU sample 184004.

Measurements.N = 2. Diameter of the vesicle 26.2–33.1 µm, diameter of protuberances 0.4–1 µm, height of protuberances 0.8–1 µm.

Description. Spheroidal to oval vesicles covered in small and solid thorn-shaped protuberances. Thorns are densely and evenly distributed over the entire vesicle surface.

Remarks.L. dubium was originally described from the East European Platform (Baltisphaeridium dubium Volkova 1968), encompassing specimens having densely distributed, evenly spaced and short thorns. Subsequent identifications of the species adopted a more relaxed definition of L. dubium, including vesicles covered in more coarsely distributed, longer thorns occasionally showing truncated terminations or other signs of abrasion (e.g. Moczydłowska 1991, pl. 3 A; Vanguestaine et al. 2002, pl. 3, fig. 12–13; Szczepanik and Żylińska 2016, pl. 3, figures 8, 10; Palacios et al. 2020, fig. 5b, d). Similar coarsely spinose vesicles were recovered here (⁰⁰⁰⁶, figures 13–19), and are interpreted as taphomorphs of Skiagia following a detailed analysis of disparity between Lophosphaeridium and Skiagia populations. We thus conform to a strict definition of L. dubium here, i.e. vesicles covered in evenly and densely distributed, thick and short thorns with pointed terminations (width of protuberance < 1 µm; height of protuberance 0.5–1 µm), and place ‘false’ spinose sphaeromorphs presumably derived from Skiagia in Skiagia? spp. The updated synonymy list provided for L. dubium reflects this approach. L. dubium (⁰⁰¹⁰, figure 1) resembles L. tentativum (⁰⁰¹⁰, figure 3) in having sculptural elements of similar width and distribution, but these sculptural elements are more elevated in L. dubium, consisting of short thorns rather than granulae. The L. tentativum specimen illustrated by Downie (1982, fig. 11n) has pointed sculptural elements clearly raising above the vesicle surface, a morphology that better conforms to L. dubium (e.g. compare with L. dubium from Jachowicz-Zdanowska 2013, pl. 16, fig. 17–18).

Occurrence and stratigraphical range. Assuming a ‘relaxed’ definition of L. dubium: Cambrian Stages 3–4 of the East European Plateform (Volkova et al. 1979; Moczydłowska 1991; Szczepanik and Żylińska 2016); Sweden (Vidal 1981a, b; Moczydłowska and Vidal 1986), Denmark (Vidal 1981b; Moczydłowska and Vidal 1986, 1992), Norway (Moczydłowska and Vidal 1986; Palacios et al. 2020), Ireland (Vanguestaine et al. 2002); Spain (Palacios and Vidal 1992) and Greenland (Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study); Cambrian Stage 3 and Miaolingian of Poland and the Czech Republic (Jachowicz-Zdanowska 2013).

Lophosphaeridium tentativum Volkova 1968 

⁰⁰¹⁰, figures 2–4 

Synonymy.non 1982 Lophosphaeridium tentativum Volkova 1968; Downie, p. 279, figure 11n

1991 Lophosphaeridium tentativum Volkova 1968; Moczydłowska, p. 62, pl. 2, figure G–I [cum syn.]

1993 Lophosphaeridium sp.; Vidal and Peel, p. 29, figure 9i

2021 Lophosphaeridium tentativum Volkova 1968; Yin et al. p. 414, figures 4H, I, 6S [cum syn.]

Material. Three specimens from GGU sample 184002, two specimens from GGU sample 184003 and one specimen from GGU sample 184004.

Measurements.N = 6. Diameter of the vesicle 20.1–39 µm (x¯ = 28.3 µm; σ = 7.6), diameter of granulae 0.3–0.8 µm, height of granulae 0.2–0.4 µm.

Description. Spheroidal to oval vesicles covered in small granulae. Granulae are densely and evenly distributed over the entire vesicle surface, and only slightly protrude beyond its outline.

Remarks. The species differs from L. dubium in having shorter (< 5 µm), rounded rather than horn-like protuberances, and from L. truncatum in having smaller granulae. In the studied population, the size of protuberances was indeed observed to be lower than those adorning L. truncatum (width generally < 0.6 µm), but showed important variations even within individual specimens (Figure 6). The observation of thin, short process-like extensions in specimens otherwise identical to L. tentativum (⁰⁰¹⁰, figure 4) suggests that the recovered L. tentativum population may include vesicles of acanthomorphic origin, possibly Skiagia or Comasphaeridium with abraded processes. Despite their uncertain origin, these specimens do show a predominance of L. tentativum features (i.e. a fine and even granulose ornamentation [Figure 6] apparently covering the entire vesicle surface) that warrants their assignment to this species. Specimens previously identified as Lophosphaeridium sp. by Vidal and Peel (1993) do not show clear evidence of process truncation, and are fully consistent with the morphological range encompassed by L. tentativum.

Occurrence and stratigraphical range. Cambrian Stages 2–3 of Poland (Asteridium tornatum–Comasphaeridium velvetum Zone; Szczepanik and Żylińska 2016, 2021); Cambrian Stages 2–4 of Canada (Asteridium tornatum–Comasphaeridium velvetum to Volkovia dentifera–Liepaina plana Zone; Palacios et al. 2011, 2017, 2018); Cambrian Stages 3–4 of the East European Platform (Holmia and Protolenus Zones; Volkova 1969; Volkova et al. 1979; Moczydłowska 1981, 1991), Sweden (Holmia Zone; Vidal 1981a, b; Hagenfeldt 1989b; Moczydłowska and Vidal 1986), Denmark (Holmia Zone; Vidal 1981a, b; Moczydłowska and Vidal 1986, 1992); Norway (Holmia Zone; Vidal 1981a, b; Moczydłowska and Vidal 1986); Scotland, Canada (Holmia Zone; Downie 1982), Spain (Palacios and Vidal 1992) and Greenland (Downie 1982; Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study); Cambrian Stage 4 of China (Yin et al. 2021); Cambrian Stage 3 to Miaolingian of Ireland (Smith 1977; Vanguestaine et al. 2002; Vanguestaine and Brück 2008); lower Cambrian of Kazakhstan (Ogurtsova 1985), Finland (Tynni 1978), India (Prasad et al. 2010); Miaolingian–Furongian of Belgium (Vanguestaine 1974, 1978).

Lophosphaeridium truncatum Volkova 1969 

⁰⁰¹⁰, figures 5–8 

Synonymy. 1991 Lophosphaeridium truncatum Volkova 1968; Moczydłowska, pp. 62–63, pl. 3, figure B [cum syn.]

2021 Lophosphaeridium truncatum Volkova 1968; Yin et al. pp. 414–415, figures 7F, 8L [cum syn.]

Material. Two specimens from GGU sample 184002, four specimens from GGU sample 184003 and six specimens from GGU sample 184004.

Measurements.N = 12. Diameter of the vesicle 22.4–42.1 µm (x¯ = 33.4 µm; σ = 6.2), diameter of granulae 0.5–3.3 µm, height of granulae 0.3–0.7 µm.

Description. Spheroidal to oval vesicles covered in coarse granulae having rounded to truncated tips. The distribution of granulae on the vesicle surface is generally irregular, but relatively dense.

Remarks. This species differs from L. tentativum in having granulae of larger size (generally > 0.6 µm; Figure 6), and from L. dubium in having truncated protuberances. The recovered L. truncatum population exhibits large variations in protuberance size and morphology (⁰⁰¹⁰, figures 5–8, Figure 6), with some vesicles showing distinct processes with truncated ends (⁰⁰¹⁰, figure 5a). Although these features are consistent with the original diagnosis of L. truncatum (defined as vesicles adorned with short cilia that are ‘often bluntly truncated’; Volkova 1969, p. 269), this high disparity may be interpreted as resulting from various degrees of abrasion in acanthomorphic acritarchs such as Skiagia. Vesicles here attributed to Skiagia? spp. indeed show identical variations in vesicle diameter and basal process width (Figure 6). Some specimens attributable to L. truncatum also show robust conical protuberances overlain by a more fragile distal portion (Figures 10, figures 6a, 6b), a configuration that is identical to the basal portion of processes in S. ciliosa (⁰⁰⁰⁶, figure 3; ⁰⁰¹⁰, figure 12). Lophosphaeridium truncatum populations from elsewhere should therefore be scrutinised on the basis of their wider within- and between-species variations to determine the validity of the taxon, particularly in settings where L. truncatum co-occurs with Skiagia or other acanthomorphs (e.g. Knoll and Swett 1987, fig. 7.15; Vidal and Peel 1993, fig. 9j; Szczepanik and Żylińska 2016, pl. 2, fig. 20–23).

Occurrence and stratigraphical range. Lower Cambrian of Kazakhstan (Ogurtsova 1985); Cambrian Stages 3–4 (Heliosphaeridium dissimilare–Skiagia ciliosa Zone) of Denmark (Moczydłowska and Vidal 1992), Spain (Palacios and Vidal 1992) and Greenland (Vidal and Peel 1993; this study); Cambrian Stage 4 of China (Yin et al. 2021); Cambrian Stage 3–Miaolingian of the East European platform (Volkova 1969; Volkova et al. 1979; Moczydłowska 1981; Moczydłowska 1991; Szczepanik and Zylinska 2016); lower Cambrian of Svalbard (Knoll and Swett 1987).

Genus Palaeosiphonella Licari 1978 

Type.Palaeosiphonella cloudii Licari 1978, Mesoproterozoic of the United States.

Palaeosiphonella sp.

Plate 5

Material. About 50 specimens from hand-picked samples; i.e. GGU 184002 and 184004.

Measurements.N = 36. Length of the filament 165.3 – 1513 µm (x¯ = 509.8 µm; σ = 329.9), maximum width of the filament 23.2–96.9 µm (x¯ = 47.4 µm; σ = 16.9).

Description. Optically dense branching filaments having a smooth surface and a relatively thick (1–2 µm) wall. Tubes have a sinuous morphology with localised constrictions and swellings. When preserved, filament terminations narrow down into a rounded, apparently closed tip.

Remarks. In the absence of internal structures providing positive evidence for true branching (Butterfield et al. 1994; Butterfield 2009), the single branching specimen recovered (⁰⁰⁰⁵, figure 5) is conservatively interpreted as a false-branching envelope attributable to Palaeosiphonella sp. Palaeosiphonella cloudii differs from the recovered Palaeosiphonella sp. population in occasionally exhibiting septa, while Ramivaginalis Nyberg and Schopf, 1984 consists of much smaller filaments (specimens at the type locality are up to 9-µm-wide and 60-µm-long; Nyberg and Schopf 1984). Although only a single specimen shows evidence of branching, all specimens in this category share a common set of features that warrant their assignment to a single species; namely, a sinuous morphology, localised constriction/swellings (⁰⁰⁰⁵, figures 17–26), degraded intracellular contents (⁰⁰⁰⁵, figures 6–13), a generally dark colour with a thick wall, a relatively large size, numerous compression folds and rounded to bulbous terminations (⁰⁰⁰⁵, figures 20–26, 28). Specimens preserving bulbous terminations may appear similar to taxa such as Baltinema (Slater et al. 2017, fig. 11A–M) or Retiranus (Slater et al. 2018b, fig. 6 A–F), which show vesicular outgrowths on their margins (Baltinema) or at their tip (Retiranus). However, close inspection of the spheroidal tip in the recovered material suggests that its cavity is continuous with the tubular portion of the filament (⁰⁰⁰⁵, figures 20–21, 25, 28), unlike in Baltinema and Retiranus, where outgrowths appear to have their own cell wall. Localised swellings and constrictions are common deformation features in cyanobacterial sheaths (Golubić and Barghoorn 1977; Bartley 1996).

Occurrence and stratigraphical range. Owing to their simple morphology, branched filaments are likely to be cosmopolitan and long-ranging (e.g. Steiner and Fatka 1996), but are only sporadically included in the Palaeosiphonella taxon. Branched filaments are known from the Meso-Neoproterozoic of the United States (Licari 1978); the Neoproterozoic of Canada (Samuelsson and Butterfield 2001) and the United States (Corsetti et al. 2003), the Tonian of Svalbard (Butterfield et al. 1994); and the lower Cambrian of China (Cui et al. 2020) and the Czech Republic (Steiner and Fatka 1996).

Genus Parmasphaeridium Jachowicz-Zdanowska 2013 

Type.Parmasphaeridium implicatum (Fridrichsone) Jachowicz-Zdanowska 2013, early Cambrian of Latvia.

Parmasphaeridium implicatum (Fridrichsone) Jachowicz-Zdanowska 2013, ⁰⁰⁰⁶, figure 20

Synonymy. 2013 Parmasphaeridium implicatum (Fridrichsone) Jachowicz-Zdanowska, pp. 64–65, pl. 21, figure 7–10, 12 [cum syn.]

Material. One specimen from GGU sample 184003.

Measurements.N = 1. Diameter of central body 27 µm, maximum process length 15.5 µm, process width 0.4–1.1 µm.

Description. Spheroidal vesicle covered in numerous, relatively straight needle-like processes. Processes vary greatly in width and length, and have either sharp-pointed or blunt terminations. The outline of the central body is evenly circular and may rarely reveal slightly expanded process bases.

Remarks. The recovered specimen differs from P. implicatum populations from elsewhere in having a clearly translucent central body lacking evidence of a double wall. The dark opaque central body of P. implicatum may be explained by the presence of a thick internal cyst, a structure that sporadically occurs in other acritarch species and is likely controlled by ontogenetic and/or ecological factors (Moczydłowska 2010; Wallet et al. 2022). This hypothesis is supported by the occurrence of a translucent, opened P. implicatum vesicle from the Miaolingian Kistedalen Formation of Norway (Palacios et al. 2022) that likely represents the excystment stage (i.e. release of the internal cyst) of the species. In this light, the recovered specimen may be interpreted as an immature cell that has not yet developed an internal cyst.

Occurrence and stratigraphical range. Cambrian Stages 3–4 to Miaolingian of Poland (Holmia kjerulfi, Protolenus and Acadoparadoxides oelandicus zones; Volkova 1969; Moczydłowska 1991; Jachowicz-Zdanowska 2013), the Czech Republic (Skiagia–Eklundia varia and Volkovia dentifera–Liepaina plana zones; Jachowicz-Zdanowska 2013); Latvia (Fridrichsone 1971; Volkova et al. 1979), Norway (Heliosphaeridium dissimilare–Skiagia ciliosa and Volkovia dentifera–Liepaina plana zones; Palacios et al. 2020; Eliasum llaniscum, Cristallinium cambriense, Adara alea and Heliosphaeridium notatum zones; Palacios et al. 2022), Sweden (Moczydłowska and Vidal 1986; Moczydłowska 1991; Moczydłowska et al. 2001) and Spain (Skiagia ciliosa, Heliosphaeridium notatum and Eliasum llaniscum zones; Fombella 1979; Moczydłowska 1991; Palacios 2015; Palacios et al. 2021); Cambrian Stage 4–Miolingian of Canada (eteminicus, hicksi and forchhammeri zones; Palacios et al. 2012; Volkovia dentifera–Liepaina plana and Eliasum llaniscum zones; Palacios et al. 2017); upper lower Cambrian–Miaolingian of Sweden and Finland (Acadoparadoxides oelandicus Superzone; Hagenfeldt 1989b); lower Cambrian of Svalbard (Knoll and Swett 1987); Cambrian Stages 3–4 of Australia (Skiagia ciliosa–Corollasphaeridium aliquolumum Zone; Zang et al. 2007), Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study), Poland and Ukraine (Volkova et al. 1979; Jachowicz-Zdanowska 2011a).

Genus Pearisphaeridium gen. nov.

Derivation of name. After Peary Land, and the spheroidal morphology of the type species.

Type species.Pearisphaeridium densispinosum comb. nov. by monotypy.

Diagnosis. As for type species.

Pearisphaeridium densispinosum (Vidal 1993) comb. nov. emend.

Plate 14

Synonymy. 1993 Comasphaeridium? densispinosum Vidal, p. 19, figure 6a

Holotype. MGUH 21.519, GGU sample 184003, England finder coordinates H.47-3, fig. 6d in Vidal and Peel (1993).

Material. Ten specimens from GGU sample 184002, two specimens from GGU sample 184003 and ten specimens from GGU sample 184004.

Measurements.N = 21. Diameter of central body 53–79 µm (x¯ = 67.7 µm; σ = 7.7), process length 1.5–4 µm (x¯ = 2.8 µm; σ = 0.7), width of the process base 0.3–0.6 µm (x¯ = 0.4 µm; σ = 0.1), width of the funnel-shaped termination 0.8–1.4 µm (N = 3; x¯ = 1.2 µm; σ = 0.3).

Original diagnosis. An acritarch species with spheroidal vesicle densely covered with short, minute, simple ciliar-like processes. The processes are generally simple, but may in some instances display a distinct, low, conical, proximal attachment to which a ciliar-like portion of the process is attached. Excystment by median split.

Emended diagnosis. Acritarch having a large spheroidal vesicle covered in short flexible processes terminating in a filmy funnel. Processes are biform, ranging from thin ciliar-like projections having a slightly expanded base, to thicker processes having robust conical bases and occasional plugs. Excystment by partial rupture, median split and possibly pylome.

Description. Optically dark spheroidal vesicles having densely distributed processes (ca. 25 processes on a 10 µm² surface) not exceeding 7% of the vesicle diameter. Cilia-like (⁰⁰¹⁴, figures 8–9) and more robust types of processes (⁰⁰¹⁴, figures 1–2, 5) may co-occur in individual specimens (⁰⁰¹⁴, figure 7) and are united by continuous morphological variations. Some processes appear to have shared bases (⁰⁰¹⁴, figures 5a, 5b), an observation that is supported by one three-dimensionally preserved specimen exhibiting what appears to be unusually long bifurcating processes (⁰⁰¹⁴, figure 6a). Openings by median split (⁰⁰¹⁴, figure 3, 5) or partial rupture (⁰⁰¹⁴, figure 2) are common; a polygonal pylome has also been tentatively identified in one specimen (⁰⁰¹⁴, figure 4).

Remarks. Vidal and Peel (1993) tentatively ascribed these specimens to the genus Comasphaeridium on the basis of their cilia-like process morphology, noting that process dimorphism is unknown from this genus. Most of the recovered vesicles only preserve the proximal portion of processes, effectively appearing as acritarchs covered in filiform ciliae. Closer inspection of this material revealed the comparatively fragile distal portion of processes in some specimens (⁰⁰¹⁴, figure 7a, 9a), which features a subtle funnel-shaped termination characteristic of the genus Skiagia. However, specimens of Pearisphaeridium densispinosum (Vidal 1993) comb. nov. form a distinct population lacking transitional forms to any other established acritarch species, which makes them an unlikely part of a Skiagia-type life cycle. The sheer size of the recovered Pearisphaeridium densispinosum (Vidal 1993) comb. nov. specimens and their clearly biform processes warrant the creation of a new taxon. The bifurcating networks resembling processes that appear on one specimen (⁰⁰¹⁴, figure 6a) have been observed on a single focal plane coinciding with the vesicle surface. Whether these structures originally extended within, outside or on the vesicle is unclear, and their morphology is obscured by overlaps with the background cover of short processes. The presence of a pylome (⁰⁰¹⁴, figure 4) in the recovered population is also uncertain, as similar irregularly shaped openings may arise from secondary fragmentation.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993; this study).

Genus Polytrichoides Hermann 1974 emend. Timofeev et al. 1976 

Type.Polytrichoides lineatus Hermann 1974, Neoproterozoic of Russia.

Polytrichoides sp.

⁰⁰⁰⁷, figure 22

Material. One specimen in GGU samples 184002.

Measurements.N = 1. Length of the filament bundle 203 µm, width of filaments 2.7–4.2 µm.

Description. Bundle of 15–20 closely packed, dark-brown filaments of relatively even width. Filaments are smooth and do not show evidence of branching or septation.

Remarks.Polytrichoides differs from Siphonophycus in consisting of filaments that occur in tight bundles lacking evidence for septation or branching (Hermann 1974; Knoll et al. 1991; Li et al. 2019; Arvestål and Willman 2020). Some of the specimens attributed to Siphonophycus also form bundles, but these are comparatively loose and consist of fewer and broader filaments. The recovered Polytrichoides specimen resembles P. lineatus, but this taxon usually exhibits a common sheath surrounding bundles of filaments (Li et al. 2019). One of the specimens recovered here consists of filaments embedded into a thinner-walled matrix (⁰⁰⁰⁹, figure 4), but their chaotic arrangement and small size (1.6 µm in width) suggests it is distinct from Polytrichoides.

Occurrence and stratigraphical range. Cosmopolitan and long-ranging, from the Mesoproterozoic to the Palaeozoic, including: Mesoproterozoic of Russia (Vorob’eva et al. 2015) and Canada (Loron et al. 2021); Meso-Neoproterozoic of Canada (Hofmann and Jackson 1994), Mauritania (Beghin et al. 2017), the Democratic Republic of Congo (Baludikay et al. 2016) and India (Srivastava and Kumar 2003); Neoproterozoic of Russia (Hermann 1974; Timofeev et al. 1976; Jankauskas et al. 1989; Mendelson and Schopf 1992; Veis et al. 2006; Golubkova et al. 2015), Svalbard (Knoll et al. 1991), India (Tang et al. 2017; Shukla et al. 2020) and China (Tang et al. 2013); Tonian of China (Tang et al. 2015; Li et al. 2019); Ediacaran of India (Joshi and Tiwari 2016), China (Xiao 2004), Estonia (Arvestål and Willman 2020) and Poland (Moczydłowska 2008); lower Cambrian of the Czech Republic (Fatka and Konzalová 1995; Steiner and Fatka 1996); Terreneuvian of Estonia (Slater et al. 2018b); Cambrian Stages 3–4 of Greenland (this study); Miaolingian of China (Harvey et al. 2011).

Genus Pterospermella Eisenack 1972 

Type.Pterospermopsis (= Pterospermella) aureolata (Cookson and Eisenack 1958) Eisenack 1972, Early Cretaceous of Australia.

Pterospermella solida (Volkova 1969) Volkova 1979 (in Volkova et al. 1979)

⁰⁰⁰⁴, figures 5–8 

Synonymy. 1991 Pterospermella solida (Volkova 1969) Volkova 1979; Moczydłowska, pp. 63–64, pl. 4, figure F–G [cum syn.]

Material. Six specimens from GGU sample 184002, one specimen from GGU sample 184003 and one specimen from GGU sample 184004.

Measurements.N = 8. Diameter of the vesicle 33.9–47.5 µm (x¯ = 40 µm; σ = 4.8), diameter of the central body 19.5–25.2 µm (x¯ = 21.5 µm; σ = 1.8), maximum width of the outer rim 7.4–11.8 µm (x¯ = 9.7 µm; σ = 1.7).

Description. Spheroidal vesicle consisting of a dark orange-brown to opaque central body surrounded by a thin, translucent, finely wrinkled equatorial rim. The outer rim is irregular in outline, up to 27% of the vesicle diameter, and is supported by irregularly distributed radial elements emerging from the central body. Radial elements range from thin, poorly defined lineations (⁰⁰⁰⁴, figure 5a) to thick rods expanding upon contact with the central body (⁰⁰⁰⁴, figures 6a, 7a).

Remarks. The species differs from P. velata in having radial elements supporting the equatorial membrane. When poorly developed, radial elements may resemble radial folds in an otherwise unsupported membrane as seen in P. velata (⁰⁰⁰⁴, figure 5). In the recovered assemblage, P. velata tends to exhibit finer-scale wrinkles that are only rarely oriented radially (⁰⁰⁰⁴, figure 2), and transitional forms between P. velata and P. solida are relatively rare (⁰⁰⁰⁴, figures 3, 5). In some cases, the poorly defined radial elements in the recovered P. solida population resemble processes expanded at their base (⁰⁰⁰⁴, figure 5a, 8a). These structures are reminiscent of the filmy muri of Fimbriaglomerella (⁰⁰⁰⁴, figure 9a) or the processes of Skiagia scottica that may have coalesced into a nearly amorphous outer rim. Regardless, the dark appearance of the central body obscures its exact relationship with the rest of the vesicle, including whether it is connected by processes or muri to a three-dimensional outer membrane (i.e. a sphere-within-a-sphere construction seen in Fimbriaglomerella and S. scottica), or simply surrounded by a two-dimensional equatorial rim adorned by faint radial elements (as seen in Pterospermella).

Occurrence and stratigraphical range. Cambrian Stages 3–4 and Miaolingian of the East European Platform (Volkova et al. 1979; Moczydłowska 1991); Cambrian Stages 3–4 of Sweden (Holmia Zone; Vidal 1981a; Moczydłowska and Vidal 1986) and Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study); lower Cambrian of Svalbard (Knoll and Swett 1987).

Pterospermella velata Moczydłowska 1988 

⁰⁰⁰⁴, figures 1–3 

Synonymy. 1993 Pterospermella sp., Vidal and Peel, p. 29, figure 9c.

1991 Pterospermella velata Moczydłowska 1988; Moczydłowska, p. 64, pl. 4, figure A–D [cum syn.]

non 2013 Pterospermella velata Moczydłowska 1988; Jachowicz-Zdanowska, 2013, pp. 64–65, pl. 21, figure 7–10, 12

Material. About 20 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 11. Diameter of the vesicle 23.4–56.4 µm (x¯ = 37.5 µm; σ = 9.8), diameter of the central body 11.2–32.1 µm (x¯ = 22 µm; σ = 6.5), maximum width of the outer rim 5.1–17.6 µm (x¯ = 9.7 µm; σ = 4.1).

Description. Spheroidal vesicle consisting of a dark central body surrounded by a thin, translucent, finely wrinkled equatorial rim. The equatorial membrane is irregular in outline, up to 36% of the vesicle diameter, and may have a spongy and/or filamentous appearance. The colour of the central body varies from dark orange-brown to clearly translucent light brown, but is invariably darker than the outer membrane.

Remarks. The species differs from P. solida in lacking radial supporting elements on the outer rim. Recovered P. velata specimens tend to have an apparently spongy texture (⁰⁰⁰⁴, figures 2–3) that may partly be imparted by the thin, wrinkled outer membrane surrounding the central body. This fabric tends to become filamentous when degraded, eventually resembling a dense cover of thin filiform processes (⁰⁰⁰⁴, figure 1). A similar filamentous rim appears to be present in P. velata specimens from the type locality (Moczydłowska 1988, pl. 1, fig. 7–8), and in a specimen from Svalbard previously assigned to Granomarginata squamacea (Knoll and Swett 1987, fig. 8.11). Granomarginata and Pterospermella may share a common wall structure and/or degradation properties, resulting in intermediate forms between these taxa (⁰⁰⁰⁴, figure 4). Nevertheless, the recovered Pterospermella vesicles generally differ from Granomarginata in showing a clear difference in optical density between the central body and the outer rim, the latter being thinner and more wrinkled. Specimens referred to as Pterospermella sp. by Vidal and Peel (1993, fig. 9c) are here interpreted as P. velata on the basis of their apparently unsupported, filamentous and thin-walled outer rim. In contrast, P. velata specimens from southern Poland and the Czech Republic (Jachowicz-Zdanowska 2013, pl. 4, fig. 11, 14) have an even outline, lack the veil-like wrinkled appearance of the outer rim of P. velata, and show irregularly distributed radial elements resembling processes or coarse folds. These characteristics are more reminiscent of Pterospermella vitalis or P. solida; or may define a separate species.

Occurrence and stratigraphical range. Terreneuvian of Poland (Asteridium–Comasphaeridium Zone; Moczydłowska 1991); lower Cambrian of Svalbard (Knoll and Swett 1987); Terreneuvian of Canada (Asteridium interval Zone; Palacios et al. 2018); Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Pterospermella sp.

⁰⁰⁰⁴, figure 10 

Synonymy. 1993 Cymatiosphaera sp., Vidal and Peel, p.21, figure 10d

Material. 2 specimens from GGU samples 184002 and 184004.

Measurements.N = 2. Diameter of the vesicle 22.8–34.8 µm, diameter of the central body 19.4–27.8 µm, maximum width of the outer rim 2.7–6.1 µm.

Description. Spheroidal vesicles consisting of a dark brown to opaque central body surrounded by a narrow, thinner-walled equatorial rim corresponding to 12–18% of the vesicle diameter. The surrounding membrane is supported by 5–10 radial elements of variable thicknesses. The outlines of the vesicle and central body are even and sharp.

Remarks. Recovered specimens resemble Pterospermella sp. from the Cambrian Stage 3 of Poland (Moczydłowska 1981, fig. 6C) and Cymatiosphaera sp. from the Cambrian Stages 3–4 of Greenland (Vidal and Peel 1993, fig. 10d). They differ from Cymatiosphaera in lacking visible polygonal fields on the central body, and from P. vitalis in having a narrower equatorial rim and uneven rod-like elements.

Occurrence and stratigraphical range. Cambrian Stage 3 of Poland (Moczydłowska 1981); Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Vidal and Peel 1993; this study).

Genus Pterospermopsimorpha Timofeev 1966 emend. Mikhailova & Jankauskas 1989 (in Jankauskas et al. 1989)

Type.Pterospermopsimorpha pileiformis Timofeev 1966, Tonian (ca. 850 Ma; Mendelson and Schopf 1992) of Siberia [lectotype selected after the loss of the holotype; Jankauskas et al. 1989].

Pterospermopsimorpha sp.

Plate 8

Material. Seven specimens from GGU sample 184004.

Measurements. Diameter of the vesicle 99–110 µm (N = 2), diameter of the inner body 63–87 µm (N = 7; x¯ = 75.7 µm; σ = 7.8).

Description. Large vesicles consisting of a brittle, thin-walled membrane enclosing an optically darker spheroidal inner body. The outer membrane is translucent, densely wrinkled and usually degraded, while the inner body has a more regular, sharp outline. The inner body appears to open by partial rupture.

Remarks. The taxonomic distinction between pteromorphic (spheres surrounded by an equatorial rim) and dispheromorphic actritarchs (spheres surrounded by a spherical membrane) can be problematic in vesicles flattened parallel to the equatorial plane (Hofmann and Jackson 1994; Riedman and Porter 2016). Nevertheless, the continuity of observed fold lines across the boundary between the inner body and the outer rim (⁰⁰⁰⁸, figures 4a–5a) suggests that a thin-walled, wrinkled envelope surrounds the entire surface of the inner body. A sphere-within-a-sphere construction is also suggested by the marginal position of the inner body in one specimen (⁰⁰⁰⁸, figure 1). Opening by partial rupture is tentatively identified in some specimens (⁰⁰⁰⁸, figures 2a, 4), but similar structures could emerge from breakage of the inner body. The complex species-level subdivision of Pterospermopsimorpha relies on details of surface texture, which often bear marks of secondary folding and rupture. We therefore limit our taxonomic assignment of dispheromorphic acritarchs to Pterospermopsimorpha sp. Cambrian occurrences of Pterospermopsimorpha (P. wolynica Kirjanov 1974; P. rugulosa Jachowicz-Zdanowska 2013; Pterospermopsimorpha sp., Hagenfeldt 1989b; Jachowicz-Zdanowska 2013) are substantially smaller than the population recovered here (e.g. diameter of the inner body is 18–45 µm in Pterospermopsimorpha wolynica; Moczydłowska 2002; 20–50 µm in P. rugulosa; Jachowicz-Zdanowska 2013). Pteropermella gigantea from the Cambrian Stages 3–4 of Poland (BAMA III Zone equivalent to the Schmidtiellus–Holmia faunal zones) is similar to the recovered Pterospermopsimorpha sp. population in its overall dimensions, but has been interpreted as a pteromorphic acritarch.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Genus Siphonophycus Schopf 1968 emend. Knoll et al. (1991)

Type.Siphonophycus kestron Schopf, 1968, Tonian of Australia.

Remarks. The species-level subdivision of the genus proposed by Knoll et al. (1991) based on filament size is not followed here, since the recovered material exhibits two clearly distinct filament populations varying in morphology and wall thickness, but having slightly overlapping size ranges (^0005,0007).

Siphonophycus spp.

⁰⁰⁰⁷, figures 1–21, 23–27; ⁰⁰⁰⁹

Material. About 60 specimens in GGU samples 184002, 184003 and 184004.

Measurements. Width of filament 0.4–37.1 µm (N = 59; x¯ = 13.1 µm; σ = 9.0); length of filaments 94.8–2312 (N = 39; x¯ = 618.2 µm; σ = 482.7).

Description. These smooth-walled, undivided and unbranched filaments likely include a polyphyletic set of tubular microfossils of prokaryotic origin, some of them occurring in dense clusters (⁰⁰⁰⁹, figure 9), fibrous bundles (⁰⁰⁰⁹, figures, 1, 6–8), loose bundles (⁰⁰⁰⁷, figures 18–21, 23–24) or mats (⁰⁰⁰⁹, figures 3–5). Other forms include filaments enclosing a central thick-walled lineation (⁰⁰⁰⁷, figures 26–27) occasionally subdivided into parallel, thin threads (⁰⁰⁰⁷, figure 27a) resembling the coalesced remains of cellular trichomes seen in Eomicrocoleus (e.g. Harvey et al. 2011, fig. 1C). Given the poor state of preservation of these internal structures, Eomicrocoleus-like specimens are here conservatively assigned to Siphonophycus.

Genus Skiagia Downie 1982 emend. Moczydłowska 1991 

Type.Skiagia scottica Downie 1982, Cambrian Stages 3–4 Holmia kjerulfi Zone of Scotland.

Skiagia ciliosa (Volkova 1969) Downie 1982 

⁰⁰⁰⁶, figures 2–3, 14–17

Synonymy. 1991 Skiagia ciliosa (Volkova 1969) Downie 1982; Moczydłowska, pp. 65–67, pl. 7, figure A–F [cum syn.]

Material. 50–100 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 42. Diameter of central body 21.5–35.9 µm (x¯ = 29.1 µm; σ = 3.5), process length 1.7–8.5 µm (x¯ = 5.4 µm; σ = 1.5), width of the process base 0.9–3.4 µm (x¯ = 1.8 µm; σ = 0.7), width of the funnel-shaped termination 0.9–3.5 µm (x¯ = 2.1 µm; σ = 0.6).

Description. Spheroidal vesicles covered in 20 to 100 (x¯ = 61) short processes (< 30% of the diameter of the central body) having funnel-shaped terminations. A continuum of forms can be observed from vesicles having thin processes with slightly widened bases (morphotype A; ⁰⁰⁰⁶, figure 2) to vesicles having wider processes with robust conical bases and occasional plugs (morphotype B; ⁰⁰⁰⁶, figure 3). A third morphotype (labelled C; Vidal and Peel 1993) consists of vesicles having very short processes (1.7–3 µm; N = 23; <10% of the diameter of the central body) joined to each other by their wide funnel-shaped terminations (⁰⁰⁰⁶, figures 14–17). Openings by partial rupture and median splits are common. Two of the recovered vesicles enclose a dark to opaque spheroidal internal body nearly occupying the entire vesicle cavity.

Remarks. The large intraspecific variability of this taxon overlaps with the morphological range of other species from the genus (Wallet et al. 2022). In particular, morphotype C is an abundant intermediate form in the recovered assemblage, exhibiting the robust conical bases of S. ciliosa and attached process tips of S. scottica. This spectrum of forms likely corresponds to one or a few biological species represented by multiple ontogenetic stages and their ecophenotypic variations. To our knowledge, morphotype C has only been reported in North Greenland (Vidal and Peel 1993), a pattern that may support an ecophenotypic origin for these forms. However, morphotype C specimens also grade into spinose sphaeromorphs when subjected to secondary abrasion (⁰⁰⁰⁶, figures 14–15), and similar morphologies are detectable among illustrated specimens of Lophosphaeridium dubium or Globosphaeridium cerinum from numerous localities (e.g. Volkova 1968, pl. 1, fig. 5, 6; Downie 1982, fig. 6e–g; Knoll and Swett 1987, fig. 7.3; Jachowicz-Zdanowska 2013, pl. 13, fig. 13; Palacios et al. 2020, fig. 5b,d, 6a,b). Skiagia ciliosa specimens of morphotype C may therefore be more widespread than previously thought, forming part of a regular Skiagia life cycle.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Australia (Heliosphaeridium dissimilare–Skiagia ciliosa and Volkovia dentifera–Liepaina plana zones; Zang et al. 2001, 2007), China (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Zang 1992), Siberia (Vidal et al. 1995); Canada (Heliosphaeridium dissimilare–Skiagia ciliosa and Volkovia dentifera–Liepaina plana zones; Palacios et al. 2011, 2017); Spain (Palacios et al. 2021); Sweden (Holmia Zone; Vidal 1981a, b; Moczydłowska and Vidal 1986; Hagenfeldt 1989b; Moczydłowska 2002), Denmark (Holmia Zone; Vidal 1981a; Moczydłowska and Vidal 1986, 1992); Norway (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; Palacios et al. 2020); Cambrian Stage 3 to Miaolingian of the East European Platform (Holmia, Protolenus, Eccaparadoxides oelandicus and Paradoxides paradoxissimus zones; Volkova 1969; Volkova et al. 1979; Moczydłowska 1981, 1991; Mens et al. 1999) and Greenland (Vidal 1979; Downie 1982; Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study); early Cambrian of England (Downie 1982) and Svalbard (Knoll and Swett 1987); early Cambrian to Miaolingian of Wales (Downie 1982).

Skiagia cf. compressa

⁰⁰⁰⁶, figure 9 

Material. Three specimens from GGU sample 184002, two specimens from GGU sample 184003 and six specimens from GGU sample 184004.

Measurements.N = 7. Diameter of central body 26.1–37.5 µm (x¯ = 30.8 µm; σ = 4.1), process length 16.1–22.5 µm (x¯ = 19.3 µm; σ = 2.1), width of the process base 2.0–3.6 µm (x¯ = 2.7 µm; σ = 0.6), width of the funnel-shaped termination 1.41–2.8 µm (x¯ = 2.1 µm; σ = 0.6).

Description. Spheroidal vesicle bearing 50 to 130 long (> 60% of the vesicle diameter) processes with funnel-shaped terminations. The basal portion of processes is flared and is occasionally separated from the tubular portion by a plug (⁰⁰⁰⁶, figure 9a). A dark to opaque inner body may occasionally be observed within the vesicle, occupying at least 90% of its cavity.

Remarks. These acritarchs combine features from multiple Skiagia species. Their robust process bases occasionally sealed by thick plugs are reminiscent of S. ciliosa, but the latter is characterised by much shorter processes (< 30% of the vesicle diameter). Skiagia ornata is adorned by longer processes, but these do not normally exceed 50% of the vesicle diameter (Volkova 1968; Downie 1982; Moczydłowska 1991, 2011; Zang 2001) and were originally described as having a relatively equal width along their entire length (Volkova 1968). Specimens previously assigned to S. ornata type 1 (Moczydłowska and Vidal 1986, fig. 11c–d) and later attributed to Elektroriskos flexuosus by Eklund (1990) exhibit robust conical process bases that are similar to those observed in studied population, but these process bases form more clearly defined, high-relief grains attached to thinner flexible processes lacking obvious plugs. Processes in S. compressa (Volkova, 1968) Downie, 1982 can occasionally be as long as in S. ornata, and have expanded bases sometimes sealed with plugs. On this basis, the recovered population is assigned to S. cf. compressa.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Skiagia ornata (Volkova 1968) Downie 1982 

⁰⁰⁰⁶, figure 5 

Synonymy. 1991 Skiagia ornata (Volkova 1968) Downie 1982; Moczydłowska, pp. 68–69, pl. 5, figure E–F, pl. 6 fig. A–D [cum syn.]

1993 Elektoriskos sp. A; Vidal and Peel, p. 23, figure 13b

Material. 50–100 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 25. Diameter of central body 20–45.9 µm (x¯ = 33 µm; σ = 5.8), process length 8.8–17.9 µm (x¯ = 12 µm; σ = 2.3), width of the process base 1–2.6 µm (x¯ = 1.7 µm; σ = 0.4), width of the funnel-shaped termination 1.1–3.5 µm (x¯ = 1.9 µm; σ = 0.6).

Description. Spheroidal vesicle consisting of a central body covered in 30 to 120 processes ending in a funnel-shaped termination. The basal portion of the processes is tubular to slightly flared. Five of the recovered vesicles enclose a dark to opaque inner body occupying at least 80% of the vesicle cavity.

Remarks. Transitional forms are evident between S. ornata and other Skiagia species (Wallet et al. 2022), particularly S. cf. compressa, S. compressa, S. orbiculare and more rarely S. scottica. A specimen previously identified as Elektoriskos sp. A shows funnel-shaped process terminations warranting its placement in Skiagia. The relative length of processes and their tubular morphology identifies this specimen as S. ornata despite its unusually small size (diameter of central body 20 µm) and sparsely distributed processes (ca. 30 in total).

Occurrence and stratigraphical range. Cambrian Stages 3–4 of the East European Platform (Talsy, Vergale and Rausve horizons; Volkova et al. 1979), Sweden (Holmia Zone; Vidal 1981a, b; Moczydłowska and Vidal 1986; Moczydłowska 2002); Denmark (Moczydłowska and Vidal 1986, 1992); Norway (Palacios et al. 2020), Greenland (Heliosphaeridium dissimilare – Skiagia ciliosa Zone; Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study) and Australia (Skiagia ornata Zone equivalent to the Skiagia ornata–Fimbriaglomerella membranacea Zone; Zang et al. 2007); Cambrian Stage 3 of Canada (Skiagia ornata–Fimbriaglomerella membranacea Zone; Palacios et al. 2011, 2018); lower Cambrian of Kazakhstan (Ogurtsova 1985) and Svalbard (Knoll and Swett 1987).

Skiagia pura Moczydłowska 1988 emend.

⁰⁰⁰⁶, figures 7–8 

Synonymy. 2011 Skiagia pura Moczydłowska 1988; Moczydłowska, pp. 132–133, pl. 15, figure 5–6 [cum syn.]

2001 Skiagia orbiculare (Volkova 1968) Downie 1982; Zang, p. 82, pl. 2, figure 3

2001 Skiagia sp. cf. pura Moczydłowska 1988; Zang, p. 83, pl. 2, figures 8–9

2022 Skiagia cf. pura; Wallet et al. p. 579, figure 2H

Material. One specimen from GGU sample 184002, two specimens from GGU sample 184003 and one specimen from GGU sample 184004.

Measurements.N = 4. Diameter of central body 31.3–38.2 µm (x¯ = 32.6 µm; σ = 4.1), process length 4.8–5.7 µm (x¯ = 5.2 µm; σ = 0.4), width of the process base 0.8–1.8 µm (x¯ = 1.2 µm; σ = 0.4); width of the funnel-shaped termination 2.1–2.7 µm (x¯ = 2.4 µm; σ = 0.2).

Original diagnosis. Acritarch with oval-shaped vesicle and numerous evenly distributed processes. The processes do not display basal attachments with the central body. The processes are funnel-shaped, the width being 2 µm. The distal funnel-shaped portions of the processes are generally in contact with each other.

Emended diagnosis. Spheroidal vesicles having tubular processes that gradually expand into a wide funnel-shaped termination. Processes may be attached to each other by their distal funnel-shaped portion. Morphotype A vesicles have relatively long, thin-walled processes of relatively brittle appearance with indistinct basal attachments. Morphotype B vesicles have shorter processes with robust, thick-walled bases that may be separated from the vesicle cavity by a plug.

Description. Spheroidal vesicles bearing 20–65 processes that gradually expand into a wide distal funnel. Vesicles of morphotype A (⁰⁰⁰⁶, figure 7) exhibit moderately long (ca. 6 µm), thin-walled processes that are weakly attached to the vesicle surface (⁰⁰⁰⁶, figure 7a). Morphotype B specimens (⁰⁰⁰⁶, figure 8) exhibit thicker process attachments, possibly reinforced by a plug (⁰⁰⁰⁶, figure 8a). In both morphotypes, processes are narrowest at their base, and expand distally at varying rates even within individual specimens. Excystment not observed.

Remarks. This species has originally been defined based on four specimens from the early Cambrian Stage 3 of Poland (Moczydłowska 1988). Since their discovery, numerous acritarchs resembling S. pura have been found worldwide and attributed to various species, many of which were later synonymised with S. pura (Moczydłowska 2011). On the basis of newly recovered material and updated synonymies, the diagnosis of the taxon was revised in order to account for this extended disparity. Many features appear to vary considerably both within and among S. pura assemblages, including the number of processes, their length and the morphology of their base. While basal process attachments are indistinct in specimens from the type locality and Australia (morphotype A; Moczydłowska 1988, pl. 2; Zang et al. 2007, fig. 13F–J), S. pura specimens from elsewhere may exhibit robust thickened bases occasionally reinforced by a plug (Morphotype B; Downie 1982, fig. 5, 8a). The presence of a separation between processes and the vesicle cavity has either been confirmed (Moczydłowska 2011) or ruled out (Zang 2001). Moreover, the length of processes in S. pura now spans a range of 4–15 µm (Vanguestaine et al. 2002; Zang 2001), and taphonomy seems to exert some control on their morphology and number. Indeed, recovered specimens tend to show unevenly shaped and irregularly distributed processes (⁰⁰⁰⁶, figures 7–8), and the presence of raised process bases lacking a distal portion (⁰⁰⁰⁶, figure 8; Wallet et al. 2022, fig. 2H) suggests that the attachment of processes to the vesicle surface is weak. Following the recognition of these sources of variation, specimens previously assigned to S. cf. pura (Wallet et al. 2022) can confidently be identified as S. pura. S. pura differs from other species of the genus in having a distinct process morphology consisting of a tubular portion that gradually expand into a wide distal funnel (Figure 12). Nevertheless, the long taxonomic history of the species reflects an abundance of transitional forms verging on the morphological range of S. insigne, S. scottica, S. ciliosa and S. orbiculare. S. pura (morphotype B) resembles S. insigne in having widely expanding process terminations and robust bases with plugs, but differs from the latter in having undivided distal funnels and septa separating process bases from their more distal portion (Fridrichsone 1971; Downie 1982, fig. 5; Moczydłowska 1988). The susceptibility of these features to taphonomic alteration may partly account for the presence of transitional forms between these two taxa (see e.g. S. cf. insigne in Moczydłowska 1991, pl. 7I and Vanguestaine et al. 2002, pl. 2, fig. 7). Such transitional forms have also been assigned to S. ciliosa before the erection of S. pura (Knoll and Swett 1987), a choice that is supported by the relatively short length and robust basal portion of their processes (Downie 1982, fig. 8a–c). Long S. pura processes that join at their tips also appear strikingly similar to those adorning S. scottica (see e.g. Downie 1982, fig. 8f; Vanguestaine et al.,2002, pl. 2, fig. 6; Moczydłowska 2011, fig. 15.5–6), and only differ from the latter in having less flattened terminations (Moczydłowska 1988). Transitional forms between S. pura and S. orbiculare are also known (Zang 2001), particularly among S. pura specimens having relatively long processes (see e.g. Zang et al. 2007, fig. 13H–J). As in other localities (Moczydłowska 1988, 2011; Moczydłowska and Vidal 1992; Zang 2001; Vavrdová 2006; Jachowicz-Zdanowska 2013), S. pura occurs in low abundance in the studied assemblage, and could represent an unusual variant in a single Skiagia-type life cycle represented by a continuous spectrum of form-species.

Occurrence and stratigraphical range. Cambrian Stage 3 (Skiagia ornata–Fimbriaglomerella membranacea Zone) of Poland (Moczydłowska 1988, 1991; Jachowicz-Zdanowska 2013), the Czech Republic (Vavrdová 2006), Sweden (Moczydłowska et al. 2001); Denmark (Moczydłowska and Vidal 1992); Cambrian Stages 3–4 (Heliosphaeridium dissimilare–Skiagia ciliosa Zone) of South Australia (Zang et al. 2007), Ireland (Vanguestaine et al. 2002), Scotland (Downie 1982) and Greenland (Wallet et al. 2022; this study).

Skiagia? spp.

⁰⁰¹⁰, figures 10–11; ⁰⁰⁰⁶, figures 10–13, 18–19

Material. More than a hundred specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 27. Diameter of central body 25.3–44.4 µm (x¯ = 34.5 µm; σ = 5.4), process length 2.2–13.8 µm (x¯ = 7.1 µm; σ = 3.0), width of the process base 0.7–2.6 µm (x¯ = 1.4 µm; σ = 0.5).

Description. Spheroidal vesicles bearing truncated processes. Processes vary in length, thickness and morphology among individual vesicles. Opening by partial rupture or median split.

Remarks. These acritarchs likely represent taphonomic variants of multiple Skiagia species having undergone variable degrees of surficial abrasion resulting in the loss of the diagnostic funnel-shaped process termination of Skiagia. Clear taphonomic series can be observed within this population, from vesicles bearing long truncated processes theoretically attributable to Baltisphaeridium (⁰⁰⁰⁶, figures 10–11), to vesicles having shorter, blunt to pointed processes similar to Globosphaeridium cerinum (⁰⁰⁰⁶, figure 13), and eventually vesicles covered in apparently solid cones or pustules characteristic of some Lophosphaeridium species (⁰⁰⁰⁶, figures 14, 18–19). All these morphotypes are here considered as parts of a taphonomic continuum likely originating from one or several Skiagia species.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study), and potentially in all assemblages featuring Skiagia (see list of occurrences in Moczydłowska 1991, 2011).

Genus Symplassosphaeridium Timofeev 1959 ex Timofeev 1969 

Type.Symplassosphaeridium tumidulum Timofeev 1959, early Ordovician ‘Obolus’-shale beds, Russia.

Symplassophaeridium sp.

⁰⁰⁰³, figure 12 

Material. 1 specimen from GGU sample 184002.

Measurements.N = 1. Diameter of vesicle 3.3–4.8 µm; maximum length of clusters 43 µm.

Description. Compact spheroidal aggregate of small cells.

Remarks. The specimen differs from E. malgica in having slightly larger cells compacted into spheroidal aggregates, presumably enclosed in a sheath. It further differs from Synsphaeridium in having smaller cells and a more compact appearance.

Occurrence and stratigraphical range. Cosmopolitan and long-ranging, from the Paleoproterozoic to the Phanerozoic.

Genus Synsphaeridium Eisenack 1965 

Type.Synsphaeridium gotlandicum Eisenack 1965, Silurian of Sweden.

Synsphaeridium spp.

⁰⁰⁰³, figures 5–11 

Synonymy. 1993 Leiosphaeridia sp.; Vidal and Peel, p. 27, figure 11f

Material. About 50 clusters in GGU samples 184002, 184003 and 184004.

Measurements.N = 24. Diameter of central body 10.7–19.3 µm (x¯ = 14.1 µm; σ = 2.1), number of cells per cluster 4–13 (x¯ = 8.8; σ = 2.5).

Description. Clusters of generally smooth spheroidal vesicles. An irregular, weakly developed ornamentation of small granulae may be present on the surface of some vesicles. Agglomerates tend to be irregular in outline and are formed of relatively loosely packed cells, with some clusters having visibly lost some specimens (⁰⁰⁰³, figure 10).

Remarks. Surface ornamentation is variably developed both within (e.g. ⁰⁰⁰³, figure 9) and among clusters of cells (compare ⁰⁰⁰³, figure 6 to ⁰⁰⁰³, figure 7). Small processes may also tentatively be recognised in some clustered vesicles (⁰⁰⁰³, figures 5a, 5b). Whether these sculptural elements result from secondary alteration or form part of the original (ontogenetic?) diversity of cell colonies is unclear. Despite this disparity, all clustered sphaeromorphs share a relatively constant cell size, which is larger than clustered Asteridium or Comasphaeridium vesicles (N = 40; x¯ = 8.9 µm; σ = 1.3; ⁰⁰⁰³, figure 1–4; Figure 13); but smaller than Lophosphaeridium specimens found in isolation (N = 20; x¯ = 31.5 µm; σ = 6.7; ⁰⁰¹⁰, figures 1–8; Figure 13). Some clustered cells exhibit intracellular inclusions that may represent internal cells (⁰⁰⁰³, figure 11) or other condensed cellular material (⁰⁰⁰³, figure 10).

Occurrence and stratigraphical range. Cosmopolitan and long-ranging, from the Paleoproterozoic to the Phanerozoic.

Genus Tasmanites Newton 1875 

Type.Tasmanites punctatus Newton 1875, Permian of Australia.

Tasmanites bobrowskae Ważyńska 1967 

⁰⁰¹¹, figure 1 

Synonymy. 1991 Tasmanites bobrowskae Ważyńska 1967; Moczydłowska, p. 70, pl. 10, figures F–G, pl. 11, figures C–E [cum syn.]

2011 Tasmanites bobrowskae Ważyńska 1967; Moczydłowska, p. 134, pl. 16, figures 1–4, pl. 17, figures 1–5, pl. 20, figure 3 [cum syn.]

Material. About 10 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 5. Diameter of the vesicle 61–101.3 µm (x¯ = 70.12 µm; σ = 20.6), minimum diameter of perforations 0.3–0.6 µm (x¯ = 0.5 µm; σ = 0.1), maximum diameter of perforations 0.9–1.2 µm (x¯ = 1.1 µm; σ = 0.1).

Description. Spheroidal vesicles pierced by irregularly distributed perforations of relatively large size. Almost all recovered specimens are fragmented. Excystment not observed.

Remarks. The species differs from T. tenellus in having larger perforations and a thicker wall (Volkova 1968; Moczydłowska 1991). In the recovered Tasmanites population, the size of perforations varies continuously both between and within individual specimens (⁰⁰¹¹, figures 1–5; Figure 6), and does not appear to correlate with the thickness of the vesicle. For this reason, an arbitrary boundary has been set to distinguish T. tenellus and T. bobrowskae populations in the studied assemblage, considering vesicles having a majority of pores with diameter 0.6 µm as T. bobrowskae (Figure 6).

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Canada (Downie 1982), Scotland (Downie 1982), Sweden (Holmia Zone; Vidal 1981a; Moczydłowska and Vidal 1986), Denmark (Holmia Zone; Vidal 1981b; Moczydłowska and Vidal 1986) and Norway (Holmia Zone; Moczydłowska and Vidal 1986; Vidal and Nystuen 1990); lower Cambrian of the East European Platform (Holmia and Protolenus zones; Lontova, Talsy, Vergale and Rausve ‘horizons’; Ważyńska 1967; Volkova et al. 1979; Moczydłowska 1981; Jachowicz-Zdanowska 2013), Finland (Tynni 1978), Greenland (Vidal 1979; Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study), Svalbard (Knoll and Swett 1987) and Ireland (Vanguestaine et al. 2002); Cambrian of Belgium (Vanguestaine 1978).

Tasmanites tenellus Volkova 1968 

⁰⁰¹¹, figures 2–5 

Synonymy. 1991 Tasmanites tenellus Volkova 1968; Moczydłowska, p. 71, pl. 11, figures A–B [cum syn.]

2011 Tasmanites tenellus Volkova 1968; Moczydłowska, p. 134, pl. 18, figures 1–6, pl. 19 figures 1–6 [cum syn.]

Material. 10–20 specimens in GGU samples 184002, 184003 and 184004.

Measurements.N = 11. Diameter of the vesicle 21.9–64.6 µm (x¯ = 38.2 µm; σ = 10.5), minimum diameter of perforations 0.3–0.5 µm (x¯ = 0.4 µm; σ = 0.1), maximum diameter of perforations 0.5–1.1 µm (x¯ = 0.9 µm; σ = 0.2).

Description. Spheroidal vesicles pierced by irregularly distributed perforations of relatively small size. Excystment by partial rupture.

Remarks. The species differs from T. bobrowskae in having smaller perforations and a thinner wall, but these characteristics show continuous variation in the recovered Tasmanites population (Figure 6). For this reason, an arbitrary boundary has been set to distinguish T. tenellus and T. bobrowskae populations in the studied assemblage, considering vesicles having a majority of pores with a diameter inferior to 0.6 µm as T. tenellus. Vesicles attributed to T. tenellus occasionally show overlapping ornaments of pores and small granulae (⁰⁰¹¹, figure 5a) and potential processes (⁰⁰¹¹, figures 4a, 5b). These characteristics may hint at an acanthomorphic origin, with pores and granulae representing variably abraded and coalesced process bases having a hollow construction. Similar, yet coarser surface elements adorn degraded vesicles of Skiagia ciliosa, with processes being abraded into pores surrounded by a darker rim (⁰⁰¹⁰, figures 10–12) that represents the contact between the conical process base and the vesicle surface.

Occurrence and stratigraphical range. Terreneuvian of Finland (Slater and Willman 2019); Terreneuvian–Cambrian Stage 3 of the East European Platform (Volkova 1969; Volkova et al. 1979; Moczydłowska 1981; Jachowicz-Zdanowska 2013); Cambrian Stages 3–4 of Sweden (Holmia Zone; Vidal 1981a, b; Moczydłowska and Vidal 1986), Denmark (Holmia Zone; Vidal 1981b; Moczydłowska and Vidal 1986), Norway (Holmia Zone; Moczydłowska and Vidal 1986; Vidal and Nystuen 1990); lower Cambrian of Svalbard (Knoll and Swett 1987); lower Cambrian of the Czech Republic (Vavdrová 2006) and Greenland (Moczydłowska and Vidal 1986; Vidal and Peel 1993; this study).

Genus Tortunema Hermann 1976 (in Timofeev et al. 1976) emend. Butterfield et al. (1994)

Type.Tortunema wernadskii (Schepeleva1960) Butterfield et al. (1994), lower Cambrian of Russia.

Tortunema wernadskii (Schepeleva 1960) Butterfield et al. (1994)

⁰⁰⁰⁹, figure 2b 

Synonymy. 1994 Tortunema wernadskii (Schepeleva 1960) Butterfield et al. (1994), p. 69, figures 24H, 27A–C [cum syn.]

Material. One specimen from GGU sample 184004.

Measurements.N = 1. Maximum width of filament 11.3 µm, preserved length of filament 70.3 µm, spacing between transversal thickenings 4.3–6 µm.

Description. Smooth-walled, unbranched filament divided by relatively equally spaced transversal thickenings. Fainter annulations may further subdivide intervals between thickenings into equal halves.

Remarks. In the absence of positive evidence for internal structures, transversal thickenings and the potential annulations between them are interpreted as pseudosepta (as opposed to true septa dividing cytoplasm into multiple cells; Butterfield et al. 1994), which supports attribution to the form-genus Tortunema. At a maximum width of 11.3 µm, the recovered sheath is identified as T. wernadskii (Butterfield et al. 1994).

Occurrence and stratigraphical range.Tortunema is cosmopolitan in the Proterozoic (Javaux and Knoll 2017), but the relatively simple morphology of tubes with transversal annulations can originate from a diversity of clades, some of which seem to have persisted in the early Cambrian of the Czech Republic (Steiner and Fatka 1996) and in the studied interval (Cambrian Stages 3–4 of Greenland; Heliosphaeridium dissimilare–Skiagia ciliosa Zone). T. wernadskii is otherwise known from the Meso–Neoproterozoic of Canada (Loron et al. 2019), Mauritania (Beghin et al. 2017) and the Democratic Republic of Congo (Baludikay et al. 2016); the Neoproterozoic of Svalbard (Butterfield et al. 1994) and Namibia (Gaucher and Germs 2007); and the Ediacaran of Estonia (Arvestål and Willman 2020).

Unknown Form 1

⁰⁰¹⁰, figure 9 

Material. One specimen from GGU sample 184002.

Measurements.N = 1. Diameter of the vesicle 21.4 µm; diameter of localised thickenings 4.5–5.8 µm.

Description. Smooth-walled vesicle covered in up to four localised thickenings.

Remarks. The optically dark localised areas covering the vesicle are reminiscent of the high-relief thickening seen in Archaeodiscina umbonulata Volkova (1968), but have a more diffuse morphology that do not show a clear star shape and is not substantially raised above the vesicle surface. Some of these dark areas have an irregular outline and may be interpreted as condensed intracellular contents rather than surficial thickenings of the wall. An additional specimen (⁰⁰¹⁰, figure 8) appears to combine localised thickenings with a L. truncatum-type ornament. Closer inspection of this vesicle suggests that most of the large, dark patches likely correspond to folded areas and/or condensed intracellular contents, which supports attribution to L. truncatum rather than Archaeodiscina.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Unknown Form 2

⁰⁰¹⁰, figure 13 

Material. One specimen from GGU sample 184004.

Measurements.N = 1. Diameter of the vesicle 19.9 µm; maximum diameter of polygonal fields 8.9 µm.

Description. Spheroidal vesicle divided into at least six polygonal fields by optically dark ridges.

Remarks. The specimen resembles Cymatiosphaera in being divided into polygonal fields, but differs from this taxon in having small dimensions and apparently low relief ridges. Similar forms could have been produced by compression of Leiosphaerida, but the sharp colour difference between ridges and fields more likely reflects differences in wall structure and/or thickness rather than variable degrees of folding.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Unknown Form 3

⁰⁰⁰¹, figure 6 

Material. One specimen from GGU sample 184004.

Measurements.N = 1. Diameter of the vesicle 137.8 µm, length of neck-like extensions 33.7–83.9 µm, maximum basal width of neck-like extensions 55.9 µm, apical width of neck-like extensions 15.3–31.2 µm, length of processes 9.4–13.8 µm, width of processes 6.3–7.3 µm.

Description. Large spheroidal vesicle covered in irregularly distributed heteromorphic processes and neck-like extensions. At least five neck-like extensions and two processes are visible on the vesicle surface. Neck-like protrusions vary greatly in size but appear to conform to the same morphology, consisting of a clearly expended base and a closed rounded termination. Processes appear to be tubular from base to tip, although their distal portion seem to have been abraded.

Remarks. Recovered specimens resemble Tappania plana Yin 1997 in having irregularly distributed processes and neck-like extensions. Tappania plana was originally described as having one trapezoidal extension and many (10–25) processes, but subsequent studies increased the morphological disparity of this taxon, now including forms exclusively having neck-like extensions (Adam et al. 2017; Javaux and Knoll 2017). Nevertheless, the recovered specimen exhibits relatively long neck-like extensions in comparison to Proterozoic Tappania populations. The absence of additional Tappania morphotypes in the studied assemblage, which is ca. 800 My younger than the last known occurrence of T. plana, further casts doubt on any potential phylogenetic relationships between T. plana and the recovered specimen. The recovered specimen also bears similarities to Majasphaeridium carpogenum in having bud-like extensions, lacking compression folds and having an overall asymmetrical morphology. However, M. carpogenum does not have processes and is usually pear-shaped or reniform.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

Unknown Form 4

⁰⁰¹¹, figures 6–9 

Material. Four specimens in GGU sample 184002 and five specimens in GGU sample 184004.

Measurements.N = 8. Diameter of the vesicle 57–163 µm (x¯ = 112.8 µm; σ = 33.6), minimum diameter of first-order perforations 1.5–5.1 µm (x¯ = 2.4 µm; σ = 1.2), maximum diameter of first-order perforations 2.1–11.2 µm (x¯ = 4.8 µm; σ = 3.1), minimum diameter of second-order perforations 0.3–0.9 µm (x¯ = 0.6 µm; σ = 0.2), maximum diameter of second-order perforations 0.9–1.8 µm (x¯ = 1.3 µm; σ = 0.3).

Description. Large, thick-walled spheroidal vesicles pierced by irregularly distributed perforations of widely varying sizes. Two orders of perforations can be distinguished; first-order perforations form large, subcircular to oval pores that are relatively sparsely distributed on the vesicle surface; while second-order pores form a background of more densely distributed, smaller perforations.

Remarks. This form is similar to Tasmanites in having a perforated thick wall; however, the large size variations in pore sizes is suggestive of taphonomic overprinting. Acritarchs are occasionally encrusted in pyrite crystals and framboids that, when removed during palynological processing, can artificially perforate acritarch walls. However, most of the recovered specimens (including ⁰⁰¹¹, figures 6–9) were hand-picked following gentle HF acid maceration without oxidation – a process that should leave pyrite crystals intact, as seen in other hand-picked organic-walled microfossils (e.g. ⁰⁰⁰⁵, figures 1; ⁰⁰⁰⁷, figure 24; ⁰⁰⁰⁹, figure 3). Regardless of the potential origin(s) of pores (e.g. mineral incrustations, microborings, original ornamentation), these perforated acritarchs form a clear population, distinct from co-occurring, generally thinner walled leiosphaerids (⁰⁰¹²) and more regularly perforated Tasmanites species (⁰⁰¹¹, figures 1–5). Fragments of spheroidal cuticle recovered from a previous study (Wallet et al. 2021, fig. 10A–I) resemble the recovered specimens in having a relatively rough surface texture, but cuticle fragments show more complex perforation patterns including elongated burrows (e.g. Wallet et al. 2021, fig. 10E–G). It is possible that the recovered Tasmanites sp. population includes remains of animal origin (e.g. fertilisation envelopes; Raff et al. 2006), but the morphological simplicity of these vesicles inevitably makes their interpretation equivocal.

Occurrence and stratigraphical range. Cambrian Stages 3–4 of Greenland (Heliosphaeridium dissimilare–Skiagia ciliosa Zone; this study).

The samples used in this study were collected during fieldwork supported by the Geological Survey of Greenland. B. J. S. acknowledges the support of the Swedish Research Council (VR grant 2020-03314). J. S. Peel (Uppsala University) is thanked for guidance and help in interpreting the geology. Two anonymous reviewers are thanked for their constructive feedback on a previous version of this paper.

No potential conflict of interest was reported by the authors.