Small carbonaceous fossils (SCFs) from the Terreneuvian (lower Cambrian) of Baltica

We describe a new assemblage of small carbonaceous fossils (SCFs) from diagenetically minimally altered clays and siltstones of Terreneuvian age from the Lontova and Voosi formations of Estonia, Lithuania and Russia. This is the first detailed account of an SCF assemblage from the Terreneuvian and includes a number of previously undocumented Cambrian organisms. Recognizably bilaterian‐derived SCFs include abundant protoconodonts (total‐group Chaetognatha), and distinctive cuticular spines of scalidophoran worms. Alongside these metazoan remains are a range of protistan‐grade fossils, including Retiranus balticus gen. et sp. nov., a distinctive funnel‐shaped or sheet‐like problematicum characterized by terminal or marginal vesicles, and Lontohystrichosphaera grandis gen. et sp. nov., a large (100–550 μm) ornamented vesicular microfossil. Together these data offer a fundamentally enriched view of Terreneuvian life in the epicratonic seas of Baltica, from an episode where records of non‐biomineralized life are currently sparse. Even so, the recovered assemblages contain a lower diversity of metazoans than SCF biotas from younger (Stage 4) Baltic successions that represent broadly equivalent environments, echoing the diversification signal recorded in the coeval shelly and trace‐fossil records. Close comparison to the biostratigraphical signal from Fortunian small shelly fossils supports a late Fortunian age for most of the Lontova/Voosi succession, rather than a younger (wholly Stage 2) range.

Abstract: We describe a new assemblage of small carbonaceous fossils (SCFs) from diagenetically minimally altered clays and siltstones of Terreneuvian age from the Lontova and Voosi formations of Estonia, Lithuania and Russia. This is the first detailed account of an SCF assemblage from the Terreneuvian and includes a number of previously undocumented Cambrian organisms. Recognizably bilaterian-derived SCFs include abundant protoconodonts (total-group Chaetognatha), and distinctive cuticular spines of scalidophoran worms. Alongside these metazoan remains are a range of protistan-grade fossils, including Retiranus balticus gen. et sp. nov., a distinctive funnel-shaped or sheet-like problematicum characterized by terminal or marginal vesicles, and Lontohystrichosphaera grandis gen. et sp. nov., a large (100-550 lm) ornamented vesicular microfossil.
Together these data offer a fundamentally enriched view of Terreneuvian life in the epicratonic seas of Baltica, from an episode where records of non-biomineralized life are currently sparse. Even so, the recovered assemblages contain a lower diversity of metazoans than SCF biotas from younger (Stage 4) Baltic successions that represent broadly equivalent environments, echoing the diversification signal recorded in the coeval shelly and trace-fossil records. Close comparison to the biostratigraphical signal from Fortunian small shelly fossils supports a late Fortunian age for most of the Lontova/Voosi succession, rather than a younger (wholly Stage 2) range.
T H E earliest Cambrian Terreneuvian Series represents one of the most transformative intervals in Earth history. Bracketed by the Ediacaran System below, and Cambrian Series 2 above, it witnessed the Cambrian 'explosion' of animals, biomineralization, bioturbation and protists, as well as the establishment of an essentially modern marine biosphere. Curiously, the most obviously 'explosive' part of the radiations occurs several million years after the c. 541 Ma base of the Cambrian, more or less in the middle of the Terreneuvian (Budd & Jensen 2000;Maloof et al. 2010). The Terreneuvian itself consists of two parts: a lower Fortunian Stage (c. , and an upper 'Stage 2' (c. , broadly equivalent to the Tommotian of Siberia (Kouchinsky et al. 2012).
Several lines of palaeontological inquiry confirm that much of the initial radiation of bilaterian animals does indeed lie in the Terreneuvian (Budd 2003(Budd , 2013. A pronounced increase in the origination rate of small shelly fossil (SSF) taxa occurs at or near the beginning of Stage 2 (Bengtson et al. 1990;Maloof et al. 2010;Kouchinsky et al. 2012). Current records of SSFs, ichnofossils and acritarchs from the Fortunian point to relatively lowdiversity ecosystems compared to Stage 2, or younger counterparts. Phosphatic SSF assemblages from this earliest Cambrian interval are dominated by enigmatic tubular forms (e.g. Anabarites, Hyolithellus, Hexaconularia) and protoconodonts (Protohertzina), typically referred to the Anabarites trisulcatus-Protohertzina anabarica Zone (Hamdi et al. 1989;Steiner et al. 2004a;Kouchinsky et al. 2012). Fortunian trace fossils record comparatively low degrees of sediment mixing, and are typified by assemblages of simple, shallow tiering treptichnid-type habits (e.g. Treptichnus, Didymaulichnus), which are joined by Rusophycus-type arthropod traces at c. 536 Ma (Jensen 2003;Mangano & Buatois 2016). The signal from Fortunian plankton is also relatively subdued, characterized by long-ranging acritarch taxa belonging to the Asteridium tornatum-Comasphaeridium velvetum Zone (Moczydłowska 1991(Moczydłowska , 1998 and the Asteridium- Heliosphaeridium-Comasphaeridium assemblage (Yao et al. 2005). No Burgess Shale-type (BST) Lagerst€ atten are known from the Terreneuvian, and apart from a handful of sites bearing phosphatized algae and metazoan larvae in South China (Steiner et al. 2004b;Liu et al. 2017), there is remarkably little accounting of the non-biomineralizing clades that dominate the diversity of most ecosystems.
One alternative source of palaeontological data comes from small carbonaceous fossils (SCFs), which have fundamentally expanded the view of non-biomineralized organisms in younger Cambrian strata (Butterfield & Harvey 2012;Harvey et al. 2012;Smith et al. 2015;Slater et al. 2017a). An extension of this SCF record into the Terreneuvian offers a novel means of tracking diversification through the 'Cambrian Explosion' interval, and a complementary record to the signal from SSFs. Indeed, at least some SCFs appear to be diagenetically de-mineralized versions of SSF elements, fossilized by virtue of their periostracum or intercrystalline organic matrix (e.g. Butterfield & Harvey 2012;Mart ı Mus 2014;Slater et al. 2017a, b); equally, some SSFs could be diagenetically mineralized remains of originally carbonaceous or lightly mineralized forms; e.g. phosphatized wiwaxiid sclerites (Porter 2000) and hallucigeniid spines (Skovsted & Peel 2001). SCFs can therefore extend the ranges of SSF-type taxa into previously unsampled depositional (or diagenetic) environments. Importantly, however, a majority of SCFs record body parts or taxa that are otherwise unrepresented in the fossil record.
A rich diversity of SCFs has recently been documented in early, but not earliest, Cambrian siliciclastic sediments of the western Baltic Basin (Slater et al. 2017a). Across this part of the basin Cambrian sediments are draped unconformably over a peneplained Proterozoic gneissic basement, and over most of the region the earliest preserved Cambrian record begins in Stage 4 (i.e. younger than 514 Ma;Nielsen & Schovsbo 2007Slater et al. 2017a). In the eastern Baltic region, however, these epicratonic successions continue down into the Terreneuvian, spanning Ogurtsova 1975Ogurtsova , 1977. In contrast, however, more recent analysis of acritarchs, trace and body fossils suggests that the bulk of the Lontovan is instead Fortunian in age (cf. Volkova et al. 1990;Jensen & Mens 1999;Suuroja & Suuroja 2010;Nielsen & Schovsbo 2011;Palacios et al. 2017). Whether these sediments represent a pre-explosion (~Fortunian) interval or were deposited during the Cambrian explosion (~Stage 2) itself is crucial for understanding early Cambrian evolution on Baltica. Here we report a unique SCF accounting of the Lontova and laterally contiguous Voosi Formations of Estonia, Lithuania and western Russia.

GEOLOGICAL SETTING
The Lontova Formation (Figs 1, 2) underlies large areas of the eastern Baltic States, extending into Russia and the Gulf of Finland. It is exposed locally in clay quarries along the north Estonian coastline and widely encountered in drillcore (Mens 2003;Nielsen & Schovsbo 2011). In the western Estonian mainland and islands of Hiiumaa and Saaremaa, contemporaneous sedimentary units are referred to as the Voosi Formation (Figs 1, 2).
Sediments of the Lontova Formation comprise a relatively homogeneous and laterally extensive sequence of poorly lithified illite-smectite rich claystones/siltstones known locally as 'Cambrian Blue Clay' (Mens 2003; Raidla et al. 2010;Budd et al. 2011). A combination of shallow burial depth and a tectonically stable setting has resulted in remarkably little lithification or thermal alteration over the past half a billion years (Kirsim€ ae et al. 1999;Kirsim€ ae & Jørgensen 2000;Winchester-Seeto & McIlroy 2006). Properties of the clay mineralogy and acritarch colour-alteration in these sediments signify maximum burial temperatures not exceeding 50°C, ideal for the preservation of SCFs (Talyzina 1998;Ivanovskaya & Geptner 2004). Across its >500 km extent, the Lontova Formation rests transgressively above sediments of the 'Rovno' (latest Ediacaran to Fortunian) or 'Kotlin' (latest Ediacaran) regional Baltic stages, or directly on Proterozoic crystalline gneissic basement (Mens et al. 1990;Jensen & Mens 1999). Despite its homogeneity, it is possible to recognize four subdivisions of the Lontova Formation, based primarily on differences in the relative ratios of sediment types. The lowermost S€ ami Member consists of a heterolithic succession of sandstones and silty claystones (Mens & Isakar 1999), and hosts a low-diversity ichnofossil assemblage including Treptichnus pedum (Palij et al. 1983). The S€ ami Member is overlain by the green-grey to red-brown silt-rich claystones of the Mahu Member. The succeeding Kestla Member is predominantly claystone, but has numerous thin fine sand and silt beds, and hosts a low-diversity SSF assemblage including the helcionelloid molluscs Aldanella kunda and Anabarella plana (Isakar & Peel 2007). The uppermost Tammaneeme Member is restricted to the western region of the Estonian Lontova Formation, and consists of coarser dark grey siltstones (Mens & Pirrus 1977;Kirsim€ ae et al. 1999;Figs 1, 2).
Sediments of the more westerly Voosi Formation are relatively sandier, though there is still a significant proportion of argillaceous material. Three members of the  , 15.4 m of , and c. 110 m of . Lontova Formation (surface sections, Tvere cius-336 core,  core and F332-Vihterpalu core); Voosi Formation (parts of F332-Vihterpalu core).
Together, the Lontova and Voosi formations encompass the bulk of the 'Lontovan' local Baltic stage of the Lower Cambrian (Nielsen & Schovsbo 2011). Sediments of the Lontova and Voosi formations lack trilobites, so biostratigraphical schemes have relied on SSFs, acritarchs and trace fossils. The SSF Aldanella kunda occurs in the uppermost parts of the Lontova Formation in Estonia and is widely considered to be a junior synonym of A. attleborensis (Landing 1988;Isakar & Peel 2007;Parkhaev & Karlova 2011). A. attleborensis has previously been suggested as a potential Stage 2 index fossil (e.g. Landing et al. 2013); however, it first appears below the Fortunian/Stage 2 boundary and within Fortunian to Stage 2 transitional beds in many Siberian sections (see Kouchinsky et al. 2017). Taken together, the Lontovan SSF assemblage is characteristic of the Platysolenites antiquissimus The Lontova and Voosi formations are overlain by the younger early Cambrian Sõru and/or L€ ukati formations, though the Sõru is missing from eastern regions (Fig. 2). The L€ ukati contains trilobites belonging to the Stage 3 Schmidtiellus mickwitzi Zone (Bergstr€ om 1973;Mens & Pirrus 1977;Ahlberg et al. 1986). The subjacent Sõru Formation has conventionally been assigned to the Rusophycus parallelum ichnofossil Zone (Mens et al. 1990), which could span Stage 2 and/or 3. Since the boundary between the Sõru/L€ ukati and Voosi/Lontova is disconformable (Nielsen & Schovsbo 2011;Meidla 2017), there is potentially a large time gap between the deposition of these units in any case. Assigning an older, Fortunian, or at least partly Fortunian age to the Lontova would help to explain the apparent discordance between the Lontova Formation and the Lontovan regional stage in certain areas. In eastern Latvia, for example, the Lontova Formation is considered to span both the earliest Cambrian Rovno and Lontovan regional stages (Birkis et al. 1972;Jensen & Mens 1999

SMALL CARBONACEOUS FOSSILS
A wide range of SCFs and acritarchs were recovered from early Cambrian sediments of the Lontova and Voosi formations (Figs 1, 2). Of 71 processed samples, a total of 1488 microfossils from 21 productive samples were permanently mounted onto 73 glass slides. Slides are housed at the Swedish Museum of Natural History, Stockholm (NRM). The recovered fossils include >10 distinct forms, two of which are clearly derived from bilaterian metazoans; the remainder are phylogenetically problematic, with most forms representing a protistan grade of organization.
Protoconodont spines. SCFs recovered from the Lontova and Voosi formations frequently include flattened carbonaceous spines with a more or less curved, horn-like shape and basal opening (Figs 3, 4A-G). The spines reach >2.1 mm in maximum length ( x = 934 lm, SD = 416 lm, n = 81), and a maximum basal width of >1 mm ( x = 323 lm, SD = 198 lm, n = 81) and an average length to base ratio of 3.4:1. Occasional examples of these spines were recovered from the Kestla Member of Tvere cius-336 and Kostovo-13, but they were particularly abundant in the Kunda quarry, where a c. 50 g sample of Kestla Member claystone yielded in excess of 200 spines, for example. The spines show a continuum of morphologies (no bimodality), varying from curved, scimitar-shaped forms, through to straight forms with sharply pointed tips. Microstructurally, the spines have an outer smooth wall (e.g. Fig. 3A-AB) and exhibit an internal fabric of longitudinal and occasionally interwoven fibres (e.g. Fig. 3AE, AL, AM, AO, AQ, BL-BS). One, or both of the margins are frequently thicker than the rest of the spine (e.g. Fig. 3AL, AR-AT). The tips terminate in an acute point, but are often broken; commonly just the outer portion is missing, exposing a thinner internal tip composed of fibrous material (e.g. Fig. 3AL). The basal opening is often incomplete, but occasionally preserves as a sharp truncation of the main spine attached to an extended portion of fibrous material (Fig. . Based on their shared construction and the continuum of the populations both within and between assemblages, we infer a single monospecific origin for these spines. Similar spines have previously been reported in Lontovan and equivalent strata. A single curved spine was described as a bedding-plane compression from the Kunda quarry ( The closest morphological comparison of these spinose SCFs lies among various phosphatized protoconodonts common in Terreneuvian-age SSF assemblages globally (e.g. Missarzhevsky 1973Missarzhevsky , 1982Chen 1982;Yang & He 1984;Bengtson et al. 1990;Azmi 1996). In particular, many of the Lontova spines bear close similarities to slender Protohertzina forms with a fibrous microstructure, such as Protohertzina anabarica Missarzhevsky, 1983(Nowlan et al. 1985Brasier 1989, fig. 7.1), P. unguliformis Missarzhevsky ,1973, P. siciformis Missarzhevsky, 1973(Pyle et al. 2006Topper et al. 2009 level taxonomy (see Brasier 1989, fig. 7.3). The flattened nature of the carbonaceous Lontova spines precludes any accurate reconstruction of their cross-sectional morphology; however, the presence of optically darker, presumably thickened portions running the length of many specimens (e.g. Fig. 3AR-AT) potentially records the original position of an analogous flattened keel. These characteristic features point to these Lontovan spines being protoconodonts. An originally non-biomineralized condition is supported by the wholly organic expression (which contrasts with the preservation of co-occurring phosphatic SSFs), combined with the lack of brittle fractures (cf. Butterfield & Nicholas 1996), and justifies the erection of a species name that suggests a biologically distinct entity, rather than a taphomorph. Despite their two-dimensional preservation, we assign them to the form-genus Protohertzina, and establish a new species P. compressa sp. nov. to accommodate the lack of mineralization (see Systematic Palaeontology below).
Szaniawski (1982,2002) has argued convincingly that protoconodonts represent the grasping spines of chaetognath worms, based on their fibrous microstructure, overall form, and evidence suggestive of secondary (diagenetic) phosphatization. The conspicuously twodimensional Protohertzina compressa sp. nov. in the Lontova assemblage strongly support an originally nonbiomineralizing habit, consistent with their interpretation as total-group Chaetognatha (see Vannier et al. 2007).
Although extant chaetognaths typically bear 1-2 rows of small rasping 'teeth' in addition to their grasping spines, we have not detected analogous structures among the Lontovan SCFs, and they have not been reported from corresponding SSF assemblages. It is notable, however, that the feeding apparatuses of fully articulated fossil chaetognaths, middle Cambrian Capinator praetermissus (Briggs & Caron 2017) and early Cambrian Ankalodus sericus (Shu et al. 2017) also lack such differentiated 'teeth'. The curved grasping spines of Capinator praetermissus are most similar to the Lontovan Protohertzina but exhibit a uniformly narrower base. Together, the considerable diversity of phosphatized/phosphatic (and now carbonaceous) Cambrian protoconodonts, coupled with the surprisingly varied grasping spine configurations in articulated Cambrian examples (e.g. Ankalodus sericus, Capinator praetermissus) point to a broader diversity of feeding apparatus among stem chaetognaths.
Scalidophorans. Other spine-shaped SCFs recovered from the Voosi Formation (F332-Vihterpalu) exhibit a fundamentally different architecture (Fig. 4H-O). These curved, hook-shaped spines lack the characteristic fibrous construction of Protohertzina, and often include a basal 'spur' oriented at a right angle to the primary spine (Fig. 4H-K, N). Some specimens also bear multiple secondary denticles in two parallel rows along the interior curve of the hook (Fig. 4I). Together, these features characterize the cuticular sclerites of many scalidophoran worms (cycloneuralian ecdysozoans). 'Sclerites' of the same general architecture adorn the integument of modern priapulids in various body positions (introvert scalids, pharyngeal teeth, and tail hooks; e.g. van der Land 1970;Wennberg et al. 2009) and also Cambrian stem-priapulids known from BSTmacrofossils (Smith et al. 2015). Actualistic taphonomic experiments on modern Priapulus have shown these scalidophoran cuticular components to be particularly robust to decay (Sansom 2016), and they are regularly preserved in Cambrian SCF assemblages (Butterfield & Harvey 2012;Harvey et al. 2012;Smith et al. 2015;Slater et al. 2017a). It is worth noting that broadly comparable hooks are also found in some sipunculans, including forms with elaborations such as a bidentate outline or a secondary spine or comb along their inner margin (e.g. Schulze  fig. 6). However, as far as we are aware, sipunculan hooks do not express double rows of more clearly separated denticles, as seen in the denticulate Lontova specimens (Fig. 4I), whereas these are a widespread feature among both modern and Cambrian priapulids (e.g. Smith et al. 2015).
In both modern and fossil scalidophorans, the sclerites of the introvert or trunk are referred to as scalids, whilst those of the pharynx are termed teeth (van der Land 1970;Smith et al. 2015). A subset of the Lontovan specimens (  fig. 8 image 7), pointing to a possible shared phylogenetic affinity. Other specimens (e.g. Fig. 4L, M, O) lack sufficient detail to resolve their particular habit, and may represent the distal portions of scalids, and/or fragments of coronal spines or tail hooks. The small numbers of these scalids (n = c. 10) precludes their reliable assignment to any known Cambrian priapulid-like worm or previously described SCF-based taxon (cf. Slater et al. 2017a

Problematica
Tubes. Tubular fossils bearing transverse annulations or flanges were recovered from throughout the Voosi Formation (F332-Vihterpalu) and from the Kestla Member of the Lontova Formation (Kunda and Tvere cius-336) (Fig. 5). The tubes are predominantly parallel-sided, but undulate slightly in places and typically taper towards one end. Transverse flanges are distributed at approximately regular intervals (spaced at c. 25% of the tube diameter) along the length of the primary tube-wall, though these flanges frequently divide and merge with neighbouring examples producing a wrinkled appearance (Fig. 5A, B, E, H). The flanges are often restricted to the expanded (presumably distal) parts of the tube and are absent from the tapered (presumably basal) portion of the tube which is smooth-walled and optically darker (Fig. 5G, P). In most cases, the flanges appear to have ragged margins (e.g. Fig. 5H, R). The maximum tube length is c. 2 mm.
These annulated tubes are broadly comparable to a number of organic-walled tubular taxa known from bedding-plane macrofossils, though they are substantially smaller. Ediacaran Saarina, for example, exhibits broadly comparable annuli (Gnilovskaya 1996), but these are more funnel-shaped, and obviously taper into the adjacent underlying cylinder. Other tubular fossils in the Lontova Formation include Platysolenites antiquissimus, P. lontova, Yanichevskyites petropolitanus, Sabellidites cambriensis and Hyolithellus sp. (Mens & Pirrus 1977, pl. 16). The transversely annulated tubular fossil Hyolithellus sp. co-occurs as rare bedding-plane compressions in the Kunda quarry (Mens & Pirrus 1977, pl. 16 figs 8-9), and is found throughout the Mahu and Kestla members of the Lontova Formation (Mens & Pirrus 1977); however, the conspicuously uniform annulations and absence of smooth portions in specimens of Hyolithellus distinguish them from the SCFs. The spacing of transverse flanges in the SCF specimens are similar to those seen in some specimens of P. lontova (e.g. Mens & Pirrus 1977, pl. 16.2), though the agglutinating construction of the latter ( McIlroy et al. 2001) undermines any close comparison. An apparently identical mode of construction, however, is found among flanged tubes described as Sokoloviina costata (Kiryanov 1968in Krandievski et al. 1968 from equivalent strata in the early Cambrian of north-west Ukraine, which likewise occur in a carbonaceous mode of preservation (both extractable and in situ) and express equivalently flanged or smooth-walled tube regions. We therefore identify the tubular Lontovan SCFs as Sokoloviina costata Kiryanov (1968), expanding the known geographical range of this taxon and adding to the conspicuously broad range of tubular microfossils already documented from the Baltic Lontovan, and from Terreneuvian assemblages more generally (e.g. Kouchinsky et al. 2012;Budd & Jackson 2016;Pang et al. 2017).
Vesicle-fringed sheets (Retiranus balticus gen. et. sp. nov.). All processed samples of the Kasari Member (Voosi Formation, F332-Vihterpalu core) and a single sample from the Tvere cius-336 core (443 m) yielded populations of distinctive sheet-like SCFs consisting of a thalloid portion fringed with anatomically connected vesicles (Fig. 6). The sheets reach >2 mm in maximum dimensions ( x = c. 1700 lm, SD = 538 lm; n = 26) with marginal spheroidal to ovoid vesicles reaching maximum diameters of c. 300 lm ( x = c. 163 lm, SD = 54 lm; n = 36). The population as a whole is conspicuously variable, with some forms consisting of narrow stalk-like thalli terminating in one or more vesicles (Fig. 6A-H), and others represented by large crenelated sheets ringed with multiple marginal vesicles (Fig. 6L, N-P). In many examples, only the widest, flabelliform (expanding fanshaped) portion of the sheet bears marginal vesicles, while the opposing part tapers towards a narrow 'base' (e.g. Fig. 6I). A possible basal holdfast structure is occasionally preserved, consisting of a flattened ovoid extension (Fig 6F, U). Biological margins can readily be distinguished from broken or torn ones, since the former align with an expanding sequence of regularly spaced concentric banding that broadens toward the vesicle-bearing margin in the largest specimens (e.g. Fig. 6P). All sheets with intact margins possess marginal vesicles. The vesicles occasionally exhibit a single spherical dark spot at the centre, typically c. 10% the total diameter of the vesicle (Fig. 6R, T). The smallest thalli, with relatively fewer vesicles, possess more or less spheroid vesicles. As the sheets attain a larger size, the margins tend to become more crenelated, and the vesicles acquire a more ovoid shape. Other characteristics of the vesicles (e.g. possession of dark spot structures; Fig. 6R) are typically consistent within a single sheet, suggesting that neighbouring vesicles are at the same ontogenetic stage. In addition, the vesicles become more evenly spaced around the sheet margins as thalli size increases.
The phylogenetic affinity and basic biology of these fossils is problematic. Some of the larger specimens (e.g. Fig. 6N-P) have an outline similar to some extant brown algae, for example Padina (e.g. Ni- Ni-Win Hanyuda et al. 2011, figs 3-5), though superficial comparisons can also be made with xenophyophoran foraminifera (e.g. Stannophyllum;Kamenskaya et al. 2015). Neither of these taxa, however, are known to differentiate marginal vesicles. In terms of grade of organization, a more comparable architecture is found among various clades of lichenized fungi which produce broad foliose thalli with marginally distributed ovoid apothecia (e.g. Peltigera). The stalked morphotypes with terminal vesicles (Fig. 6A-H) also resemble the overall morphology of various matchstick lichen (e.g. Pilophorus, Cladonia; Jahns 1981). Broadly comparable grades of architecture can also be found among several photosynthetic epiphytes, for example, the kidney fern (Hymenophyllum nephrophyllum; Brownsey & Perrie 2016, figs [78][79][80][81][82][83] and the green alga Phycopeltis epiphyton (Zhu . Even so, the lack of any clear cellular arrangement in the sheets, along with the non-aquatic nature of the analogous modern plants and lichens, precludes a more detailed comparison. Among early fossils, a useful comparison can be drawn with the Ediacaran macroalgae Flabellophyton, which occurs abundantly as carbonaceous compressions from the Lantian biota of South China (Yuan et al. 2013). Although substantially larger, Flabellophyton exhibits a similar overall thallus shape and holdfast structure, but lacks the distinguishing marginal vesicles (Wan et al. 2013, figs 3-4). The vesicles could conceivably represent parasites or exosymbionts attached at the margins of the thalli, but this seems unlikely given the consistent relationship between the shape and arrangement of the vesicles and thallus size. Even distinguishing whether these fossils are multicellular is problematic (see Zhu et al. 2016), though they probably represent eukaryotes of a protistan grade. Despite their phylogenetic ambiguity, these fossils are sufficiently distinctive to be recognized as a true biological taxon, adding to the records of early Cambrian diversity. We establish a new genus and species, Retiranus balticus gen. et. sp. nov. to circumscribe these problematic Cambrian organisms (see Systematic Palaeontology below).
Although some of the processes are preserved with a distally pointed tip (Fig. 8F, I), most are open distally, indicating breakage and taphonomic shortening; this is best illustrated in specimens with relatively complete processes ( Fig. 8C). The distribution of processes on host vesicles is apparently random, but with a tendency towards more even distribution with increasing process number. The presence of small bump-like protrusions on some vesicles may represent incipient processes suggesting continuous ontogenetic acquisition (Fig. 8F). Apparently paired processes (e.g. Fig. 8I) are more likely to be a product of taphonomic flattening/superposition than an original biological feature. There is often a darkened mass within the vesicle, presumably degraded cytoplasm or inner layers (Fig. 7B, F-I, L, O; cf. Grey 2005; Pang et al. 2013). Several specimens appear to have developed outgrowths from the vesicle, which occasionally form smaller, contiguously attached vesicles (e.g. Fig. 8D); others exhibit a more irregular hemispherically bulging habit (Figs 7G-H, 8F).
These early Cambrian microfossils are fundamentally larger and more irregular than typical Cambrian acanthomorphic acritarchs. They are, however, broadly comparable to various Proterozoic acanthomorphs that display a continuum of vegetative growth forms (see Discussion below); e.g. Palaeo-/Mesoproterozoic Tappania, the Germinosphaera phase of the Wynniat Formation ' Tappania Though there are some broad similarities, the Lontovan specimens display significant differences to these Proterozoic forms; in Trachyhystrichosphaera, for example, the processes may be polarized on the vesicle, a pattern not observed in any of the Lontovan specimens. Further, no traces of any outer mucilaginous membrane or discrete intracellular body occurs in any Lontovan example despite the large number of well-preserved specimens. The absence of such features is therefore likely to be original. The suite of features seen in these large Cambrian forms are sufficient to warrant classification as a new acritarch form-taxon. We establish a new taxon, Lontohystrichosphaera grandis gen. et. sp. nov. (see Systematic Palaeontology below). Other carbonaceous forms. Lontova/Voosi samples yielding metazoan SCFs invariably yielded a background of other organic-walled microfossils. Nearly all fossiliferous horizons, for example, also preserve relatively large (>200 lm diameter) spheroids (Leiosphaeridia sp.; Fig. 9K-Q). These more conventional acritarchs, filaments and other forms are conspicuously more common in samples producing Protohertzina, suggesting that their preservation is subject to similar ecological or taphonomic controls. Filamentous forms are the most common, typically consisting of single filaments but also including compound Polythrichoides-like specimens in the Kestla Member (Kunda quarry) (Fig. 9B, C), which most likely represent cyanobacteria based on their lack of branching and bundled habit (cf. Samuelsson & Butterfield 2001, fig. 9;Tang et al. 2013, figs 13-14). Other, pseudo-segmented filaments in the Kestla Member are assignable to the formtaxon Rugosoopsis (Fig. 9D; see Butterfield et al. 1994). A single large oval fossil, a millimetre long, 0.46 mm wide, with a distinctive 'cross-hatch' microstructure was recovered from the Kasari Member of the Voosi Formation (Fig. 9A). Comparably sized ovoid cross-hatched forms from the early Cambrian of the East European Platform were described as Leiovalia Eisenack 1965(Volkova et al. 1979. These specimens (as well as the new Voosi Formation example), however, fall well outside the morphology of the form-taxon Leiovalia, which (aside from being substantially smaller) is a smooth-walled acritarch that lacks any comparable surface texture (e.g. _ Zyli nska & Szczepanik 2009, pl. 7. [31][32][33]. Masses of frasslike organic material with compacted margins are also abundant in Protohertzina-bearing samples, and consist of either cylindrical forms, or lengths of adjoined spheres, ranging from c. 300-800 lm in length x = c. 550 lm, SD = 159 lm) (Fig. 9F-J). Given their broadly cylindrical to lobate form, they are most likely to represent microcoprolites (see Harvey & Butterfield 2011;Slater et al. 2017b).

DISCUSSION
These Lontovan data are the first systematic accounting of SCFs from the Terreneuvian Series, substantially complementing and expanding upon the more conventional records of shelly fossils, acritarchs and trace fossils. In many aspects, the biological signal is comparable to that of Fortunian SSFs; for example, the prevalence of protoconodonts/Protohertzina, tubular forms and simple scalidophoran-derived spines (Kouchinsky et al. 2012;Guo et al. 2014;Yang et al. 2014a;Nagovitsin et al. 2015;Budd & Jackson 2016). But there are also forms that have no counterparts among the SSF record, representing previously undetected aspects of Terreneuvian ecology, in particular thalloid Retiranus balticus gen. et sp. nov. and vesicular Lontohystrichosphaera grandis gen. et sp. nov.

SCF vs SSF signals
Congruence between Lontovan SCFs and typical Fortunian SSF assemblages is revealing; taken together with examples of wiwaxiids, hyoliths, hallucigeniids and palaeoscolecids from younger Cambrian SCF/SSF assemblages, these records point to a substantial overlap between the two taphonomic modes. In the case of originally mineralized structures found as SCFs, preservation is usually best explained by the fusion of internal and external organic constituents into a single carbonaceous layer following the dissolution of mineral components (cf. Mart ı Mus 2014). The converse scenario, in which wholly non-biomineralized structures are replicated in phosphate, is a more complex and perhaps less well understood process. While the majority of phosphatic fossils are the products of secondarily replaced carbonate skeletons, or components originally constructed from phosphate, others were originally organic (Brasier 1990). Examples of the latter include cyanobacterial remains (Spirellus, Cambricodium;Brasier 1990), tubes (e.g. crumpled Hyolithellus;Pyle et al. 2006, fig. 8.6-7), cnidarians (e.g. Olivooides; Dong et al. 2016), metazoan cuticle (e.g. Pyle et al. 2006, fig. 8.11-19), articulated larval panarthropods (e.g. Orsten-style preservation;Maas & Waloszek 2001;Eriksson et al. 2012) and a rich variety of coprolites (e.g. Peel 2015).
Protoconodonts are among the most common SSFs from Fortunian strata (Kouchinsky et al. 2012). Structural and chemical analysis of protoconodont spines has demonstrated that they were originally constructed of three primarily organic layers, and only secondarily phosphatized (Szaniawski 2002), though their marked abundance as SSFs would suggest that they were histologically prone to phosphatization, possessed a lightly biomineralized component, or that some forms were originally biophosphatic (Brasier 1990;Kouchinsky et al. 2017). Detection of abundant carbonaceous Protohertzina compressa sp. nov. in Lontovan sediments supports a primarily organic habit for at least some protoconodont grasping spines, a premise further reinforced by the preservation of carbonaceous grasping spines among Burgess Shale-type fig. 7). It is possible that a cryptic record of smaller scalids is being selectively removed from SSF assemblages during processing.
The profusion of secondary phosphatization in early Cambrian fossils probably relates to broad scale palaeoceanographic controls on the abundance of phosphate in Ediacaran and early Cambrian marine environments (Porter 2004). Why originally organic scalids, and protoconodont elements in particular, are so readily replicated in phosphate though, is unclear. One factor may simply be their shape: protoconodonts and scalidophoran cuticular spines both share a broadly conelike habit. It is well-known that the size, shape and orientation of shells affects their replication in phosphate (Brasier 1990). Indeed, there is evidence to suggest that narrow, conical and tubular morphologies may be preferentially phosphatized (e.g. the prevalence of molluscan steinkerns), since they act as traps in which P-saturated pore waters can become concentrated (Brasier 1990;Creveling et al. 2014). Analogous microenvironments could feasibly form in the interiors of a range of hollow bioclasts, whether or not they possessed originally mineralized walls.
Preservation of protoconodonts, tubes and scalids among both SCF and SSF records offers further grounds for recognizing yet more SSF taxa as the diagenetically phosphatized remains of non-biomineralizing organisms (as opposed to the phosphatized remains of aragonitic or calcitic forms, or phosphatic steinkerns). In addition to capturing entirely new aspects of early Cambrian palaeobiology, SCFs also offer a degree of continuity with SSF, and potentially even Doushantuo and Orsten-type phosphatic preservation. This overlap is particularly important in the Cambrian, since detection of comparable fossils across multiple taphonomic modes is crucial for distinguishing any underlying evolutionary signal from that of an ecological/taphonomic megabias.
Such taphonomic correspondence between SSFs and SCFs also has taxonomic implications; both SSFs and SCFs are commonly the isolated parts of originally multielement scleritomes (Bengtson 1985;Smith et al. 2015). Since mineralized and non-mineralized components can potentially be preserved under either of these taphonomic pathways, this raises the possibility of different taphonomic expressions of the same component (see Sciotaxon;Bengtson 1985); or even similar expression of different components. The problems presented by dissociated remains and contrasting preservation states is not unlike the situation faced by palaeobotanists dealing with 'organ-taxa' found in states of permineralization versus flattened compression fossils (e.g. Bateman & Hilton 2009). In all cases where it can be confidently verified, the goal should ultimately be to integrate form classifications into biological taxa, either by establishing that co-occurring sclerites derive from the same scleritome (see Smith et al. 2015), through recognition of the parent organism(s) (see Caron et al. 2013), or ideally via the identification of articulated scleritomes (cf. Conway Morris & Peel 1990). As potential connections arise, an increased awareness of the taphonomic overlaps in SCF/SSF preservation will aid interpretation of latent synonymies as these respective records continue to grow. In this study we justify the establishment of a new species of Protohertzina, P. compressa sp. nov., on the basis of its distinct carbonaceous, two-dimensional habit, which is consistent between and within large populations. Although it may eventually prove to be a junior synonym of a phosphatized 3-D Protohertzina species, the name usefully delineates the fossil morphology.

New insights into early Cambrian protistan diversity
The Lontovan SCFs shed new light on the diversity of early Cambrian protists, and perhaps signal the existence of 'firmground' seafloor conditions more usually associated with pre-Cambrian environments; Fortunian SSFs (and now SCFs) demonstrate that these poorly mixed and firm early Cambrian seafloor environments (Jensen et al. 2005), like their Ediacaran precursors, were host to numerous sedentary organisms, many in the form of enigmatic mineralized, unmineralized and agglutinating 'tubes' (Budd & Jackson 2016;Yang et al. 2016) and thalloid forms (LoDuca et al. 2017). Other components of the benthos recovered as SCFs appear to be metabolically active, protistan-grade organisms of a kind not previously documented in the Cambrian and perhaps more suitably compared to Proterozoic forms (Javaux et al. 2001;Adam et al. 2017;Javaux & Knoll 2017). Lontohystrichosphaera grandis gen. et sp. nov., for example, is substantially larger and more irregular than typical Cambrian acanthomorphic acritarchs, regularly exceeding 200 lm, and occasionally more than 500 lm in diameter (see Cohen et al. 2009, fig. 1). Previously described Lontovan acanthomorphic acritarchs have been uniformly small (5-50 lm) and symmetrical (e.g. Asteridium tornatum; Vidal & Moczydłowska 1992), probably reflecting the differing processing techniques: whereas conventional palynological processing recovers smaller cyst-like forms, our low manipulation technique preferentially recovers relatively larger, more delicate fossils.
Unlike more conventional Cambrian acanthomorphic acritarchs Lontohystrichosphaera grandis gen. et sp. nov. also shows no evidence of excystment structures, while its conspicuously variable distribution and expression of processes and secondary vesicles points to a continuously growing, metabolically active organism (cf. Butterfield 1997(cf. Butterfield , 2005(cf. Butterfield , 2007Knoll et al. 2006;Javaux & Knoll 2017). Moreover, such large irregular morphologies are functionally incompatible with a planktic habit. As such, L. grandis can be reliably interpreted as a metabolically active member of the Lontova shallow-water benthos. Although most likely a photosynthetic alga (a constituent of the 'microphytobenthos'; MacIntyre et al. 1996), it is difficult to rule out alternative habits, such as the heterotrophy/osmotrophy of fungi and oomycetes.
Retiranus balticus gen. et sp. nov. similarly represents a benthic (large size and basal holdfast), probably photosynthetic, protist (thalloid habit and shallow-water setting), in this instance almost certainly capturing a larger part of its life cycle as it differentiates what appear to be marginal propagules. The potential recovery of such vesicular propagules among spheroidal acritarchs once again raises potential taxonomic issues, whereby alternate generations of the same organism, when found in isolation, may lead to the establishment of multiple taxa. In any event, it is clear that SCF processing also offers entirely new insights into both metazoan and protistan-grade components of early Cambrian ecosystems, complementing other records of ' soft-bodied' Cambrian 'algae' (cf. LoDuca et al. 2017).

Age of the Lontova/Voosi succession
Like many Terreneuvian strata worldwide, the precise stratigraphic position of the Lontova and Voosi formations has been difficult to constrain. It has been particularly problematic to resolve sequence stratigraphic boundaries in the Lontova Formation, since nearshore sections are almost ubiquitously eroded. Acritarchs and SSFs have been more promising, identifying the Lontovan as Terreneuvian; however, the long ranging biostratigraphic schemes do not conclusively resolve the intra-Terreneuvian stratigraphy of these sediments (i.e. whether Fortunian or Stage 2 in age). The predominance of protoconodonts and tubular fossils among the new SCFs data adds further weight to the signal from compression fossils (Sabellidites cambriensis, which were probably restricted to the Fortunian (Kouchinsky et al. 2012)), and trace fossils (treptichnid assemblage (Jensen & Mens 2001)) that the Lontovan is primarily Fortunian in age (Volkova et al. 1990;Palacios et al. 2017).
The principal phase of 'explosive' diversification in the SSF record is broadly coincident with the Fortunian/Stage 2 boundary. Though all fossil biotas are to a greater or lesser degree facies-specific, the correspondence of Lontovan SCFs and trace fossils to 'pre-explosion' SSF biotas and bioturbation indices is revealing. SCF biotas from comparable environments in younger (Stage 4) Baltic and Laurentian strata typically produce a richer diversity of metazoans; together with the Lontovan SCFs, this appears to echo the traditionally recognized 'explosion' signal established from carbonate hosted SSFs, giving credence to a broad-reaching diversification pulse during this interval, as opposed to a peculiarity of the biomineralization record. In concert with acritarchs and trace fossils, these SCFs contribute novel data to a longstanding stratigraphic conundrum in a difficult-to-date sequence relatively devoid of age-diagnostic shelly fossils. Further, the broad geographic and stratigraphic coverage of this study establishes a foundation for comparison with interregional Terreneuvian SCF assemblages. Future sampling of other Cambrian cratons will reveal the degree to which this assemblage is endemic to Baltica or, alternatively, part of a globally characteristic Terreneuvian SCF biota.  fig. 57, f1). Protohertzina compressa sp. nov. is on average smaller than P. anabarica (which reaches up to 3 mm in length), and the latter is straighter from base to tip. The (lost) holotype of Protohertzina anabarica (Missarzhevsky 1973, pl. 9.1-2) is probably missing the basal portion, but comparison with other P. anabarica and topotype material (Kouchinsky et al. 2017) reveals that the base is typically broader in P. compressa sp. nov., particularly in larger specimens. Protohertzina compressa sp. nov. is consistently stouter than P. anabarica and other broad forms such as P. robusta (Qian 1977).
LSID. urn:lsid:zoobank.org:act:DB7A1685-  Derivation of name. Named for its resemblance to a weighted-casting-net, after the gladiator class retiarius, who were armed with a weighted net fringed with stones, and the Norse sea deity R an, who used a net to capture sailors.
Diagnosis. As for type species.
Retiranus balticus sp. nov. Diagnosis. Sheet-like or funnel-shaped organisms consisting of a main 'thallus' bearing apically/marginally distributed spheroidal to ovoid vesicles with a unilayered apparently acellular wall. Apical portion may terminate in a single vesicle, branch to multiple vesicles, or expand to a wide flabelliform sheet fringed with many vesicles. Vesicles may exhibit a darkened spot c. 10% the size of the parent vesicle. Complete specimens up to 2.5 mm in maximum dimension, with regularly spaced concentric banding emanating from the narrow base and widening toward the flabelliform terminus. Narrow base emanates from an ovoid basal holdfast. Marginal vesicles are typically darker than the main sheet. Larger thalli may exhibit a crenelated margin, where the majority of vesicles are borne on the outermost lobes and the depressions house comparatively fewer vesicles. Vesicle size does not scale to the overall size of the sheet; however, in the largest specimens with crenelated margins the vesicles are dominantly ovoid.
Remarks. The biological affinity of these fossils is unresolved.
Diagnosis. As for type species.
Lontohystrichosphaera grandis sp. nov. Diagnosis. Large spheroidal or oval vesicles, with one to many irregularly distributed heteromorphic hollow processes. Vesicle wall is smooth. Processes open basally directly into the main vesicle. Processes may be tubular, conical or consist only of small protrusions of the vesicle wall. When complete, longer processes terminate in a pointed or rounded tip. Contiguous secondary vesicles occasionally occur, smaller than the primary vesicle. An optically darker internal mass often occurs within the vesicle(s), typically around two-thirds the size of the main vesicle.
Distribution. Early Cambrian (Terreneuvian) Kestla Member of the Lontova Formation, north-west Estonia (known from the Kunda Quarry).
Remarks. Specimens of Lontohystrichosphaera grandis appear to be particularly delicate, and do not survive conventional palynological processing. The biological affinity of these microfossils is unclear; however, the variability of outline and process distribution and length, and presence of contiguous sub-vesicles suggest that Lontohystrichosphaera grandis represents the remains of an ontogenetically and metabolically active eukaryotic organism, and not a dormant protistan cyst.

DATA ARCHIVING STATEMENT
This published work and the nomenclatural acts it contains, have been registered in ZooBank: http://zoobank.org/References/ C5EB3F45-98C9-  Additional high resolution images of the holotypes described in this paper are available in the Dryad Digital Repository: https://doi. org/10.5061/dryad.8hn63