High diversity and early radiation of organic-walled phytoplankton in southern Baltica during the Middle-Late Ordovician – evidence from the Borenshult-1 drillcore of southern Sweden

ABSTRACT Highly diverse and well preserved organic-walled phytoplankton were recorded from the Darriwilian–early Katian interval of the Borenshult-1 drillcore. We identified 154 species in 53 genera, and three assemblages were distinguished; Assemblage A of a late Darriwilian age, Assemblage B of a Sandbian age (further subdivided into sub-assemblages B1 and B2), and Assemblage C dated as Katian. Taxa with “Silurian affinities” with previous first appearance datum in the early Silurian, Hirnantian, such as Metaleiofusa and Visbysphaera, are here recorded from the late Darriwilian and Sandbian respectively. These occurrences question the relationship between the appearance of pioneering phytoplankton morphotypes and the Hirnantian glaciation. Other taxa with no pre-Silurian records such as Visbysphaera pirifera subsp. minor, Petaloferidium cazurrum and Dorsennidium cf. D. estrellitae are here present in the Sandbian, where bentonite beds are intercalated. The diversity curve of acritarchs shows similarities with those proposed for the Darriwilian-Katian of Baltica with main differences in the interval with bentonite beds representing an intense volcanic activity. The species Metaleiofusa arcuata Wall is here emended and a new combination is proposed: Petaloferidium cazurrum (Cramer) comb. nov. The genus Fankea is recorded for the first time from Swedish strata, suggesting a dominant high- to middle latitudinal distribution instead of a Perigondwanan distribution. We contend that the location of paleo-southern Sweden contributed to the great diversity seen, since a middle-low latitude provided a suitable habitat with warm, shallow water, rich in nutrients.


Phytoplankton
Acritarchs, an informal and likely polyphyletic group with unknown biological affinity, make up the majority of Paleozoic organic-walled phytoplankton (Playford 2003 and references therein).The majority of acritarch taxa probably represent cyst stages of different algal groups (e.g., dinophycea, prasinophycea, chlorophycea, or zygnemaphycea; Mays et al. 2021), however, a few acritarch taxa may instead represent fungi, crustacean eggs, or exoskeletal remains of higher organisms (Playford 2003 and references therein).Acritarchs were a major constituent of the marine microplankton during the Paleozoic (Servais et al. 1997(Servais et al. , 2004;;Vecoli & Le Hérissé 2004).They contributed by providing oxygen to the atmosphere (Knoll et al. 2006), and by controlling the accumulation of greenhouse gases in the atmosphere and consequently contributing to the global climatic changes (Vecoli & Le Hérissé 2004).As primary producers, they play an essential role in the marine food chain, modifying the ecosystems and biogeochemical cycles in the oceans (Servais et al. 2008;Moczydłowska 2011;Kroeck et al. 2022).
Although their affinity is uncertain, acritarchs have proved to be very useful for dating and correlation of Paleozoic strata; thus they are an important tool for biostratigraphy.This study aims to provide a phytoplankton biostratigraphy for the Mid-Upper Ordovician of Sweden.The changes in phytoplankton diversity are discussed in light of environmental changes, including climate change, eustatic changes and we correlate and compare the acritarch assemblages with invertebrate faunas, conodonts, and carbon isotope chemostratigraphy.

Geological setting
The paleocontinent Baltica constitutes a large part of northern Europe, bounded by the British and Scandinavian Caledonide mountains to the northwest, the Ural Mountains to the east, and the Trans-European Suture Zone to the south (Cocks & Torsvik 2005;Torsvik & Cocks 2017).Baltica separated from Gondwana during the Neoproterozoic (Torsvik & Cocks 2017), forming an isolated continent until its amalgamation with Avalonia during the Late Ordovician, when the Caledonides were uplifted (Cocks & Torsvik 2005).This convergence of continental plates and subduction produced an extensive system of volcanic arcs with exceptional volumes of volcanic ash by the Late Ordovician (Parnell & Foster 2012;Torsvik & Cocks 2017).The Tornquist Sea was formed across the southern margin of Baltica as the continent drifted northwards, away from Gondwana during the Ordovician, and formed a basin favourable for marine life (Bergström et al. 2013).By the end of the Middle Ordovician, Baltica was located at c. 35-40°S (Torsvik & Cocks 2017), thus the climate was comparable to that of the modern subtropical belt, with the extensive formation of reefs in the shallower areas (Cocks & Torsvik 2021).
The Borenshult-1 drillcore is located in the vicinity of Motala, east of Lake Vättern in Östergötland, central Sweden (Fig. 1).The strata are well-dated, representing a nearly complete succession of Middle to Upper Ordovician (Darriwilian to Rhuddanian) marine marly carbonates deposited relatively proximal to land (Bergström et al. 2011;Rubinstein & Vajda 2019).
The studied interval of the Borenshult-1 drillcore encompasses the interval 71.33-30.75metres, spanning the upper part of the Furudal Limestone of Darriwilian age (Dw3), the entire Sandbian, represented by the Dalby Limestone, the Kinnekulle K-bentonite, and the lower Skagen Limestone.The topmost part of the core 31-30.75m comprises the topmost part of the Skagen Limestone of Katian age (Ka1), just below the Moldå Limestone, which together with the Skagen Limestone, in Östergötland are included within the Freberga Formation (Bergström et al. 2011(Bergström et al. , 2012;;Nielsen et al. 2023 and references therein).
The Furudal Limestone (71.33-56.51m) is a relatively uniform succession of grey, nodular limestone, interbedded with thin partings of calcareous mudstone.It is conformably overlain by the Dalby Limestone (56.51-38.35 m), which consists of grey, fine-grained to coarse-grained, and in some intervals nodular, limestone with intercalations of dark-grey calcareous mudstone and thin bentonite beds.The Dalby Limestone is in the Borenshult-1 drillcore succeeded by the Kinnekulle K-bentonite that here reaches c. 1.55 m (Bergström et al. 2011), though there are several, much thinner, K-bentonite beds below and above the Kinnekulle K-bentonite within the Dalby Limestone.The Kinnekulle K-bentonite overlies the Dalby Limestone and is followed by the Skagen-and Moldå Limestone both included in the Freberga Formation (Ebbestad et al. 2015;Nielsen et al. 2023).These successions represent a stable and moderately deep depositional environment close to the continent, based on the presence of land-derived cryptospores and trilete spores (Rubinstein & Vajda 2019).
In summary; the age of the studied section is well-constrained to the late Darriwilian (Stage slice Dw3)-early Katian (Stage slice Ka1) based on conodonts (Bergström et al. 2011(Bergström et al. , 2018)), and 206 Pb/ 238 U dating of volcanic ash deposits (Bauert et al. 2014) providing a stratigraphical framework for the ranges of phytoplankton taxa.

Materials and methods
For this study, 34 core samples from the Borenshult-1 drillcore, across the interval 71.33-30.75m, were analysed for palynology (Fig. 2(A, B); Fig. 3).The samples, each of c. 20 g were processed at the GeoLab Ltd, Canada following standard palynological methods.The sedimentary rock samples were treated with 20% HCl to remove the carbonates, followed by treatment with 37% HF in order to remove the mineral matter.The remaining organic residue was sieved using a 5 µm mesh and two slides for light microscopy were prepared from each sample residue.One to two entire slides from each sample were analysed for palynology by light microscopy under Nomarski differential interference contrast illumination.
All the identified organic-walled phytoplankton taxa are listed and illustrated .An emended species is fully described and taxonomical remarks are given for some relevant forms, for taxa left in open nomenclature and questionable assignments.
The total organic-walled phytoplankton diversity per sample was calculated and complemented with the normalized diversity (Fig. 3) which is the sum of species ranging through an interval, plus a half of those which originate or became extinct in the interval (Cooper 2004).The evolutionary change is estimated based on the total number of originations (appearances) and extinctions (disappearances) and the turnover, by summing originations and extinctions per sample (Hints et al. 2010) (Fig. 3).

Occurrence and stratigraphic interpretation
Palynomorphs are generally well-preserved, and their abundance and diversity strongly vary throughout the studied interval.The 34 analysed samples yielded predominantly marine organic-walled phytoplankton, mainly acritarchs, with subordinate chitinozoans, scolecodonts, and fragments of graptolites.Sparse terrestrial palynomorphs, represented by cryptospores and trilete spores, were also found in 23 of the samples, treated in Rubinstein and Vajda (2019).
A total of 154 acritarch species in 53 genera were identified, as well as low percentages of abnormal acritarchs (teratological forms) at some levels.The stratigraphic distribution of genera and species allows for three palynological assemblages to be distinguished, from older to younger, informally named A, B, and C. The assemblage B is, in turn, divided into two subassemblages, B1 and B2 (Fig. 2(A, B)).
Twenty-nine species were assigned to Baltisphaeridium, a genus especially common in the Baltic assemblages and common throughout the studied section.However, most species within this genus were previously erected based on a few or single specimens, generally poorly illustrated or with incomplete descriptions, and without considering intraspecific variations.As a result, as several authors have already stated (e.g., Loeblich & Tappan 1978;Wicander et al. 1999), some of these are probably synonyms.Therefore, it is evident that this genus suffers from taxonomic oversplitting, which hinders its use in biostratigraphy.
Pterospermella tranvikensis is an important taxon in Assemblage A, this species has only been found in the Darriwilian of Estonia and the Middle Ordovician of Finland (Tynni 1982;Uutela & Tynni 1991), where it is the index species of the Pterospermopsis tranvikensis provisional zone of the Lasnamägi Regional Stage (Raevskaya & Hints 2019).Taxa such as Aremoricanium rigaudae, Excultibrachium concinnum, Ordovicidium elegantulum, and Orthosphaeridium rectangulare var.quadricornis first appear globally in Darriwilian successions, while Pachysphaeridium christianii, P. mochtiense, and P. robustum are Darriwillian marker species in Baltica.
Pachysphaeridium robustum and Aremoricanium rigaudiae are diagnostic species of the following provisional biostratigraphical zones in the Darriwilian of Estonia; P. robustum, P. balticum, A. rigaudiae, of the Kunda to lower Aseri Regional Stages (Raevskaya & Hints 2019).Orthosphaeridium rectangulare is one of the index species of the O. rectangulare, Electoriskos?pogonius, and Navifusa similis provisional zones, of the late Lasnamägi to Uhaku Regional Stages, Darriwilian of Estonia (Raevskaya & Hints 2019).On the other hand, Excultibrachium concinnum first appears in the eponymous provisional zone of the Haljala Regional Stage, middle-late Sandbian of Estonia, although there are doubtful records in the late Darriwilian (Raevskaya & Hints 2019).
The genus Gyalorhethium was also identified in Assemblage A, making its first appearance elsewhere in the Middle Ordovician of Poland (Górka 1987) and becoming a common Late Ordovician taxon of different paleoplates such as Laurentia, Baltica, and the Tarim block (Playford & Wicander 2006;Wallin & Hagenfeldt 1996;Li et al. 2006).The presence of Nanocyclopia aspratilis in the lowermost sample of the Borenhult-1 core is interesting.This genus was originally described from the Devonian of USA (Loeblich & Wicander 1976), but was subsequently recorded from the Middle and Upper Ordovician successions of Finland and Estonia (Uutela 1989;Uutela & Tynni 1991), Hirnantian of Canada (Delabroye 2010), and as? Nanocyclopia sp. from the Darriwilian of northern Russia (Raevskaya et al. 2006).
It is noteworthy that the genus Tylotopalla is found here in strata as old as Darriwilian.This genus is one of the morphotypes of Silurian affinity that first appeared during the Hirnantian glacial and post-glacial regressive deposits, but evolved and diversified during the Silurian ( Le Jaanusson (1995) and Bergström et al. (2012).The Caledonian fronts are marked in the west and in the south.The Borenshult-1 drillcore is marked with a yellow circle.Herein the use of the international term ecofacies belt (a combination of faunal assemblages and lithological characteristics) is preferred over the local term "confacies belt" as defined by Jaanusson (1995) and Hagenfeldt (1995).

2013).
It is important to highlight that Tylotopalla was already recorded in the upper Darriwilian to lower Katian successions of Sweden (Kjellström 1976;Górka 1987), Katian of Iran (Ghavidel-syooki et al. 2011), and from the Katian/Hirnantian boundary of Poland (Sullivan et al. 2018).The most striking occurrence is that of the species Metaleiofusa arcuata, extending the range of this species to the Darriwilian, the previous oldest record being the early Silurian, Llandovery (Hill 1974b;Hill & Dorning 1984;Le Hérissé 1989).From the revision of  with previous oldest record from the Lower Carboniferous of Saudi Arabia (Hemer & Nygreen 1967).

Assemblage B
The subsequent Assemblage B encompasses the entire Sandbian (stage slices Sa1 and Sa2), from the lowermost sample at 48.2 m to the uppermost at 33.05 m (30 samples).This assemblage is divided into sub-assemblages B1 and B2 (Fig. 2A, B).Sub-assemblage B1 spans the interval 48.2 m to 41.9 m of the Dalby Limestone, corresponding to the Baltoniodus variabilis and the B. gerdae subzones of the Amorphognathus tvaerensis Conodont Zone (Bergström et al., 2011).Sub-assemblage B2 spans the interval 41.1 m to 33.05 m, comprising the upper part of the Dalby Limestone, the Kinnekulle K-bentonite, and the lower part of the Skagen Limestone, equivalent to the Sandbian part of the Baltoniodus alobatus Conodont Subzone.
In addition to the presence of Visbysphaera brevifurcata throughout sub-assemblage B1, the presence of Baltisphaeridium spp., Ordovicidium spp., Orthosphaeridium spp.and Pachysphaeridium spp., is still notable.Particularly noteworthy in sub-assemblage B1 is the presence of a specimen of Frankea cf.F. sartbernardensis at 43.1 m, clearly reworked due to its poor preservation and darker colour.Frankea is a genus characteristic of Middle Ordovician Perigondwanan assemblages, with doubtful records in the Late Ordovician.It is considered a temperature-sensitive genus that is restricted to the western and eastern margins of Gondwana, at high to mid-paleolatitudes (Servais et al. 2018;Rubinstein et al. 2021).Frankea is present in Avalonia and Saudi Arabia, located at mid-southern paleolatitudes like Baltica.Therefore, according to Molyneux et al. (2013), other factors, besides paleolatitudes, could control its distribution.
Consequently, its first record in Sweden supports the paleolatitudinal factor as dominant.Interestingly, Frankea has been reported from the Hirnatian of Poland (Trela & Szczepanik 2009), and it was interpreted as possibly transported from Avalonia during the collision with Baltica, which took place around the Ordovician/Silurian boundary, at 443 Ma (Torsvik & Cocks 2013).
Sub-assemblage B2 (upper Sandbian, interval 41.1 m to 33.05 m) is characterized by the appearance of Chlamydosphaeridia sp., at 41.1 m, whose range extends up to the top of the Sandbian, at 33.05 m.The monospecific genus Chlamydosphaeridia was erected by Eisenack (1971), with the species Chlamydosphaeridia baltica identified in erratic boulders of Baltic Limestone.Subsequently it was recorded from successions of late Darriwilian-late Sandbian age of Estonia (Uutela & Tynni 1991) and possibly also from Russia (Raevskaya, pers. com. 2021).This rare taxon, with a restricted stratigraphic and geographic distribution, but widespread and relatively common in the Sandbian of the Borenshult-1 drillcore, could become a valuable biostratigraphic and biogeographic marker.
The first appearance of Cheleutochroa cf. C. clandestina, in the lowermost Sandbian sample (41.1 m) of sub-assemblage B2 is the earliest record globally of this genus.Although Cheleutochroa is widely distributed in Katian successions (Vecoli & Le Hérissé 2004;Playford & Wicander 2006;Ghavidel-Syooki et al. 2011;Delabroye et al. 2011a;Le Hérissé et al. 2015), the oldest record so far is from the late Sandbian of England and Estonia (Turner 1984;Uutela 2008).In addition, Cheleutochroa is the index taxon of the eponymous provisional zone of the Keila Regional Stage, corresponding to the late Sandbian-early Katian in Estonia (Raevskaya & Hints 2019).
The LAD of the genus Pachysphaeridium occurs at 40.2 m, with Pachysphaeridium robustum representing the last species.
Sub-assemblage B2 spans the Kinnekulle K-bentonite (Fig. 2(A, B)), the thickest bentonite deposit of the Ordovician.Although the acritarch diversity decreases within the main bentonite beds, which extends from 39.20 m to 38.98 m and 38.06 m to 36.7 m (Fig. 2B; Fig. 3), almost all genera reappear following the bentonite.
Ampullula suetica, recorded herein from Assemblage A and B2 (40.35 m) has previously not been reported from strata younger than late Darriwilian (Yan et al. 2010), except for a Katian record from Iran, where it is considered reworked (Ghavidel-Syooki & Borji 2018).
Dateriocradus asturiae was originally described from the upper Silurian (Ludlow) of Spain (Cramer 1964) and later reported from the Katian of Iran (Ghavidel-Syooki & Borji 2018).Other examples of taxa with extended ranges in this study are; Visbysphaera pirifera subsp.minor with previous oldest records from the early Silurian (Telychian, late Llandovery) of Gotland and from the Silurian of UK and Russia (Le Hérissé 1989); Comasphaeridium aurora, with previous first appearance in the Middle Silurian of the USA (Loeblich 1970a) and Petaloferidium cazurrum with previous first appearance in the Lower Devonian of Spain (Cramer 1964).In the samples above the Kinnekulle K-bentonite, 36.6 m to 33.05 m, up to the upper boundary of Assemblage B (late Sandbian), FAD of several taxa occur, among them, Dorsennidium cf.D. estrellitae.This taxon was previously identified from the Upper Silurian (Ludlow) of Spain (Cramer 1964) and subsequently recorded from the Middle Ordovician of Poland (Górka 1969) and the Early Ordovician of China (Yan et al. 2013).However, none of the two latter species agrees with the description of the Cramer's species and may be misidentified.Other interesting FADs are those of Evittia cf.E. porkuniensis and Cheleutochroa cf. C. elegans, previously recorded from the Katian to the Hirnantian of Estonia and Finland (Uutela & Tynni 1991;Delabroye et al., 2011b) which extend their ranges to the Sandbian in the Borenhult-1 core.Likewise, Cheleutochroa diaphorosa, which ranges globally from the Katian to the Hirnantian, is positively assigned in the uppermost level of this assemblage.
Over 50% of the species occurring in the uppermost sample of sub-assemblage B2, at 33.05 m (which displays the highest diversity of the entire studied interval), do not range into Assemblage C, of Katian age.

Organic-walled phytoplankton diversity
Assemblage A (of late Darriwilian age) spans the interval 71.1 m to 59.1 m.This assemblage displays one of the highest diversities of the studied section (Fig. 3), with 41 species occurring in the lowermost sample (71.1 m).At 67.6 m, a pronounced diversity drop occurs but a recovery is seen at 59.1 m, keeping diversity relatively high at the sub-assemblage B1, which corresponds to the Sandbian (Sa1 and the beginning of Sa2).In the uppermost sample of sub-assemblage B1, at 41.9 m, there is a significant drop in diversity, and the species are reduced by c. 50%.From 67.6 m to 43.1 m (Fig. 3), the turnover is remarkable, with many more originations than extinctions, and tends to decrease in the Sandbian until it drops sharply, together with the originations, at the end of the sub-assemblage B1 at 41.9 m (Fig. 3).The basal sample of sub-assemblage B2, at 41.1 m, evidences a considerable change with a marked increase in diversity and species originations.From this sample upward, the diversity decreases, reaching its minimum values in the proximities and within the Kinnekulle K-bentonite.The originations also decrease in this stratigraphic interval, reaching their minimum values near or within the bentonite beds, while the extinctions remain relatively low and constant and; therefore, a low turnover rate characterizes this interval.After the minimum value at 35.2 m, the diversity increases, attaining its highest value in the uppermost Sandbian sample at 33.05 m, while extinctions outnumber originations, giving the highest peaks in extinctions and turnover of the whole studied interval at the same sample.
At the end of the Sandbian, more than 50% of the species went extinct.The early Katian (Ka1) Assemblage C, represented by a single sample at 30.75 m, shows a significantly low diversity (Fig. 3).

Discussion
In this study on acritarch assemblages from the Darriwillian to Katian successions from the Borenshult-1 drillcore of south, central Sweden, well-preserved and abundant acritarch communities were identified.The species diversity is striking, as are the many taxa with a First Appearance Datum, extending the range of several genera and species.Three major contributing factors are commonly put forward to explain diversity-changes; genetic innovations, ecological innovations, and environmental changes.Although diversity is highest in periods of high sea level and extended flooded continental regions, and the diversity curves clearly correlate with suitable habitats such as shelf areas, it can also be affected by sediment input, upwelling and volcanism (Kroeck et al. 2022;Vajda et al. 2020).We argue that the paleolocation of paleo-southern Sweden was a contributing factor to the high diversity encountered, where a middle-low latitude provided a favourable environment with a warm, shallow sea rich in nutrients.This is corroborated by studies showing that variations in the latitudinal distribution of phytoplankton diversity throughout the early Paleozoic can be related to long-term climatic changes and plate tectonics, while at the stage scale, they were affected by annual fluctuations in sea-surface temperatures and, subordinately, by salinity and available areas of the continental shelf (Zacaï et al. 2021).Global diversity curves of phytoplankton (acritarchs) and chitinozoans combined with regional curves, one of them from Baltica, display fluctuating diversities, with peaks in the Middle Ordovician, similar to chitinozoans trends (Harper et al. 2021).
The phytoplankton taxa with Silurian affinities, previously known from the Hirnantian and related to paleoclimatic and paleoenvironmental disturbances that led to the Late Ordovician glaciation, appear for the first time in the Late Darriwilian part of the Borenshult-1 drillcore (Dw3) interval.Important taxa occurring are Tylotopalla and Metaleiofusa which are definitively established from the beginning of the Sandbian (early Late Ordovician), together with the first appearance of the genus Visbysphaera.The FAD of Metaleiofusa arcuata occurs at the boundary between the Eoplacognathus robustus / E. lindstroemi subzones within the Pygodus serra Conodont Zone.
The genus Frankea is documented for the first time from the Ordovician of Sweden.This record, together with its occurrence in the Hirnantian of Poland, both of them from Baltica, reinforces the hypothesis of a latitudinal control rather than a Perigondwanan distribution for this genus.
The genera Baltisphaeridium, Ordovicidium, Pachysphaeridium and Orthosphaeridum, common in Middle Ordovician phytoplankton assemblages from Baltica, are well represented, with several species, mainly in Assemblage A and sub-assemblage B1, up to the lower Sandbian.However, the presence of a majority of taxa with worldwide distribution supports the cosmopolitanism of the studied assemblages, already proposed to begin near the Darriwilian-Sandbian transition (Molyneux et al. 2013).
Several speciation events can be distinguished across the studied interval.The appearance of the genus Metaleiofusa in the Darriwilian would mark the inception of new, advanced morphological characters in the phytoplankton assemblages, at a depth where diversity drops but with relevant values of originations and turnover.Although evidence of lithological or environmental changes is lacking at this level, it lies stratigraphically close to the speciation event of the Pygodus lineage at the Pygodus serra /P.anserinus Zone boundary.
A new speciation event corresponding to the advent of the genus Visbysphaera accompanied by the first probable trilete spore occurs within the lower Sandbian at 48.2 m, approximately 5 m above the speciation event at the top of the P. anserinus Zone, in coincidence with high originations and turnover that persist until the sharp drop in coincidence with a lithological change (skeletal limestone to argillaceous limestones) and the onset of the SAICE (Sandbian isotope carbon excursion) (Lehnert et al. 2014).
The last acritarch speciation event within the studied interval initiates at 38.6 m, within the Kinnekulle K-bentonite and extends into the Skagen Limestone up to the uppermost Sandbian successions.This speciation event is characterized by the first appearances of species including Dateriocradus asturiae, Comasphaeridium aurora, Petaloferidium cazurrum, Visbysphaera pirifera subsp.minor and Dorsennidium cf.D. estrellitae, with previous records no older than Silurian.This event coincides with the onset of the Keila Regional Stage, the major Keila Drowning Event, the peak of the SAICE within the Kinnekulle K-bentonite (Lehnert et al. 2014), followed by a gradual increase of diversity after reaching its minimum values.
Interestingly, low numbers of teratological acritarchs occur from the uppermost sample of Assemblage A, at 59.1 m, to the lower part of sub-assemblage B1, and close to or within the Kinnekulle K-bentonite, in sub-assemblage B2.The presence of abnormal acritarchs has been related to environmental stress conditions such as pollution by volcanic ashes (Delabroye et al. 2012).
The diversity curve roughly coincides with the diversity pattern proposed for the Darrivillian-Katian of Baltica (Hints et al. 2010), with the highest values of diversity in the Darriwilian and partly in the Sandbian assemblages, followed by a significant decline in the Katian.The differences with the "Viru plateau", evidenced by the significant drop in diversity, are probably related to volcanic activity represented by the bentonite beds.Diversity, originations, and turnover rates exhibit their lowest values through the interval bearing the suite of K-bentonites, particularly, the thickest ones.Consequently, these decreases can be related to volcanic ashes.
The marked drop in diversity is noticeable in the Katian part of the succession, which is characterized by both low originations and abundance (Badawy et al., 2014).This is possibly related to a regression at the onset of the GICE (Guttenberg isotope carbon excursion), with less favourable environmental and climatic conditions (Ainsaar et al. 2010).
Although phytoplankton could have been more sensitive to carbon isotope excursions than other fossil groups, most likely, the phytoplankton innovations were related to the intense volcanic activity during this time interval in the region.Importantly, the first appearance of taxa with Silurian affinities in the late Darriwilian and Sandbian successions of Sweden extends the range of these taxa with nearly 15 Ma and shows that the acritarch diversity in the shallow marine ecosystems was larger than previously thought and that Baltica provides an area for future paleogeographical investigations.

Conclusion
The Darriwilian-early Katian interval of the Borenshult-1 drillcore yields well-preserved, rich and diverse organic-walled phytoplankton, enabling the recognition of three biostratigraphically significant assemblages; Assemblage A of a late Darriwilian age, Assemblage B of a Sandbian age and further subdivided into sub-assemblages B1 and B2, and Assemblage C dated to the Katian.Importantly, several taxa with Silurian affinities, which first occur in Hirnantian glacial and post-glacial deposits are here identified in the late Darriwilian and the early Sandbian.Among them, Metaleiofusa, Visbysphaera, and Tylotopalla are the most remarkable.These speciation events roughly coincide with conodont speciation events.
The first appearances in the Sandbian of Dateriocradus asturiae, Comasphaeridium aurora, Petaloferidium cazurrum, Visbysphaera pirifera subsp.minor and Dorsennidium cf.D. estrellitae that were not previously recorded in strata older than the Silurian indicate a last speciation event, probably related to the ash deposits, a major drowning event and/or the peak of the SAICE.
Changes in phytoplankton assemblages with the onset of innovative morphologies in acritarchs were previously interpreted as a consequence of environmental and climatic perturbations during the Ordovician glaciation.Therefore, our findings push back the first appearances of these advanced taxa with c. 15 Ma, suggesting that a possible combination of factors such as sea level, volcanism, and carbon excursions, instead of a major event such as the Hirnantian glaciation, triggered these changes.
The diversity curve of the Borenshult-1 phytoplankton is similar to the diversity pattern already observed for the Darrivillian-Katian of Baltica, except for the "Viru plateau", where differences could be related to the presence of the bentonite beds in the Borenshult-1.The new findings challenge previous models of evolution and radiation of the Ordovician phytoplankton and set up Baltica as a new key area for paleogeographical research.

Figure 4A
Description: Vesicle circular in outline, apparently bi-layered.7-11 processes in optical section (probably more from scars).Central body-wall relatively thick, densely ornamented with grana or spines, some of them with blunted or irregular distal terminations.Processes smooth, filmy, formed by the outer layer, separated from vesicle cavity, scan rate to finely granulated, with circular bases, conical, acuminate.
Dimensions: vesicle: 47-51 µ; length of processes 9-11 µ, width of bases: 3-3.5µ Remarks: It is similar to the figured isotype of Loeblich (1970a) (fig.3, B), with short conical processes, but smaller (it fits the dimensions in Playford & Wicander 2006).It is also similar to Actipillion sp.A in Wicander et al. (1999) due to a heavily granulate central body, but it has fewer and shorter processes than A. sp.A and more similar in shape to those of A. druggi.
Occurrence: Sandbian, sub-assemblage B, 40.2 m, 40.1 m.Ammonidium cf. A. minimum Delabroye 2010 nomen nudum Figure 4B Remarks: It is similar to A. minutum with a small vesicle, granulate with short, flexible, hair-like processes.It is not clear if process-cavities communicate with the interior of the vesicle.Processes seem to bifurcate distally and probably some of them are simple.

Ammonidium sp. A Figure 4C
Remarks: Similar to A. cf.minimun but larger.The processes communicate with the vesicle cavity and are slightly wider at the base.
Aremoricanium cf. A. carolineae Kjellström 1976 Figure 4G Remarks: It is similar to this species in the shape of the vesicle and processes, although it is not clear whether processes are petenoid, as indicated in the original description.The vesicle wall is not reticulated, but in the only specimen illustrated by the author, it seems to be an artefact of preservation.
Occurrence: Sandbian, sub-assemblage B1, 43.1 m.  1978, but with wider processes and ornamented vesicle, possibly a junior synonym of B. perclarum.The Swedish specimens show some transitional forms between both species.However, we do not consider these species synonyms awaiting a detailed taxonomic revision.

Baltisphaeridium bystrentos Loeblich & Tappan 1978
Figure 4P Remarks: This species is very similar to Baltisphaeridium calicispinae Górka 1969, except for the thickening of the vesicle wall at the base of the processes and the presence of a plug at the basal part of the processes.However, the specimens illustrated by Górka also exhibit basal plugs and, consequently, the distinction between the two species is not evident.
Baltisphaeridium cf. C. pseudocalicispinum Górka 1980 Figure 5I Remarks: It is close to Baltisphaeridium calicispinae Górka 1969, but without constriction at the junction of the processes with the vesicle, which is not evident in all the specimens.
Occurrence: Late Darriwilian to early Katian, assemblages A to C, 71.1 m to 30.75 m.
Baltisphaeridium cf.B. trichophorum (Eisenack 1965a) Kjellström 1971b Figure 5K Remarks: Although the shape and dimensions of the vesicle and processes and the number of processes are in agreement with those of Baltisphaeridium trichophorum, it differs in the ornamentation of the vesicle, which seems to be psilate instead of shagrinate, and lacking pylome.Occurrence: Late Darriwilian to Sandbian, assemblage A to sub-assemblage B2, 71.1 m to 36.3 m.

Baltisphaeridium cf B. varsoviensis Górka 1969
Figure 5N Remarks: Positive assignment to this species is hampered by insufficient description and illustration of the type material.
cf. ?Baltisphaeridium sp. in Delabroye et al. (2011b) Figure 5O Remarks: The Swedish specimens have short, conical processes, with bulbous bases and acuminate tips that are apparently separated from the vesicle cavity and solid at their bases, but it is not clear if the processes are solid throughout.
The vesicle wall seems to be granulated.Although the specimens in this study are similar to?Baltisphaeridium sp. in Delabroye et al. (2011b), recorded from Estonia (plate 2: 1-4), the authors did not provide a description (only illustrations) which hampers comparison.

Baltisphaeridum? sp. A Figure 5P
Remarks: It is doubtfully assigned to Baltisphaeridum due to the apparent presence of a pylome.It differs from Ferromia based on its larger size, the distribution of processes and the smaller ornamentation of the vesicle.
Baltisphaeridium spp.Remarks: Based on the diagnosis of Cheleutochroa elegans, the Borenshult specimen would correspond to this species.However, the single SEM photograph in Uutela & Tynni (1991) of this species, is not sufficient to allow a positive assignment.

Cheuletochroa homoia Turner 1984
Figure 5W Occurrence:  (1991) is also very similar, but only one SEM picture was included in the publication.

Dictyotidium biscutulatum
Remarks: Cramer (1964) erected the species Veryhachium estrellitae without providing a description, but based on the figures it seems smaller compared to the Borenshult-1 specimens.The specimens of Veryhachium?estrellitae from the Lower Ordovician of China (Yan et al. 2013) and Veryhachium estrellitae from the Middle Ordovician of Poland (Górka 1969) appear morphologically different from Cramer's (1964)  Remarks: Although the specimen is roughly similar to some specimens illustrated by Loeblich (1970a, Fig. 35), it does not present the typical bell-shape of Dorsennidium hamii (Loeblich 1970a) Sarjeant & Stancliffe 1994.

Dorsennidium spp.
Enneadikosocheia sp.A Figure 6W Remarks: It differs from the only species of this genus, Enneadikosocheia granulata Colbath 1979, in having a psilate vesicle instead of granulated.
Estiastra sp.A Figure 6X Description: Small specimen, stellate in outline, psilate to shagrinate, with 8 conical processes that merge to form the vesicle.
Interior of processes connected with the vesicle cavity.
Remarks: It is similar to Estiastra minima Volkova 1969, from the Cambrian of Russia, however the poor description and illustration hinder the comparison.
Evittia cf.E. denticulata denticulata (Cramer 1970) Le Hérissé 1989 Figure 7E Remarks: This species exhibits a high variability that led Mullins (2001) to erect the Diexallophasis remota Group.Although similar to some specimens illustrated in the literature, the Swedish ones do not strictly conform to the description.Therefore, they are doubtfully assigned to this species.
Evittia cf.E. porkuniensis Delabroye et al. 2011b Figure 7F Remarks: Evittia porkuniensis has a less polygonal vesicle outline and a more granulate vesicle wall.
Evittia sp.A Figure 7G-J Description: Vesicle polygonal to subquadrate, with 5-6 processes, commonly 4 at the corners, in the same plane, and 2 arising from one face of the vesicle.Processes broad at the base, simple, with rounded or blunted tips, or digitate or forked up to second order at the distal termination.Processes ornamented with small, scattered grana, some of them spinose at their distal end.
Remarks: It is included in Evittia due to its more polygonal shape and more simple processes that are also granulate.Multiplicisphaeridium has a more complex ramification of the processes.It is left in open nomenclature due to the few specimens recorded.
Evittia sp.B Figure 7K Description: Vesicle triangular, psilate to shagrinate, with straight to convex sides.Three broad-based processes arise from the corners.They bifurcate distally, and may bifurcate again up to second order giving short spines.
Remarks: It is similar to Dateriocradus josefae (Cramer 1964) Sarjeant & Vavrdová 1997 but it is not clear from the original description if the vesicle may be triangular.
Evittia sp.C Figure 7L-P Description: Vesicle polygonal in outline, with 7-9 broad and stout processes, not well delimited from the vesicle, freely communicated with the vesicle interior.The processes are straight to conical, with digitate distal termination or furcate up to second order, in some cases with spines on the distal branches.Some processes can be simple.Vesicle and processes are smooth or ornamented with scattered grana or spines.
Remarks: It is similar to Dateriocradus asturiae (Cramer 1964) Sarjeant & Vavrdová 1997, but apparently (the number of processes is not specified in the description) D. asturiae has fewer processes.This species is left in open nomenclature due to the few specimens recorded.
Gorgonisphaeridium sp.A Figure 7U Description: Vesicle smooth to shagrinate, bearing numerous processes not in connection with the vesicle cavity, apparently solid.Processes broadly conical, acuminate to slightly expanded at the distal end.Gyalorhethium cf.G. chondrodes Loeblich & Tappan (1978) Figure 7V Remarks: The present specimens have fewer processes than Gyalorhethium chondrodes.
Gyalorhethium sp. in Loeblich & Tappan (1978) Figure 7W Remarks: It has the same vesicle shape, number of processes, and ornamentation, typically with spinules that grade to grana towards the distal part of the processes.However, the Borenhult specimens are larger than the dimension provided by Loeblich & Tappan (1978).
Occurrence Remarks: Due to the characteristics of Lophosphaeridium shaveri, it is difficult to distinguish this species from L. edenense.
Melikeriopalla cf.M. amydra Tappan and Loeblich 1971 Figure 8P Remarks: The present specimen is doubtfully assigned to Melikeriopalla amydra because it is difficult to differentiate whether the polygonal fields are subdivided in discontinuous ridges or have small grana.
Dimensions: vesicle length: 23-38 μm; vesicle width: 23-30 μm, process length: 14-40 μm and process basal width: 1.5-6 μm; (24 specimens measured) Remarks: Hill (1974a) classified the Silurian specimens from the UK as Metaleiofusa cf.arcuata based only on the temporal gap between them and the original species of Wall (1965) from the Jurassic.Therefore, they are here positively assigned to the species.He recognized three different forms based on the shape of the vesicle and the third process lying on a different plane.However, due to the transitional forms between them, he did not create different species or varieties but instead separated them into Forma 1, 2 and 3.The Swedish specimens show transitional shapes between all three forms and, accordingly, they are considered here as part of the intraspecific variability and left in Metaleiofusa arcuata Wall (1965) s.l.
Le Hérissé (1989) recorded?Metaleiofusa sp.A from the Telychian of Sweden.His doubtful assignment to the genus is due to the morphological similarity between Metaleiofusa arcuata and Domasia trispinosa.He considered that some forms in which one posterior process is located too high over the vesicle are intermediate between Domasia and Metaleiofusa.
The specimen from the Caradoc (Sandbian-Katian) of Canada (Martin 1983), assigned to Sylvanidium? sp. (p. 28, pl. 7, fig. 9) would correspond to Metaleiofusa.However, due to the presence of two additional processes near one pole, it should be assigned to Metaleiofusa diagonalis Wall 1965.If this record could be confirmed, it would be the first occurrence of the genus in the Late Ordovician (Sandbian-Katian).
Sylvanidium? hawbanense Miller and Al-Ruwaili (2007), from the Hirnantian of Saudi Arabia may be a junior synonym of Metaleiofusa arcuata, due to the similarity in morphology and dimensions, especially to the Forma 2 and 3 of Hill (1974a).

Metaleiofusa? sp. A
Figure 9B Dimensions: Vesicle length: 30 μm; vesicle width: 27 μm, process length: 23-25 μm, process basal width 3.5 -7 μm; polar bifurcate process: length 18 μm, basal width: 3.5 μm (1 specimen) Remarks: A single specimen, similar in shape and dimensions to Metaleiofusa arcuata but with one of the polar processes shorter and bifurcated near the base.Vesicle wall covered by hair-like spines that decrease towards the processes to become grana and tend to disappear distally.Similarities between Metaleiofusa and Domasia Downie 1960 were already pointed out by Wall (1965) and other authors (Hill 1974b;Le Hérissé 1989).According to the original diagnosis of Downie (1960), the genus Domasia consists of a vesicle ellipsoidal in outline, psilate to ornamented, with hollow processes, two of them relatively long emerging adjacent to one pole and the other one, variable in length, at the opposite pole.Hill (1974b) emended Domasia to include specimens with three processes or one that bifurcates at one pole.Considering that the studied specimen could be also included in Domasia, it is provisionally doubtfully assigned to Metaleiofusa.
In view of the partial overlap of Metaleiofusa and Domasia, these genera should be reviewed.
Micrhystridium cf.M. taeniosum Uutela & Tynni 1991 Figure 9D Remarks: The present specimens agree in dimensions and morphology of processes with the original description of Uutela & Tynni (1991), although they only provided a SEM photograph.They are also similar to the specimen illustrated by Delabroye et al. (2011b).However, lacking a full description and illustration, we doubtfully assign our specimens to Micrhystridium taeniosum.Sarjeant and Stancliffe (1994) transferred the species to Baltisphaeridium? taeniosum as they consider that the processes and vesicle cavities may not be continuous.
Remarks: It resembles Veryhachium cazurrum Cramer 1964 in the tetrahedral shape and number of processes.Although Cramer described the processes as having rosette-like structures at their tips, the only figured specimen (Plate XIII: 1*; text figure 30: 4) appears to have rosette-like, petaloid to cualiflorate distal terminations.This would justify the assignment to Petaloferidium Jacobson 1978.
The reassignment of this species to Villosacapsula Loeblich & Tappan 1976 by Sarjeant & Vavrdová (1997), is not accepted here due to the distal morphology of the processes.
Petaloferidium florigerum (Vavrdová 1977) Fensome et al. 1990 Figure 10S Occurrence: Late Darriwilian, assemblage A, 59.1 m.Remarks: It differs from other species of the Rhopaliophora by the numerous, densely packed short and thin processes that join to give a reticulate appearance to the vesicle.If the ornamentation is interpreted as rugulate, it could be assigned to Dicommopalla.Vecoli et al. (2015) proposed the Dicommopalla-Rhopaliophora acritarch plexus to comprise these transitional forms.

Stellechinatum helosum Turner 1984
Figure 11D Occurrence: Sandbian, sub-assemblages B1 to B2, 48.2 m to 34 m.Remarks: The Borenshult specimens are similar to T. tzadiensis based on the shape of the vesicle and the ornament which consists of low verrucae with a central pore, homogenously distributed on the vesicle.However, vesicles are 20-40% smaller compared to those in the original description.
Tylotopalla sp.A Figure 11H-J Description: Vesicle polygonal, with 8-10 short processes that may join at their bases.Processes are simple, with acuminate to rounded endings or distally ornamented with a rosette of small spines.Vesicle and process wall psilate, shagrinate to slightly granulate.

Figure 1 .
Figure 1.Map of Sweden illustrating lower Paleozoic deposits modified fromJaanusson (1995) andBergström et al. (2012).The Caledonian fronts are marked in the west and in the south.The Borenshult-1 drillcore is marked with a yellow circle.Herein the use of the international term ecofacies belt (a combination of faunal assemblages and lithological characteristics) is preferred over the local term "confacies belt" as defined byJaanusson (1995) andHagenfeldt (1995).

Figure 2 .
Figure 2. A-B.Stratigraphic distribution of organic-walled phytoplankton within the analysed succession of the Borenshult-1 drillcore, set in the framework of the International Chronostratigraphic time scale.Identified species are indicated by black circles; cf.species by white circles.Recognized palynological assemblages (A, B and C).
the Borenshult-1 specimens and the literature on M. arcuata, the species is herein emended.Other interesting occurrences in Assemblage A are those of Helosphaeridium tongiorgii, previously only recorded from the late Katian-Hirnantian of Canada and Estonia (Delabroye et al. 2011a; Raevskaya & Hints 2019), and Lacunalites?sp.

Figure 2
Figure 2 Continued

Figure 3 .
Figure 3. Correlation chart for Ordovician regional biozones for the Borenshult-1 drillcore (from Rubinstein & Vajda 2019).The regional zonations are linked to the international standard time scale (Cohen et al. 2023); local stratigraphic units are calibrated against the conodont zonation established for the Borenshult-1 drillcore.The conodont zonation was subsequently tied to graptolite and chitinozoan biostratigraphy.Ages (red) are a combination of data from the ISC (Cohen et al. 2023), zircon dates from the Kinnekulle bentonite (Bell et al. 2013; Bauert et al. 2014) and 206Pb/238U radiometric zircon dates from Lindskog et al. (2017).Organic-walled phytoplankton data; Left: Diversity = blue line; Normalized diversity = dotted blue line; Right: Origination = dotted green line, Extinction = red line, Turnover = grey line.
Wicander et al. 1999 similar to B. adiastaltumWicander et al. 1999, but may have fewer processes, and one of them can be bifurcated.Hence, subject to further revision, B. adiastaltum could be a junior synonym of B. delicatum.Late Darriwilian to Sandbian, assemblage A to sub-assemblage B2, 71.1 m to 40.02 m.Sandbian to early Katian, sub-assemblage B1 to assemblage C, 43.1 m to 30.75 m.
Loeblich & Tappan 1978urner (1984)Cheleutochroa meionia only differs from Cheleutochroa gymnobrachiataLoeblich & Tappan 1978in being smaller.Consequently, C. meionia could be considered a junior synonym of C. gymnobrachiata, which is a more paleogeographically widespread species.Remarks: Although the numerous specimens recorded in this study are similar to Chlamydosphaeridia baltica Eisenack 1971, the single species of this rare genus and the poor description and illustration of the type material hinder the classification at a species level.Chlamydosphaeridia sp.inUutela & Tynni Description: vesicle stellate in outline, apparently psilate, with the bases of the conical 6-7 processes that merge to form the vesicle.Processes interior connected with the vesicle cavity.Remarks: It is similar to Estiatra sp.A, but larger.

:
Sandbian, 36.6 m to 36.4m.The Swedish specimens, although smaller, are compared to Leiosphaeridia voigtii due to the presence of a rimmed pylome Occurrence: Sandbian, sub-assemblage B2, 40.2 m to 33.05 m.The present specimen is similar in shape and dimensions to Loeblichia nambeetense but has fewer processes.