Miaolingian (Cambrian) trilobite biostratigraphy and carbon isotope chemostratigraphy in the Tingskullen drill core, Öland, Sweden

The Cambrian succession of the Tingskullen drill core from northern Öland comprises Cambrian Series 2 and Miaolingian (Wuliuan Stage) siliciclastic strata. The major portion of the succession is represented by the Miaolingian Borgholm Formation, which, in ascending order, is subdivided into the Mossberga, Bårstad and Äleklinta members. The Äleklinta Member is barren of body fossils, whereas the Mossberga and Bårstad members are moderately to highly fossiliferous and biostratigraphically reasonably well constrained. Trilobites and agnostoids from the Bårstad Member are indicative of the Acadoparadoxides pinus Zone. The Mossberga Member has not yielded any zonal guide fossils but is tentatively assigned to the Eccaparadoxides insularis Zone. A δCorg curve throughout the Borgholm Formation shows a general positive trend up­section without any distinctive excursion, suggesting that the Wuliuan Acadoparadoxides (Baltoparadoxides) oelandicus Superzone (the ‘Oelandicus beds’) of Öland is younger than the negative Redlichiid–Olenellid Extinction Carbon isotope Excursion (ROECE), which is known from near the top of Stage 4 and close to the traditional ‘Lower–Middle Cambrian boundary’ in several parts of the world.


INTRODUCTION
In most parts of the world, the Cambrian has traditionally been divided into lower, middle and upper parts (corresponding to series/epochs). The boundary of each series (epoch) was, however, placed at a chronostrati graphic level that varied from region to region (e.g. Robison et al. 1977;Geyer & Shergold 2000). In order to resolve this problem, the International Subcommission on Cambrian Stratigraphy is working towards a global subdivision of the Cambrian System into four series and ten stages (e.g. Babcock et al. 2005), as opposed to the traditional tripartite subdivision into three series. Hence, the chronostratigraphic subdivision of the Cambrian System is currently undergoing substantial changes and the traditional series 'Lower', 'Middle' and 'Upper' Cambrian will be abandoned (see Peng et al. 2012 for a brief review). Each new global chronostratigraphic unit in the current conceptual model of the Cambrian System will have definitions based on the boundarystratotype concept (Salvador 1994), in which a Global boundary Stratotype Section and Point (GSSP) is used to define the base of a series or stage.
So far, six Cambrian stages (Fortunian, Wuliuan, Drumian, Guzhangian, Paibian and Jiangshanian) and three series (Terreneuvian, Miaolingian and Furongian) have been defined and formally named (Peng et al. 2004(Peng et al. , 2009Babcock et al. 2007;Landing et al. 2007;Zhao et al. 2012Zhao et al. , 2019Geyer 2019). The remaining undefined series and stages have received provisional numerical designations. The Miaolingian Series was ratified in 2018 and is divided into three global stages, in ascending order, the Wuliuan, Drumian and Guzhangian stages. Although of considerably greater stratigraphic thickness, it replaces and includes the traditional Middle Cambrian of Baltica and Avalonia. The conterminous base of the Miaolingian Series and the Wuliuan Stage is defined by a GSSP coinciding with the first appearance datum of the oryctocephalid trilobite Oryctocephalus indicus in the Kaili Formation of Jianhe County, eastern Guizhou, China .
In Scandinavia, the boundary between provisional Cambrian Stage 4 (traditional upper Lower Cambrian) and the Wuliuan Stage (lower Miaolingian Series) is generally marked by a prominent unconformity ascribed to non deposition and erosion during a eustatic sealevel fall that correlates partly with the regressive 'Hawke Bay Event' (Nielsen & Schovsbo 2015;Nielsen & Ahlberg 2019;Fig. 1 herein). The Wuliuan Stage of southeastern and south central Sweden comprises the silt and mudstone dominated Borgholm Formation (sensu Nielsen & Schovsbo 2007), followed by dark grey to black, organic rich siliciclastic mudstones of the Drumian through the Lower Ordovician (Tremadocian) portion of the Alum Shale Formation. In southern and southcentral Sweden, the upper or topmost part of the Wuliuan Stage is generally developed as a bioclastic limestone, the Exsulans Limestone Bed, assigned to the Ptychagnostus gibbus Zone. The Borgholm Formation is well developed in the Öland-Gotland area, southeastern Sweden, and in Närke and Östergötland, southcentral Sweden (Fig. 1).
Wuliuan strata are widely distributed in Scandinavia, but exposures are relatively few and small. Hence, the stratigraphic succession has to be pieced together using information from several outcrops and available drill cores. This paper focuses on the biostratigraphy and carbon isotope chemostratigraphy (δ 13 C org ) of the Cambrian succession in the Tingskullen drill core from the island of Öland, southern Sweden. It is one of the very few δ 13 C org investigations in the Miaolingian of Baltoscandia and also one of the few δ 13 C org studies dealing with this interval in the world.

GEOLOGICAL SETTING
During much of the early Palaeozoic, the Baltic (or Baltoscandian) Basin was submerged under a shallow to moderately deep epeiric sea that covered most of present day Scandinavia, the East Baltic area and northeastern Poland. The island of Öland is located in the western part of the Baltic Basin ( Fig. 2A) and a classical geological outcrop area for Cambrian and Ordovician research in Baltoscandia (for reviews, see Martinsson 1974;Stouge 2004). The sedimentary rock succession rests uncon formably on a Proterozoic crystalline basement and consists of Cambrian-Ordovician strata, which dip slightly, generally less than 2º, towards the east. Thus, the oldest formations are exposed in the westfacing cliffs along the western shore, and the youngest ones in the east.
The File Haidar Formation attains a thickness of 112 m (Hagenfeldt 1994;Nielsen & Schovsbo 2011) and is dominated by fine to mediumgrained, light grey quartz arenites with subordinate silt and mudstone beds. The silt and mudstonedominated Borgholm Formation is 114 m thick on southern Öland and thins out to 43 m on northern Öland (Hessland 1955;Hagenfeldt 1994). It generally rests on the File Haidar Formation with the prominent 'Hawke Bay' unconformity (Nielsen & Schovsbo 2007. In the Öland-Gotland area, the Borgholm Formation is subdivided into four members, in ascending order: the Grötlingbo, Mossberga, Bårstad and Äleklinta members (Nielsen & Schovsbo 2007). Nielsen & Schovsbo (2015) and Nielsen & Ahlberg (2019) indicated the presence of the Grötlingbo Member on northern Öland, but that part of the Öland succession is herein assigned to the När Sandstone Member of the upper File Haidar Formation. The Mossberga Member consists of medium to dark grey siltstones with thin sandstone interbeds, whereas the Bårstad Member is generally more finegrained and dominated by greenishgrey siliciclastic mudstones alternating with thin siltstone beds (Nielsen & Schovsbo 2007). The Mossberga and Bårstad members were deposited in an intracratonic basin (see Nielsen & Schovsbo 2015, fig. 51) and are generally richly fossi liferous. The fossil faunas are dominated by trilobites, agnostoids (including condylopygids) and phosphatic shelled brachiopods. In older literature, the Mossberga and Bårstad members have been referred to as shales with Paradoxides oelandicus, Oelandicus beds/shales or the Oelandicus Stage (e.g. Sjögren 1872; Linnarsson 1876aLinnarsson , 1877Westergård 1936;Hessland 1955).  briefly discussed the depositional setting and faunal diversity of the Mossberga and Bårstad members and noted that the diversity is higher (~30 taxa) than in the equivalent outer shelf-shelfedge deposits of Jämtland and Ångermanland, northcentral Sweden.
The Äleklinta Member, in older literature referred to as the Paradoxissimus Siltstone or Tessini Sandstone (see Martinsson 1965Martinsson , 1974Weidner & Nielsen 2009), consists of grey siltstones with thin sandstone inter calations. It has a thickness of up to 70 m on southern Öland and thins out to ~1 m on northern Öland (Nielsen & Schovsbo 2007). The Äleklinta Member is un conformably overlain by the Alum Shale Formation, which is incomplete with several substantial gaps of variable magnitudes. The Alum Shale Formation grad ually thins out towards the NNW of Öland  (Westergård 1944(Westergård , 1947Erlström 2016;Ahlberg et al. 2019; Fig. 2B herein). The exposed Ordovician succession of Öland has a thickness of up to 46 m and consists predominantly of coolwater carbonates ('orthoceratite limestone') with numerous hardgrounds (Wu et al. 2017). It is highly condensed and a typical representative of the Central Baltoscandian Confacies Belt of Jaanusson (1976Jaanusson ( , 1979Jaanusson ( , 1995. The lowermost Ordovician (Tremadocian) consists of dark grey mudstones and shales belonging to the 0.35-2.80mthick Djupvik Formation, previously referred to as the Ceratopyge Shale (Stouge 2004), which reflects the end of siliciclastic sedimentation in the Central Balto scandian Confacies Belt and is overlain by Lower-Middle Ordovician grey or red 'orthoceratite limestone'. The youngest exposed strata are of early Sandbian age and belong to the Dalby Limestone (Jaanusson 1960).

MATERIALS AND METHODS
The Tingskullen core drilling was performed in 2010 at a site (WGS coordinates N 57.116172, E 16.993013) approximately 700 m northeast of Källa Hamn and south of Tingskullsgatan on northeastern Öland (Fig. 2B). It was made by the Engineering Geology Group of the Department of Measurement Technology and Industrial Electrical Engineering, Lund University, with an Atlas Copco CT20 drill rig (Riksriggen). Drilling with continuous coring continued to a depth of ∼107.0 m below the ground level, which corresponds to the bedrock surface, and ended within the upper part of the File Haidar Formation (provisional Cambrian Series 2; Dahlqvist et al. 2013). The drill core is currently housed at the Department of Geology, Lund University, and comprises Cambrian Series 2 (107.00-102.28 m), Miaolingian (102.28-46.50 m), ?Furongian (46.50-46.00 m) and Lower-Middle Ordovician (46.00-0 m) strata (Fig. 3). The core diameter is 39 mm and recovery of the tectonically undisturbed, essentially horizontal, core rock succession is close to 100%. The succession shows no major late diagenetic alteration and has not been buried below the oil window (cf. Buchardt et al. 1997, fig. 19). The pristine preservation of conodonts from the 46m thick Ordovician limestone succession strata overlying the Cambrian strata described in this paper showing a Colour Alteration Index (CAI of Epstein et al. 1977) of 1 is also indicative of shallow burial (Wu et al. 2017). The major portion of the Cambrian succession in the core is represented by the lower Miaolingian (Wuliuan Stage) Borgholm Formation. Middle and upper Miaolingian (Drumian-Guzhangian) strata are lacking in the core. Dahlqvist et al. (2013) provided a brief first overview of the drill core and its lithostratigraphic succession. Calner et al. (2014) and Wu et al. (2017) discussed the δ 13 C carb chemostratigraphy of the Ordovician portion of the drill core and its correlation with the conodont bio stratigraphy.
The Cambrian and lowermost Ordovician (Trema docian) succession of the Tingskullen core was split up and examined. Fossils were collected and specimens of characteristic taxa were painted with opaque matt black and then lightly coated with a sublimate of ammonium chloride prior to being photographed using a digital camera (Canon 550D) mounted on a tableset camera holder with four external light sources.
A total of 116 samples, most of which taken at 0.5m or 1.0m intervals, were collected from a 56.11 m thick rock interval (102.12-46.01 m) of the Tingskullen drill core. All samples were subjected to processing for δ 13 C org following the procedure described by Ahlberg et al. (2009) and Terfelt et al. (2014). Carbon isotope analyses of organic carbon were performed using a Flash EA 2000 elemental analyser connected online to a ThermoFinnigan Delta V Plus mass spectrometer at the Geocentre of Northern Bavaria, FriedrichAlexander University of ErlangenNürnberg in Erlangen, Germany. All carbon isotope values are reported in the conventional δnotation in per mil relative to VPDB (ViennaPee Dee Belemnite). The accuracy and reproducibility of the analyses were monitored by replicate analyses of laboratory standards calibrated to the international standards USGS 40 and 41 and were ±0.05 (1σ). The obtained δ 13 C org values are listed in Table 1 and used for the isotope curve described and discussed below.

LITHOLOGICAL SUCCESSION
The Cambrian succession of the Tingskullen core has a thickness of 61.00 m ( The Mossberga Member is succeeded by a 17.25m thick interval (70.40-53.15 m) with medium to dark greenishgrey, bioturbated siliciclastic mudstones alter nating with thin siltstone beds. This interval is assigned to the Bårstad Member. The boundary between the Mossberga Member and the Bårstad Member is tentatively placed at ~70.40 m, where there is a fairly abrupt shift to more greenish and generally darker mudstones. The Bårstad Member is overlain, probably conformably, by an interval (53.15-46.50 m) comprising light grey finely laminated silt to very finegrained sandstones alternating with medium greenishgrey mud stones. Bioturbation and smallscale crosslamination occur frequently throughout this interval, which is as signed to the Äleklinta Member. It is in turn overlain by a 0.5mthick light grey, limestonedominated succession (46.50-46.00 m) with three 1-2cmthick intervals with wellpreserved palisadic crystals of calcite in the lower most part (46.50-46.40 m; Fig. 4B) and a light grey crystalline limestone in the upper part. It is covered by a suc cession of variegated greenish to dark grey, in part very coarsely sparitic limestone, which has a thickness of 0.60 m (46.00-45.40 m) and shows palaeokarst structures (Dahlqvist et al. 2013;Calner et al. 2014;Wu et al. 2017). This interval is interpreted as an equivalent to the lower Tremadocian 'Obolus conglomerate' (e.g. Westergård 1922Westergård , 1947Tjernvik 1956

DEPOSITIONAL ENVIRONMENTS AND RELATIVE SEALEVEL
The cored part of the När Sandstone Member is a massive to faintly laminated quartz arenite that represents rapid deposition in an inner shelf environment. The pre dominance of planar to slightly undulating lamination and inclusions of mudstone suggests deposition below the influence of daily wave action (shoreface) and more likely in the most shallow part of the lower shoreface. The upper unconformable boundary of this unit and the associated conglomerate (Fig. 4A) imply the shallowing of the depositional environment followed by prolonged exposure of the succession and reworking of already lithified sandstone. No obvious shallowing upward suc cession (i.e. shoreface deposits) is preserved in the core, indicating rapid regression and a substantial erosion of the suc cession. The presence of phosphoritic clasts in a glauconiterich matrix in the boundaryconglomerate, as well as the immediately overlying dark and organicrich mudstones, further implies sediment starvation and low depositional rates in this area during the ensuing transgression. Based on the temporal shift from sandstone, reworked subaerially and in the shoreline environment, to mudstone deposited well below the wavebase (see below), the transgression likely changed depositional depth with several tens of metres. This major regressivetransgressive cycle can partly be attributed to the 'Hawke Bay Event' (cf. Nielsen & Schovsbo 2007, p. 78, 2015.
The relatively minor facies changes and conformable transitions through the overlying Mossberga, Bårstad and Äleklinta members exclude more abrupt changes to relative sealevel and instead continuous sedimentation in deep to moderately shallow waters through the remainder of the Wuliuan Age. It is worth noting, however, that the Bårstad Member is unconformably overlain by the Äleklinta Member in most other areas of Öland (Nielsen & Schovsbo 2007, p. 80, 2015. The thin and finely laminated siltstone interbeds in the Mossberga Member were deposited through lowdensity currents and interpreted as the distal expression of storm reworked sediments. The laminated beds are common and well preserved due to the absence of bioturbation and infauna during deposition, which in turn may reflect relatively low levels of oxygenation in the sediment pore waters in the outer shelf areas. The transition to the overlying Bårstad Member is fairly distinct but con formable in the Tingskullen succession. This member lacks the finely laminated siltstone beds typical of the underlying Mossberga Member, suggesting continuous drowning of source areas during the peak of the post 'Hawke Bay' transgression (cf. Nielsen & Schovsbo 2015). The transition to the overlying Äleklinta Member also appears to be conformable. The facies of this member is distinct with abundant laminated or crosslaminated siltstone beds, interbedded with greenish mudstone, and abundant bioturbation. The Äleklinta Member marks a renewed shallowing of the environment and deposition in the lower shoreface. The upper portion of the Äleklinta Member is exposed at Älekinta approximately 22 km southwest of the Tingskullen drill site. Here abundant sedimentary structures and the overall sedimentary facies imply deposition in a wave and stormdominated inner shelf setting. The Äleklinta Member is truncated by a major unconformity (sequence boundary).

BIOSTRATIGRAPHY
The Terreneuvian and Cambrian Series 2 succession of Scandinavia is generally unfossiliferous or poorly fossiliferous except in its uppermost part. By contrast, the Miaolingian and Furongian strata of Scandinavia are generally richly fossiliferous with faunas commonly dominated by polymerid trilobites and agnostoid arthropods. Both groups have been widely applied as the principal tools for biostratigraphical and chronostrati graphical classification of Miaolingian through Furongian strata of Scandinavia since at least the 1870s. The Miaolingian of Scandinavia is currently subdivided into three superzones, which in ascending order are the Acadoparadoxides oelandicus, Paradoxides paradoxissimus and P. forchhammeri superzones Babcock et al. 2017). These superzones were considered to represent regional stages by Westergård (1946). The A. oelandicus Superzone corresponds to the Bödan regional Stage, and the P. paradoxissimus Superzone corresponds to the Almbackenian regional Stage (Nielsen & Schovsbo 2015). The Wuliuan Stage of Scandinavia is now being subdivided into three polymerid trilobite zones, in ascending order: the Eccaparadoxides insularis, Acadoparadoxides pinus and Ctenocephalus exsulans zones (e.g. Nielsen et al. 2014). The A. pinus and C. exsulans zones are equivalent to the Ptychagnostus praecurrens and P. gibbus agnostoid zones, respectively (Ahlberg 1989;Nielsen et al. 2014).
The När Sandstone Member in the Tingskullen core is unfossiliferous and not biostratigraphically constrained. The overlying Borgholm Formation is partly bioturbated with horizontal and oblique burrows in several intervals (Fig. 5A). Indeterminate pyritized or carbonaceous fila ments are common in both the Mossberga and the Bårstad members. Body fossils collected from these members include phosphaticshelled brachiopods, trilobites, agnos toids and a few hyoliths. The Mossberga Member is generally poorly fossiliferous, especially in its lower half, whereas the Bårstad Member has yielded a rich and fairly diverse shelly fauna. Brachiopods are most common in the Mossberga Member and represented by obolids, acrotretids and acrothelids (Fig. 5B-D). Acrothele cf. granulata Linnarsson, 1876 (Fig. 5B) was recorded in the core interval 99.78-79.49 m.
The lowest observed occurrence of trilobites is in the upper half of the Mossberga Member at 82.59 m. Polymerid trilobites are very common in the Bårstad Member, but the fauna is of low diversity and restricted to paradoxidids and Ellipsocephalus polytomus Linnarsson, 1877. The latter species appears at 69.82 m and ranges throughout the entire Bårstad Member. All paradoxidid trilobites belong to the genus Acadoparadoxides (s.l.) and the most common species is seemingly A. (Baltoparadoxides) oelandicus (Sjögren, 1872) s.l. In the Bårstad Member, the trilobites are accompanied by generally rare agnostoids of four taxa: Condylopyge cf. carinata Westergård, 1936 (68.97-67.64 m), Ptychagnostus praecurrens (Westergård, 1936) The Äleklinta Member in the Tingskullen succession is largely unfossiliferous except for trace fossils, but following Weidner & Nielsen (2009) it is assigned to the Ptychagnostus gibbus Zone. The overlying 0.5mthick light grey, carbonatedominated succession (46.50-46.00 m) with wellpreserved palisadic crystals of calcite is unfossi liferous, but may represent a thin wedge of the uppermost Miaolingian and/or Furongian (Dahlqvist et al. 2013). The top of the studied interval, i.e. the Tremadocian Djupvik Formation, is poorly fossiliferous but phosphaticshelled brachiopods (Broeggeria sp.; Fig. 5E, F) were observed at 45.31 and 45.30-45.28 m, and two cranidia of the small trilobite Shumardia (Conophrys) pusilla (Sars, 1835) were recorded at 45.20 m (Fig. 6O, P On the basis of the succession on Öland, Westergård (1936) subdivided the 'Oelandicus beds/stage', com prising the Mossberga and Bårstad members, into a lower zone with Paradoxides (now Eccaparadoxides) insularis Westergård, 1936 and an upper zone with Paradoxides (now Acadoparadoxides) pinus Westergård, 1936. The E. insularis Zone is also characterized by the occurrence of Bailiella emarginata (Linnarsson, 1877) and Condylopyge regia (Sjögren, 1872) (Westergård 1936(Westergård , 1953. The trilobite and agnostoid fauna in the A. pinus Zone is more diverse and, in addition to the eponymous species and forms of the A. oelandicus plexus, it includes, among others, Acadoparadoxides torelli (Westergård in Asklund & Thorslund, 1935), Burlingia laevis Westergård, 1936, Ptychagnostus praecurrens and Condylopyge carinata (e.g. Westergård 1936, 1946, 1953Geyer & Corbacho 2015). In the Öland-Gotland area, the E. insularis Zone grades downwards into a fairly thick succession with no other body fossils than linguliformean brachiopods. This succession has generally been assigned to the E. insularis Zone (e.g. Westergård 1936), but should rather be regarded as unzoned (cf. Nielsen & Ahlberg 2019).
The zonal index fossils have not been identified in the Tingskullen core and the biostratigraphical data are inadequate for subdividing the 'Oelandicus beds/stage' into biozones. The record of Acadoparadoxides (Baltoparadoxides) oelandicus forma bidentatus (Westergård, 1936), Condylopyge cf. carinata and Ptychagnostus praecurrens? in the Bårstad Member suggests, however, that this member belongs to the Acadoparadoxides pinus Zone (Fig. 3). The Mossberga Member has not yielded any zonal guide fossils, but its upper trilobitebearing part (82.59 to ~70.40 m) is tentatively assigned to the Eccaparadoxides insularis Zone. The only shelly fossils recorded from the lower half of the Mossberga Member are phosphaticshelled bra chiopods.

SYSTEMATIC NOTES
All illustrated specimens are stored in the Palaeontological collections of the Museum of Evolution, Uppsala University, Sweden (PMU).

Family ACROTHELIDAE Walcott & Schuchert in
Walcott, 1908 Genus Acrothele Linnarsson, 1876 Acrothele cf. granulata Linnarsson, 1876 Figure 5B Remarks. Circular to subcircular valves with concentric growth lines and a granulated external surface from the Mossberga Member (99.78-79.49 m) are identified as Acrothele cf. granulata Linnarsson, 1876. The ventral valve is virtually flat to gently convex with a prominent raised umbo and a foramen situated excentrically, adjacent to posterior margin. Acrothele granulata was originally described from the 'Oelandicus beds/stage' (probably the A. pinus Zone) cropping out at Lillviken and Hackås in Jämtland, central Sweden. It has subsequently been described from the 'Oelandicus beds/stage' and younger Miaolingian strata in various parts of Scandinavia, e.g. on Öland (Linnarsson 1877) and in southwestern Norway (e.g. Henningsmoen 1952;Bruton & Harper 2000).
Family CONDYLOPYGIDAE Raymond, 1913 Genus Condylopyge Hawle & Corda, 1847 Remarks. Condylopyge is a distinctive and geographically widely distributed genus characterized by a very large, expanded and almost semicircular anterior glabellar lobe and a cylindrical posterior glabellar lobe. It ranges from the upper part of Cambrian Series 2 through the Miaolingian and was most recently discussed by Fatka et al. (2015) and Naimark & Pegel (2017). The former authors assigned 16 species to the genus. Miaolingian representatives of Condylopyge seem to be most common in inner shelf settings that were located at high latitudes and they are largely restricted to West Gondwana, Avalonia and Baltica, albeit with a few occurrences in Siberia (Conway Morris & Rushton 1988;Fatka et al. 2015;Naimark & Pegel 2017).
Condylopyge cf. carinata Westergård, 1936 Figure 5G-I Remarks. Westergård (1936Westergård ( , 1946 described two species of Condylopyge from the 'Oelandicus beds/stage' of Öland: C. regia (Sjögren, 1872) from the E. insularis Zone and C. carinata Westergård, 1936 from the A. pinus Zone. The cephala of these species are more or less indistinguishable, whereas the pygidium of C. carinata differs from C. regia in having unfurrowed pleural fields and a longer axial node (see Westergård 1936;Rushton 1966;Rushton & Weidner 2007). Three condylopygid cephala were recorded from the lower Bårstad Member (68.97-67.64 m) in the Tingskullen core. They are identified under open nomenclature as C. cf. carinata. A slightly distorted pygidium from 68.18 m may also represent this species, but it has a short axial node and is herein referred to as Peronopsis? sp.
Family PTYCHAGNOSTIDAE Kobayashi, 1939 Genus Ptychagnostus Jaekel, 1909 Ptychagnostus praecurrens (Westergård, 1936)? Figure 5J Remarks. One small, nonspinose and nonscrobiculate pygidium from the middle Bårstad Member in the Tingskullen core (60.79 m) has a prominent postaxial median furrow, a lanceolate posteroaxis and an unconstricted acrolobe. The median node is of moderate size and slightly deflects the posterior axial furrow (F2). The pygidium appears to represent an early holaspid and seems to fall within the range of variation seen in juveniles of P. praecurrens illustrated by . The posteroaxis is, however, more sharply pointed and the pygidium at hand is therefore questionably assigned to P. praecurrens.
Family PARADOXIDIDAE Hawle & Corda, 1847 Genus Acadoparadoxides Šnajdr, 1957 Remarks. Acadoparadoxides (type species Paradoxides sacheri Barrande, 1852) is difficult to diagnose and the concept of the genus has been widely discussed in the literature (e.g. Fletcher et al. 2005;Żylińska & Masiak 2007;Esteve 2014;Geyer & Vincent 2015;Nowicki & Żylińska 2019). We follow the concept of Acadoparadoxides discussed by  and Geyer & Vincent (2015), who listed junior synonyms. The nominate subgenus is characterized by a wellmarked frontal border, transglabellar S1 and S2 furrows, weakly developed or obsolete S3 and S4 furrows, recurved palpebral lobes, 19 thoracic segments (in adults) with relatively short pleural furrows, and a weakly segmented pygidium that is subequal in length and width or slightly wider than long and has a rounded or subangular posterior margin without spines. Species assigned to the subgenus Acadoparadoxides (Baltoparadoxides) (type species Paradoxides oelandicus Sjögren, 1872) differ in having a more broadly oval exoskeletal outline, a medially wellincised S2, 17 thoracic segments in adults and a hexagonal pygidium with 2-4 pairs of short marginal spines (Geyer & Vincent 2015).  (Westergård, 1936) (cf. Ahlberg 1989Fig. 6F, G herein). Their posterior pygidial margin is broadly rounded and one of the pygidia ex hibits a pair of short marginal spines. The zonal index paradoxidines for the Eccaparadoxides insularis and Acadoparadoxides pinus zones were not identified in the Tingskullen core.
Family ELLIPSOCEPHALIDAE Matthew, 1887 Genus Ellipsocephalus Zenker, 1833 Ellipsocephalus polytomus Linnarsson, 1877 Figure 6I-N Remarks. Ellipsocephalus polytomus was described in some detail by Linnarsson (1877), Westergård (1936Westergård ( , 1950 and Ahlberg (1989). It is one of the most com mon and most widely distributed trilobites in the Acadoparadoxides oelandicus Superzone of Sweden, in which it ranges from the lower-middle part of the E. insularis Zone to the top of the A. pinus Zone (Westergård 1936). Possibly reworked specimens were reported by Westergård (1936Westergård ( , 1950 from the overlying Granulata Conglomerate Bed (Nielsen & Schovsbo 2007) of the basal Paradoxides paradoxissimus Superzone. Ellipsocephalus polytomus is very common in the Tingskullen core, but appears to be confined to an interval (69.82-53.23 m) assigned to the A. pinus Zone.

CARBON ISOTOPE CHEMOSTRATIGRAPHY
Studies during the past three decades have demonstrated the great value of δ 13 C for both intercontinental and intracontinental correlation of Cambrian successions (e.g. Peng et al. 2012 and references therein;Zhu et al. 2019).
Most of these studies have focused on δ 13 C carb using samples from carbonatedominated successions. During recent years, however, δ 13 C org has been utilized for Cambrian successions dominated by shales and sil ici clastic mudstones, and it has been shown that δ 13 C org chemostratigraphy is useful for approximate correlations between organicrich siliciclasticdominated and car bonate successions (e.g. Ahlberg et al. 2009, 2019 andreferences therein;Saltzman et al. 2011;Woods et al. 2011). Although δ 13 C chemostratigraphy has emerged as an increasingly powerful tool for regional and global correlation of Cambrian strata, it has become evident that the stratigraphical pattern of carbon isotope curves can vary laterally (e.g. Edwards & Saltzman 2016;Henderson et al. 2018;Lindskog & Young 2019) and δ 13 C events may be facies dependent and diachronous (Schiffbauer et al. 2017;Barili et al. 2018). Zhu et al. (2006) compiled previous studies on Cambrian δ 13 C chemostratigraphy and provided a composite δ 13 C curve with ten distinct and named excursions, most of which seem to coincide with im portant biotic events (see also Babcock et al. 2015). The negative Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE; Zhu et al. 2006) is associated with the extinction of redlichiid and olenellid trilobites at the end of Stage 4 (Fan et al. 2011;Wotte et al. 2011;Faggetter et al. 2017Faggetter et al. , 2018Lin et al. 2019;Zhu et al. 2019). Although considered as one of the largest negative carbon isotope excursions known from the Cambrian (Zhu et al. 2006;Peng et al. 2012), isotopic data from vastly different regions provide inconclusive evidence as to the precise definition and placement of the ROECE (cf. Lin et al. 2019) and further studies are needed to establish if it is a globally identified perturbation in the Cambrian isotope curve.
A δ 13 C org curve based on 116 samples from the Mossberga, Bårstad and Äleklinta members in the Tingskullen drill core shows values between -32.1‰ and -28.9‰ (Table 1, Fig. 3). The high value of -26.6‰ at 64.01 m is explained by incomplete decarbonatization prior to analysis or by the source and type of organic matter. This anomalously high value is excluded from the curve. Despite some scatter in the δ 13 C org values, a trend to higher δ 13 C org values is recorded upsection without any distinctive excursion.
The isotope curve can be broadly divided into two segments, one restricted to the Mossberga Member and the other largely to the Bårstad and Äleklinta members. The lowermost part of the Mossberga Member (core depth 102.1 to ~95.0 m) shows δ 13 C org values varying between -32.1‰ and -30.6‰. It is followed by an interval (95-76 m) with values between -31.7 and -30.6‰. Although the δ 13 C org values exhibit a considerable scatter, the isotope curve from near the top of the Mossberga Member to the top of the Bårstad Member (76-52 m) shows an overall positive trend (c. +1.5‰). The consistent shift to more positive values starts near the base of the Bårstad Member (at c. 70.25 m). Maximum δ 13 C org values of around 29.0‰ are measured in the lower half of the member (at 64.5-62.9 m) and are followed by a shift to more negative values in the middle part of the Bårstad Member. Thus, though not well expressed, a minor positive excursion with an amplitude of c. +1‰ seems to be present in the lower-middle Bårstad Member (lower Acadoparadoxides pinus-Ptychagnostus praecurrens Zone). The isotope values from the Äleklinta Member are variable and range from -30.1‰ to 29.4‰.
No distinctive negative excursion was captured, suggesting that the entire Acadoparadoxides (Baltoparadoxides) oelandicus Superzone of Scandinavia, including the lower nontrilobitic part of the Mossberga Member, is younger than the ROECE event, which is known in many areas from near the top of Cambrian Stage 4 and close to the traditional 'Lower-Middle Cambrian boundary'. The absence of the ROECE in the Wuliuan succession of the Tingskullen core was expected because studies during the past two decades have shown that the Acadoparadoxides (Balto para doxides) oelandicus Superzone of Scandinavia is younger than the oldest paradoxididbearing strata in West Gondwana (Morocco and Spain) and Poland (Żylińska & Masiak 2007;Nowicki & Żylińska 2019;Sundberg et al. 2020).

DISCUSSION
The Wuliuan succession (the Borgholm Formation) in the Tingskullen drill core is lithologically and strati graphically similar to coeval intervals in other drill cores from Öland (e.g. Westergård 1929Westergård , 1936Waern 1952;Hessland 1955;Erlström 2016;Ahlberg et al. 2019). The succession has a thickness of at least 100 m on southern Öland (114 m in the Kvinnsgröta1 well and 106 m in the Segerstads Fyr drill core ;Hagenfeldt 1994;Nielsen & Schovsbo 2007;Erlström 2016), and thins out to the north of Öland (55.8 m in the Tingskullen core and 43.2 m in the Böda Hamn core ;Waern 1952;Hessland 1955). On Öland, the maximum recorded thickness of the Mossberga Member is 26 m in the Kvinnsgröta1 well (Nielsen & Schovsbo 2007). This member is, however, thicker (31.9 m) in the Tingskullen core. The thickness of the Bårstad Member in the Tingskullen and Böda Hamn cores is remarkably similar (~17 m). The Äleklinta Member is up to 68 m thick on southern Öland (Hagenfeldt 1994;Nielsen & Schovsbo 2007 and thins out towards the north. In the Tingskullen and Böda Hamn cores it is 6.65 and 1.50 m thick, respectively. The Äleklinta Member of Öland is generally dominated by a succession of light grey siltstones with thin interbeds of sandstone and rests unconformably on the Bårstad Member (Martinsson 1965;Nielsen & Schovsbo 2007). In the Tingskullen core, this member rests conformably on the Bårstad Member and is dominated by light grey siltstones alternating with medium grey mudstones. A thin con glomerate (the Granulata Conglomerate Bed of Nielsen & Schovsbo 2007) suggestive of an unconformity was, however, described from the base of the Äleklinta Member (42.45 m) in the Böda Hamn core (Waern 1952). This conglomerate is inferred to reflect a rapid sealevel rise, the Forsemölla Drowning, at the beginning of the Ptychagnostus gibbus Chron (Nielsen & Schovsbo 2015).
Locally on northern Öland, the Borgholm Formation is taken to include a silt and mudstone succession that rests conformably on the File Haidar Formation and is truncated at the top by the 'Hawke Bay' unconformity (Nielsen & Schovsbo 2007). This succession is recognized as the Grötlingbo Member and inferred to be latest Cambrian Epoch 2 in age (Hagenfeldt 1994;Nielsen & Schovsbo 2011. However, the Grötlingbo Member cannot be identified in the Tingskullen core. Nielsen & Schovsbo (2007, p. 77) suggested that the Grötlingbo Member is disconformably overlain by the Mossberga Member and has a thickness of ~7 m in the Böda Hamn core (90.8-84.1 m). The succession below the basal conglomerate of the Mossberga Member in the Böda Hamn core is, however, dominated by a finegrained, whitish sandstone with mudstone interbeds (Hessland 1955), and it agrees in all essential lithological characteristics with the När Sandstone Member of the File Haidar Formation. Thus, the Grötlingbo Member cannot be positively identified on Öland. Nielsen & Schovsbo (2015, fig. 60) presented a gen eral sealevel curve for the Borgholm Formation with a trans gression at the base of the Mossberga Member and a renewed and prominent transgression (the Oelandicus Drowning) at the base of the Bårstad Member. In the Tingskullen succession, the Bårstad Member facies indicates deposition in distinctly deeper waters than the Mossberga Member and provides confirming evidence for the renewed transgression during the deposition of the Bårstad Member.
On Öland, the lower half of the Mossberga Member, corresponding to subsequence MC32A of Nielsen & Schovsbo (2015), is nontrilobitic, but has yielded various phosphaticshelled brachiopods. Nielsen & Schovsbo (2015) speculated that the dissolution of sclerites due to variable synsedimenatry dysoxic/oxic conditions on the sea floor (cf. Schovsbo 2001) might be responsible for the absence of trilobites in this interval. However, redox proxies and the oxicdysoxic conditions on the seafloor have not been investigated, and it is also worth noting that the lower, nontrilobitic Mossberga Member is litho logically closely comparable to the trilobitebearing upper part of the Mossberga Member. Thus, the reason for the absence of trilobites in the lower Mossberga Member remains unknown.

CONCLUSIONS
The Cambrian succession of Öland, southern Sweden, records several facies changes, largely because of prom inent sealevel changes during deposition. The recent Tingskullen core drilling penetrated 60.50 m of Cambrian Series 2 and Miaolingian (Wuliuan Stage) siliciclastic strata. The lowermost part of the drill core belongs to the upper När Sandstone Member (File Haidar Formation; provisional Cambrian Series 2), which is disconformably overlain by the Miaolingian Borgholm Formation. In ascending order, the Borgholm Formation is subdivided into the Mossberga, Bårstad and Äleklinta members. The Äleklinta Member is barren of body fossils, whereas the Mossberga and Bårstad members are moderately to richly fossiliferous with a shelly fauna largely represented by linguliformean brachiopods, paradoxidid and ellipso cephalid trilobites, and agnostoids. Trilobites and agnostoids from the Bårstad Member are indicative of the Acadoparadoxides pinus Zone. The Mossberga Member has not yielded any zonal guide fossils but its upper trilobitebearing part is tentatively assigned to the Eccaparadoxides insularis Zone.
The Tingskullen drill core offers a unique opportunity to calibrate the Wuliuan δ 13 C org curve with the Wuliuan trilobite and agnostoid zone succession of Baltoscandia. Although the data exhibit some scatter, δ 13 C org values through the Borgholm Formation show a positive trend up section without any characteristic excursion. This suggests that the Wuliuan A. oelandicus Superzone is younger than the negative Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE), which is known from near the top of Stage 4 and close to the traditional 'Lower-Middle Cambrian boundary' in several parts of the world.