The locality Storeklev (N58°17’38’’; E12°25’53.5’’) is the type locality of the Scandinavian Hunneberg stage of Tjernvik (1956). The trilobite faunas were described by Tjernvik (1956) who also discovered the important ‘dichograptid horizon’, later described to belong to the Hunnegraptus copiosus Biozone and found widely distributed in southern Scandinavia (Lindholm 1991a, b). Tjernvik (1956, p. 117) defined the base of his Hunneberg Group on the presence of the trilobite Megistaspis (Ekeraspis) armata in this section (Fig. 4).

The Tøyen Shale interval differs considerably from the localities on the NE corner of the mountain. Here the shale interval is much thicker and greyish green instead of black, but this color may be largely due to the contact metamorphism. Graptolites are uncommon and poorly preserved, but still identifiable even in the metamorphosed upper part of the succession. Individual limestone layers can be correlated with Mossebo and Diabasbrottet by their trilobite and conodont faunas (Tjernvik 1956; Maletz et al. 1996).

Tjernvik (1956) discovered the main layer with the Hunnegraptus copiosus Biozone fauna at Storeklev, but rare graptolites can be found at a number of levels of the whole approximately 2 m thick interval (Fig. 4). The Hunnegraptus copiosus Biozone fauna was found at only three localities in the Hunneberg area. It is present at Holsbrotten and Storeklev in green shales, but in a small pit a few tens of meters west of Storeklev, Hunnegraptus copiosus was discovered in an interval of black shales. The presence of Tetragraptus phyllograptoides and Prothorthis hunnebergensis indicate an increased thickness of the Tetragraptus phyllograptoides Biozone interval at Storeklev. Paratetragraptus approximatus was found on loose slabs in the higher part of the section, exposed above the cliff section in the forest, where it is associated with Tetragraptus phyllograptoides.

The locality Holsbrotten (N58°19’12.9’’; E12°23’42’’), on the western side of Hunneberg (Fig. 2A) has a nearly identical lithology to the Storeklev locality. The succession ranges from the upper Cambrian Peltura scarabaeoides Trilobite Zone to the Megistaspis (Paramegistaspis) planilimbata Trilobite Zone (Fig. 5). Löfgren (1993) documented the conodont faunas from the section and from the comparable Prästeklev section a little further south. Graptolites are present at a few levels at Holsbrotten, but have not been described previously. Except for the main Hunnegraptus copiosus band, graptolites are rare and poorly preserved in the grey shales of the Tøyen Shale Formation and only a few specimens have been collected from the contact metamorphosed shales.

Tremadocian graptolites can be found in a thin band of shale at the base of the upper Alum Shale Formation (previously Dictyonema Shale). Numerous specimens of Rhabdinopora flabelliformis occur crowding a thin layer of shale. In other parts of the interval, few metres along strike, specimens of Adelograptus tenellus occur at roughly the same level. These graptolites, however, are extremely rare and mainly preserved as small fragments. Rhabdinopora flabelliformis and Adelograptus tenellus do not occur together and must have originated from different biostratigraphic and lithostratigraphic intervals (see Egenhoff & Maletz 2012, fig. 2).

The Nygård locality on the western slope of Hunneberg, north of Holsbrotten, was first noted by Linnarsson (1869) and the graptolites were subsequently described by Linnarsson (1871a) as the first graptolites to be described and illustrated from Hunneberg. The precise origin of this material, however, is unknown, as there are a number of small pits along a long line of caves (Fig. 1B) on the western to northwestern slopes of the mountain, over a distance of more than two km, where Tremadocian graptolites can occasionally be found. Moberg (1892) described Adelograptus tenellus (Linnarsson, 1871a) from ‘Skaktet No. 1 at Nygård’ and added Adelograptus hunnebergensis, now a junior synonym (Maletz & Erdtmann 1987) from the same layers. Sidenbladh (1870, fig. 4) illustrated the highly uneven surface of the lower Tøyen Shale Formation (his unit hvitberget) under the dolerite in the area between Nygård and Holsbrotten.

Rasmussen et al. (2016) provided precise locality data for their Nygård section, but the location on their map does not fit the information and is much closer to the section here identified as Holsbrotten. Their succession at Nygård starts in the late Cambrian (Furongian) Olenus Superzone, overlain by the Parabolina, Leptoblastus, Protopeltura and Peltura superzones (Rasmussen et al. 2016). The Ctenopyge bisulcata Biozone is unconformably overlain by the Ordovician succession with Adelograptus tenellus or Rhabdinopora flabelliformis, indicating probably a time of erosion during the latest Cambrian time interval. The Ordovician strata usually only reach the Megistaspis (Ekeraspis) armata Biozone, and faunas of the Tøyen Shale are not preserved under the dolerite sill at this locality.

The modern Diabasbrottet section (N58°21’47’’; E12°29’46.5’’) (Fig. 6) is the GSSP (Global Stratotype section and Point) of the Floian Stage of the Ordovician System (Maletz et al. 1995, 1996; Bergström et al. 2004, 2006) at the NE edge of Hunneberg, an important international level for geological correlation. The section is adjacent to a now inactive quarry (Fig. 2D), in which the overlying dolerite was mined for road construction material and other purposes.

Based on the description in Sidenbladh (1870, p. 72) this locality is the old Lilla Mossebo, probably also identified as Mossebo by early graptolite workers (e.g. Holm 1881; Törnquist 1901, 1904). The dolerite sill described and illustrated by Sidenbladh (1870, fig. 10) is recognizable in the modern section (Fig. 6) and can be followed for some distance from the section to the NW, where it merges with the overlying massive dolerite cap. The lowermost levels of the succession at Diabasbrottet can be found in the mines below the vertical cliffs. These were made during the exploration for limestones in the upper Cambrian Alum Shales. The GSSP at the base of the Floian is positioned at the 2.10 m level in the section initially described and figured by Maletz et al. (1995, 1996), shown as the 0.0 m level here (Fig. 6). This level is directly above marker bed E (see Tjernvik 1956, Fig. 5), yielding conodont faunas of the Oelandodus elongatus–Acodus deltatus deltatus Conodont Subzone of the Paroistodus proteus Conodont Zone. Directly above the limestone, the earliest specimens of Paratetragraptus approximatus and Tetragraptus phyllograptoides were discovered. The level can also be recognized by trilobites from marker bed E, which have yielded faunas of the Megistaspis (Paramegistaspis) planilimbata Trilobite Zone.

Figure 6 shows the ranges of all taxa found in the Diabasbrottet section. The range chart was modified from Egenhoff & Maletz (2007, fig. 3). Identifications of all species have been revised from the available collections. Information has been added from museum collections and approximate biostratigraphic occurrence is added, based on faunal assemblages. Thus, for example, the presence of Holograptus expansus in the higher part of the Tetragraptus phyllograptoides Biozone is shown, as the type specimen is associated with four-stiped Tetragraptus amii and Cymatograptus demissus. The youngest occurrence of this species is uncertain, as the available material consists of very large four-stiped specimens without further branching. These are referred to Holograptus expansus only with reservation. The presence of Paratemnograptus magnificus is based on the record of a single specimen showing the characteristic plaited overlap of thecae. It is associated with a faunal assemblage typical of the Tetragraptus phyllograptoides Biozone including its index species.

The modern outcrop Mossebo (cf. Maletz et al. 1996; Bergström et al. 2001) (N58°21’22.2’’; E12°30’44’’) (Fig. 7) was shown as Stora Mossebo on the map of Sidenbladh (1870). A number of smaller exposures of the Lower Ordovician succession can be found poorly exposed to the SE of the Diabasbrottet quarry, where the base of the dolerite forms an uneven surface. Here the Ordovician succession is less extensively exposed and often only a few meters of strongly contact-metamorphosed shales are exposed, bearing a very poor graptolite fauna.

The modern Mossebo section was first described by Tjernvik (1956), who documented the trilobite fauna from the limestone layers in detail and regarded this section as one of his key sections. Löfgren (1993) described the conodont faunas from the limestone layers and differentiated a number of zones. There are minor differences in thickness of the individual shale layers from the nearby Diabasbrottet section, but all limestone intervals can be correlated easily. The metamorphism from the overlying dolerite sill is more pronounced than at Diabasbrottet and the graptolites are generally poorly preserved except for the Tetragraptus phyllograptoides Biozone fauna. Thus, the section has been less collected and many species found at Diabasbrottet have not been documented from the Mossebo section.

Another fairly complete succession through the upper Cambrian to Lower Ordovician, reaching the Baltograptus vacillans Biozone, can be found in the Floklev section of Maletz (1987) (Fig. 2A), but is not described herein, as it was not investigated in detail. Egenhoff & Maletz (2007, fig. 8) illustrated a tepee structure in the upper Megistaspis (Paramegistaspis) planilimbata limestone bed (bed F/G in Maletz et al. 1996) from this locality, quoted only as ‘south of the locality Mossebo’. The section is to the south of the eastern road ascending to the top of the mountain.

Tjernvik (1956) introduced the Hunnebergian substage, based on the succession at Storeklev, SW Hunneberg (Fig. 4), where he found a fauna of ‘undescribed dichograptids’, now known as the Hunnegraptus copiosus Biozone fauna (Lindholm 1991a, b). Tjernvik (1956) understood the Hunneberg and Billingen groups to represent the lower Arenig of Sweden. Jaanusson (1960, table 3) introduced the Latorp Stage for the same interval in limestone facies and regarded the Hunnebergian and Billingenian intervals as substages. Lindholm (1991a) suggested introducing a Hunneberg Series between the Tremadoc and Arenig Series, following Erdtmann (1988) and differentiated a lower and upper Hunnebergian. She defined the interval through the use of conodonts, comprising the Paroistodus proteus and Prioniodus elegans conodont zones. Lindholm (1991b) provided a detailed discussion on the correlation of the Hunnebergian beginning with the Sagenograptus murrayi Graptolite Biozone. Cooper & Lindholm (1990, fig. 1) provided a worldwide correlation of Ordovician graptolite sequences and indicated the Hunnebergian to include the Sagenograptus murrayi to Tetragraptus phyllograptoides biozones. The Hunnebergian is overlain by the Billingenian including the Cymatograptus balticus to Pseudophyllograptus angustifolius elongatus biozones. The Hunnebergian thus starts in the late Tremadocian and ranges into the early Floian of the modern global chronostratigraphy (Goldman et al. 2020). Bergström et al. (2009, fig. 1) erroneously correlated the Hunneberg and Billingen stages of Baltoscandia with the Floian of the international chronostratigraphy and indicated the presence of a GSSP at the base of the Hunneberg stage. They defined the base of the Billingen Group by the Megalaspides dalecarlicus trilobite zone and the Pseudophyllograptus densus graptolite Biozone (see Jaanusson 1960).

Tjernvik (1956) introduced the Billingen Group based on the succession in Västergötland with the name derived from Billingen Mountain, and regarded the Pseudophyllograptus densus Biozone as its base. Tjernvik & Johansson (1980) introduced the Billingen substage and regarded the Cymatograptus balticus Biozone as its basal graptolite biozone. Lindholm (1991a, b) preferred to use the original definition of the Hunneberg and Billingen substages, however, while Maletz & Ahlberg (2018), in their discussion of the Tøyen Shale Formation in Scania, did not discuss the Hunnebergian and Billingenian stages at all. Based on the poor graptolite record and the lack of trilobites in the succession, it is not clear whether the Tøyen Shale Formation at Hunneberg reaches up into the Billingenian stage. If the definition of Tjernvik & Johansson (1980) is used, the uppermost part of the Hunneberg succession, including the Cymatograptus balticus bearing interval, would have to be included in the Billingenian.

The term Alum Shale has commonly been used in Scandinavia for the Cambrian succession of black shales with its typical ‘Orsten Limestone’, but is now understood to also include the lowermost Ordovician black shales. Buchardt et al. (1997) provided the formal definition of the Alum Shale Formation as currently used in the literature and designated a type section in the Gislövshammer-2 drill core of southeastern Scania. Nielsen & Buchardt (1994) described the successsion of the drill core in some detail. The definition of the Alum Shale Formation follows the suggestions of Gee (1972, 1987) and used by Andersson et al. (1985) and Vejbæk et al. (1994) to include the Ordovician alum shales of the Dictyonema and Ceratopyge shales with the Cambrian Alum shales in a single lithostratigraphic unit. Thus, the upper part of the Alum Shale Formation includes the continuation of the alum shale facies into the Ordovician and now includes the ‘Dictyonema Shale’ and ‘Ceratopyge Shale’ of earlier Scandinavian literature. The unit is noteable for the ‘Orsten fauna’ of small phosphatized arthropods (Müller 1982, 1983, 1985; Andres 1989; Maeda et al. 2011), but these faunas have not been discovered in the Hunneberg region so far.

The upper part of the Alum Shale Formation at Hunneberg bears thin layers of black shale with Rhabdinopora specimens or with monospecific faunas of Adelograptus tenellus, indicating a strong condensation (Fig. 8). The graptolites are quite fragmentary and have been discovered only at the western side of the mountain between the localities Nygård and Västra Tunhem/Holsbrotten. Their presence indicates that the uppermost part of the Alum Shale Formation at Hunneberg is Tremadocian in age, and also indicates a considerable gap in the succession, encompasing the upper Cambrian to basal Ordovician. These graptolites may have been preserved in pockets in the undulating surface of the late Cambrian erosion surface within the Alum Shale Formation.

The Bjørkåsholmen Formation (formerly Ceratopyge Limestone) is a thin succession of limestones overlying the Alum Shale Formation (Fig. 8) and separates the Alum Shale Formation from the overlying Tøyen Shale Formation (Owen et al. 1990; Egenhoff et al. 2010). It is well known from the Hunneberg area (Tjernvik 1956; Egenhoff & Maletz 2012). The limestones bear a rich fauna of trilobites referable to the Apatokephalus serratus Biozone (Moberg & Segerberg 1906; Tjernvik 1956; Ebbestad 1999) and conodonts (Löfgren 1993), but other faunal elements have not been described. Graptolites are absent from the unit.

Erdtmann (1965b) initially defined the Tøyen Shale Formation in a temporary tunnel section at Tøyen in the city of Oslo and differentiated the Hagastrand, Galgeberg and Slemmestad members. Owen et al. (1990) defined a neostratotype at Hagastrand in Asker, Oslo Region, but did not accept the Slemmestad Member and differentiated only a lower Hagastrand Member and an upper Galgeberg Member. The name Tøyen Shale Formation has also been used for equivalent lithologies in southern Sweden (e.g. Lindholm 1991a, b; Maletz & Ahlberg 2011; Egenhoff & Maletz 2012). Egenhoff et al. (2019) discussed the highly complex mudstone diagenesis of the Tøyen Shale Formation of Scania, southern Sweden. The authors recognized tempestite laminae and an abundance of trace fossils, indicating that the black shales might not have been deposited in anoxic conditions.

The base of the Tøyen Shale Formation is defined by the base of the unit overlying the Bjørkåsholmen Formation limestones (Owen et al. 1990) and appears to be abrupt in the Norwegian successions in the Oslo Region. Biostratigraphically, the correlation is difficult, as graptolites are generally not present in the Bjørkåsholmen Formation. Maletz et al. (2010) described the late Tremadocian graptolite faunas of the top of the Alum Shale Formation and referred the graptolites to the Kiaerograptus kiaeri/Aorograptus victoriae Biozone. Erdtmann (1965a) described Kiaerograptus stoermeri from a level directly above the main limestone of the Bjørkåsholmen Formation (then the Ceratopyge Limestone), shown as the Kiaerograptus stoermeri Biozone in Maletz et al. (2010). This interval may be regarded as the base of the Tøyen Shale Formation in the Oslo Region of Norway. It is followed by the Kiaerograptus supremus Biozone, which is usually regarded as the lowermost graptolite biozone of the Tøyen Shale Formation in southern Scandinavia (Lindholm 1991a, b; Maletz & Ahlberg 2018).

The Tøyen Shale Formation is quite variably developed in the localities of Hunneberg (Fig. 8), recognizable above the Bjørkåsholmen Formation limestones with its trilobite fauna of the Apatokephalus serratus Trilobite Biozone (Moberg & Segerberg 1906; Tjernvik 1956; Ebbestad 1999). It includes a number of limestone beds, the ‘Armata’ and ‘Planilimbata’ limestones, individual layers of carbonate, up to 10–12 cm thick, bearing moderately diverse Floian trilobite and conodont faunas (Tjernvik 1956; Löfgren 1993; Maletz et al. 1996). The ‘Armata’ and ‘lower Planilimbata’ limestones are quite compact in the Mossebo–Diabasbrottet region and form a unit largely consisting of carbonates more than 1 m thick. Individual beds can be differentiated from the trilobite and conodont faunas. Thin shale beds may be present, but do not bear any recognizable faunas.

The lower part of the Tøyen Shale Formation is a carbonaceous mudstone to marlstone and can be differentiated from the typical Tøyen Shale Formation through its light color. This pale interval is identified as the Hagastrand Member of the Tøyen Shale Formation and is similar to the equivalent interval in the Oslo Region of Norway. It bears a graptolite fauna of the Hunnegraptus copiosus Biozone (Lindholm 1991a), from a single level and few localities at Hunneberg, but initially recognized by Tjernvik (1956) who discussed a ‘large dichograptid’ as the most important faunal element. Lindholm (1991a) described this taxon as Hunnegraptus copiosus. The Hagastrand Member is found only in the western outcrops at Storeklev, Nygård and Holsbrotten (Fig. 2), as the succession is more condensed in the eastern outcrops where little remains of shales of this lower interval, which appears to lack graptolites.

The overlying succession of black shales is rich in graptolites at many levels. These can be referred to several graptolite biozones (Egenhoff & Maletz 2007). The graptolites are well preserved and often found in full relief, filled by pyrite. The Tøyen Shale succession on the NE side of Hunneberg is up to ca 12 m thick, but the Hagastrand Member is not present and the graptolitic succession begins with the Tetragraptus phyllograptoides Biozone.

Maletz (2020a) discussed the preservation of graptolites in some detail and provided examples from the Hunneberg sections. The material shows the profound effects of contact metamorphism. Most of the graptolites are preserved as silvery shining fusellum, contrasting markedly from the black shale (see Bates et al. 2015). Material from the Tetragraptus phyllograptoides Biozone of Diabasbrottet shows the increasing coalification. Higher up in the succession, close to the contact with the dolerite sill, growth of metamorphic minerals has modified the sediment and the fusellum of the graptolites disappears. The baked shales are hard and their color is light grey to yellowish, with graptolites preserved as faint outlines when remains of the fusellum is preserved. Pyrite-filled tubaria are still evident but may no longer be identifiable.

Graptolite faunas with Rhabdinopora flabelliformis have not previously been illustrated from Hunneberg. Neither Linnarsson (1869) nor Sidenbladh (1870) in the initial mapping mentioned the presence of the ‘Dictyonema Shale’ at Hunneberg. However, Lindström (1887, p. 39) and Westergård (1909, p. 34) discussed the record of Dictyonema flabelliforme from Skaktet No. 1 near Nygård by the collector Gustaf von Schmalensee. Fearnsides (1907) mentioned this record, but stated that he was unable to find the species. Maletz (1987) illustrated a single fragment with its typical mesh from Nygård (actually Holsbrotten) on the western side of the mountain, confirming the presence of the Rhabdinopora flabelliformis interval. Due to the fragmentary preservation of the material, it is uncertain which part of the early Tremadocian succession is present. Cooper et al. (1998) and Cooper (1999) indicated the range of Rhabdinopora flabelliformis and its subspecies through the entire Lower Tremadocian, and ranging into the Adelograptus interval of the Upper Tremadocian.

Linnarsson (1871) recognized the level with Adelograptus tenellus at Hunneberg, but referred it the Cambrian Olenus interval (regio Olenorum). Moberg (1892) described the species in some detail and added Bryograptus? hunnebergensis and Bryograptus? sarmentosus from the same level. Maletz & Erdtmann (1987) revised the material and considered the fauna to be based on a single, highly variable species. The authors considered the morphs distinguished by Moberg (1892) to be astogenetic variants of Adelograptus tenellus. Egenhoff & Maletz (2012) showed that the layer with Adelograptus tenellus is a part of the highly condensed Tremadocian succession at Hunneberg. Adelograptus tenellus may be from the basal part of the upper Tremadocian and is clearly younger than the Rhabdinopora flabelliformis fauna, but a precise biostratigraphic assignment of the interval is impossible from the poor record and Maletz & Erdtmann (1987) considered many records of the species in various regions as misidentifications.

Tjernvik (1956) first recognized the Hunnegraptus copiosus fauna from Hunneberg as a separate and special fauna. Lindholm (1991a, b) described it in some detail from southern Sweden and Norway. Initially known only from Storeklev at the south-western side of Hunneberg (Tjernvik 1956), it is now known from two localities at the western and south-western side of the mountain. Identical faunas occur at Storeklev and Holsbrotten/Västra Tunhem at a single level of the Hagastrand Member of the Tøyen Shale (Egenhoff & Maletz 2007, figs 5, 6). A few specimens of Hunnegraptus copiosus are also present in black shale at Nygård and specimens of Paradelograptus norvegicus and Tetragraptus longus have been discovered at other levels at Holsbrotten and Storeklev. Altogether, however, the Hagastrand Member is very poor in graptolites and the typical Hunnegraptus copiosus fauna is restricted to a single layer, less than 0.5 cm thick. Only at Storeklev is the Hagastrand Member overlain by graptolite-bearing shales of the Tetragraptus phyllograptoides Biozone (Egenhoff & Maletz 2007, fig. 6), providing the evidence for the correlation of the faunal succession around this table mountain.

The Floian Stage is based on the rocks overlying the GSSP at Diabasbrottet, one of the most important sections of the Hunneberg region. Maletz et al. (1995, 1996) proposed the First Appearance Datum (FAD) of Paratetragraptus approximatus in the section for the GSSP of the second stage of the Ordovician System and this was subsequently accepted by the International Commission on Stratigraphy. The name Floan Stage was initially proposed (Bergström et al. 2004, p. 271), but later changed to Floian Stage (Bergström et al. 2006). The GSSP of the Floian Stage is correlated with the first appearance of the graptolite Tetragraptus phyllograptoides, a common faunal element in cold-water faunas worldwide, thus useful for wider, global correlation of the Paratetragraptus approximatus Biozone across temperature gradients in the Early Ordovician.

Recently, Bergström et al. (2020) provided the first geochemical data for the GSSP section and identified a δ¹³C excursion that correlated with biostratigraphic data. The authors interpreted their data to show the base of the Floian between the LTNICE and BFICE δ¹³C excursions recognized in other regions. The work, thus provides new chemostratigraphic evidence for the worldwide correlation of the Diabasbrottet GSSP section in addition to the well established biostratigraphic correlation.

The Paratetragraptus approximatus Biozone was introduced by Maletz et al. (1996) in the discussion of the proposed GSSP at Diabasbrottet, but the local Tetragraptus phyllograptoides Biozone is preferred here as this index species is more common and is easily recognized. The Paratetragraptus approximatus Biozone is a widely used Lower Ordovician graptolite biozone (Berry 1992; Maletz 1999), but has rarely been used in Scandinavia in the past, even though the species has been reported from Sweden by Törnquist (1901) and was also found in the Oslo region of Norway by Monsen (1937). The biozonation of Monsen (1937) for the Oslo sequence was not based on observation of precise biostratigraphic ranges of the graptolite species, but largely on faunal associations. She named the Tetragraptus phyllograptoides and Paratetragraptus approximatus biozones as the lowermost graptolite zones of the Lower Didymograptus Shale. In the lower zone, the Tetragraptus phyllograptoides Zone, she also found specimens of Sagenograptus murrayi.

The base of the Tetragraptus phyllograptoides Biozone can be recognized in the Diabasbrottet and Mossebo sections (Figs 6, 7), where the species occurs together with Paratetragraptus approximatus. The connection to the underlying Hunnegraptus copiosus Biozone is documented from the Storeklev and Holsbrotten sections (Figs 4, 5), in which the Hunnegraptus copiosus Biozone was recognized in the greenish shales at the base of the Tøyen Shale Formation. Tetragraptus phyllograptoides and Paratetragraptus approximatus can be found in the higher part of the succession, here in strongly metamorphosed strata.

Paratetragraptus approximatus ranges through most of the succession in the Diabasbrottet section and the highest records are from the Baltograptus vacillans Biozone (Fig. 6). It is uncertain whether this is the top of its range in Västergötland, but the younger fossil record is very poor due to the contact metamorphism from the overlying dolerite sill. Tetragraptus phyllograptoides is restricted to the lower part of the Paratetragraptus approximatus interval. A number of two-stiped graptoloids are common in the lower part of the Tetragraptus phyllograptoides Biozone, including the highly characteristic Cymatograptus undulatus and a number of other species referred to the genus Cymatograptus. The two-stiped taxa may be the earliest didymograptids in the fossil record and demonstrate the diversification of Cymatograptus in the higher part of the Tetragraptus phyllograptoides Biozone. A single-stiped form also appears here, assigned to Azygograptus in the past but here identified as Cymatograptus validus (Törnquist, 1904).

Rare specimens of large dichograptids, including Holograptus expansus and Paratemnograptus magnificus, have been discovered in the Tetragraptus phyllograptoides Biozone. Horizontal to somewhat reclined and declined tetragraptids are common, including the highly characteristic three-stiped Tetragraptus gerhardi n. sp. Tshallograptus simplex may be regarded as a predecessor of the slightly younger Tshallograptus fruticosus and can be differentiated from Tetragraptus specimens in the same interval by a proportionally longer and more slender sicula.

Egenhoff & Maletz (2007) used the concept of the Cymatograptus protobalticus Biozone for the Hunneberg sections and the interval was subsequently defined by Maletz & Ahlberg (2011). The Cymatograptus protobalticus Biozone is defined by the first appearance of the index species in the Diabasbrottet section (Fig. 6), where it is associated with Baltograptus geometricus. The interval is widely distributed in southern Scandinavia and is easily recognizable through its graptolite fauna. The higher part of the zone is characterized by the appearance of Expansograptus species, while in the lower part only members of the genera Cymatograptus and Baltograptus are found. The species Baltograptus geometricus is the most common two-stiped graptoloid in the interval and ranges into the lowermost part of the Baltograptus vacillans Biozone. A number of multiramous members of the Dichograptidae and Sigmagraptidae have been discovered in the Cymatograptus protobalticus Biozone, but are usually uncommon.

Maletz & Ahlberg (2011) erected the Baltograptus vacillans Biozone that had been used informally by Egenhoff & Maletz (2007) for the Hunneberg succession. The base of the zone is defined by the first occurrence of its index species at Diabasbrottet (Fig. 6) that quickly becomes one of the most common members of the fauna. The zone shows a further increase in diversity of the Early Ordovician graptolite faunas and a number of new taxa appear in the interval. The Baltograptus vacillans Biozone is also characterized by a rich and diverse fauna of expansograptids. The intraspecific variation of these expansograptids has not yet been evaluated, but a number of taxa can be differentiated and more have been described from other regions in the world. The species of the genus Expansograptus are uncommon elements in the earlier zones, but here become the dominant faunal elements. The most common dichograptid is Clonograptus multiplex, but other species of the genus, as well as Schizograptus and the sigmagraptine Trichograptus are not uncommon in some layers. Slender two-stiped taxa that are here referred to the sigmagraptine genus Acrograptus are present in the higher part of the Baltograptus vacillans Biozone.

Maletz & Ahlberg (2011) introduced the Baltograptus sp. cf. Baltograptus deflexus Biozone for the interval following the Baltograptus vacillans Biozone in the Lerhamn drill core of Scania. The base of the interval is recognized by the first appearance of truly deflexed Baltograptus specimens. As this taxon is now known as Baltograptus jacksoni (Rushton, 2011), the name of the zone has been changed to the Baltograptus jacksoni Biozone (Fig. 6). The base of the zone is difficult to define in the Diabasbrottet section, as the faunas of the interval are poorly preserved and largely destroyed through contact metamorphism. Only parts of the graptolite colonies that were filled with pyrite are preserved in the highest layers of the section and the apertural parts of the thecae are missing, so that the true shape and dimension of the tubaria is not recognizable. A few poorly preserved graptolites have been found in the highly contact metamorphosed shales, including a number of multiramous taxa. At a single level, specimens of the genus Goniograptus were found, a taxon that is extremely rare in Scandinavia.

The graptolite biostratigraphy of the Tøyen Shale Formation has recently been described and correlated for Scania, southern Sweden in some detail (Maletz & Ahlberg 2018), but the succession at Hunneberg includes only the central part, from the latest Tremadocian to the mid-Floian (Fig. 10). The Scanian succession is much more complete than the Hunneberg one. It ranges from the late Tremadocian Kiaerograptus supremus Biozone to the Levisograptus sinicus Subzone of the Levisograptus austrodentatus Biozone and includes 14 biozones and subzones. It is known largely from a number of drillcores and has been pieced together from incomplete and tectonically disturbed successions. Late Floian to Dapingian graptolite faunas are known from other successions in Västergötland, but appear to be more strongly condensed.

The succession at Kinnekulle (Västergötland) is incompletely known. Graptolite faunas of the formation have been noted by Linnarsson (1866, 1869), indicating a thickness of the unit of ca 30 feet. Törnquist (1901, 1904) listed the zone of Phyllograptus densus from Martorps-klef, where the graptolites occur in thin black shales within a green shale unit. This material, however, was not illustrated, nor was Didymograptus hirundo in the upper part of the formation at Hällekis (Tjernvik 1956, fig. 14), which probably indicates a mid to late Dapingian age. Skoglund (1961) was the first to describe and illustrate graptolites from Kinnekulle. He erected the genus Kinnegraptus and described two species, Kinnegraptus kinnekullensis and Kinnegraptus multiramosus from chemically isolated material showing growth lines and further details of the development of the colonies. He listed a number of additional genus level taxa, but these were not described. Lindholm (1992) noted the presence of Tetragraptus phyllograptoides at Kinnekulle, but the material was never described and has not been found in the collections at Lunds University. The presence of the Tetragraptus phyllograptoides Biozone at Kinnekulle therefore, remains questionable.

The succession in Dalarna (Siljan impact) is poorly known and the faunas have not been described in detail. Törnquist (1876) described a section at Skattungbyn (later known as the Talubäcken section). Törnquist (1879, 1890) documented the graptolite fauna and erected a number of new species from the succession. Two of these, Baltograptus minutus (Törnquist, 1879) and Pseudophyllograptus densus (Törnquist, 1879), subsequently became index taxa for Floian graptolite biozones. Törnquist (1883) listed a number of taxa from the Tøyen Shale Formation of Dalarna and Warburg (1910) extended the list considerably. Skoglund (1968) indicated the presence of chemically isolatable graptolites at Skattungbyn. Maletz & Slovacek (2013) and Schulze (2018) provided the first descriptions of the material. Maletz (2023) described the remaining material from the Skoglund collection of isolated graptolites. There is no evidence so far for the presence of early Floian graptolites in Dalarna.

Graptolites from the Tøyen Shale Formation are present in Jämtland, central Sweden, but have rarely been discussed or illustrated, so that the completeness of the succession is unknown. Wiman (1893, p. 263) regarded the formation at Tossasen, Jämtland as the ‘classical area of the Phyllograptus Shale in Jämtland’ (see Bulman, 1950a, p. 389) and cited a number of taxa, but did not describe or illustrate them. Wiman (1899) suggested that the formation (Unterer Graptolithenschiefer) may be present everywhere in the region and listed a number of taxa, including Tshallograptus fruticosus. Bulman (1950a) described the well-preserved fauna and Maletz (2004) re-identifed the species described as Holmograptus callotheca by Bulman (1950a) with the mid-Floian Maeandrograptus sinosus Maletz, 2004. Tjernvik (1956) indicated the presence of graptolites in the Tøyen Shale Formation of Jämtland and even listed Tetragraptus phyllograptoides from Anderson Island in Lake Storsjön, providing evidence for the age of the base of the formation. Beckly & Maletz (1991, pl. 1, fig. 1) illustrated Azygograptus ellesi Monsen, 1937 from Nipan, Jämtland and Maletz (1994) illustrated a specimen of Tshallograptus flagellifer from the same locality. The presence of Tshallograptus fruticosus, Pseudophyllograptus and Maeandrograptus sinosus indicates that the higher part of the Floian or even the Dapingian may be present in the region.

Monsen (1937) described the graptolite fauna of the Tøyen Shale succession of Norway and erected numerous new species. She recognized the Tetragraptus phyllograptoides Biozone at the base of the unit and listed the Phyllograptus angustifolius elongatus Biozone as the youngest biozone of the region. The Tetragraptus phyllograptoides Biozone included specimens of Dictyonema cf. D. murrayi and Dictyonema macgillivrayi (now Sagenograptus) and Tetragraptus (now Paratetragraptus) approximatus. P. approximatus, together with T. phyllograptoides, ranges upwards into the P. approximatus Biozone, so that the two intervals correlate with the Tetragraptus phyllograptoides Biozone at Hunneberg. The overlying Didymograptus validus and Didymograptus balticus biozones can be correlated with the Cymatograptus protobalticus to Baltograptus vacillans biozones. It needs to be pointed out that the Monsen material was not collected from precise levels and the assignment to a certain biozone is based in many cases on the interpretation of the taxa present on the same slabs. Precise correlation of the Tøyen Shale Formation in the Oslo Region therefore remains difficult. Erdtmann (1965b) described a new section in the city of Oslo, but did not provide precise ranges of taxa and did not describe the faunas. Maletz (2011) discussed the late Floian to Dapingian succession of the Oslo region by using isograptid faunal elements. He recognized the Isograptus mobergi & Maeandrograptus schmalenseei Biozone as the youngest graptolite zone of the Tøyen Shale Formation below the Komstad Limestone.

The British standard succession (Fig. 10) is well known and has been revised most recently by Zalasiewicz et al. (2009). It appears to be fairly incomplete and has few faunal elements, so that biostratigrapically useful successions showing precise ranges and boundaries between the individual biozones are unknown. The late Tremadocian faunas are sparse and only the Sagenograptus murrayi Biozone is recognized in the Skiddaw Slates (Cooper et al. 1995), where specimens of Sagenograptus (previously Araneograptus: Maletz et al. 2017) are associated with possible didymograptids (Molyneux & Rushton 1988). A more precise identification of these is impossible due to their poor preservation and tectonic deformation. Cooper et al. (1995) identified the Tetragraptus phyllograptoides Biozone through the presence of Cymatograptus protobalticus, Cymatograptus rigoletto and Clonograptus multiplex, but this interval is here correlated with the Cymatograptus protobalticus Biozone (Fig. 10), as Tetragraptus phyllograptoides is not present and in any case would be found at a lower biostratigraphic horizon. A considerable biostratigraphic gap therefore occurs between the late Tremadocian and early Floian faunas, as no graptolite faunas of this interval have been found. Paratetragraptus approximatus occurs at Ballantrae, Scottland, associated with Tshallograptus fruticosus (Stone & Rushton, 1983), indicating probable correlation with the Baltograptus vacillans to Baltograptus jacksoni Biozone, as the Paratetragraptus approximatus Biozone does not include Tshallograptus fruticosus.

The Corymbograptus varicosus Biozone fauna of Zalasiewicz et al. (2009) can be correlated with the Baltograptus jacksoni Biozone (Rushton 2011). The base of the interval in Britain is not defined, but the fauna shows a higher diversity with the appearance of a number of new faunal elements. The Expansograptus simulans Biozone shows a high diversity and yields a number of characteristic faunal elements (e.g. Azygograptus, Baltograptus minutus), indicating a long late Floian to early Dapingian time interval (Fig. 10). Zalasiewicz et al. (2009) indicated that the interval straddles the Floian‒Dapingian boundary. The overlying Isograptus victoriae Biozone is characterized by the index species, but the species is rare in the Lake District. The specimen illustrated by Zalasiewicz et al. (2009, fig. 66) may be Isograptus rigidus Maletz, 2011, known mainly from Scandinavia (Spjeldnaes 1953; Maletz 2011). The specimen was first illustrated as Isograptus victoriae by Fortey et al. (1990, fig. 6K).

Gutiérrez-Marco et al. (2017) discussed the chronostratigraphy of the region and provided some general information on the faunal successions (Fig. 10). They did not, however, discuss the graptolite succession of the Tremadocian to Arenigian (Floian‒Dapingian) interval in any detail. The lower Tremadocian interval with Rhabdinopora has been identified in a number of papers in North Africa. Legrand (1974) described the quadriradiate proximal development in intermediate forms of Rhabdinopora socialis from the Lower Ordovician of the Algerian Sahara and noted the presence of somewhat younger strata with Adelograptus (Legrand 1964a, b). Gutiérrez-Marco & Martin (2016) discussed the succession of the Tremadocian‒Floian interval of southern Morocco and provided an overview of the faunas, including the important faunas of the Fezouata Biota. In general, the successions are fairly incomplete and the fossil record has to be pieced together from many localities. Thus, precise ranges have not been established for the faunas. Further study, however, may show that the graptolite sequence is more complete. Two specimens with subhorizontal, slender stipes have been identified as Hunnegraptus copiosus by Martin et al. (2016), but the identification remains in doubt until better material is available. Gutiérrez-Marco et al. (2016) discussed the Hunnegraptus copiosus Biozone of the Fezouata lagerstätte, but did not illustrate the material. Gutiérrez-Marco & Martin (2016) inserted the Tetragraptus phyllograptoides Biozone in their scheme for a barren interval in the Fezouata Biota, even though the taxon has not been collected and the interval is known only from a single specimen of Tetragraptus phyllograptoides collected in the Sinclinal de Valle (Sevilla, Spain) (Gutiérrez-Marco et al. 1984). These authors also illustrated a poorly preserved specimen as Cymatograptus cf. C. protobalticus from Fezouata, representing the Cymatograptus protobalticus Biozone of Scandinavia. Deflexed and pendent Baltograptus specimens may represent the Baltograptus jacksoni to Baltograptus minutus biozones. An interval with Azygograptus eivionicus may represent the early Dapingian, but a more precise determination of the age of this interval is not possible. Typical examples of the presence and preservation of Lower Ordovician graptolite faunas of the Bohemo–Iberian area can be found in the Czech Republic (see Bouček 1973), but also include isolated records like Azygograptus undulatus in northern Spain (Gutiérrez-Marco & Rodriguez 1987).

Phillipot (1950) illustrated a deflexed didymograptid under the name Didymograptus falco Phillipot, 1950 from the French ‘Massif Armoricain’. The species is here regarded as synonymous with Baltograptus deflexus. The illustrations in Phillipot (1950, p. 239) indicate considerable tectonic deformation of the material, making the identification difficult. This is also true for material considered as belonging to the Didymograptus murchisoni group (Phillipot, 1950, fig. 21). Other Early Ordovician graptolites were not illustrated.

Iran as part of Gondwana (cf. Ghorbani 2021) is not well known for the presence of early Ordovician graptolite faunas. Rushton et al. (2021) described small faunas from the Rhabdinopora flabelliformis, Hunnegraptus copiosus and Cymatograptus protobalticus biozones, including the widely distributed Baltograptus geometricus. The faunas indicate that the successions could be more complete and new records can be expected.

Kaljo (1974) discussed the Tremadocian‒Arenigian succession of the Peribaltic and Moscow syneclises and recognized the succession as similar to the one in southern Sweden. Following the Clonograptus cf. tenellus Biozone of late Tremadocian age is the Paratetragraptus approximatus + Tetragraptus phyllograptoides Biozone, overlain by the Didymograptus balticus Biozone and the Phyllograptus densus Biozone (probably correlating with the Azygograptus volkhovensis Biozone of Koren’ et al. 2004). As the faunas were not illustrated, some doubt remains about the identity of the taxa and the biostratigraphic ranges (Fig. 10). The information was supported by the findings of Ulst (1976), but the fauna of the Tetragraptus phyllograptoides Biozone of this general region was first described and illustrated by Tolmacheva et al. (2001). The authors documented the Tetragraptus phyllograptoides Biozone of the St. Petersburg area. Tetragraptus phyllograptoides and Didymograptus rigoletto are found in the lower part of the interval, while Tetragraptus phyllograptoides is associated with Cymatograptus sp. cf. C. protobalticus in the higher part of the zone. Younger Floian graptolite faunas have not been found. Paškevičius (2011) discussed Floian‒Dapingian graptolites from Lithuania, but the material has not been illustrated. Koren’ et al. (2004) described the Dapingian succession of the St. Ptersburg region. The oldest faunal elements include Azygograptus volkhovensis Koren et al., 2004, but the majority of the faunas are younger and can be referred to the Pseudophyllograptus angustifolius elongatus to Expansograptus hirundo biozones. The presence of Azygograptus volkhovensis may indicate an early Dapingian age, but a comparison with other species of Azygograptus is not possible and the species must be regarded as endemic at the moment.

Williams & Stevens (1988) described the Lower Ordovician graptolites of the Cow Head Group of western Newfoundland (Fig. 10), the most complete succession known from North America. The faunas are documented in great detail and numerous species are described from chemically isolated material. The sequence begins with the late Tremadocian Aorograptus victoriae Biozone (Williams & Stevens 1991), followed by the early Floian Paratetragraptus approximatus Biozone. Maletz et al. (1996) discussed the presence of a biostratigraphic gap between these zones, based on the faunal succession at Hunneberg, covering the Sagenograptus murrayi to Tetragraptus phyllograptoides Biozone interval. A part of this gap may be closed in eastern North America by the Sagenograptus murrayi Biozone found in Québec (Hall 1865; Maletz 1997a). However, the Hunnegraptus copiosus Biozone may be missing. Hunnegraptus novus has been recognized in the succession of Texas (Maletz 2006) and Hunnegraptus copiosus has been described from northern Yukon by Jackson & Lenz (2003). The Paratetragraptus approximatus Biozone of the Cow Head Group bears a poor and low diversity fauna in which Paratetragraptus approximatus is the most important faunal element. Interestingly, Cymatograptus sp. cf. Cymatograptus protobalticus is present in the upper part of the interval, recorded as Didymograptus (Expansograptus) latus (Hall, 1907). It is the only species of Cymatograptus found in North America and indicates correlation with the Cymatograptus protobalticus Biozone of Scandinavia. The Tetragraptus akzharensis Biozone may correlate with the upper part of the Cymatograptus protobalticus Biozone and the Baltograptus vacillans Biozone because of the presence of the common Expansograptus specimens. The base of the Tshallograptus fruticosus Biozone is based on the FAD of its index species, but Williams & Stevens (1988) indicate that the species may have originated earlier. This fits with the occurrence of earlier Tshallograptus in the Paratetragraptus approximatus Biozone (see also VandenBerg 2017). Proximal ends of these may be impossible to distinguish from true Tshallograptus fruticosus. The overlying Didymograptellus bifidus Biozone should in general correlate with the Baltograptus minutus Biozone of Scandinavia. The correlation of the Isograptus lunatus and Isograptus victoriae biozones was discussed by Maletz (2010).

The succession in the western part of North America (Fig. 10) seems quite different, at least in the late Tremadocian. Jackson & Lenz (1999, 2000, 2003, 2006) described the late Tremadocian to Floian graptolites of the Yukon territory in some detail and established a quite detailed biostratigraphy. The Psigraptus–Adelograptus Biozone of Yukon (Jackson & Lenz 1999) can be correlated with the Kiaerograptus kiaeri Biozone based on the presence of Ancoragraptus, even though Psigraptus has never been discovered in Scandinavia. Jackson & Lenz (2000) differentiated a number of local biozones in the late Tremadocian based on species of the genus Paradelograptus. Jackson & Lenz (2003) made the first record of Hunnegraptus copiosus in Yukon and placed a Hunnegraptus copiosus Biozone below the Lignigraptus kinnegraptoides Biozone. Jackson & Lenz (2006) identified the Paratetragraptus approximatus Biozone followed by the Tshallograptus fruticosus Biozone in the Yukon sections. The faunas show a relatively low diversity of Tetragraptus and Paradelograptus species. Expansograptus appears in the lower part of the Lower Canyon of Peel River indicating the presence of an interval correlating with the Tetragraptus phyllograptoides or Cymatograptus protobalticus Biozone, while the higher part with its characteristic Expansograptus species may represent the Baltograptus vacillans Biozone or a higher interval. The Tshallograptus fruticosus Biozone is indicated by its index species associated with a number of new taxa appearing in this interval.

Very little is known of the graptolitic succession of Vietnam and few taxa have been listed from the region, but Ordovician to Devonian graptolite faunas have been mentioned from scattered outcrops in the region (see Van Phuc 2002; Thanh & Khuk 2011). Hung et al. (2017) described a poorly preserved Floian graptolite fauna including Paratetragraptus approximatus, Expansograptus urbanus and Expansograptus abnormis from the Dinh Ca Formation of NE Vietnam. The fauna shows a low diversity and few taxa are present. It was referred to a local Tetragraptus quadribrachiatus Biozone and may be correlated with the Baltograptus vacillans Biozone of Scandinavia due to the presence of the Expansograptus species. Rushton et al. (2018) described a similar small fauna of Floian age from the Than Sa Formation of NE Vietnam, bearing Paratetragraptus approximatus associated with a number of expansograptids. A slender declined species was identified as ‘Didymograptus’ sinensis? Its poor preservation precludes a more precise identification, but the specimens may be a declined Baltograptus species intermediate between Baltograptus geometricus and Baltograptus vacillans. The material is certainly from a level above the Tetragraptus phyllograptoides Biozone, in which no horizontal Expansograptus species occur. The authors suggest a correlation with the Be1 of Australasia, even though Tshallograptus fruticosus is not present. Further graptolite records are not known from the region.

Zhang et al. (2019) discussed the Ordovician time scale for China and provided the latest information on the graptolite biozonation of the region (Fig. 10). However, only some of the biostratigraphic intervals were discussed in the paper. Zhang et al. (2005) recognized the Aorograptus victoriae, Acanthograptus sinensis and Hunnegraptus copiosus biozones in the late Tremadocian of South China, with a gap between the Aorograptus victoriae and Acanthograptus sinensis biozones. Feng et al. (2009) revised the succession based on the Nanba section, Hunan Province, and recognized the Adelograptus tenellus, Aorograptus victoriae, Araneograptus (now Sagenograptus) murrayi and Hunnegraptus copiosus biozones in the late Tremadocian, followed by the basal Floian Paratetragraptus approximatus Biozone. Wang & Wang (2001) introduced the Hunnegraptus copiosus–Clonograptus s.s. Biozone to the succession of South China and Zhang et al. (2003, 2004) first recognized the genus Hunnegraptus in China. A number of small specimens have been identified as Hunnegraptus novus (Berry, 1960) and Feng et al. (2009) identified other juvenile specimens from the Nanba section as Hunnegraptus copiosus. Mature specimens of Hunnegraptus copiosus supporting the identification are unknown from China.

Zhang et al. (2019) provided an overview on the Floian succession with five biozones (Fig. 10), which were not discussed in detail. The basal Floian Paratetragraptus approximatus Biozone is widely distributed in South China and bears a diverse graptolite fauna of tetragraptids and dichograptids (see references in Chen et al. 1983). The lack of extensiform didymograptids (genus Expansograptus) in many faunas may indicate that the interval can be correlated with the Tetragraptus phyllograptoides to lower Cymatograptus protobalticus biozone interval of Scandinavia. The Pendeograptus fruticosus–Acrograptus filiformis Biozone can be correlated with the Baltograptus vacillans to Baltograptus jacksoni biozones as both index species are common here.

Zhang & Zhang (2014) described deflexed Baltograptus species from the Baltograptus varicosus Biozone of Eastern Yunnan, South China and correlated it with the mid-Floian Tshallograptus fruticosus to Didymograptus eobifidus Biozone of other regions of South China. The authors indicated the presence of a Baltograptus deflexus Biozone above the interval. The preservation of many specimens is quite good, even though the specimens are flattened and details of the proximal development are rarely visible. The material may originate from an equivalent of the Baltograptus jacksoni Biozone based on the presence of Expansograptus similis and Expansograptus constrictus, but a precise correlation is impossible, as these wide-stiped Baltograptus taxa are unknown from Scandinavia and may be endemic. However, wide-stiped deflexed taxa are also present in Argentina and Bolivia (see below). The Didymograptellus eobifidus Biozone can be correlated with the Didymograptus bifidus Biozone of North America or the Didymograptellus kremastus Biozone of Australasia.

North China, including the Tarim region, has very little record of Lower Ordovician graptolite faunas (Fig. 10). Zhang & Erdtmann (2004) and Zhang et al. (2005) discussed the Tremadocian graptolite succession of North China, especially of Jilin Province, and introduced the Aorograptus victoriae Biozone above the Psigraptus jacksoni Biozone. The Aorograptus victoriae Biozone is recognized by the presence of the index species, associated with numerous dendroids. No other planktic graptoloids are present and a precise correlation of this interval is not possible. The higher part of the biozone is characterized by the genus Kiaerograptus and probably correlates with the Bryograptus ramosus to Kiaerograptus kiaeri biozones of Scandinavia (Maletz et al. 2010). The top of the biozone cannot be determined with certainty, leaving a gap in the late Tremadocian faunal succession. Zhang et al. (2019) named the latest Tremadocian as the Clonograptus s.s. Biozone, but did not discuss it.

Paratetragraptus approximatus has not been recognized in North China (Zhang et al. 2019), where the base of the Floian is therefore difficult to identify. Xu & Huang (1979) described the Paratetragraptus approximatus Biozone from Xinjiang, but the succession appears to be highly incomplete and the next younger fauna belongs to the late Dapingian Oncograptus upsilon–Cardiograptus morsus Biozone. The fauna of the Paratetragraptus approximatus Biozone includes a number of tetragraptids, clonograptids and expansograptids, some of them described as new species. The presence of the typical extensiform Expansograptus holmi may indicate correlation with the Baltograptus vacillans Biozone of Scandinavia.

Chen et al. (2012) indicated a considerable biostratigraphic gap in the eastern Tienshan Mountains, where the Kiaerograptus kiaeri–Aorograptus victoriae Biozone is folllowed by the Expansograptus abnormis Biozone of latest Floian to early Dapingian age in the Heituao Formation. Thus, the Floian and some of the Dapingian biozones are not present. Zhang et al. (2019) combined the succession of North China and Tarim and indicated the presence of the Paratetragraptus approximatus Biozone in the Liangchiashan Formation, but noted the next younger graptolite biozone as that of Expansograptus abnormis from the latest Floian (Fig. 10).

The Early Ordovician graptolite faunas of South America can be referred to two distinct realms. Those of the Argentine Precordillera belong to the Pacific faunal realm, whereas the remaining South American faunas are of the the Atlantic faunal realm (Maletz & Ortega 1995). Goldman et al. (2013) preferred the terms high and low latitude realms instead of Atlantic and Pacific, as the latter refer to modern biogeography and Goldman’s terms will be used herein. The presence of the Didymograptellus bifidus Biozone in a number of sections of the Eastern Cordillera and the Famatina Range of Argentina (Toro 1994, 1997a, b; Toro & Brussa 1997) may indicate a more differentiated picture, but a number of questions remain unresolved.

The Early Ordovician graptolite faunas of the eastern Cordillera of Argentina have been covered in numerous publications and only few of these are discussed here. Paratetragraptus approximatus has been reported from several localities in South America, especially from the Cordillera Oriental of Argentina (Martin et al. 1987), but it seems that the presence of Tetragraptus phyllograptoides is the most indicative evidence of the base of the Floian in South America. Moya et al. (1994) described a number of faunal associations from the eastern Cordillera of Argentina that range from the basal Tremadocian to the early Arenig (Floian). Specimens identified as Isograptus sp. (Moya et al. 1994: pl. 4, figs 9, 10) and referred to the mid-Arenig Isograptus Biozone (assemblage 10) were supposedly among the youngest graptolites recorded from the region. However, they must be reassigned to Tetragraptus phyllograptoides of basal Floian age, as is the specimen (Moya et al. 1994, pl. 4, fig. 12) found in assemblage 11. Faunal assemblage 9 contains Sagenograptus murrayi, identified as Dictyonema yaconense by Moya et al. (1994). Moya et al. (1998) used graptolites of the Tetragraptus phyllograptoides Biozone to date the Tumbaya unconformity in the Eastern Cordillera of the Argentinian Andes. Ortega et al. (1998) described further graptolite faunas from the Tetragraptus phyllograptoides Biozone of the eastern Cordillera of Argentina.

Toro (1993, 1994, 1996, 1997a) initially documented the succession from the Eastern Cordillera in more detail and Toro et al. (2015) provided the most complete overview of the succession (Fig. 10), that is easily correlated with Scandinavia trough the presence of Tetragraptus phyllograptoides and the indicative Baltograptus species of the Floian. The presence of Baltograptus turgidus (Lee) and Baltograptus kunmingensis (Ni) in the Baltograptus deflexus Biozone of Bolivia is important for the correlation of this interval and the first time that these conspicuous faunal elements were described from outside China. The species of the Baltograptus calidus group (cf. Maletz 1994) were previously known only from southwest China, but not described from established successions. Toro (1999a, b) also described the peculiar Baltograptus bolivianus (Finney & Branisa, 1984) for the first time from Argentina. Toro (1994, 1997a) recognized the Didymograptellus bifidus zone faunas as the youngest faunas from the Eastern Cordillera. The presence of the low-latitude type Didymograptellus bifidus in this region of apparent high latitude faunas still needs to be explained. Further work has refined the biostratigraphy of the Eastern Cordillera (Toro & Vento 2013), showing the presence of the late Tremadocian Hunnegraptus copiosus Biozone (Toro & Brussa 2003; Toro et al. 2010) to the mid-Floian deflexed Baltograptus species (cf. Toro & Maletz 2007, 2008; Toro et al. 2011, 2015).

Toro (1997b) described two faunal associations from the La Alumbrera section in the Famatina range, but did not assign zonal names. The lower fauna includes a number of Tetragraptus species, of which Tetragraptus phyllograptoides is the most important one. The specimens are relatively slender, but appear otherwise typical of this species. The association with Didymograptus (Expansograptus) sp. (Toro 1997b, pl. 1, fig. 3) and Tetragraptus akzharensis might indicate that the fauna comes from a high level in the Tetragraptus phyllograptoides Biozone. The occurrence of Tetragraptus phyllograptoides here may be slightly younger than the Scandinavian occurrences. Toro & Brussa (1997) described slightly younger faunas of the Baltograptus deflexus and Didymograptellus bifidus biozones from the Saladillo River gorge. Unfortunately, there is no good succession of faunas between both zones. However, the presence of Phyllograptus sensu stricto and Didymograptellus bifidus clearly indicates the shift from high to low latitude region faunas in the higher part of the succession.

The Lower Ordovician graptolite faunas of southern Bolivia have also been studied more recently. Maletz et al. (1995, 1999) initially described the lower Ordovician graptolites from southern Bolivia and realized the similarity of the faunas to those of southern Scandinavia. Egenhoff (2000) discussed the sedimentology of the extremely thick succession of southern Bolivia and, with the biostratigraphical investigation and correlation by Maletz & Egenhoff (2001, 2003) and Egenhoff et al. (2004), was able to interpret the basin evolution based on the widely scattered and poorly preserved graptolite faunas. The faunas range from the basal Ordovician Rhabdinopora flabelliformis Biozone to the lower Dapingian interval with Azygograptus lapworthi. The index of the basal Floian Paratetragraptus approximatus has been reported from several localities (Maletz & Egenhoff 2001; Egenhoff et al. 2004) and is associated with Tetragraptus phyllograptoides. Altogether the succession was considered to be closely comparable to the Scandinavian successsion and many of the faunal elements suggested to be endemic for Scandinavia by Egenhoff & Maletz (2007) were recognized in the region. Toro & Maletz (2018) provided a current overview on the graptolite faunas of Bolivia, including also the upper Ordovician to Silurian faunas.

The Lower Ordovician graptolite succession of Australasia (Fig. 10) has been revised by VandenBerg & Cooper (1992) and updated by Percival et al. (2011). It consists largely of a number of isolated levels with distinct graptolite faunas, but a precise and detailed biostratigraphic succession is not established. Thus, precise boundaries between the faunal associations cannot be provided. The late Tremadocian is represented by the Aorograptus victoriae Biozone (La2), an interval often identified as the Sagenograptus murrayi Biozone in other regions (Lindholm 1991a; Gutiérrez-Marco & Martin 2016) due to this more characteristic faunal element. The Floian Paratetragraptus approximatus Biozone (La3) is characterized by its index species and marks also the first occurrence of extensiform didymograptids (VandenBerg & Cooper 1992, p. 39). It might therefore be correlated with a level in the Cymatograptus protobalticus or even Baltograptus vacillans Biozone of Scandinavia, but not with the basal Floian, as in this interval no extensiform didymograptids have been discovered. The Bendigonian is differentiated into four intervals, based on the number of stipes in Tshallograptus fruticosus (VandenBerg & Cooper 1992; VandenBerg 2017). The differentiation is impossible to follow in most regions, however, but the whole interval may be correlated with the Baltograptus vacillans to Baltograptus jacksoni biozones of Hunneberg, in which specimens of Tshallograptus fruticosus are uncommon. The higher interval is not discussed herein, but it is noted that the lower Chewtonian (mid-Floian) of Australasia is now known as the Didymograptellus kremastus Biozone (VandenBerg 2018). Didymograptellus kremastus VandenBerg, 2018 is very similar to Yutagraptus mantuanus, but not to Didymograptellus bifidus, but the FAD of both taxa might have been at approximately the same level (see Maletz 2010, text-fig. 7). Didymograptellus bifidus is extremely rare in the Chewtonian of Australasia contrary to common understanding (VandenBerg 2018). Didymograptus protobifidus Elles, 1933 was previously used as the index for the lower Chewtonian, but considerable concerns exist about the identity of this species (see Williams & Stevens 1988) and its type material may have originated from the lower Darriwilian.

Diagnosis. – Graptolites united by the retention of the nematophorous sicula in the adult stage as the defining synapomorphy (Maletz et al. 2017a, p. 1).

Remarks. – The diagnosis is in accordance with the definition of Fortey & Cooper (1986), who were the first to define the Graptoloidea to include all planktic graptolites. The presence of bithecae has long been taken as an important character to differentiate the benthic Dendroidea from the planktic Graptoloidea, but many early planktic taxa of the Anisograptidae and even the Sigmagraptidae bear bithecae along the stipes or at least a sicular bitheca.

Diagnosis. – Quadriradiate to biradiate planktic, nematophorous graptolites with a triad budding system of the thecae; bithecae reduced or even lost in stratigraphically younger taxa; colonies multiramous to biramous, pendent to reclined; dissepiments present in some taxa; autothecae simple, widening tubes with or without short ventral rutellum; thecal apertures may be isolated; prosicula with distinct differentiation of conus and cauda; origin of th1¹ in the middle part of the conus; proximal end may possess one to three successive (proximal) dicalycal thecae with th1² as the first dicalycal theca; later dicalycal thecae adventitiously or regularly distributed (Maletz et al. 2017, p. 2).

Remarks. – Mu & Lin in Lin (1981) introduced the Graptodendroidina for planktic forms showing intermediate thecal features between the benthic Dendroidea and the planktic Graptoloidea, assembled in the family Anisograptidae. All taxa bear a nematophorous sicula and bithecae along the stipes, thus show typical dendroid characters combined with derived graptoloid characters. Fortey & Cooper (1986) redefined the Graptoloidea as including all planktic, nematophorous forms in the attempt to define a monophyletic unit, but stated that the bithecae were lost separately and independently in a number of lineages. It is here advocated to lessen the emphasis on the presence of bithecae and define upper level taxa based on proximal development types only. This interpretation lends considerable support to the independent loss of bithecae in a number of groups of planktic Graptolithina (see also Maletz et al. 2017, fig. 1).

Diagnosis. – Planktic nematophorous, multiramous graptoloids with a triad budding system; colony shape ranges from reclined through horizontal to declined and bell-shaped or pendent; bithecae distinctly smaller than autothecae; bithecae initially regular, but in later taxa irregular and often reduced or absent; autothecae simple, widening tubes, sometimes aperturally isolated; ventral rutella common; dissepiments present in a few taxa; proximal development type isograptid, quadriradiate to biradiate, variably dextral or sinistral; maeandrograptid type proximal symmetry with inclined sicula; prosicula with distinct differentiation of conus and cauda; origin of first theca in median part of the conus; th1² is first dicalycal thecae, later dicalycal thecae adventitiously or regularly distributed (Maletz et al. 2017a, pp. 2, 3).

Remarks. – The family Anisograptidae are the earliest planktic graptoloids and are used in the sense of Maletz et al. (2017a). All members of the taxon bear at least a single sicular bitheca, but most members possess bithecae in alternating positions along the stipes. The identification of anisograptid taxa is often problematic, especially in flattened material, in which the bithecae cannot be observed. Anisograptidae appear to be restricted to the Tremadocian and are replaced by the Dichograptina and Sinograptina in the basal Floian. The transition is, however, poorly known and the Anisograptidae must be regarded as a paraphyletic taxon. Sadler et al. (2011, fig. 14) indicated a distinct extinction event within the clade of the Anisograptidae in the late Tremadocian, after which they recovered and survived into the late Floian. Maletz (2017a) considered this extinction event as a gap in the investigation of Tremadocian graptolite faunas, stating that late Tremadocian faunas are very poorly known.