Middle Ordovician Drepanoistodus (Vertebrata, Conodonta) from Baltica, with description of three new species

Drepanoistodus basiovalis (Sergeeva, 1963) is a common conodont species in Middle Ordovician strata of Baltica. For many years it has been widely accepted that the species encompasses a wide range of morphological plasticity. Hence, several different morphotypes that significantly deviate from the holotype have nonetheless been included in the broad species concept. In this study, we performed a detailed taxonomical study on 112 predominantly well-preserved specimens (geniculate elements) from the St. Petersburg region of Russia; 37 of these were selected for morphometric analyses together with 21 wellillustrated specimens from the published literature. The results demonstrate that, among the morphotypes that share some characteristics with D. basiovalis sensu lato, at least five species can be readily distinguished. Hence, three new species – Drepanoistodus iommii sp. nov., D. svendi sp. nov. and D. viirae sp. nov. – are here added to the previously known D. basiovalis and D. contractus (Lindström, 1955). In addition, some specimens were left under open nomenclature and assigned to Drepanoistodus aff. basiovalis and D. cf. suberectus (Branson & Mehl, 1933). In order to objectively compare the Drepanoistodus taxa and test the validity of the new species, we performed a Principal Component Analysis combined with nonparametric (PERMANOVA) tests based on 21 morphological characters.


Introduction
Conodonts are generally interpreted as an extinct clade of marine vertebrates (Aldridge et al. 1993;Donoghue et al. 2000;Murdock et al. 2013), although also other hypotheses regarding their affinity have been suggested (Turner et al. 2010). These primitive 'fish' first appeared in the Late Cambrian and went extinct in the Late Triassic. Whereas body fossils are very rare, the microscopic tooth-like elements (known as conodont elements) are composed of calcium phosphate and have great preservation potential. Therefore, these microfossils can be extracted in large quantities from calcareous, sedimentary rocks, and have proven widely useful, e.g., for biostratigraphy, palaeogeography, palaeobathymetry, and assessing thermal maturity (by means of colour alteration index; CAI). Conodont microfossils have been known for more than 160 years with the first description of these phosphatic elements published being that of Pander (1856), who based his study on material from the St. Petersburg region, Russia. Since then, several studies have dealt with Ordovician conodonts from this area (e.g., Sergeeva 1962Sergeeva , 1963Sergeeva , 1974Bergström 1988;Tolmacheva et al. , 2003aTolmacheva et al. , 2003b. The present study follows this research tradition and is based on conodonts from the Lynna River section of Russia (Fig. 1).
Ever since the first multi-element reconstruction of the conodont genus Drepanoistodus Lindström, 1971 was made by Lindström (1971), it has been generally accepted that Drepanoistodus basiovalis (Sergeeva, 1963), a common species in the Middle Ordovician of Baltoscandia, includes quite a wide range of different morphotypes. Whereas some of these morphotypes were subsequently distinguished as distinct species, e.g., Drepanoistodus contractus (Lindström, 1955) (see Stouge & Bagnoli 1990), most have been tentatively assigned to Drepanoistodus under open nomenclature, as D. cf. basiovalis and D. aff. basiovalis (e.g., Rasmussen 2001;Mellgren & Eriksson 2010;Lindskog et al. 2020), or accepted as morphological variations fitting into the broad species concept of D. basiovalis. Originally, Sergeeva (1963) based the characterisation of the new species Oistodus basiovalis on geniculate elements from the Volkhovian and Kundan regional stages (Dapingian to middle Darriwilian global stages) of the St. Petersburg region. The holotype was recovered from Volkhovian strata. The two original D. basiovalis specimens figured by Sergeeva (1963) were incorporated in the analyses performed herein, alongside other elements assigned to D. basiovalis, D. cf. basiovalis or D. aff. basiovalis by various authors.
In this study we assess D. basiovalis-like specimens from Dapingian through middle Darriwilian (Middle Ordovician) strata of the Lynna River in the St. Petersburg region, Russia, and demonstrate that these unarguably comprise a collection of separate species, which are readily distinguished based on morphological characters of the geniculate elements. The validity of all taxa has been tested by use of Principal Component Analysis followed by the non-parametric PERMANOVA similarity test comprising 21 morphological characters.

Geological setting
The St. Petersburg region of Russia has an extensive history of geological exploration, and the Ordovician strata have been well studied (Fig. 1;e.g., Pander 1830;Schmidt 1882;Lamansky 1905;Raymond 1916;Sergeeva 1962;Ivantsov 2003;Dronov & Mikuláš 2010;Rasmussen & Harper 2008;Lindskog et al. 2020). During the Ordovician this region formed part of the palaeocontinent Baltica, at the time situated in the southern hemisphere and moving northwards (Cocks & Torsvik 2006). Baltica was largely covered by a shallow sea, in which laterally extensive sedimentary strata were being deposited (e.g., Männil 1966;Lindström 1971;Jaanusson 1973). Today, the rocks of the St. Petersburg region form part of the natural escarpment known as the Baltic-Ladoga Klint ( Fig. 1; Dronov & Mikulaš 2010). The regional Ordovician succession is ca 100-200 m thick and composed predominantly of carbonate rocks. East of the city of St. Petersburg, several natural exposures have been cut into the Palaeozoic rocks by rivers that drain into Lake Ladoga, providing ample opportunity for quarrying as well as scientific studies (e.g., Popov 1997;Dronov & Mikulaš 2010).
South-southwest of the village of Kolchanovo, ca 150 km east of St. Petersburg, Middle Ordovician sedimentary rocks crop out along the Lynna River. For this study, we sampled an exposure in the valley close to the mouth of the river, where it drains into the Syas River ( Fig. 1; WGS 84 coordinates 60°00′39″ N, 32°33′49″ E). The relatively expanded local succession, comprising the upper Volkhov, Lynna, Sillaoru, and lower Obukhovo formations, is approximately 10 m thick and mainly composed of alternating limestone and variably silty-sandy marl (e.g., Lindskog et al. 2020). Chronostratigraphically, it spans the uppermost middle Volkhovian to lowermost upper Kundan regional stages, which corresponds approximately to the uppermost Dapingian through lowermost middle Darriwilian global stages (Fig. 2).
For additional detailed information on the geology and (bio)stratigraphy of the Lynna River section, readers are referred to the recent study by Lindskog et al. (2020).

Material and methods
For a study on carbonate sedimentology and conodont stratigraphy at Lynna River (see Lindskog et al. 2020), 22 samples were treated with buffered acetic acid for the retrieval of microfossils according to standard procedures (e.g., Jeppsson et al. 1999). These collections formed the basis also for the present study. After sieving of the acid-insoluble residues, conodont elements were electrostatically handpicked from the >63 μm heavy fractions. In total, the collections comprise tens of thousands of conodont elements, most of which are well to excellently preserved. A consistent colour alteration index (CAI) of ca 1 indicates insignificant heating of the local rocks (cf. Epstein et al. 1977).
All conodont specimens derive from the Volkhov, Lynna, Sillaoru and Obukhovo formations at Lynna River (Fig. 2). In terms of conodont biostratigraphy, the sampled strata span the Lenodus antivariabilis Zone to the base of the Lenodus pseudoplanus Zone (see Lindskog et al. 2020), which correlate with the upper Volkhovian and Kundan regional stages, and the Darriwilian global Stage (Dw1-lower Dw2 stage slices ;Ivantsov 2003;Bergström et al. 2009;Lindskog et al. 2020).
Photographs of conodont elements were taken with an Olympus SC30 digital camera attached to an Olympus SZX16 light microscope. All images were subsequently processed and stacked to increase focal depth, using cellSens software. Lighting conditions and magnification were kept at identical settings for all specimens.

Material repository
All sample materials are stored at the Department of Geology, Lund University, Sweden, where the 13 figured conodonts belong to the type collection and have repository numbers LO (for Lund Original) followed by five digits and a t for "type specimen" and capital T for holotype.

Morphometric analyses
Principal Component Analysis (PCA) helps to identify patterns in large datasets with many variables, and to objectively highlight similarities and differences. Hence, PCA plots concentrate variance into relatively few axes compared to the high dimensionality (many variables) of the original data, and thereby aid in distinguishing between different morphotype groupings. Herein, we used this method to visualize the most typical characters in each species and also to show which species are most morphologically similar. PCA was first described by Pearson (1901) and has since then been used in numerous morphometrical/ phenetic taxonomical studies. We refer readers to e.g., Davis (1986) and Hammer & Harper (2006) for more detailed descriptions of PCA, the latter including a palaeontological perspective. All statistical analyses and initial plots were performed using the free PAST software package (Hammer et al. 2001).  Lindskog et al. 2020, and references therein;additions after Pärnaste et al. 2013;Bauert et al. 2014;Gradstein et al. 2020). The pink area indicates the approximate stratigraphic range of the outcrop at the Lynna River, with ranges of selected Drepanoistodus species indicated. For more detailed information on the local conodont biostratigraphy and ranges of individual species, see Lindskog et al. (2020: fig. 6).
The PCA approach used herein was based on 21 unweighted morphometric characters from the geniculate element in Drepanoistodus (see below in "Species characters and coding" and Fig. 3 for explanation of the 19 binary (presence/absence) and two numerical characters included). All selected morphological characters were observable in 58 of the examined specimens of Drepanoistodus, making them applicable for the PCA and statistical tests (Table 1). In the present work, we have reduced the visual result to two dimensions, where only the two most significant principal axes were plotted (Fig. 4). Because the plot involves morphological characters of different types (presence/absence, measured angle, and distance ratio), the correlation coefficient matrix was selected instead of the more common variance-covariance matrix, which means that variables are centred and standardized at the same time. The first principal axis (PC 1, the horizontal axis) accounts for 18.4% of the total data variance and the second principal axis (PC 2, the vertical axis) accounts for 16.1% of the variance.
The result of the PCA is visualized in a so-called biplot (Gower & Hand 1996), where both objects (individual geniculate conodont elements) and variables (selected morphologic characters) are presented simultaneously. The variables are shown in Fig. 4 as blue lines (vectors), where both the direction and length of the vectors are important. For example, the vectors representing b/c (ratio  (Sergeeva, 1963) element demonstrating the essential morphological characteristics utilized in the present study. Two characters were measured quantitatively: Angle A between the median line (usually carina) of the cusp and the lower margin of the keel situated on the upper margin of the cusp (A); and the b/c ratio, which is the ratio between the length of the upper margin of the base (b) and the length of the free cusp (c).

Species characters and coding
In total, the data matrix at hand comprises 21 discrete morphological characters related to the geniculate element in Drepanoistodus (Table 2; Fig. 3). Whereas 19 of these characters are binary (presence = 1; absence = 0), two are measured values, i.e., the angle between the upper margin and the cusp, and the ratio between the length of the upper margin and the cusp, respectively. If a character is missing because of damage (fragmentation), the character has been coded with a question mark (?).

Remarks
Drepanoistodus is here interpreted as quinquemembrate and comprises four nongeniculate coniform elements and one geniculate coniform element that collectively make a curvature-transition series from erect to recurved element types (e.g., Stouge & Bagnoli 1990;Rasmussen 1991). The nongeniculate elements comprise a suberectiform element associated with drepanodontiform type-l, type-2 and type-3 elements. In general, Middle Ordovician nongeniculate Drepanoistodus elements from Baltica can be described as follows: the suberectiform element is characterised by a straight, erect cusp. The drepanodontiform type-1 element has a strongly recurved cusp, which is keeled both anteriorly and posteriorly. The anterior keel is twisted strongly inwards. An extension, sometimes triangular in outline, may occur at the anterobasal corner. The drepanodontiform type-2 element has a recurved cusp which is keeled. It is separated from the drepanodontiform type-l element by the straight or only weakly twisted anterior margin, and by the consistent presence of an anterobasal flare, commonly with a triangular outline. The drepanodontiform type-3 element is typified by a slightly recurved cusp, which is anteriorly and posteriorly keeled and not twisted. As opposed to the drepanodontiform type-1 and type-2 elements, it lacks the anterior triangular flare. For a more comprehensive description, see Rasmussen (1991).
Many coniform conodont apparatuses are not easily placed in the locational PMS notation scheme favoured by Sweet (1981Sweet ( , 1988, or the more biologically correct terminology advocated by Purnell et al. (2000), because it is very difficult or even impossible to identify locational homologues with the ozarkodinid notation (Smith et al. 2005). This is primarily a consequence of the lack of natural assemblages in many conodont genera and families, including Drepanoistodus.

In most cases, it is extremely difficult to distinguish between individual Middle Ordovician
Drepanoistodus species based on the largely homeomorphic nongeniculate elements (van Wamel 1974;Dzik 1983;Stouge 1984;Rasmussen 2001), and this is indeed also the case with respect to the species studied herein. Because our material includes nothing but isolated conodont elements (as opposed to articulated clusters or natural multi-element assemblages), the identification of Drepanoistodus at the species level is solely based on the geniculate element, which are described below. The stratigraphical distribution of the studied specimens of Drepanoistodus is shown in Table 3. (Sergeeva, 1963)   Inclined conodonts, almost symmetrical, with a wide shortened base, the edge of which is rounded.

Material examined
33 geniculate elements including 24 from the Lynna section.

Original description, slightly shortened (translated from Sergeeva, 1963 [in Russian])
Medium-sized conodonts (0.52-0.92 mm), inclined; the degree of inclination of the cusp is 45-60°, sometimes up to 80°. Base high, not very long, elongated along the CD; base length 2.5-3 times its height (comment by the authors: "we find the meaning of the latter measure ambiguous"). Base wall slightly transparent near the edge, rounded. The angle between the sides AC is more than 90°; angle between AD 40-45°; corners are smoothly obtuse. Transverse in cross section, the base is oval, elongated along CD and compressed along L1L2. From the C side, the base is compressed, sometimes with a thin keel near the tip, with a small keel on side D. The sides of the base L1 and L2 are smooth and flat. Basal cavity is not always visible, it is wide, but not deep, without visible tops. The cusp is long, straight or slightly curved towards L1, sharply tapering towards the tip; compressed. The sides of the cusp are almost flat, with a welldeveloped longitudinal, wide carina on L1 and less developed carina on the side L2. The carinae usually run from the base to the tip of the cusp. Thin keels occur on the lower (D) and upper (C) parts of the cusp.

Remarks
In her original diagnosis, Sergeeva (1963) only included geniculate elements with a rounded basal margin in "Oistodus" basiovalis, which is also evident from the species epithet: basiovalis (meaning oval base). This interpretation of the geniculate element in Drepanoistodus basiovalis is followed here. Additional typical characters that may be added to the original species description include: anterior margin and upper anterior corner rounded or weakly rounded; cusp usually straight; a median or median to lower, longitudinal carina present on the inner (sometimes slightly concave) side of the element. Carina is more distinct in Darriwilian specimens than in Dapingian ones. Whereas angle A (Fig. 3) between the cusp and the upper margin is 29.6° with a standard deviation at 5.6, the mean ratio between the length of the free upper margin and free cusp (b/c ratio) reaches 0.40 with a standard deviation of 0.1 (Fig. 5B).

Occurrence
Drepanoistodus basiovalis occurs from the L. antivariabilis Zone (sample LY12-9) to the interzone ("uncertain interval") between the L. variabilis Zone and the Y. crassus Zone sensu Lindskog et al. (2020) in the Lynna River section (sample LY12-21b; between LY12-21 and LY12-22). In addition, D. basiovalis has been reported from several other localities in Baltoscandia and Poland, and also outside the Baltica palaeocontinent, e.g., New Brunswick, Argentina, Australia and China (for references, see the synonymy list above).

Diagnosis
A Drepanoistodus species characterised by a geniculate element with distinct keels on the cusp and upper margin of the base; a straight basal margin; a straight to weakly rounded (convex) anterior margin and cusp which is approximately twice the length of the upper margin of the base.

Etymology
Named in honour of legendary guitarist Tony Iommi, founding member of heavy metal band Black Sabbath.

Material examined
Ten geniculate elements including eight from the Lynna section.

Type stratum
Approximately 10 cm above the local base of the Lynna Formation, sample LY12-16. Lower part of the Lenodus variabilis Zone.

Description
Cusp reclined and straight with distinct keels developed on the anterior (upper) and posterior (lower) margins. A median, longitudinal carina is developed on both sides of the cusp, but it is especially distinct on the inner side. Base is characterised by a straight or almost straight basal margin and a distinct keel on the upper margin. Whereas this keel is slightly convex, the upper margin below the keel is straight. Anterior margin is usually straight or weakly rounded (convex), but occasionally, it is strongly rounded. Angle A between the cusp and upper margin of the base is ca 30° (mean) with a standard deviation at 4.2 (Fig. 5B), and the mean ratio between length of the free upper margin (b) and the free cusp (c) is 0.54 (standard deviation 0.10).

Remarks
In the PCA plot (Fig. 4), the population of D. iommii sp. nov. is situated in the upper right corner, separated from the D. basiovalis population as well as the other two new species populations described herein. The vectors in the biplot demonstrate that this is mainly due to the straight basal margin, the relatively long upper margin (high b/c values), and the usually straight anterior margin in D. iommii sp. nov., which is in accordance with the characters diagnosed above. The hypothesis that the population of D. iommii sp. nov. is morphologically different from the D. basiovalis population is supported by the PERMANOVA test (Fig. 5A), which shows that the probability that the two populations are the same is exceedingly low (p (same) = 1.00E-04).

Diagnosis
A Drepanoistodus species characterised by a geniculate element with a recurved cusp and distinct keels on both the cusp and the upper margin of the base. Weakly curved but distinct carinas are developed on both sides of the cusp, especially well developed on the inner side.

Etymology
Named after the Danish palaeontologist and conodont specialist Svend S. Stouge, Natural History Museum of Denmark, University of Copenhagen.

Material examined
Five geniculate elements including three from the Lynna section.

Type stratum
Approximately 40 cm below the local top of the Volkhov Formation, sample LY12-14, ca 20 cm above the base of the Lenodus variabilis Zone sensu Lindskog et al. (2020).

Description
Cusp is recurved (bent weakly downward), with distinct keels developed along the anterior (upper) and posterior (lower) margins. A median, longitudinal, weakly curved carina is developed on both sides of the cusp, most distinct on the inner side of the cusp. Cusp is almost twice as long as the upper margin of the cusp; the mean ratio between the length of the free upper margin and the free cusp (Fig. 3) is ca 0.55 with a standard deviation of 0.13. Basal margin varies from rounded (convex) to almost straight. A distinct keel is developed on the upper margin. Anterior margin is rounded or weakly rounded (convex). Angle A (Fig. 3) between the cusp and upper margin of the base varies considerable with a mean of 24° and standard deviation of 6.9 (Fig. 5B).

Remarks
Drepanoistodus svendi sp. nov. is distinguished from all the other Drepanoistodus species in the present study by the recurved cusp and the curved carina on each side of the cusp. Like D. iommii sp. nov., it is characterised by a clearly longer upper margin of the base compared to the cusp length than in D. basiovalis. The D. svendi sp. nov. population is located in the lower, right quadrangle of the PCA plot, far from any other species of Drepanoistodus, and the biplot vectors representing the recurved cusp and the curved carina point in this direction (Fig. 4). The PERMANOVA test on the first seven PCA axis shows that the probability that the D. basiovalis and D. svendi sp. nov. populations are the same, is exceedingly low (p (same) = 1.00E-04).

Diagnosis
A Drepanoistodus species characterised by a geniculate element with a wide, straight, compressed cusp and a very short base, where the free cusp typically is ca 4 times longer than the upper margin of the base.

Etymology
Named after the Estonian palaeontologist and conodont specialist Viive Viira, Tallinn University of Technology, Estonia.

Material examined
Nine geniculate elements including five from the Lynna section.

Type stratum
Approximately 15 cm above the local base of the Sillaoru Formation, sample LY12-31. Lower part of the 90 cm thick interzone ("uncertain interval") between the Lenodus variabilis Zone and the Yangtzeplacognathus crassus Zone sensu Lindskog et al. (2020).

Description
Cusp is reclined, wide (from upper to lower margin) and straight, with keels developed along the anterior (upper) and posterior (lower) margins. A weak, median, longitudinal carina is developed on the inner side of the cusp. Occasionally, the carina may be distinct. Basal margin is weakly rounded or straight. A distinct keel is developed on the upper margin. Anterior margin is rounded or weakly rounded (convex). Angle A (Fig. 3) between the cusp and upper margin of the base is ca 30° (mean) with a standard deviation of 4.3 (Fig. 5B), and mean ratio between length of the free upper margin and the free cusp is ca 0.25 with a standard deviation of 0.05.

Remarks
Drepanoistodus viirae sp. nov. is situated in the lower left quadrangle of the PCA plot (Fig. 4). Like D. basiovalis, it is clearly separated from D. iommii sp. nov. and D. svendi sp. nov., whereas it partly overlaps with the D. basiovalis population, when only the PC 1 (x) and PC 2 (y) axis is plotted. The vectors in the biplot reinforce that D. viirae sp. nov. is characterised by a convex basal margin, a weakly developed carina and a short upper margin on the base (= low b/c value), the latter because it is situated in the opposite direction of the b/c vector, as seen in Fig. 4. The partial overlap with D. basiovalis occurs because the two species share some characters. A significant difference, however, is that D. viirae sp. nov. has a relatively shorter upper margin of the base, where the mean b/c ratio is 0.40 in D. basiovalis but only 0.25 in D. viirae sp. nov. (Fig. 5B). Moreover, D. viirae sp. nov. is characterised by a wider cusp when viewed from the side and, typically, a less developed carina on the cusp. The hypothesis that the D. viirae sp. nov. population is morphologically separate from the D. basiovalis population is supported by the PERMANOVA test (Fig. 5A), which shows that the probability that the two populations are the same is low (p (same) = 8.00E-03). Drepanoistodus viirae sp. nov. is distinguished from the stratigraphically older Drepanoistodus contractus on the relatively wider and more compressed cusp and the usually less distinct longitudinal carina, and from D. cf. suberectus on the markedly smaller angle between the cusp and the upper margin of the base (mean angle = 46° in D. cf. suberectus, 30° in D. viirae sp. nov.).

Occurrence
The lower part of the L. variabilis Zone (sample LY12-13) to the lower part of the interzone ("uncertain interval") between the L. variabilis Zone and the Y. crassus Zone (sample LY12-31) sensu Lindskog et al. (2020). In addition, D. viirae sp. nov. has been recorded from the B. norrlandicus and basal Y. crassus zones at Gillberga, Sweden (Löfgren 2000b(Löfgren , 2003; the uppermost part of the P. rectus -M. parva Zone at Steinsodden, Norway, which correlates with the uppermost P. originalis Zone (as D. cf. stougei sensu Rasmussen 2001); the lower part of the B. medius -H. holodentata Zone at Andersön, Sweden, correlating with the uppermost part of the L. variabilis Zone (as D. cf. basiovalis sensu Rasmussen 2001), and the L. variabilis Zone at Hällekis, Sweden (as D. aff. suberectus sensu Mellgren & Eriksson 2010). Moreover, it shares some characteristics with the geniculate element from strata correlated with the L. pseudoplanus Zone of the Canning Basin, Australia, which was included in D. basiovalis (Zhen 2020: fig. 7b), but this identification is questionable.

Material examined
Seven geniculate elements including five from the Lynna section.

Remarks
The specimens assigned to Drepanoistodus aff. basiovalis herein share some characters with D. basiovalis, D. iommii sp. nov. and D. stougei, but do not meet the full criteria of any of these species. The most significant character is an extended upper keel anteriorly, which may be angular (see Fig. 6P-Q) or rounded. Cusp is reclined and straight with distinct carinas on both sides of the cusp. Anterior margin varies from almost straight to rounded (convex). The upper anterior corner is angular or weakly rounded. The basal margin is usually slightly convex. The cusp is often nearly twice as long as the upper margin of the base, but it varies considerably (b/c ratio mean = 0.46, standard deviation 0.14). Similarly, angle A is very variable with a mean near 28° and standard deviation at 6.55 (Fig. 5B). Specimens with a rounded anterior edge share similarity with Drepanoistodus stougei Rasmussen, 1991

Remarks
Drepanoistodus stougei was first described by Rasmussen (1991) and further morphological details for all the four element types were subsequently added by Rasmussen (2001). The most distinct features are the rounded anterior margin and the relatively long upper margin on the base. In the material at hand, the b/c ratio (see Fig. 3) mean is 0.50 and angle A has a mean of 27.4°. In some cases, D. svendi sp. nov. is characterized by a similarly rounded anterior margin but is distinguished by its (weakly) curved cusp and carina. Drepanoistodus aff. basiovalis morphotypes with a rounded anterior margin are separated from D. stougei by the characteristically extended keel in the upper anterior corner of the former taxon. Because D. stougei is included here just for comparison with the new species described, synonymy or further descriptive details have not been incorporated, but can be found in Rasmussen (1991Rasmussen ( , 2001.

Occurrence
From the Lenodus antivariabilis Zone (sample LY12-5) to the basal part of the interzone ("uncertain interval"; sample LY12-29) between the L. variabilis Zone and the Y. crassus Zone sensu Lindskog et al. (2020) in the Lynna River section.

Material examined
Four geniculate elements including three from the Lynna section.

Remarks
Drepanoistodus cf. suberectus is included in the present work because it superficially resembles Drepanoistodus viirae sp. nov. Originally, D. suberectus was described as Oistodus suberectus from the Upper Ordovician strata of Missouri, USA, by , but it was not until 1966 that conodont specialists included the geniculate element in the apparatus (see Bergström &Sweet 1966 andWebers 1966, for details Drepanoistodus cf. suberectus occurs only sporadically in the Lynna River section samples. It is characterised by a short upper margin of the base compared to the free cusp (b/c ratio near 0.20 in the three specimens found). Angle A between the upper margin of the cusp and the carina on the cusp (see Fig. 3) varies considerably (41-52°) but it is wider than that of the other Drepanoistodus species described here. Moreover, it is typified by a convex basal margin; weakly rounded anterior margin, and a weakly developed carina on the straight cusp, which is located on the lower half part of the cusp.
Superficially, D. cf. suberectus resembles D. viirae sp. nov. because of the relatively short base, but the latter species is distinguished by a narrower angle A (see Fig. 3); wider sides anteriorly on the cusp; laterally compressed cusp with distinct keels, and a median, as opposed to a lower, carina.

Discussion
The present study not only assesses the debated Ordovician conodont genus, Drepanoistodus, but also adds to our general knowledge of this useful microfossil group from the St. Petersburg region -an area with an extensive historical research tradition. Herein, we show that morphotypes formerly interpreted as Drepanoistodus basiovalis include at least five separate species. Whereas two of these species were previously known as D. basiovalis and D. contractus, three are newly established herein; D. iommii sp. nov., D. svendi sp. nov. and D. viirae sp. nov. Notably, the establishment of D. iommii sp. nov. means that a conodont species is now rightfully inducted into the "rock fossil hall of fame", and that D. iommii sp. nov. will become part of the Rock Fossils travelling exhibition (see Eriksson 2019). As such, it will also benefit palaeontological outreach.
Drepanoistodus basiovalis appears to have been stratigraphically (Fig. 2) and geographically more widespread than the four other Drepanoistodus species studied. Whereas the three new species seem to have been restricted to Baltica, D. basiovalis probably also inhabited other palaeocontinents in low numbers. Note, however, that we consider many of the records from outside Baltica (see the D. basiovalis synonymy above) to be questionable. In the Lynna River section, D. svendi sp. nov. is restricted to the lowermost part of the L. variabilis Zone, while D. viirae sp. nov. ranges through most of the L. variabilis Zone to a level just above the top of the L. variabilis Zone (Fig. 2). Drepanoistodus iommii sp. nov. appears earlier than D. svendi sp. nov. and D. viirae sp. nov. in the Lynna River Section (in the L. antivariabilis Zone) and this seems also to be the case in a regional scale, where the species has been recorded already from P. originalis Zone. More material is needed to assess if these range differences could be related to the cooling event and associated shallowing recorded in the lower Kundan (Rasmussen et al. 2016;Rasmussen & Stouge 2018), or it results from other causes.