Emended Sandbian (Ordovician) conodont biostratigraphy in Baltoscandia and a new species of Amorphognathus

Abstract

Conodonts are an important biostratigraphic tool for many Phanerozoic stages. Along with graptolites, they define all global Ordovician Stage boundaries. Within the Upper Ordovician interval, a known species of Amorphognathus tvaerensis (Bergström) is present in both Sandbian and Katian stratotype sections. Study of changes in the succession of A. tvaerensis revealed that elements in the upper part of its range differ morphologically quite distinctly from those in its lower part. Here, they are described as a new conodont species, A. viirae sp. nov. This new species is recognized in several Estonian and Swedish sections, with apparent occurrence also in Mójcza Quarry, Holy Cross Mountains, Poland and Black Knob Ridge, Oklahoma, USA. Detailed analysis of early Amorphognathus elements from Estonian and Swedish sections revealed the absence of A. inaequalis (Rhodes) in both regions, although a conodont subzone based on this species was identified earlier by some authors. Both the absence of A. inaequalis (Rhodes) and recognition of the new species A. viirae sp. nov. resulted in the revision of the conodont zonation, and a new version of it is proposed for the Sandbian Stage in the Atlantic Realm. The new zonation includes (from below) Pygodus anserinus, Baltoniodus variabilis, A. tvaerensis, B. gerdae, A. viirae and B. alobatus Conodont zones.

1. Introduction

Originally, the genus Amorphogathus was defined as a formal taxon (Branson & Mehl, 1933) based on an element which was later recognized as Pa of the Amorphognathus apparatus. Identification of other elements of the apparatus revealed that the M element can also be used as a diagnostic for different species of the genus Amorphognathus (Bergström, 1971). The structure of the apparatus and phylogeny of Amorphognathus has been discussed in several papers (Bergström, 1983; Dzik, 1999 a; Bergström & Leslie, 2010; Ferretti et al. 2014 a). It is commonly accepted that S elements of the Amorphognathus apparatus are the most conservative; they hardly change during evolution of the genus and are of limited assistance in the identification of the species (Ferretti et al. 2014 a).

As a rapidly evolving genus with nearly global distribution, Amorphognathus is especially useful for stratigraphic purposes (Bergström & Ferretti, 2017); further complementary research on the topic is therefore essential. Among the different species of genus Amorphognathus, Amorphognathus tvaerensis (Bergström, 1962) has stratigraphic importance in Baltoscandia and Argentine Precordillera (Albanesi & Ortega, 2016). Additionally, A. tvaerensis is present in stratotype sections of both the Sandbian (Baltoscandia, Sweden, Fågelsång; Bergström et al. 2000) and the Katian (Laurentia, Oklahoma, Black Knob Ridge; Goldman et al. 2007) stages, making it a useful species for global correlation. Occurrences of A. tvaerensis in the sections of Poland and Scotland are also known (Bergström, 1962). Several authors have discussed variations in elements of A. tvaerensis (Dzik, 1990, 1994; Viira, 2008; Xu et al. 2010; Paiste et al. 2022) but have not drawn any conclusions, probably because of a lack of representative material for such a study. In the present study, which is based on a rich collection of conodonts from the Mehikoorma drillcore section from Estonia, the morphological variation of the P and M elements of the species of genus Amorphognathus in the Sandbian and lower Katian interval is analysed in detail, with the aim of clarifying species variation and the resulting implications to stratigraphy.

2. Geological setting

During Sandbian and early Katian (Late Ordovician) time, the Baltoscandian Ordovician Palaeobasin on the western Baltica palaeocontinent was situated close to the tropical realm (Cocks & Torsvik, 2005). The basin is commonly divided into three distinct facies belts, sometimes called also basins (Fig. 1). Western Sweden and northwestern Poland comprise the Scandinavian Basin with graptolitiferous black shales and grey shales. Eastern Sweden, northeastern Poland, the western part of Latvia and Lithuania, and southern Estonia are addressed together as the Livonian Basin, characterized by marls and argillaceous limestones. The eastern part of Latvia and Lithuania together with northern Estonia are combined as the Estonian/Lithuanian Shelf, characterized by the shelf limestones (Harris et al. 2004).

Fig. 1. General facies structure of the Baltoscandian Ordovician Palaeobasin. Locations of the sections discussed or referred to in the text are marked with circles (modified after Männik et al. 2021; Paiste et al. 2022).

Conodont stratigraphy of the Sandbian and Katian Stage of Baltoscandia is predominantly based on evolutionary lineage of the genus Amorphognathus (Bergström, 2007; Meidla et al. 2014). The base of the A. inaequalis Conodont Subzone (CSz) is used as the best approximation of the lower boundary of the Sandbian Stage (Bergström et al. 2017; Goldman et al. 2020). This species is succeeded by the long-ranged A. tvaerensis defining a Conodont Zone (CZ) with three CSz based on successive ranges of species of the genus Baltoniodus. The younger A. superbus defines the next CZ, with the boundary in the lower part of Katian Stage (Bergström et al. 2017; Goldman et al. 2020). This sequence of conodont zones is also present in sections of the United Kingdom (Bergström & Ferretti, 2018) and the Holy Cross Mountains in Poland (Dzik, 1990).

3. Materials and methods

This study revisits previously a described conodont succession from the Mehikoorma-421 drill core (Männik & Viira, 2005). A detailed core description and distribution of Ordovician conodonts is presented in Põldvere (2005) and Männik & Viira (2005). The Mehikoorma-421 borehole, drilled in the course of geological–hydrogeological mapping in 1972, is located in SE Estonia, on the SW coast of Lake Peipsi (Põldvere, 2005; 58° 14' 34.4" N, 27° 27' 08.3" E; Fig. 1). It penetrates the upper part of the Palaeoproterozoic crystalline basement and spans Ediacaran, Cambrian, Ordovician and Devonian sedimentary rocks, comprising a total thickness of 548.5 m. The studied Ordovician interval of the core (288–333 m) is represented by various marl and limestones from the Uhaku Regional Stage (RS) (Darriwilian) up to the Oandu RS (Katian) (Fig. 2; Põldvere, 2005). With respect to palaeogeography, the section represents Livonian Basin (Fig. 1) (Harris et al. 2004). The study interval is dominated by marl and argillaceous limestone of shelf origin. A geochemical comparison of δ¹⁸O_phos and δ¹³C_carb curves, along with litho- and biostratigraphy from the Mehikoorma core, were published recently (Männik et al. 2021), but conodonts from the Mehikoorma core are visualized in only a few samples (Xu et al. 2010; Paiste et al. 2020).

Fig. 2. Distribution of selected conodont taxa in the Sandbian strata of Mehikoorma-421 core section. From left to right: global series, global stage, regional stage, formation, lithological log (after Põldvere, 2005), biostratigraphical samples (hollow boxes, with depth, contained genus Amorphognathus elements), δ¹³C_carb record (Kaljo et al. 2007, table 2; Bergström et al. 2009), distribution of the graptolite Nemagraptus gracilis after Männik et al. (2021), zonation in Männik et al. (2021), zonation modified from Webby et al. (2004), zonation proposed in this study. Ord. – Ordovician; 1 – argillaceous limestone; 2 – limestone with fine-grained skeletal detritus; 3 – limestone with coarse pyritized skeletal detritus; 4 – kerogen; 5 – discontinuity surface; 6 – bed of altered volcanic ash (K-bentonite); 7 – limestone nodules; 8 – dolomitized marlstone; 9 – calcareous marlstone; lower – Sagittodontina kielcensis; upper – Amorphognathus inaequalis; * – Amorphognathus viirae sp. nov.; K – Kinnekulle K-bentonite; G – Grefsen group K-bentonites (Bergström et al. 1995); CZ – conodont zones; CSz – conodont subzones. Lower boundaries of A. tvaerensis and A. superbus conodont zones are based on appearance of the first M elements of the species.

Conodont samples were collected in 2003/2004, weighing 250–1220 g. All samples yielded conodonts, quite well preserved, with conodont alteration index (CAI) < 1.5. The number of specimens per sample varied from less than 10 up to several thousands, the richest samples coming from the lower part of the succession (Kõrgekallas, Dreimani and Tatruse formations). The studied interval (Fig. 2) includes 45 samples from the depth interval of 288.2–328.6 m, which yielded the elements of Amorphognathus. The illustrated specimens are housed in the Institute of Geology at Tallinn University of Technology, Estonia (collection GIT870).

Most representative and intact elements of Amorphognathus from all 45 analysed samples were photographed with SEM and partitioned by element types (see online Supplementary Figs S1–S17, available at https://doi.org/10.1017/S0016756822001005) in order to record the full range of changes in morphology of Amorphognathus elements in the study interval, and to provide raw data for further taxonomic and stratigraphic analysis. Our identification of taxa is based on empirical comparison of specimens. A statistical approach for species delineation (e.g. Guenser et al. 2022) is currently problematic because most Pa elements in our material are only partly preserved.

4. Systematic description

All descriptions below are compiled without considering the positions of conodont elements in the apparatus, as their anatomical orientation is not known; biological terminology (ventral, dorsal, caudal etc.) advocated by Purnell et al. (2000) therefore could not be adopted. Instead, the traditional Pa, Pb, Pc, M, Sa, Sb, Sc, Sd notation introduced by Sweet & Schönlaub (1975) and modified by Cooper (1975) and Sweet (1981, 1988) has been followed. In descriptions of elements the terms anterior, posterior, lateral, inner, outer, upper and lower are used in the conventional sense for isolated conodont elements (see Sweet, 1981, 1988), and do not refer to biological orientation in the animal. The use of these terms in descriptions of P elements of Amorphognathus is illustrated in Figure 3s, t, al and an. The use of cusp, posterior denticle and anterior, posterior and lateral processes in M elements is demonstrated in Figure 3h.

Fig. 3. Amorphognathus tvaerensis (Bergström, 1962) elements from the Mehikoorma core: (a) sinistral M element, lateral view, sample 328.6–328.7 m, specimen GIT870-363; (b) dextral M element, lateral view, sample 327.6–327.7 m, specimen GIT870-364; (c) sinistral M element, lateral view, sample 326.6–326.7 m, specimen GIT870-365; (d) sinistral M element, lateral view, sample 325.5–325.6 m, specimen GIT870-366; (e) dextral M element, lateral view, sample 325.5–325.6 m, specimen GIT870-367; (f) probable fragment of dextral Pa element, posterior process, upper view, sample 328.6–328.7 m, specimen GIT870-1; (g) dextral Pa element, posterior process, upper view, sample 325.5–325.6 m, specimen GIT870-4; (h) sinistral M element, lateral view, sample 324.6–324.7 m, specimen GIT870-370; (i) sinistral M element, lateral view, sample 324.6–324.7 m, specimen GIT870-375; (j) sinistral M element, lateral view, sample 324.6–324.7 m, specimen GIT870-376; (k) dextral Pa element, upper view, sample 320.8–320.9 m, specimen GIT870-30; (l) dextral Pa element, upper view, sample 319.5–319.6 m, specimen GIT870-44; (m) dextral Pa element, upper view, sample 316.4–316.55 m, specimen GIT870-82; (n) dextral Pa element, upper view, sample 323.8–323.9 m, specimen GIT870-14; (o) dextral Pa element, posterior process, upper view, sample 319.0–319.05 m, specimen GIT870-51; (p) dextral Pa element, posterior process, upper view, sample 319.75–319.85 m, specimen GIT870-36; (q) dextral Pa element, posterior process, upper view, sample 322.8–322.9 m, specimen GIT870-20; (r) dextral Pa element, posterior process, upper view, sample 316.9–317 m, specimen GIT870-72; (s) dextral Pa element, upper view, sample 316.4–316.55 m, specimen GIT870-79; (t) sinistral Pa element, upper view, sample 323.8–323.9 m, specimen GIT870-17; (u) Pa element, posterior process, upper view, sample 323.8–323.9 m, specimen GIT870-18; (v) sinistral Pa element, posterior process, upper view, sample 320.8–320.9 m, specimen GIT870-34; (w) sinistral Pa element, posterior process, upper view, sample 319.75–319.85 m, specimen GIT870-39; (x) sinistral Pa element, upper view, sample 316.4–316.55 m, specimen GIT870-85; (y) sinistral M element, lateral view, sample 323.8–323.9 m, specimen GIT870-377; (z) dextral M element, lateral view, sample 323.8–323.9 m, specimen GIT870-379; (aa) dextral M element, lateral view, sample 319.75–319.85 m, specimen GIT870-406; (ab) dextral M element, lateral view, sample 319.75–319.85 m, specimen GIT870-408; (ac) sinistral M element, lateral view, sample 317.95–318.05 m, specimen GIT870-451; (ad) dextral M element, lateral view, sample 324.6–324.7 m, specimen GIT870-373; (ae) sinistral M element, lateral view, sample 321.7–321.8 m, specimen GIT870-396; (af) dextral M element, lateral view, sample 319.75–319.85 m, specimen GIT870-413; (ag) sinistral M element, lateral view, sample 316.9–317 m, specimen GIT870-476; (ah) sinistral M element, lateral view, sample 316.4–316.55 m, specimen GIT870-486; (ai) dextral M element, lateral view, sample 317.7–317.8 m, specimen GIT870-460; (aj) dextral Pb element, upper view, sample 323.8–323.9 m, specimen GIT870-250; (ak) dextral Pb element, inner-lateral view, sample 323.8–323.9 m, specimen GIT870-251; (al) dextral Pb element, upper view, sample 320.8–320.9 m, specimen GIT870-260; (am) sinistral Pb element, outer-lateral view, sample 320.8–320.9 m, specimen GIT870-257; (an) sinistral Pb element, upper view, sample 321.7–321.8 m, specimen GIT870-255; (ao) sinistral Pb element, upper view, sample 319.5–319.6 m, specimen GIT870-266; (ap) Sa element, lateral view, sample 324.6–324.7 m, specimen GIT870-622; (aq) Sd element, lateral view, sample 323.8–323.9 m, specimen GIT870-623; (ar) Sb element, lateral view, sample 317.7–317.8 m, specimen GIT870-655; (as) Sc element, lateral view, sample 318.5–318.6 m, specimen GIT870-649; (at) Sc element, lateral view, sample 319.5–319.6 m, specimen GIT870-644. All scale bars are 100 μm.

Material count for each species is based on illustrated and/or examined elements of most representative and intact material. Complete amount is higher but not counted.

Genus Amorphognathus Branson & Mehl (1933)

Type species. Amorphognathus ordovicicus Branson & Mehl (1933).

Diagnosis. Bergström (1971, p. 131–4).

Remarks. Species diagnostics are based on general morphology of the Pa elements and on the features of denticulation on the upper edge of the M element (Ferretti et al. 2014 a).

Occurrence in the Baltoscandian region: Sandbian–Hirnantian.

Amorphognathus inaequalis (Rhodes, 1953)

1953 Amorphognathus inaequalis sp. nov.; Rhodes (1953, p. 283–4, pl. 22: 204).

1974 Amorphognathus inaequalis Rhodes (1953); Lindström et al. (1974, p. 16–17, pl. 1: 8–11; pl. 2: 1, 2, 7).

1985 Amorphognathus inaequalis Rhodes (1953); Bergström & Orchard (1985, pl. 2.2: 14).

1987 Amorphognathus inaequalis Rhodes (1953); Bergström et al. (1987, pl. 18.1: 8–10).

2022 Amorphognathus inaequalis Rhodes (1953); Ferretti & Bergström (2022, p. 467–9, fig. 9A–M).

Diagnosis. Rhodes (1953, p. 283–4).

Remarks. The holotype of A. inaequalis (Rhodes, 1953, pl. 22, fig. 204) is a sinistral Pa element. The dextral Pa element, illustrated by Lindström et al. (1974, pl. 1, figs 9–11), has an outline similar to that of a sinistral Pa element. Both Pa elements have straight posterior and anterior processes (in upper view), with an angle between them of c. 150° for the dextral Pa element and c. 165° for the sinistral Pa element. None of the dextral Pa elements of A. inaequalis illustrated so far (Lindström et al. 1974, pl. 1, figs 9–11; Bergström et al. 1987, pl. 18.1, fig. 8; Ferretti & Bergström, 2022, pl. 9, figs A, B) have an extra postero-lateral process on the outer side as is known in the dextral Pa elements of the younger species, A. tvaerensis. Additionally, both Pa elements of A. tvaerensis have a slight curvature on the posterior process (Ferretti & Bergström, 2022, pl. 11, figs B, C), while no recognizable curvature is recognized in A. inaequalis (Ferretti & Bergström, 2022, pl. 9, figs A, C). While the Pb and S elements of A. inaequalis are similar to those of younger species of Amorphognathus (Ferretti & Bergström, 2022), the M element can be differentiated mainly by lack of a prominent posteriorly directed denticle, which is characteristic of the M element of A. tvaerensis. However, the illustrated M elements of A. inaequalis are broken (Bergström et al. 1987, pl. 18.1, fig. 10; Ferretti & Bergström, 2022, fig. 9J, K) or shown in lateral view only (Lindström et al. 1974, pl. 2, fig. 7), making it difficult to determine their actual outline. The Pa elements provide the best opportunity to identify A. inaequalis with certainty, and differentiate it from A. tvaerensis. A probable dextral Pa element that has intermediate morphology between those of A. inaequalis and A. tvaerensis is figured by Ferretti & Bergström (2022, fig. 11D). This is the oldest specimen in this section that bears an extra postero-lateral process on the outer side, a small lateral expansion next to it and a weak sinusoidal curvature in the main row of denticles.

Specimens identified in an earlier study as A. inaequalis (Dzik, 1976, fig. 27a–f) were later attributed to an earlier form of A. tvaerensis (Dzik, 1994). Likewise, Viira (2008) stated that the earliest M elements of A. tvaerensis are very similar to those of A. inaequalis, as both have a larger straight anteriorly directed denticle on the anterior end. Published illustrations of A. inaequalis are very rare; no specimens are figured from the Baltoscandian sections so far, although both Swedish (Bergström, 2007) and Estonian (Meidla et al. 2014) papers contain references to the A. inaequalis CSz underlying the A. tvaerensis CZ.

According to Ferretti & Bergström (2022, table 1), A. inaequalis occurs together with Baltoniodus prevariabilis. However, some images of the specimens of B. prevariabilis provided by Ferretti & Bergström (2022, fig. 8M, N) from the samples, including also A. inaequalis, appear quite similar to B. variabilis. Specifically, the main feature of B. variabilis, the ‘prominent triangular lateral expansion of the inner side of the posterior process’ (Bergström, 1971, p. 148), is present in these specimens, making them distinct from the element illustrated next to them (Ferretti & Bergstrom, 2022, fig. 8O), representing a typical B. prevariabilis with no triangular lateral expansion. This indicates that A. inaequalis probably has a longer range and also occurs together with B. variablis. In the Mehikoorma core section, the lowermost elements of Amorphognathus have been found from the interval corresponding to the B. variabilis range in the Mehikoorma core, and none of the Pa elements occurring here can be assigned to A. inaequalis. The lowermost M elements of Amorphognathus in the studied interval of this section (samples from interval 325.5–328.7 m, Fig. 3a–e) are typical A. tvaerensis. Pa elements (depth 325.5–328.7 m, Fig. 3f, g) with curvature on the posterior process at the same depth supports the latter. The only single M element in the Mehikoorma section that was earlier identified as A. inaequalis by Männik & Viira (2005; sample 327.6–327.7 m, Fig. 3b) has two anteriorly directed denticles with a larger denticle as the anteriormost and a small posteriorly directed one. A morphologically somewhat similar M element is also present a few samples higher in the section (sample 324.6–324.7 m, Fig. 3h). As Pa elements indicative of A. inaequalis have not been found in these samples both M elements are tentatively attributed to A. tvaerensis, and the strata corresponding to the range interval of A. inaequalis (326.6–324.6 m) in the sense of Viira (2008, fig. 7) are here attributed to the A. tvaerensis CZ as its lower part.

From comparison of the first Amorphognathus elements in the Mehikoorma, Ruhnu (Männik, 2003) and Velise (Paiste et al. 2022) drill cores of Estonia, and in the Fjäcka main section and in the Smedsby Gård drillcore in Sweden, along with a personal communication (S. M. Bergström, 2022), it is evident that there is currently no evidence that A. inaequalis could be present in Estonian or Swedish sections. It is therefore likely that A. inaequalis was not present on the Baltica Palaeocontinent during early Sandbian time. A. inaequalis has been reported from the Bliudziai-150 drillcore in Lithuania (Stouge et al. 2016) and the Kovel-1 drillcore in Ukraine (Saadre et al. 2004), but no specimen was illustrated making it impossible to check these indentifications.

Distribution. South Wales (Ffairfeach, Golden Grove, Dynevor Park), Flags and Grits Formation, Llandeilo flags (uppermost Darriwilian, lowermost Sandbian, Ferretti & Bergström, 2022); West France (Armorican Massif): Upper Postolonnec Formation (Sandbian or Darriwilian, Lindström et al. 1974).

Amorphognathus tvaerensis Bergström (1962)

1962 Amorphognathus tvaerensis sp. nov.; Bergström (1962, p. 36–7, pl. 4: 7–10).

1962 Amorphognathus ordovicica Branson & Mehl ssp. simplicior spp. nov.; Bergström (1962, p. 34–5, pl. 4: 2, 5, 6).

1971 Amorphognathus tvaerensis Bergström (1962); Bergström (1971, p. 135–6, pl. 2: 10–11).

1971 Amorphognathus tvaerensis Bergström (1962); Sweet et al. (1971, pl. 1: 24).

1974 Amorphognathus ordovicica spp. nov. 3.; Viira (1974, p. 59, 60, pl. 7: 23, not including 18, 19, 24).

1976 Amorphognathus inaequalis Rhodes (1953); Dzik (1976, fig. 27c–f).

1976 Amorphognathus tvaerensis Bergström (1962); Dzik (1976, fig. 27g–q).

1981 Amorphognathus tvaerensis Bergström (1962); Nowlan (1981, p. 11, pl. 5: 13, 14, 16).

1985 Amorphognathus tvaerensis Bergström (1962); Bergström & Orchard (1985, pl. 2.3: 8, 9, 11, 16).

1994 Amorphognathus tvaerensis Bergström (1962); Dzik (1994, p. 91–3, pl. 22: 8–22; pl. 23: 1, 2).

?2000 Amorphognathus tvaerensis Bergström (1962); Leslie (2000, fig. 7: 14–19).

2006 Amorphognathus tvaerensis Bergström (1962); Viira et al. (2006, p. 223–5, fig. 1: 1–9, 11, 13–15).

2008 Amorphognathus tvaerensis Bergström (1962); Viira (2008, p. 35, 36, fig. 5A–I, O–R, not including J, K, L, M, N; fig. 6A–E, G–I, not including F; fig. 8A–S).

2016 Amorphognathus tvaerensis Bergström (1962); Stouge et al. (2016, fig. 3E, J, K, Q, R).

?2018 Amorphognathus cf. A. tvaerensis Bergström (1962); Bergström & Ferretti (2018, fig. 6g–n).

2020 Amorphognathus tvaerensis Bergström (1962); Paiste et al. (2020, fig. 4E, F, not including A–D; fig. 5A–D).

2022 Amorphognathus tvaerensis Bergström (1962); Ferretti & Bergström (2022, p. 8–14, pl. 11: A–W).

2022 Amorphognathus tvaerensis Bergström (1962); Paiste et al. (2022, fig. 4A–C, H–J, M–Q, S–U, Y–AG, AO–AU, AW, not including D–G, K, L, R, V–X, AH–AN, AV).

Diagnosis. Bergström (1971, p. 135–6).

Remarks. Detailed description of the dextral Pa holotype and paired sinistral Pa element (designated as A. ordovicica ssp. simplicior) are provided by Bergström (1962), and the multi-element species is discussed by Bergström (1971). The main difference from the younger species of Amorphognathus, A. superbus, is the occurrence of an extra (second) postero-lateral process on the outer side of the dextral Pa element that is missing in the same element of A. superbus (Bergström, 1971). This extra process also has a small lateral anterior lobe next to it, originally described as ‘…a slight curvature outwards of the aboral margin which might be interpreted as a rudimentary development of a new process…’ (Bergström, 1962, p. 37). A typical M element of A. tvaerensis has a characteristic prominent reclined posteriorly directed denticle of variable size located posteriorly of usually poorly developed, and often indistinguishable in size from, other denticles cusp (Bergström, 1971, pl. 2, fig. 11). This characteristic denticle (referred to here as posterior denticle) gradually disappears and the cusp becomes dominant in younger species, in A. superbus and A. ordovicicus (Bergström, 1971, pl. 2, figs 7, 9).

The lowermost dextral Pa element with features characteristic of A. tvaerensis in the Mehikoorma section comes from the interval 325.5–326.6 m (Fig. 3g). It is a posterior process of a broken dextral Pa element with the characteristic sinusoidal curvature in the main row of denticles and with a postero-lateral process. However, all three lower samples from the interval 328.7–326.7 m yield M elements that are characteristic of A. tvaerensis (Fig. 3a–c), and mark the probable first appearance of A. tvaerensis in the Mehikoorma section at 328.7 m. The elements of A. tvaerensis range in the Mehikoorma section up to depth 316.4 m.

Variation of A. tvaerensis is prominent in Pa (online Supplementary Figs S1a–ai; S2a–u, w–aq; S3a–i), Pb (online Supplementary Fig. S7a–ax) and M elements (online Supplementary Figs S9 a–az; S10a–s, ab–bb; S11a–s), but S elements (online Supplementary Fig. S14a–bd) show no significant variation. While the sinusoidal outline of the dextral Pa element in the upper view is a characteristic feature of A. tvaerensis (Fig. 3k), a few samples also yielded elements with almost straight posterior process (Fig. 3l, m). All these specimens come from the upper part of the A. tvaerensis range, from the B. gerdae CSz, and are similar to the specimen illustrated by Bergström & Orchard (1985, pl. 2.3, fig. 11) from the Girvan area, southern Scotland. Configuration of the extra postero-lateral process is highly variable. The length of it can change from a small node (Fig. 3n) or bulge (Fig. 3o) to a prominent process (Fig. 3p), while denticulation on it may be absent (Fig. 3q), rudimentary (in a form of ridge; Fig. 3r) or prominent (Fig. 3p). The small anterior lobe next to the extra postero-lateral process is usually merged to the process (Fig. 3s) but, occasionally, may be located separately (Fig. 3n). Sinistral Pa elements of A. tvaerensis in the lowermost samples show no clearly recognizable lateral expansion on the outer side of its posterior process (Fig. 3t, u), but it appears on elements from the uppermost B. variabilis CSz (Fig. 3v). In the lowermost B. gerdae CSz, the lateral expansion on the sinistral Pa elements becomes prominent (Fig. 3w) and this feature is present throughout the upper range of A. tvaerensis. The youngest sinistral Pa elements of A. tvaerensis also show additional denticulation on that lateral expansion (Fig. 3x).

The variation of M elements is expressed in the changing number of anteriorly oriented denticles, including cusp and orientation of the prominent posterior denticle. Previously described morphotypes of M elements in A. tvaerensis (Viira, 2008) are also present in Mehikoorma section. However, their proposed stratigraphic importance is not evident. The first morphotype is described as ‘…low in the range of A. tvaerensis, is similar to A. inaequalis…’ and ‘…has a large straight denticle in the anterior part of the oral denticle row…’ (Viira, 2008, p. 36, fig. 8C). Another morphotype is similar to the third described as ‘…late specimens of A. tvaerensis is characterized by an orally directed large denticle in the anterior part and occurs on the level of the B. gerdae range…’ (Viira, 2008, p. 36, figs 5K, 8Q). M elements that fit with the previous descriptions (Viira, 2008, figs 5K, 8C, Q) are present from the lower part of A. tvaerensis range (Fig. 3b), in the B. alobatus CSz (Fig. 3h, y, z), in the lower part of the B. gerdae CSz (Fig. 3aa, ab) and within the uppermost range of A. tvaerensis (Fig. 3a, c). Similarly, the proposed second morphotype described as ‘…with a very characteristic posteriorly directed denticle in the M element occurs in the middle part of the A. tvaerensis range…’ (Viira, 2008, p. 36, figs 5E, 8H) is present in almost every sample from the B. alobatus CSz (Fig. 3a, e, i, ad, ae), from the lower B. gerdae CSz (Fig. 3af) and from the upper range of A. tvaerensis (Fig. 3ag, ah).

The prominently reclined posterior denticle of variable size of the M element between the denticle and the posterior processes forms an angle of c. 50° within B. variabilis CSz (Fig. 3e, i, ad) and c. 75° within B. gerdae CSz (Fig. 3af, ah). No stratigraphic significance is found within the change of the number of anteriorly directed denticles. The number of these denticles ranges from two (Fig. 3b, ag) up to four or more (Fig. 3ah, ai) varying in size.

No distinct variation pattern is recognized in Pb elements (Fig. 3aj–ao; online Supplementary Fig. S7a–ax) and S elements (Fig. 3ap–at; online Supplementary Fig. S14a–bd) of A. tvaerensis.

Material. Mehikoorma section: 47 dextral Pa, 33 sinistral Pa, 25 dextral Pb, 23 sinistral Pb, 118 M and about 200 S elements; Velise section: 10 dextral Pa, 4 sinistral Pa, 5 dextral Pb, 5 sinistral Pb, 28 M and about 100 S elements; Ruhnu section: 11 dextral Pa, 9 sinistral Pa, 9 dextral Pb, 10 sinistral Pb, 20 M and about 80 S elements; Fjäcka Main section: 14 dextral Pa, 12 sinistral Pa, 12 dextral Pb, 11 sinistral Pb, 8 M and about 60 S elements; Smedsby Gård drillcore: 20 dextral Pa, 12 sinistral Pa, 14 dextral Pb, 10 sinistral Pb, 14 M and about 50 S elements.

Distribution. Baltoscandian region: lower half of the Sandbian; Fjäcka Main section: Dalby limestone (6.05–13.10 m above the base of Dalby limestone); Mehikoorma section: Dreimani and Tatruse Formation (316.4–328.7 m); Ruhnu section: Dreimani and Adze Formation (653.8–662.8 m); Smedsby Gård drillcore: Dalby limestone (95.35–130.49 m); Velise section: Pihla and Tatruse Formation (202.95–211.83 m).

Amorphognathus viirae sp. nov.

1974 Amorphognathus ordovicica spp. nov. 1.; Viira (1974, p. 59–60, pl. 7: 15, 16).

1974 Amorphognathus ordovicica spp. nov. 2.; Viira (1974, p. 59–60, pl. 7: 17, 20–22).

1974 Amorphognathus ordovicica spp. nov. 3.; Viira (1974, p. 59–60, pl. 7: 18, 19, 24, not including 23).

1990 Amorphognathus superbus Rhodes (1953); Dzik (1990, p. 24, text-figs 16, 17: depth 2–3 m).

1994 Amorphognathus superbus Rhodes (1953); Dzik (1994, p. 93, 94, pl. 23: 3–5; text-fig. 22: samples 93–96).

1999a Amorphognathus aff. ventilatus Ferretti & Barnes (1997); Dzik (1999 a, pl. 1: 1–21).

1999a Amorphognathus tvaerensis Bergström (1962); Dzik (1999 a, pl. 1: 22–26).

1999b Amorphognathus tvaerensis Bergström (1962); Dzik (1999 b, text-fig. 5: depth 1.5–2.5 m).

?2007 Amorphognathus tvaerensis Bergström (1962); Goldman et al. (2007, fig. 7: 17–22).

2008 Amorphognathus tvaerensis Bergström (1962); Viira (2008, fig. 5J, K, L, M, N, not including A–I, O–R; fig. 6F, not including A–E, G–I; not including fig. 8A–S).

2020 Amorphognathus tvaerensis Bergström (1962); Paiste et al. (2020, fig. 4A–D, not including E, F; not including fig. 5A–D).

2022 Amorphognathus tvaerensis Bergström (1962); Paiste et al. (2022, fig. 4D–G, K, L, R, V–X, AH–AN, AV, not including A–C, H–J, M–Q, S–U, Y–AG, AO–AU, AW).

Holotype. GIT870-99 (Fig. 4a), from Mehikoorma-421 core section sample 314.9–315 m, Tatruse Formation, Sandbian Stage, Upper Ordovician, Estonia.

Fig. 4. Amorphognathus viirae sp. nov. elements from the Mehikoorma core: (a) holotype, dextral Pa element, upper view, sample 314.9–315 m, specimen GIT870-99; (b) dextral Pa element, upper view, sample 315.5–315.7 m, specimen GIT870-95; (c) dextral Pa element, outer-lateral view, sample 313.1–313.2 m, specimen GIT870-121; (d) dextral Pa element, lower view, sample 312.6–312.7 m, specimen GIT870-128; (e) dextral Pa element, upper view, sample 312.6–312.7 m, specimen GIT870-130; (f) dextral Pa element, upper view, sample 316.0–316.1 m, specimen GIT870-90; (g) dextral Pa element, upper view, sample 314.9–315 m, specimen GIT870-101; (h) dextral Pa element, upper view, sample 313.1–313.2 m, specimen GIT870-120; (i) dextral Pa element, upper view, sample 313.1–313.2 m, specimen GIT870-123; (j) sinistral Pa element, upper view, sample 315.5–315.7 m, specimen GIT870-97; (k) sinistral Pa element, upper view, sample 312.6–312.7 m, specimen GIT870-135; (l) sinistral Pa element, upper view, sample 312.2–312.3 m, specimen GIT870-140; (m) sinistral Pa element, lower view, sample 312.6–312.7 m, specimen GIT870-134; (n) sinistral Pa element, posterior process, upper view, sample 313.6–313.7 m, specimen GIT870-117; (o) sinistral Pa element, upper view, sample 312.6–312.7 m, specimen GIT870-131; (p) sinistral Pa element, upper view, sample 313.1–313.2 m, specimen GIT870-126; (q) dextral Pb element, inner-lateral view, sample 316.0–316.1 m, specimen GIT870-288; (r) dextral Pb element, upper view, sample 316.0–316.1 m, specimen GIT870-289; (s) dextral Pb element, upper view, sample 312.2–312.3 m, specimen GIT870-314; (t) dextral Pb element, inner-lateral view, sample 314.9–315 m, specimen GIT870-292; (u) dextral Pb element, upper view, sample 314.9–315 m, specimen GIT870-293; (v) dextral Pb element, upper view, sample 314.9–315 m, specimen GIT870-294; (w) sinistral Pb element, outer-lateral view, sample 313.1–313.2 m, specimen GIT870-305; (x) sinistral Pb element, upper view, sample 314.2–314.3 m, specimen GIT870-300; (y) dextral M element, lateral view, sample 316.0–316.1 m, specimen GIT870-489; (z) sinistral M element, lateral view, sample 315.5–315.7 m, specimen GIT870-497; (aa) sinistral M element, lateral view, sample 314.9–315 m, specimen GIT870-512; (ab) sinistral M element, lateral view, sample 314.2–314.3 m, specimen GIT870-522; (ac) sinistral M element, lateral view, sample 313.6–313.7 m, specimen GIT870-537; (ad) sinistral M element, lateral view, sample 313.1–313.2 m, specimen GIT870-538; (ae) sinistral M element, lateral view, sample 312.6–312.7 m, specimen GIT870-550; (af) sinistral M element, lateral view, sample 312.6–312.7 m, specimen GIT870-558; (ag) sinistral M element, lateral view, sample 312.6–312.7 m, specimen GIT870-559; (ah) sinistral M element, lateral view, sample 312.2–312.3 m, specimen GIT870-566; (ai) sinistral M element, lateral view, sample 312.2–312.3 m, specimen GIT870-568; (aj) Sa element, lateral view, sample 312.6–312.7 m, specimen GIT870-705; (ak) Sd element, lateral view, sample 312.2–312.3 m, specimen GIT870-711; (al) Sb element, lateral view, sample 312.2–312.3 m, specimen GIT870-712; (am) Sb element, lateral view, sample 314.2–314.3 m, specimen GIT870-691; (an) Sd element, lateral view, sample 312.2–312.3 m, specimen GIT870-710; (ao) Sc element, lateral view, sample 312.2–312.3 m, specimen GIT870-717; (ap) Sc element, lateral view, sample 312.2–312.3 m, specimen GIT870-718; (aq) Sc element, lateral view, sample 313.6–313.7 m, specimen GIT870-699; (ar) Sc element, lateral view, sample 314.2–314.3 m, specimen GIT870-696. All scale bars are 100 μm.

Etymology. The species is named after conodont specialist Viive Viira who published the first illustrations of this new species (Viira, 1974).

Diagnosis. A species of Amorphognathus in which the main denticle row on the dextral Pa element in upper view is shaped as sinuous curve. A small and distinct lateral lobe occurs on the outer side of the posterior process, at the starting point of the sinuous curvature. The element bears bifurcated lateral processes on both sides.

Description. Dextral Pa element. Inner lateral process is connected to the main blade just posteriorly of the cusp, at the starting point of the sinuous curvature (Fig. 4a). The shorter (posterior) branch of the process is almost perpendicular to the main row of denticles. The longer, anterior branch (at least twice as long the posterior branch) is directed away from the anterior process at an angle of c. 40°. The bifurcated outer lateral process is smaller than the inner process and is positioned just anteriorly of the cusp (Fig. 4b, c). In all available specimens this process is only partly preserved but, as much as is visible, its posterior branch seems to be longer than the anterior branch. The anterior process of the dextral Pa element is straight in upper view (Fig. 4a) and tilted somewhat downwards (at c. 25°) in lateral view (Fig. 4c). The posterior process is slightly arched in lateral view. A distinct higher denticle (smaller than the cusp) is positioned in the proximal part of the main row of denticles on the posterior process, just above the small platform on its outer side at the beginning of the sinuous curvature. A distinct rim surrounds the basal platform of the element (Fig. 4c). The basal cavity is wide and deep below the posterior and lateral processes, but narrows sharply below the anterior process and continues as a narrow groove up to its distal end (Fig. 4d). The variation in morphology of the dextral Pa element (online Supplementary Figs S2v; S3j–m, o, q, r, u–w, aa, ab, ae–ag; S4a–c, g–j, n–q, w–z) is mainly expressed in configuration of its posterior process. In upper view, the distal end of it may be located in the line of the anterior process (Fig. 4a) or turned outwards (Fig. 4b). Additionally, a few elements bear a single denticle on the small outer platform lobe (Fig. 4b, e). Some elements studied bear a quite distinct lateral expansion on the inner sides of their posterior processes. This feature appears sporadically at different levels in the species’ range interval: in the lowermost sample studied (Fig. 4f), in the middle part of the range (Fig. 4g) and also in its upper part (Fig. 4h). A probable juvenile example of a dextral Pa element (Fig. 4i) has slender processes with no lateral expansion or clearly developed lateral lobe on the outer side of the posterior process.

The sinistral Pa element is a platform element with predominantly straight (Fig. 4j) or slightly curved (Fig. 4k) main row of denticles. The element bears a bifurcated lateral process on both sides. The inner lateral process joins the main blade somewhat posteriorly of the cusp (Fig. 4k). Its anterior branch is twice as long or even slightly longer than posterior branch. Branches of the outer lateral process located just below the cusp are about the same size (Fig. 4l). Similarly to the dextral Pa element, the anterior process of the sinistral Pa is tilted downwards and a distinct rim surrounds its basal platform (Fig. 4k, m). The main variation in morphology of the sinistral Pa element (online Supplementary Figs S3n, p, s, t, x–z, ac, ad, ah, ai; S4d–f, k–m, r–v, aa–ac) is related to the width of the basal platform below its posterior process that is narrower on specimens from older samples (Fig. 4j, k, n) and becomes wider on those from younger samples (Fig. 4o). The small outer lateral expansion on its posterior process becomes more common on specimens from younger samples (Fig. 4l, p).

The dextral Pb element is dominated by a conspicuous central suberect pointed cusp (Fig. 4q) with three diverging denticulated processes (Fig. 4r). The posterior process is straight, with a slightly curved to inner side row of denticles and a distinct basal lateral expansion (Fig. 4s). The outline of this expansion is wavy in lateral view (Fig. 4q). The angle between the downwards oriented anterior process and posterior process is c. 90°. The anterior process is bent to inner side, crosswise to posterior process (Fig. 4t), slightly longer than the posterior process and with an inner edge and pointed end (Fig. 4u). The outer lateral process is short and emerges with anterior process just below the cusp (Fig. 4v). This short process bears 1–3 discrete pointed denticles and is laterally compressed. The cusp of the dextral Pb element (Fig. 4q) is twice as high as that of the sinistral Pb element (Fig. 4w). No significant variation in morphology of the dextral Pb element was found (online Supplementary Figs S7az, ba, bd–bf, bi, bj, bm; S8c, f, g, l, m).

The sinistral Pb element resembles its dextral counterpart and differs in only a few noticeable ways. The cusp of the sinistral Pb (Fig. 4w) is smaller in size compared with the dextral Pb (Fig. 4t), and the angle between the downwards directed anterior process and posterior process is c. 120° (Fig. 4x). Additionally, both posterior and anterior process are of a similar length and possess basal lateral expansions. No significant variation in morphology of the sinistral Pb element was found (online Supplementary Figs S7ay, bc, bg, bh, bk, bl; S8a, b, d, e, h–k).

The M element is characterized by three variously denticulated processes. The anterior edge of the anterior process is sharp, rarely denticulated and may bear a different number (usually 2–5) of anteriorly directed denticles (Fig. 4y–ai). Cusp (interpreted here as the first prominent denticle anterior to the posterior denticle) may be prominent (Fig. 4y) or undistingushable in size (Fig. 4ae, ag). The backwards directed posterior denticle on the proximal part of the posterior process also varies greatly in size but, as a rule, is very prominent, sharply turned backwards, and with its distal end curved anteriorly (Fig. 4ac, af–ai). The posterior process is usually poorly denticulated (Fig. 4ab, ad) and form an angle of c. 40° with the anterior process. The inner lateral process is directed at an angle of c. 45° with the posterior process. Rare denticles occur in the distal part of the lateral process. All processes are sharp-edged and bear ridges (keels) between denticles. The number, size and location of denticles on their anterior end of M element vary greatly (online Supplementary Figs S10t–aa; S11t–ba; S12a–av), for example, from specimens with almost unrecognizeable posterior denticle (Fig. 4ab) to those with a very distinct denticle (Fig. 4ac, af–ai), and from those with only two larger denticles on the anterior process (Fig. 4ae, ai) to those with several (Fig. 4y, ag, ah). Additionally, the denticle and the posterior processes form an angle of c. 90° (Fig. 4z, aa) in older specimens and c. 110° in younger specimens (Fig. 4ah, ai). The latter variation described in the studied section seems to have stratigraphic value, allowing the older and younger populations of A. viirae sp. nov. to be distinguished.

The S elements of A. viirae sp. nov. (Fig. 4aj–ar; online Supplementary Figs S14be–bu; S15a–ag) are typical of the genus Amorphognathus and similar to those in the apparatus of its ancestor A. tvaerensis (Fig. 3ap–at). No significant differences in their morphology could be noted. However, some noticeable differences were present within Sc elements. The orientation of the cusp varies from proclined (Fig. 4ao–ap) to erect (Fig. 4aq–ar). Denticulation on the anterior process may vary from rare denticles (Fig. 4ao–ap) up to a row containing numerous denticles (Fig. 4aq–ar). A bulge on the outer side of the element between the posterior and anterior processes may be visible (Fig. 4ao), but the same area can also be round and smooth (Fig. 4ar).

Comparison. The absence of an extra postero-lateral process on the outer side of dextral Pa element was the main distinguishing feature for erecting A. viirae sp. nov. as a new species and differentiating it from its predecessor A. tvaerensis. This process is one of the main characteristic features of the dextral Pa element of A. tvaerensis, and its absence has been used previously for differentiating dextral Pa elements of A. superbus. The small lateral lobe anterior to the extra postero-lateral process in the dextral Pa element of A. tvaerensis (Fig. 3s) persists in dextral Pa elements of A. viirae sp. nov. (Fig. 4a).

The main central row of denticles is shaped as sinuous curve in both A. tvaerensis and A. viirae sp. nov.; the curvature is smooth in elements of A. tvaerensis (Fig. 3k) but considerably much more distinct in A. viirae sp. nov., in which it resembles a cubic function (Fig. 4a). Both Pa elements of A. viirae sp. nov. have distinctly longer anterior branches on their inner lateral processes (Fig. 4a, k) than the elements of A. tvaerensis (Fig. 3k, t). Additionally, the anterior part of the posterior process (in front of the cusp) is aligned in a straight line with the anterior proccess (Fig. 4a) in dextral Pa elements of A. viirae sp. nov., while there is a curvature in this part of the posterior process in A. tvaerensis (Fig. 3k).

Pb elements of these two species are almost identical, with the only subtle differences in morphology of the dextral Pb elements. The posterior process in the dextral Pb element of A. tvaerensis bears a straight row of denticles (Fig. 3al), while it is slightly curved inwards just behind the cusp in A. viirae sp. nov. (Fig. 4s). A narrow basal platform-like edge is present in the posterior processes of both species. Additionally, elements of A. viirae sp. nov. possess a distinctive inner lateral expansion (Fig. 4s) that in lateral view has a wavy outline (Fig. 4q). The anterior process of the sinistral Pb element of A. viirae sp. nov. bears narrow basal platform ledges on both sides of the denticle row (Fig. 4x), while no noticeable outwards extensions are present on analogous elements of A. tvaerensis (Fig. 3ao).

The M elements of A. viirae sp. nov. and A. tvaerensis are morphologically very similar. The most distinct difference is related to the configuration of their posterior denticle: on elements of A. viirae sp. nov. the distal part of it is erect or curved anteriorly (Fig. 4z), while on elements of A. tvaerensis it is reclined (Fig. 3af, y). The processes of the M element in A. tvaerensis also often tends to be more densely denticulated than those of A. viirae sp. nov. but, as a rule, this feature becomes apparent in a comparison with larger samples in which M elements are well represented.

A detailed comparison of A. viirae sp. nov. with younger species of Amorphognathus is problematic based on material from the Mehikoorma core section. In the upper part of the studied interval, above the range of A. viirae sp. nov., Amorphognathus is missing or represented by rare, poorly preserved specimens. This phenomenon is characteristic of the upper Sandbian – lower Katian interval in Baltoscandian sections (Bergström, 2007; Viira, 2008). However, based on published data, some comparisons are still possible.

The dextral Pa element of A. superbus shows a relatively straight main row of denticles with the tip of the posterior process turned to the outer side (Bergström, 1971, pl. 2, fig. 8; Dzik, 1976, fig. 28a; Goldman et al. 2007, fig. 7: 15), while A. viirae sp. nov. has a recognizable sinuous curve in the central part of the main row of denticles (Fig. 4a). Additionally, no extra postero-lateral process (or lateral lobe) on the outer side of the posterior process is present in the dextral Pa elements of A. superbus. Furthermore, the posterior branch of inner bifurcated processes of the dextral Pa element of A. superbus is considerably shorter than the anterior process (difference 4–5 times) than in A. viirae sp. nov. (only 2–3 times). In A. complicatus (Rhodes, 1953) the inner lateral process is simple, without a second branch. The posterior process on sinistral Pa element of A. viirae sp. nov is 2–3 times longer than the anterior process, while its size difference in younger A. superbus and A. complicatus decreases and is only c. 1.5 times (Dzik, 1976, fig. 28g; Goldman et al. 2007, fig. 7: 16). The dextral Pb element of A. superbus lacks the slight curvature of denticle row towards the inner side of the posterior process and the distinctive inner basal platform-like lobe (Fig. 5q–s) recognized in elements of A. viirae sp. nov. Additionally, the basal platform on the inner side of the posterior process of the dextral Pb element in A. viirae sp. nov. has a wavy edge (in lateral view, Fig. 4q), while in A. superbus it is straight with distal ends of the process pointing upwards (Fig. 5r) or downwards (Fig. 5s). The denticles on posterior and anterior processes on both Pb elements of A. superbus are more prominent (Fig. 5r, t) compared with the denticulation on Pb element of A. viirae sp. nov. (Fig. 4q, w). Additionally, the cusps of the sinistral Pb (Fig. 5t) and dextral Pb elements (Fig. 5s) of A. superbus are of similar size, but in A. viirae sp. nov. the cusp of the sinistral Pb (Fig. 4w) is half the size of that of the dextral Pb element (Fig. 4t). The processes of the S elements in A. viirae sp. nov. (Fig. 4al) are more densely denticulated and denticles laterally compressed to a higher degree than in A. superbus (Fig. 5aa). The strongly backwards-turned posterior denticle characteristic of the M element of A. viirae sp. nov. (Fig. 4z, aa, ac, ah, ai) is missing in A. superbus (Fig. 5j) or only occurs as a small barb-like tooth (Fig. 5i). Additionally, the number of anteriorly directed denticles including an often undistinguishable cusp on the anterior end of the M element vary noticeably in A. viirae sp. nov. (Fig. 4y–ac), whereas the element of A. superbus has only two anteriorly directed denticles a cusp and an additional denticle (Fig. 5i–l).

Fig. 5. Amorphognathus in the Mehikoorma core section. (a–e) Amorphognathus viirae sp. nov.: (a) dextral Pa element, posterior process, upper view, sample 318.5–318.6 m, specimen GIT870-57; (b) dextral M element, lateral view, sample 318.5–318.6 m, specimen GIT870-439; (c) sinistral M element, lateral view, sample 318.5–318.6 m, specimen GIT870-434; (d) sinistral M element, lateral view, sample 318.5–318.6 m, specimen GIT870-435; (e) dextral M element, lateral view, sample 318.5–318.6 m, specimen GIT870-437. (f–h) Amorphognathus tvaerensis (Bergström, 1962): (f) dextral Pa element, upper view, sample 318.5–318.6 m, specimen GIT870-55; (g) sinistral M element, lateral view, sample 318.5–318.6 m, specimen GIT870-446; (h) sinistral M element, lateral view, sample 318.5–318.6 m, specimen GIT870-447. (i–am) Amorphognathus superbus (Rhodes, 1953): (i) sinistral M element, lateral view, sample 294.7–294.8 m, specimen GIT870-573; (j) dextral M element, lateral view, sample 288.2–288.3 m, specimen GIT870-613; (k) sinistral M element, lateral view, sample 290.1–290.2 m, specimen GIT870-595; (l) dextral M element, lateral view, sample 292.8–292.9 m, specimen GIT870-577; (m) dextral Pa element, posterior process, upper view, sample 290.1–290.2 m, specimen GIT870-196; (n) sinistral Pa element, posterior process, upper view, sample 289.7–289.8 m, specimen GIT870-209; (o) probable fragment of sinistral Pa element, posterior process, upper view, sample 296.3–296.4 m, specimen GIT870-149; (p) dextral M element, lateral view, sample 296.3–296.4 m, specimen GIT870-570; (q) dextral Pb element, upper view, sample 290.1–290.2 m, specimen GIT870-348; (r) dextral Pb element, inner-lateral view, sample 290.7–290.8 m, specimen GIT870-342; (s) dextral Pb element, inner-lateral view, sample 291.2–291.3 m, specimen GIT870-338; (t) sinistral Pb element, outer-lateral view, sample 292.2–292.3 m, specimen GIT870-330; (u) sinistral Pb element, upper view, sample 290.1–290.2 m, specimen GIT870-346; (v) sinistral Pb element, upper view, sample 291.2–291.3 m, specimen GIT870-335; (w) Sa element, lateral view, sample 290.7–290.8 m, specimen GIT870-790; (x) Sd element, lateral view, sample 292.8–292.9 m, specimen GIT870-765; (y) Sb element, lateral view, sample 292.8–292.9 m, specimen GIT870-766; (z) Sd element, lateral view, sample 290.1–290.2 m, specimen GIT870-803; (aa) Sd element, lateral view, sample 295.8–295.9 m, specimen GIT870-741; (ab) Sd element, lateral view, sample 292.8–292.9 m, specimen GIT870-767; (ac) Sb element, lateral view, sample 290.7–290.8 m, specimen GIT870-793; (ad) Sc element, lateral view, sample 290.1–290.2 m, specimen GIT870-809; (ae) Sc element, lateral view, sample 290.1–290.2 m, specimen GIT870-810; (af) Sc element, lateral view, sample 294.7–294.8 m, specimen GIT870-755; (ag) Sc element, lateral view, sample 292.8–292.9 m, specimen GIT870-768; (ah) sinistral M element, lateral view, sample 295.8–295.9 m, specimen GIT870-571; (ai) sinistral M element, lateral view, sample 291.9–292 m, specimen GIT870-586; (aj) dextral M element, lateral view, sample 291.2–291.3 m, specimen GIT870-590; (ak) sinistral M element, lateral view, sample 290.1–290.2 m, specimen GIT870-594; (al) sinistral M element, lateral view, sample 289.3–289.4 m, specimen GIT870-607; (am) sinistral M element, lateral view, sample 289.3–289.4 m, specimen GIT870-608. (an–at) Amorphognathus ventilatus (Ferretti & Barnes, 1997): (an) sinistral M element, lateral view, sample 292.8–292.9 m, specimen GIT870-580; (ao) dextral M element, lateral view, sample 292.8–292.9 m, specimen GIT870-581; (ap) dextral M element, lateral view, sample 292.8–292.9 m, specimen GIT870-582; (aq) sinistral M element, lateral view, sample 292.2–292.3 m, specimen GIT870-583; (ar) sinistral M element, lateral view, sample 292.2–292.3 m, specimen GIT870-584; (as) sinistral M element, lateral view, sample 292.2–292.3 m, specimen GIT870-585; (at) dextral M element, lateral view, sample 291.9–292 m, specimen GIT870-587. (au–az) Amorphognathus complicatus (Rhodes, 1953): (au) probable fragment of dextral Pa element, posterior process, upper view, sample 288.2–288.3 m, specimen GIT870-229; (av) sinistral Pa element, upper view, sample 288.6–288.7 m, specimen GIT870-223; (aw) sinistral M element, lateral view, sample 288.6–288.7 m, specimen GIT870-609; (ax) dextral M element, lateral view, sample 288.6–288.7 m, specimen GIT870-610; (ay) sinistral M element, lateral view, sample 288.2–288.3 m, specimen GIT870-614; (az) sinistral M element, lateral view, sample 289.7–289.8 m, specimen GIT870-605. All scale bars are 100 μm.

Remarks. One dextral Pa element (Fig. 5a) and some probable M elements (Fig. 5b–e) of A. viirae sp. nov. were found together with typical dextral Pa and M elements of A. tvaerensis (Fig. 5f and Fig. 5g, h, respectively) in the interval 318.5–318.6 m, well below the last occurrence of A. tvaerenis at 316.4 m. The continuous range of A. viirae sp. nov. starts at 316.55 m. Because such an earlier occurrence of A. viirae sp. nov. in a single sample below the interval of continuous range of this taxon has not been recorded in the Ruhnu and Velise core sections (restudy of collections published by Männik, 2003 and Paiste et al. 2022, respectively), this is currently considered to be as a result of contamination. Further studies of other sections are required to determine the actual situation.

From the B. gerdae and B. alobatus CSzs in the Tartu core section, from the interval coeval with that yielding A. viirae sp. nov. in the Mehikoorma core section, Amorphognathus cf. tvaerensis, Amorphognathus n. sp. and Amorphognathus sp. A were identified by Stouge (1998). However, as no description and illustrations of these taxa were provided, it is not possible to confirm whether (some of) these conodonts might belong to A. viirae sp. nov.

The variation in morphology of P and M elements of A. tvaerensis has been noticed by many authors (Dzik, 1990, 1994; Stouge, 1998; Viira, 2008; Xu et al. 2010; Paiste et al. 2022). The first illustrations of the elements described here as A. viirae sp. nov. come from the Äiamaa, Are and Ohesaare core sections in Estonia (Viira, 1974). No additional description or information was provided. The first reference outside Estonia is from the Mójcza section in Poland. From this section, Dzik (1990, figs 17, 18, depth c. 2 m) illustrated the evolution of Amorphognathus in the Mójcza section with figures of transitional Pa elements between A. tvaerensis and A. superbus. These elements lack the small lateral lobe anterior of the extra postero-lateral process, noticeable shortening of the extra postero-lateral process and elongation of the anterior branch on inner lateral process. An additional illustration of the evolution of Amorphognathus in the same section by Dzik (1999 b, fig. 5) shows a dextral Pa element that is more similar to A. viirae sp. nov. In a more detailed description of the Mójcza section, Dzik (1994) shows a general outline of a fragmented dextral Pa element (Dzik, 1994, fig. 22: sample 93), describing this element as an early form of A. superbus, morphologically transitional between the typical representatives of A. tvaerensis and A. superbus. However, later transitional elements were reidentified as A. aff. ventilatus (Dzik, 1999 a). Additional samples from the Mójcza section yielded elements that were designated as A. aff. ventilatus or assigned as late forms of A. tvaerensis (Dzik, 1999 a, pl. 1, figs 22–26). The illustrations of the Pa elements of A. viirae sp. nov. from the Mójcza section are morphologically more variable than those from the Mehikoorma section. The lateral lobe of posterior process of the dextral Pa elements in the Mehikoorma section is rarely denticulated (Fig. 4b, e), but this feature is very common on elements from the Mójcza section (Dzik, 1994, text-fig. 22: 93, 96; Dzik, 1999 a, pl. 1, figs 1–26).

The strata in the Mehikoorma section containing A. viirae sp. nov. are overlain by an interval of depth almost 13 m (310.6–297.75 m) yielding no specimens of Amorphognathus. For that reason, the full range of A. viirae sp. nov. and its relationship to the younger representatives of Amorphognathus cannot be identified precisely; however, it is evident that A. tvaerensis is the ancestor of A. viirae sp. nov. and A. superbus is its descendant. This conclusion is supported by data from the stratotype section of the base of the Katian Stage (Black Knob Ridge Section; Goldman et al. 2007). The uppermost illustrated Pa elements of A. tvaerensis from the Black Knob Ridge Section, 4.3 m below the lower boundary of the Katian Stage (Goldman et al. 2007, fig. 7: 17, 18), are similar to those of A. viirae sp. nov. and, before direct study of these specimens, can be tentatively considered as conspecific. Most noticeably, the dextral Pa element (Goldman et al. 2007, fig. 7: 18) from the Black Knob Ridge Section lacks the small lateral lobe anterior to the extra postero-lateral process inherent to A. tvaerensis (Ferretti & Bergström, 2022, fig. 4a–h), and the anterior branch on the inner lateral process is as long as the anterior process, similar to that of A. viirae sp. nov. elements (Fig. 4a). Other illustrated Pa elements of Amorphognathus from the Black Knob Ridge Section, from 1.7 m above the Katian boundary, were originally identified as Amorphognathus sp. with probable affiliation to A. superbus (Goldman et al. 2007, fig. 7:15, 16). These illustrated specimens resemble elements of typical A. superbus, suggesting that A. superbus might really be a successor of A. viirae sp. nov. However, as in the Black Knob Ridge Section, ranges of A. viirae sp. nov. and A. superbus are separated by an unsampled interval of thickness c. 5 m and the morphological changes in transition between A. viirae sp. nov. and A. superbus are currently not observed.

Fig. 6. Relationships between the Sandbian and Katian boundaries with conodont zones within Atlantic Realm Conodont Zone successions (Nõlvak et al. 2006; Bergström et al. 2007; Männik et al. 2021). Mid. – Middle; Dar. – Darriwilian; Kat. – Katian.

Fig. 7. Variations of A. tvaerensis and A. viirae sp. nov. elements in Mehikoorma section within the distribution interval of Baltoniodus variabilis, B. gerdae and B. alobatus. Elements are not to scale. Numbers with letters next to elements refer to the illustrations in the Supplementary Material. Double-headed arrow points to the posterior process of dextral Pa element. Square marks the lateral extension on the outer side of the posterior process on sinistral Pa elements. Circles indicate ends of the inner edge of posterior process of dextral Pb elements, and the line below the figure repeats the shape of the edge in lateral view.

The reinvestigation of formerly published conodont collections from the Swedish sections carried out by TP with the kind permission of S. Bergström confirmed that collections from the Fjäcka main section and the Smedsby Gård drillcore (distribution of taxa characterized in Bergström, 2007 and Bergström et al. 2011, respectively, but specimens not illustrated), contain A. viirae sp. nov.

Only one M element of A. viirae sp. nov., with distinctive curved posteriorly directed denticle, was found in the Smedsby Gård drillcore at depth 93.04–93.10 m. The upper part of the previously described range of A. tvaerensis contains mostly elements determinable as Amorphognathus sp. only. In the Fjäcka main section A. viirae sp. nov. was identified at depth 15.05–17.30 m (according to sample labels), above the base of the Dalby Limestone, where recognizable fragments of the dextral Pa elements occur. A single M element in this section also comes from the sample 15.05 m above the base of the Dalby Limestone. It is noteworthy that between the uppermost identifiable A. tvaerensis (13.05–13.10 m above the base of Dalby Limestone) and the lowermost A. viirae sp. nov. sample (15.05–15.10 m above the base of Dalby Limestone) there is a 1.95 m interval with samples that yielded only rare and poorly preserved elements that could be identified only as Amorphognathus sp.

Material. Mehikoorma section: 31 dextral Pa, 25 sinistral Pa, 14 dextral Pb, 14 sinistral Pb, 90 M and about 100 S elements; Velise section: 10 dextral Pa, 8 sinistral Pa, 7 dextral Pb, 10 sinistral Pb, 13 M and about 50 S elements; Ruhnu section: 5 dextral Pa, 4 sinistral Pa, 4 dextral Pb, 5 sinistral Pb, 10 M and about 40 S elements; Fjäcka Main section: 5 dextral Pa, 1 sinistral Pa, 1 M and about 50 Pb and S elements; Smedsby Gård drillcore: 1 M element.

Distribution. In all sections where A. viirae sp. nov. can be identified, the first appearance datum (FAD) of the species falls within the upper range of B. gerdae or within the lowermost range of B. alobatus (when B. gerdae is absent or when Amorphognathus can only be identified on genus level within B. gerdae range). Baltoscandian region: upper half of the Sandbian; Abja section: Adze Formation (429.3 m); Äiamaa section: Tatruse Formation (196.08 m); Are section: Tatruse Formation (291.7 m); Fjäcka Main section: Dalby limestone (15.05–17.85 m above the base). Holy Cross Mountains: Mójcza Quarry, Mójcza limestone (samples 93–97 in Dzik, 1994, text-fig. 22); Mehikoorma section: Tatruse and Kahula formations (310.6–316.2 m); Ohesaare section: Tatruse Formation (473.8–471.8 m); Ruhnu section: Adze Formation (656.8–653.8 m); Smedsby Gård drillcore: Dalby limestone (93.04–93.10 m); Velise section: Tatruse and Kahula formations (202.54–197.74 m).

Amorphognathus superbus (Rhodes, 1953)

1953 Holodontus superbus sp. nov.; Rhodes (1953, p. 304, pl. 21: 125–127).

1964 Holodontus superbus Rhodes (1953); Bergström (1964, p. 26–7, text-fig. 11).

1971 Amorphognathus superbus Rhodes (1953); Bergström (1974, pl. 2: 8, 9).

1976 Amorphognathus superbus Rhodes (1953); Dzik (1976, fig. 28a–e, ?f–i).

?1980 Amorphognathus superbus Rhodes (1953); Merrill (1980: fig. 5: 9, 10, not including fig. 4:1).

1980 Amorphognathus complicatus Rhodes (1953); Merrill (1980, fig. 4:10 not including 2–9, 11–23).

1980 Amorphognathus superbus Rhodes (1953); Orchard (1980, 16, 17, pl. 4: 19, 20, 24).

1985 Amorphognathus superbus Rhodes (1953); Savage and Bassett (1985, p. 692, 694, pl. 83: 1–19).

1994 Amorphognathus superbus Rhodes (1953); Dzik (1994, p. 93–94, text-fig. 22: samples 100–112; not including pl. 23: 3–5).

1999a Amorphognathus superbus Rhodes (1953); Dzik (1999a, p. 246, pl. 1: 27–30).

2007 Amorphognathus sp.; Goldman et al. (2007, fig. 7: 15, 16).

2014a Amorphognathus superbus Rhodes (1953); Ferretti et al. (2014a, p. 819–21, fig. 12Z, AA).

2014b Amorphognathus superbus Rhodes (1953); Ferretti et al. (2014b, p. 114–6, pl. 1: 13–15).

2017 Amorphognathus superbus Rhodes (1953); Männik (2017, fig. 4M, R, S, U, ?P).

Diagnosis. Rhodes (1953, p. 304).

Remarks. The type specimen of A. superbus is a poorly preserved M element (Rhodes, 1953, pl. 21, figs 125–127), refigured by Bergström (1964, text-fig. 11). The holotype has a central cusp with two smaller denticles on both sides of it, the larger (roughly half the size of the cusp) on the anterior process and a tiny denticle on the posterior side. Only the posterior process of the holotype is denticulated. The M element illustrated by Bergström (1971, pl. 2, fig. 9) in the multi-element description of A. superbus seems to have a discrete denticle on all processes. The dextral Pa element of A. superbus can be distinguished from those of A. tvaerensis by lack of an ‘extra’ postero-lateral process on its outer side (Bergström, 1971).

The M element from the Mehikoorma section, interval 294.7–294.8 m (Fig. 5i), is similar to the holotype of A. superbus in having a central cusp with two smaller denticles on both sides of it, the larger on the anterior process and a smaller, barb-like denticle on the posterior side. Only the posterior process of the specimen is denticulated. Elements of similar morphology occur up to the uppermost studied sample in the section (288.2–288.3 m; Fig. 5j) and both types of specimens, with (Fig. 5i, k) and without (Fig. 5j, l) the barb-like denticle, are present. No complete Pa element of A. superbus was found, and only a few larger fragments of it could be assigned to this species (Fig. 5m, n). The lowermost of such elements comes from the sample 296.3–296.4 m (Fig. 5o) where it occurs together with an M element of morphology characteristic of A. superbus (Fig. 5p).

Variation in morphology of A. superbus elements in the Mehikoorma section is difficult to address as no complete Pa were found, and Pb (Fig. 5q–v) and S (Fig. 5w–ag) elements are too poorly preserved. The ‘strongly sinuous aboral inner margin’ (Savage & Bassett, 1985, p. 692) considered to be characteristic of the sinistral Pb element of the A. superbus was not recognized in sinistral Pb elements from the studied samples. The M elements that are morphologically similar to those of A. superbus, but have a denticulated anterior process (Fig. 5p, ah–ak) or a prominent barb-like denticle on their posterior processes (Fig. 5al, am), are considered here as varieties of the M elements of A. superbus. This conclusion is based on the fact that at the time of wide morphological variation of M elements (Fig. 5i–l, p, ah–am; online Supplementary Fig. S13e, g, i, j, s, t, v–x, aa–ai, ap, aq) no distinct changes in morphology of other elements were observed (Fig. 5m–o, q–ag; online Supplementary Figs S4ad–an, S5a–aw, S6a–ah, S8n–bi, S15ah–br, S16a–bv, S17a–q). Several M elements earlier identified as Amorphognathus sp. (aff. A. duftonus Rhodes) by Männik (2017, fig. 4M, R, S, U) are evidently elements of A. superbus as understood here.

Material. Mehikoorma section: 6 dextral Pa, 2 sinistral Pa, 15 dextral Pb, 14 sinistral Pb, 23 M and about 200 S elements; Velise section: 2 dextral Pa, 1 sinistral Pa, 1 dextral Pb, 2 sinistral Pb and 4 S elements; Ruhnu section: 4 dextral Pa, 3 sinistral Pa, 7 dextral Pb, 9 sinistral Pb, 3 M and about 20 S elements; Fjäcka Main section: 1 M.

Distribution. The first M element of the A. superbus that is similar to the element shown in Figure 5i was recovered 4.9–5.0 m above the base of the Skagen Formation in the Fjäcka main section, determined by TP. As demonstrated by Goldman et al. (2007, see also remarks to A. viirae sp. nov.), the FAD of A. superbus is located at or near the lower boundary of the Katian Stage in the boundary stratotype section. Baltoscandian region: lower half of the Katian Stage; Mehikoorma section: Variku Formation (288.2–296.4 m in the studied part of the section, Fig. 2); Ruhnu section: Mossen and Mõntu formations (645–633 m); Velise section: Hirmuse and Rägavere formations (176.75–173.0 m); Fjäcka Main section: 4.9–5.0 m above the base of the Skagen Formation.

Amorphognathus ventilatus (Ferretti & Barnes, 1997)

1974 Holodontus sp. nov.; Viira (1974, p. 90, fig. 110).

1997 Amorphognathus ventilatus sp. nov.; Ferretti & Barnes (1997, p. 28–29, pl. 2: 14–17).

2014a Amorphognathus ventilatus Ferretti & Barnes (1997); Ferretti et al. (2014a, p. 821–3, figs 7V, 12S–U).

2014b Amorphognathus ventilatus Ferretti & Barnes (1997); Ferretti et al. (2014 b, p. 116, pl. 1: 17, 18).

Diagnosis. Ferretti & Barnes (1997, p. 28–9).

Remarks. Originally, A. ventilatus was established based on the morphology of its M element from the A. ordovicicus CZ from Kalkbank limestone, Schmiedefeld area, Thuringia, Germany (Ferretti & Barnes, 1997). In the remarks to their new species, Ferretti & Barnes (1997) referred to the specimen from the Ohesaare core section illustrated by Viira (1974, fig. 110, depth 461.95 m, A. superbus CZ) as a probable element of A. ventilatus. Additional M elements identified as A. ventilatus were also recognized by Ferretti et al. (2014 a, Whitland A40 road cut, Wales, A. ordovicicus CZ; 2014b, Portrane Limestone at Portrane, Ireland, A. ordovicicus CZ). Only the M element of A. ventilatus is currently known.

In the Mehikoorma section, probable M elements of A. ventilatus, morphologically similar to those described and figured by Viira (1974, fig. 110), occur together with A. superbus in the interval 292.9–291.9 m (Figs 2, 5an–at). No P or S elements that could be distinguished from those of A. superbus were found. It is currently unclear whether A. ventilatus recognized in Estonian sections is conspecific with that described by Ferretti & Barnes (1997). Estonian specimens come from an older interval (the A. superbus CZ) than the type material that is described from the A. ordovicicus CZ. Furthermore, there is no indication of probable P or S elements of A. ventilates, raising the question of whether the M elements in Estonian material assigned to A. ventilatus may simply represent a variety of A. superbus.

Material. Mehikoorma section. 7 specimens.

Distribution. Estonia: lower half of the Katian Stage; Mehikoorma section: Variku Formation (292.9–291.9 m, Fig. 2); Ohesaare section: Paekna Formaiton (461.95 m).

Amorphognathus complicatus (Rhodes, 1953)

1953 Amorphognathus complicatus sp. nov.; Rhodes (1953, p. 282, pl. 20: 42, 45, 46).

?1966 Amorphognathus complicata Rhodes (1953); Hamar (1966, p. 53, pl. 7:2–4).

?1980 Amorphognathus superbus Rhodes (1953); Merrill (1980, fig. 4:1, not including fig. 5: 9, 10).

1980 Amorphognathus complicatus Rhodes (1953); Merrill (1980, fig. 4:2–9, 11–23, not including 10).

1980 Amorphognathus aff. complicatus Rhodes (1953); Orchard (1980, p. 16, pl. 6: 30).

2017 Amorphognathus complicatus Rhodes (1953); Männik (2017, figs 4Q, 5A).

Remarks. Both Pa elements were illustrated by Rhodes (1953). They are characterized, additionally to anterior and posterior processes, by a smaller bifurcated outer lateral process next to the cusp and an inner posterolateral process of almost equal size to the posterior process. The simple, unbranched inner lateral process is the main feature allowing the Pa elements of A. complicatus to be separated from those of A. ordovicicus (Rhodes, 1953). The S elements of A. complicatus are figured by Merrill (1980) and are generally similar to those of other representatives of the genus Amorphognathus. The sinistral Pa element illustrated by Merrill (1980, fig. 4:10) possesses an inner lateral process with two branches. This suggests that it most probably does not belong to A. complicatus but instead to A. superbus. Another sinistral Pa element described as A. superbus (Merrill, 1980, fig. 4:1) probably does belong to A. complicatus as the inner lateral process is unbranched. The M element of A. complicatus (Merrill, 1980, fig. 4:22) has a prominent proclined cusp with a single smaller anterior denticle. The posterior and inner lateral processes bear a single denticle in their distal parts. The main difference between the M elements of A. complicatus and A. superbus is that the cusp of the element in the former taxon is considerably larger (taller) in comparison to the denticle located just anterior of it (Fig. 5aw, ax) and to that of A. superbus (Fig. 5k). The Pa elements of A. complicatus have characteristic wide basal platforms (Rhodes, 1953, pl. 20, fig. 42; Merrill 1980, fig. 4:3; Orchard 1980, pl. 6, fig. 30; Männik 2017, fig. 4Q). Denticles on the processes are connected by ridges (Fig. 5av). In some cases, the denticles may also be elongated perpendicularly to the main axis of the process (sometimes so strongly that there seems to be two denticles instead of one) and bear sharp ridges oriented in the same direction (Fig. 5au). Apices on the processes are not very sharply pointed. It is currently unclear if the complex denticle morphology is characteristic of A. complicatus or shared with other species of similar age. Furthermore, Pb and S elements of A. complicatus are indistinguishable from those of A. superbus apparatus within studied sections.

In the Mehikoorma core section, A. complicatus has been found in the three samples studied from the interval 288.2–289.4.7 m (Fig. 2), where it co-occurs together with A. superbus.

Material. Mehikoorma section: 1 sinistral Pa, 5 M; Velise section: 1 dextral Pa; Ruhnu section: 2 dextral Pa, 8 M.

Distribution. Estonia: lower–middle parts of the Katian Stage; Mehikoorma section: Variku Formation (288.2–288.7 m in the studied part of the section, Fig. 2); Ruhnu section: Mossen and Mõntu formations (640.5–635 m); Velise section: Rägavere Formation (173.0–173.07 m).

5. Discussion

5.a. Concluding taxonomic remarks

Morphologically distinct A. inaequalis has been reported and illustrated only from the sections of Wales and France (Ferretti & Bergström, 2022). However, a subzone established based on this species is also included in regional conodont zonation in Sweden (Bergström, 2007) and Estonia (Meidla et al. 2014). A re-study of collections from the Fjäcka (Bergström, 2007) and Smedsby Gård (Bergström et al. 2011) sections from Sweden, and from the Ruhnu (Männik, 2003) and Mehikoorma (Männik & Viira, 2005) core sections from Estonia, from where A. inaequalis was reported earlier, revealed that all these identifications are highly problematic; most probably, all specimens identified as A. inaequalis actually belong to A. tvaerensis.

The morphological variation of elements of A. tvaerensis in its upper range has been discussed by several authors. Specimens from this interval have been assigned to an early form of A. superbus (Dzik, 1994), to A. aff. ventilatus or to a late form of A. tvaerensis (Dzik, 1999 a), and to A. cf. tvaerensis, Amorphognathus n. sp. or Amorphognathus sp. A. (Stouge, 1998). The present study of rich material from the Mehikoorma core section demonstrates that, in the interval starting from the uppermost B. gerdae CSz up to the level of temporal disappearance of Amorphognathus in the section in the B. alobatus CsZ, a distinct species of Amorphognathus exists, morphologically different from A. tvaerensis and described above as Amorphognathus viirae sp. nov. Its most distinct, dextral Pa element is characterized by a sinuous curvature of the main blade, an occurrence of a distinct lateral lobe on the outer side of the posterior process and bifurcated lateral processes on both sides of the element. The M element of A. viirae sp. nov. has a prominent, posterior denticle, the distal part of which is curved anteriorly. This new species has so far been found in Estonian and Swedish sections, with probable occurence also in Polish (Holy Cross Mountains) and Oklahoma (Black Knob Ridge) sections based on published figures.

The high morphological variation of the M element in A. tvaerensis (online Supplementary Figs S9a–az; S10a–s, ab–bb; S11a–s), A. viirae sp. nov. (online Supplementary Figs S10t–aa; S11t–ba; S12a–av) and A. superbus (online Supplementary Fig. S13a, b, d–j, q, s–ai, al, am, ap–ar) indicates that definition/identification of species in the Amorphognathus lineage based only on this element might be highly problemtic.

5.b. Biostratigraphical considerations

The absence of A. inaequalis and presence of A. viirae sp. nov. within Estonian and Swedish sections necessitates a revision of the conodont zonations of Baltoscandia. The concept of the A. tvaerensis CZ and its subzones, based on the succession of species of the Baltoniodus lineage and with the lower boundary drawn at the appearance level of A. inaequalis, would be difficult to maintain in the situation where the formerly identified specimens of A. inaequalis in Estonia and Sweden are attributed to A. tvaerensis and the specimens in the upper part of the range of A. tvaerensis are assigned to the new species, A. viirae sp. nov. While the former B. gerdae CSz and B. alobatus CSz are easily recognizable and seemingly reliable, the appearance of B. variabilis is reported as a gradual transition from its predecessor B. prevariabilis (e.g. Dzik, 1978). The prevariabilis–variabilis transition has not been examined in detail in the Baltoscandian sections, but the appearance of B. variabilis may be the best approximation of the lower boundary of the Sandbian (and Upper Ordovician) strata in the conodont succession. This boundary is currently drawn within the Pygodus anserinus CZ, and the appearance of B. variabilis is documented from the first samples above the lower boundary of the Sandbian Stage in its stratotype section (Bergström et al. 2000). Further careful investigation of the morhpological transition between B. prevariabilis and B. variabilis is essential for clarifying the potential of B. variabilis as a possible marker of the lower boundary of Sandbian Stage in the Baltoscandian conodont succession. The fact that the Fågelsång Phosphate in the Sandbian stratotype section is considered to mark a sedimentary cap in the succession (Goldman et al. 2020) would make the use of A. tvarensis CZ as the marker of lower boundary of the Sandbian Stage rather arbitrary. In several sections of Argentina the appearance of B. variabilis coincides with the appearance of Nemagrapthus gracilis, but again right above a hiatus in the succession (Serra et al. 2015; Feltes et al. 2018).

As there is no A. inaequalis in the sections of Estonia and Sweden, it would be reasonable to abandon the previous A. inaequalis CZ (Viira, 2008) or A. inaequalis CSz (Bergström, 2007; Meidla et al. 2014). The last recorded elements of genus Pygodus in the Velise and Mehikoorma sections are succeeded upsection by the first appearence of Amorphognathus. An overlap between the ranges of P. anserinus and A. tvaerensis is recorded from the Smedsby Gård (Bergström et al. 2011), Kovel-1 (Saadre et al. 2004) and Bliudziai-150 (Stouge et al. 2016) sections. A small gap between the ranges is present in Fjäcka (Bergström, 2007), Ruhnu (Männik, 2003) and Viki (Männik, 2010; Hints et al. 2014) sections, probably because of poor yield and/or poor preservation of conodonts. In the sections without a notable hiatus, the base of the A. tvaerensis CZ is located within the upper range of P. anserinus.

A transition between A. viirae sp. nov. and A. superbus is not documented in the Ordovician succession of Baltoscandia because of a scarcity of Amorphognathus in the Sandbian–Katian boundary interval. This barren interval, poorly represented by specimens of this genus, has been documented in all known sections in the region from Sweden, Estonia, Latvia and Poland. Previously, this interval had been considered to correspond to the Mid-Caradoc Event (Männik, 2003, 2004; Männik & Viira, 2005), with noticeable changes in conodont succession occurring along with an episode of biotic, climatic, sea-level and facies changes.

It is possible that A. tvaerensis reported earlier from the stratotype section of the Katian Stage (Goldman et al. 2007) and from Argentina (Ortega et al. 2008) also includes A. viirae sp. nov. The lowermost elements of Amorphognathus in the Katian stratotype (Goldman et al. 2007, fig. 7:15, 16), somewhat above the base of the Katian Stage, are identified as Amorphognathus sp. cf. A. superbus. The figured A. tvaerensis from this section (Goldman et al. 2007, fig. 7:17–22) is similar to A. viirae sp. nov., as discussed here in Section 4 (Systematic description). Additionally, a high-resolution conodont–graptolite study from Argentina (Ortega et al. 2008) revealed a conodont succession simlar to that in the boundary interval of type section of the Katian Stage. The uppermost A. tvaerensis in the La Invernada Range (probably A. viirae sp. nov. based on information from Batloscandian and Katian stratotype successions) occurs below the FAD of D. caudatus in the section and therefore also below the base of the Katian Stage (Ortega et al. 2008, fig. 2). It is noteworthy that the latest B. alobatus disappears at the same level and the appearance of A. superbus is recorded above the FAD of D. caudatus.

Considering the data above, we propose an updated conodont zonation for the Sandbian – lower Katian interval (Fig. 6) that probably has the potential to be applied all over the Atlantic Realm in the sense of Pyle & Barnes (2002). The former subzone of the A. tvaerensis CZ, A. inaequalis CsZ, is removed from the scheme because, according to current knowledge, A. inaequalis has a very limited geographical distribution; it has only been reliably identified from Wales and France (Ferretti & Bergström, 2022). The former B. variabilis, B. gerdae and B. alobatus subzones of the A. tvaerensis CZ are treated here as CZs. The A. tvaerensis CZ is redefined and, as a new one, the A. viirae CZ is included in the scheme. Lower boundaries of all zones in the revised version of the scheme correspond to the FADs of the nominate taxa (Fig. 6). The last appearance datum (LAD) of B. alobatus and the FAD of A. superbus are separated by an interval that does not yield biostratigraphically diagnostic conodonts. In the proposed scheme this interval is tentatively included in the B. alobatus CZ. The Sandbian–Katian boundary lies in the upper part of this interval.

The morphological variation of the elements of A. tvaerensis and A. viirae sp. nov. in the succession may offer some clues for a more detailed stratigraphic subdivision (Fig. 7). The P and M elements of A. tvaerensis show a noticeable variation in the B. gerdae CZ. Typically, posterior process of the dextral Pa element of A. tvaerensis is curved (Fig. 7:S1ae, S3a). However, in the B. gerdae CZ elements with a straight posterior process are also present (Fig. 7, double-headed arrow pointing to S2i). Additionally, a sinistral Pa element of A. tvaerensis in the B. gerdae CZ possesses a distinct lateral lobe on the outer side of its posterior process (Fig. 7, square on the S3g) which is missing on the elements below this level, in the B. variabilis CZ (Fig. 7, square on S1q). Furthermore, the dextral Pb element of A. tvaerensis has, in lateral view, a slightly undulating inner edge of posterior process in the B. variabilis CZ (Fig. 7:S7q, indicated by a circles and line below the figure), it becomes more evident on elements from the upper part of A. tvaerensis range in the B. gerdae CZ (Fig. 7:S7ah) and is particularly distinct on, and characteristic of, elements in A. viirae sp. nov. (Fig. 7:S8f). In the B. gerdae CZ, the angle between the posterior process and the posterior denticle of the M elements of A. tvaerensis increases from c. 50° (Fig. 7:S9m) in the lower part of the CZ to 75° (Fig. 7:S10g) in the upper part. The P and M elements of A. viirae also changed in time. Most noticeably, the angle between the posterior process and posterior denticle of the M elements increases from c. 90° (Fig. 7:S11ac) to 110° (Fig. 7:S12au). Most of the sinistral Pa elements in the upper part of the A. viirae sp. nov. range have a poorly developed lateral extension on the outer side of the posterior process (Fig. 7, square on S4t), a feature missing from elements of the lower range interval of the species (Fig. 7, square on S3s). Additionally, the shape of the outer edge of the process of the sinistral Pb elements become more laterally expanded within the upper range of A. viirae sp. nov. (Fig. 7:S8k), a trend of gradual morphological change from elements of A. tvaerensis (Fig. 7:S7ad) to those of A. viirae sp. nov. (Fig. 7:S7bh).

6. Conclusions

The re-study of rich collections of Sandbian – lower Katian conodonts from Estonia (Mehikoorma and Ruhnu core section) and Sweden (Fjäcka and Smedsby Gård sections) revealed that A. inaequalis is missing in this part of the Baltoscandian region. However, the probable occurrence of this species has been reported from Lithuania (Bliudziai-150 core section) but also from Ukraine (Kovel-1 core section). Further studies are required to prove the occurrence of A. inaequalis in these regions.

The re-study of A. tvaerensis revealed that the specimens of Amorphognathus, previously considered to belong to the younger representatives of this taxon, are actually morphologically quite different. Here, they are described as a new species: Amorphognathus viirae sp. nov. To date, outside Estonia the new species has been recognized in some sections in Sweden (re-study of collections by TP). Based on an analysis of published figures, it also evidently occurs in Poland (Mójcza Quarry, Holy Cross Mountains) and the USA (Black Knob Ridge, Oklahoma).

The absence of A. inaequalis in the Baltoscandian conodont succession and recognition of the new species A. viirae sp. nov. resulted in the revision of the Sandbian stratigraphic scheme in the region. The zonation proposed here yields (from below) the P. anserinus, B. variabilis, A. tvaerensis, B. gerdae, A. viirae and B. alobatus CZs.