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22 October 2013 The Hirnantian (Late Ordovician) Brachiopod Fauna of the East Baltic: Taxonomy of the Key Species
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Abstract

Rhynchonelliformean brachiopods, belonging to 17 genera, are described from the East Baltic Porkuni Regional Stage, correlated with the global Hirnantian Stage. The brachiopod genera Paromalomena, Proboscizambon?, Kinnella, Drabovia, and Coolinia, which are described from the region for the first time demonstrate together with characteristic Hirnantian species of the genera Hirnantia, Dalmanella, Plectothyrella, Eostropheodonta, and Hindella a greater than previously thought commonality of the Baltic fauna with the terminal Ordovician Hirnantia brachiopod fauna of the Kosov Province. The samples containing brachiopods were collected from 43 drill core sections in Central East Baltic. The study area belongs to the Livonian Tongue of the Central Baltoscandian Facies Belt in the Baltic Basin. The brachiopods occur mainly in the skeletal and silty limestone of the Kuldiga Formation within the lower half of the Porkuni Stage. Few brachiopods are known from the sandy or oolitic limestone of the Saldus Formation in the upper part of the stage. Due to excellent preservation some brachiopod species (e.g., Cliftonia psittacina and Dalmanella testudinaria) yield key morphological information, relevant to their classification and phylogeny. This taxonomic study of the East Baltic brachiopods presents essential groundwork for analysis in progress on the distribution and onshore-offshore successions of the Hirnantia brachiopod fauna within both a Baltoscandian and global context.

Introduction

The latest Ordovician Hirnantia brachiopod fauna is a unique phenomenon in the evolution of Palaeozoic benthic faunas, which has received much attention in the last decades. During the latest Ordovician, restructuring of facies patterns commenced in the late Pirgu (end of the Katian) with a diachronous, upward shallowing across Baltoscandia. These changes prompted the extinction of many shallow-water shelf faunas due to habitat destruction (first phase of the mass extinction; Rong and Harper 1999; Harper et al. 2014). Due to the lowering of sea level at the beginning of the Porkuni (Hirnantian) the stromatoporoid-coral reefs developed in the shoals across northern Estonia containing specialized faunas (Ärina Formation, Fig. 1) (Nestor and Einasto 1997; Hints 2012). The previously deeper part of the basin (the Livonian Tongue), containing in places the Foliomena fauna (Sheehan 1973, 1979; Rong et al. 1999), also shallowed and the Hirnantia brachiopod fauna invaded as far as the easternmost parts of the basin. Changes in carbon isotope composition and the distribution of zonal chitinozoans (Kaljo et al. 2008; Hints et al. 2010) indicate that the development of reefs in the Estonian facies belt started at about the same time with the invasion of the Hirnantia brachiopod fauna into the Livonian Tongue.

In Baltoscandia, the Hirnantia brachiopod fauna has been studied taxonomically in Sweden (Bergström 1968) and Norway (Brenchley and Cocks 1982; Cocks 1982). Data on the distribution of Hirnantian brachiopods in Poland are presented by Temple (1965) and Podhalańska (2009). Bergström (1968) described 18 species of rhynchonelliform brachiopods from southern Sweden (Västergötland) representing the majority of the typical elements of the Hirnantia fauna (e.g., species of genera Dalmanella, Hirnantia, Kinnella, Paromalomena, Eostropheodonta, Cliftonia, Plectothyrella, and Hindella; Rong and Harper 1988; Jin and Bergström 2010). Additional data on the distribution of the Hirnantia fauna have been reported from other parts of Sweden (Bergström and Bergström 1996; Smelror et al. 1997; Dahlquist et al. 2010). The most diverse Hirnantia fauna (the Hindella-Cliftonia Association comprising a brachiopod fauna of up to 20 genera) has been identified in Norway (Brenchley and Cocks 1982). It includes some brachiopods (e.g., Thebesia and Brevilamnulella) indicating a similarity to the Uppermost Ordovician Midcontinent (Edgewood) fauna of North America (Amsden 1974; Rong and Harper 1988).

Fig. 1.

Correlation and distribution of Late Ordovician—early Silurian faunal communities (C.) and associations (As.) (modified from Kaljo et al. 2008). Ä, Ärina Formation; K, Kuldiga Formation; S, Saldus Formation.

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The distribution of some representatives of the Hirnantia brachiopod fauna and associated trilobites in the East Baltic has been known since the 1960s (Männil 1966, Männil et al. 1968) and refined by more recent investigations (Ulst et al. 1982; Brenchley et al. 2003; Kaljo et al. 2004; Hints et al. 2010, 2012). The most diverse Hirnantia brachiopod fauna occurs in westernmost Latvia, where the Porkuni Regional Stage (Hirnantian) has the thickest development in the East Baltic (over 20 m; Hints et al. 2010). However, this fauna occurs in the skeletal and silty limestone of the Kuldiga Formation, which constitutes most of the stage in the westernmost East Baltic. The oolitic and sandy limestone, and microlaminated marl of the Saldus Formation in the upper part of the Porkuni Stage comprise fragments of a few brachiopod shells possibly belonging to Hindella. Hirnantia cf. sagittifera (M'Coy, 1851) is the only well-preserved brachiopod of note from the Saldus Formation (Hints et al. 2012). Correlation and overviews of the lithostratigraphical units and faunas of the uppermost Ordovician in the East Baltic are presented in several publications (Ulst et al. 1982; Kaljo et al. 2001, 2004; Hints et al. 2010) and summarized in Fig. 1.

The Hirnantia brachiopod fauna is not known in the uppermost Ordovician Porkuni Regional Stage in northern Estonia (Ärina Formation, Fig. 1) where the corresponding strata crop out (Hints and Meidla 1997; Hints and Rõõmusoks 1997; Rõõmusoks 2004). The depositional model formulated for the Baltic Basin (Männil 1966; Jaanusson 1976) indicates that the benthic faunas of the Central East Baltic (westernmost Latvia, southern Estonia and northern Lithuania) within the Livonian Tongue of the Central Baltoscandian Facies Belt are quite different from those in the more onshore Estonian and Lithuanian belts (Hints and Harper 2003; Kaljo et al. 2011; Fig. 1). The Ordovician brachiopod fauna of the latter two belts comprises largely the common taxa, described primarily from northern Estonia and neighbouring areas (Rõõmusoks 1970, 2004; Paškevišius 1997, 2000). The species of the Hirnantia brachiopod fauna have not been reported from the uppermost Ordovician Porkuni Regional Stage in northern Estonia (Ärina Formation, Fig. 1).

Hirnantian brachiopods from the Central East Baltic described in this study are conspecific or closely related to brachiopods previously reported from the western parts of the Baltic Basin (in Sweden, Norway, and northern Poland) and from several other regions. But in spite of the occurrence of well-known species of brachiopods, the Hirnantia brachiopod fauna in the study area provides new morphological and phylogenetic data, due to excellent preservation, and new information on their distribution during the latest Ordovician. Some well-known species of the Hirnantia brachiopod fauna, which are abundant in the Baltic (e.g., Cliftonia psittacina and Dalmanella testudinaria), yield key morphological information, relevant to their classification and phylogeny. New palaeontological studies and taxonomic revision of Hirnantian brachiopods (Rong et al. 2008; Jin and Bergström 2010; Jin 2012; Benedetto et al. 2013) have improved our understanding of the latest Ordovician faunal provinces and documented the spatial and temporal variation of the component species. The taxonomic identification of the key species of the Hirnantia brachiopod fauna “may play a key role in our understanding of brachiopod faunal provisionalism during the Late Ordovician” (Jin 2012: 206). This study presents the foundation for analyses in progress on the significance of the Baltic Hirnantia brachiopod fauna in a more global context.

Institutional abbreviations.—GIT, Institute of Geology at Tallinn University of Technology, Tallinn, Estonia; LDM G, geological collections, Natural History Museum of Latvia (Latvijas Dabas muzejs), Riga, Latvia.

Other abreviations.—L, length; W, width.

Material and geological setting

The brachiopod samples were collected from 43 drill core sections, located in the Central East Baltic (western Latvia, southern Estonia, and northern Lithuania) (Fig. 2). The most fossiliferous unit, the Kuldiga Formation was sampled in some core sections (Stirna-18, Hints et al. 2010; Mežmali-16, Riekstini-15; Brenchley et al. 2003) with a view to acquiring as many fossils as possible. The other sections were sampled in less detail. However, the sandy and oolitic limestone of the Saldus Formation in the upper half of the stage were randomly sampled and thus the data available are more incomplete.

Fig. 2.

A. The facies belts in the Baltic Basin and the study area. B. The location of drill core sections. C. Enlarged Jurkalne area. Black dots mark the locality of the drill core with brachiopods of the Hirnantia Fauna; empty dots indicate occurrences of the Hirnantia Fauna (from Paškevičius 1997, 2000). Drill cores: 1, Stirnas-18; 2, Pliekalni-14: 3, Dizrungi-17, 4, Vilcini-19; 5, Adze-6; 6, Dreimaņi-11; 7, Riekstini-15; 8, Mežmaļi-16; 9, Mežvagari-13, 10, Ēdole-60; 11, Anši. Distribution of the Hirnantian rocks: a, outer limit the the Porkuni Stage, including of the Saldus Formation on the NE areas; b, outer limit of the Saldus Formation in northern part of region; c, outer limits of the distribution area of the Kuldiga Formation.

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Very few brachiopod specimens from the drill cores in Lithuania have been used in this study. Their distribution is known from published data (Paškevičius 1997, 2000). Some brachiopods were collected from the drill cores in the Gulf of Gdańsk (S-7 and S-8, Ulst 1992; Fig. 1A), where the Hirnantian sequence is most similar to those in northern Poland (Podhalańska 2009).

The brachiopods are in general well preserved, although it is difficult to extract the shells from the non-weathered, carbonate rocks. Crushing the rock samples splits the brachiopods along the shell into two parts commonly obscuring the valve exterior and interior surfaces. The best-preserved specimens occur in the clay interlayers. Amongst the rare and poorly-preserved specimens are possible atrypides, which are not described in this study. Routine methods have been used (mechanical and ultrasonic techniques in some cases) for the preparation of specimens. The material studied is housed at the Institute of Geology at Tallinn University of Technology (collection 542, institutional abbreviation GIT) and the Latvian Museum of Natural History in Riga (various collections, mainly from rock samples of individual wells; institutional abbreviation LDM G). The complete data on individual specimens are accessible online in the Estonian geocollections database ( http://geocollections.info) and in the Latvian national collection database ( http://nmkk.lv/). The initial depths of the samples are calculated according to the drilling intervals mentioned in the core boxes, not adjusted later by geophysical data. The revised depths usually mark a somewhat higher level in the core in comparison with the initial data. For example, the lower boundary of the Porkuni Regional Stage in the Cicere drill core is identified by Ulst et al. (1982) at a depth of 900 m, which is supported by the occurrences of the Hirnantian brachiopods (Cliftonia, Eostropheodonta, and Dalmanella) at a depth of 899.2–899.4 m in the samples housed at the Latvian Natural History Museum. However, the geophysical studies of the borehole (Pomeranceva 1997) suggest a depth of 897 m for that boundary.

Systematic palaeontology

Phylum Brachiopoda Duméril, 1806
Subphylum Rhynchonelliformea Williams, Carlson, Brunton, Holmer and Popov, 1996
Class Strophomenata Williams, Carlson, Brunton, Holmer and Popov, 1996
Order Strophomenida Öpik, 1934
Superfamily Strophomenoidea King, 1846
Family Rafinesquinidae Schuchert, 1893
Subfamily Leptaeninae Hall and Clarke, 1894
Genus Leptaena Dalman, 1828

  • Type species: Leptaena rugosa Dalman, 1828; Dalmanitina Beds, Loka Formation, Hirnantian (Upper Ordovician); Borenshult, Östergötland, south Sweden.

  • Fig. 3.

    Leptaenin brachiopods from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia (A–D, F–G), southwestern Estonia (E). A, B, D, F, G. Leptaena (L.) rugosa (Dalman, 1828). A. Shell, LDM G 273-1, Ēdole-60 drill core, depth 840.7 m, ventral (A1) and ventro-lateral (A2) views. B. Shell, GIT G 542-2, Vilcini-15, 910.35 m (figured in Kaljo et al. 2008), ventral (B1) and posterior (B2) views. D. Shell, GIT 542-17, Stirnas-18, 908.6 m, dorsal exterior view. F. Shell, GIT 542-18, Stirnas-18, 909.4 m, ventral view. G. Dorsal valve, GIT 542-197, Sturi-8, 941.8 m, view of interior (G1) and cruralium (G2). C, E. Leptaena sp. C. Dorsal valve, GIT 542-189, Vilcini-15, 909.3 m, exterior view. E. Incomplete ventral valve, GIT 542-192, Ikla, 536.5 m, exterior view. Scale bars 10 mm.

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    Subgenus Leptaena (Leptaena) Dalman, 1828
    Leptaena (Leptaena) rugosa Dalman, 1828
    Fig. 3A, B, D–F.

  • 1828 Leptaena rugosa Dalman; Dalman 1828: 106, pl. 1: 1.

    1968 Leptaena rugosa Dalman, 1828; Bergström 1968: 14–15, pl. 5: 8, 9; text-fig. 7.

    2008 Leptaena (Leptaena) rugosa Dalman; Cocks 2008: 59 (see synonymy therein).

  • Material.—30 specimens (among them 15 fragmentary); mostly embedded in rock. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia, southwestern Estonia. Drill core and depth (in meters) of sample intervals with brachiopods. Depth of fragmentary specimens is marked by “?”. Collection GIT 542: Adze-6, ?844.5–845.2; Aizpute-41, 997.1–?997.8; Engure, 882.9–?884.4; Ikla, ?536.5; Mežmaļi-16, ?912.8–?913.2; Riekstini-15, ?857.3; Ruhnu-500, 610.1–?617.25; Stirnas-18, 908.6–909.4; Sturi-8, 941.8; Vilcini-15, 909.3–910.35; collections LDM G: Anši-12, 922.5; Cicere (from the sample with Hirnantia sagittifera; depth unknown); Ēdole-60, 840.7; Kandava-52, 932.5; Kuili-9, 948.0–?948.2; Priekule-23, 1363.0; Pliekalni-14, ?884.8; Talsi-55, ?866.5.

  • Remarks.—The East Baltic specimens of Leptaena are almost identical with L. (L.) rugosa from Sweden and Norway (Bergström 1968: text-fig. 7; Spjeldnæs 1957: pl. 7: 1–2, 4; Cocks and Rong 2000: fig. 150, 1d) having a transversely subquadrate shell outline, weakly convex ventral disc with up to 11 concentric rugae, multicostellate ornament with few accentuated ribs on median fascicle and 5–9 costellae per 2 mm at anterior part of the disc; the exterior of the dorsal valve is similar to the material from Norway and Sweden. The shell size, up to 45 mm wide at alate hinge line, seems to be more similar to the Norwegian specimens of the species. However, the Baltic specimens possibly have a shorter trail and less geniculated profile. The Baltic specimens have ventral interarea about twice as high as the dorsal interarea; foramen opening apical. Chilidium, up to 4 mm wide, triangular, convex with the median groove, which is characteristic for the Swedish specimens (Bergström 1968). The single dorsal interior with trapezoidal notothyrial platform and median septa on anterior half of disc (Fig. 3G1, G2) does not differs from the Swedish specimen (Bergström 1968: text-fig. 7). An exception is the cardinal process of the Baltic specimen where the lobes seem to be less divergent.

    The species affiliation of one ventral and one dorsal valve (Leptaena sp.; see Fig. 3C, E) is unclear due the discontinuous nature of the rugae. Nevertheless, the latter valve has a peripheral ridge, similar to L .(L.) rugosa, but the ventral valve has smooth transition from the disc to trail, similar to the specimens of L. (L.) rugosa from the Killey Bridge Formation (Katian) of Ireland (Mitchell 1977).

    The Baltic specimens differ from the older representatives of L. (L.) rugosa from Pomeroy, N. Ireland (Candela 2003) in having a more transverse disc and fewer rugae.

    The early Hirnantian leptaenids Leptaena acuteplicata (Schmidt, 1908) and L. friedrichi (Rõõmusoks, 2004) (assigned to the genera Schmidtomena and ?Similoleptaena in Rõõmusoks 2004) in the reef complex (Ärina Formation) of North Estonia differ markedly from L. (L.) rugosa. Both Estonian species have much smaller shells; L. acuteplicata differs in having less prominent rugae; the L. friedrichi has a more robust ornament. A comparison of several Hirnantian species of Leptaena in Baltoscandia was presented by Cocks (2005).

  • Stratigraphic and geographic range.—This widespread species is mainly restricted to the Hirnantian Stage, Upper Ordovician, in Europe (Czech Republic, France, Ireland, and Wales), China (Rong and Harper 1988; Rong et al. 2002), and in North and South America. In Baltoscandia besides Norway and Sweden, it occurs in Estonia and Latvia (this paper) and also in Lithuania (Paškevičius 1997). Closely related forms occur in the highest Katian in, for example, Northern Ireland and Scotland.

  • Fig. 4.

    Strophomenoid brachiopods from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia (A–F, H) and southwestern Estonia (G). A, B, FH. Paromalomena polonica (Temple, 1965). A. Dorsal valve, GIT 542-299, Mežmali-16, depth 911.8 m, exterior view. B. Dorsal valve, GIT 542-352-1, Adze-6, 844.5 m, split into two parts: exterior view (B1) and impression (B2). F. Dorsal valve, GIT 542-377, Aispute-41, 884.4 m, exterior view. G. Dorsal valve, GIT 542-361, Ikla, 535.3 m, exterior view. H. Dorsal valve, GIT 542-355, Mežmali-16, 912.2 m, exterior view (H1) and view of the chilidium (H2). C. Eostropheodonta cf. parvicostellata Rong, 1984. Dorsal valve, GIT 542-365, Aispute-41, depth 988.7 m, exterior view. DE. Proboscisambon? sp. D. Mould of dorsal valve, GIT 542-53, Stirnas-18, depth 908.2 m, exterior view. E. Dorsal valve, GIT 542-322-1, exterior view; Riekstini-15, 858.6 m. Scale bars 2 mm.

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    Family Glyptomenidae Williams, 1965
    Subgenus Glyptomeninae Williams, 1965
    Genus Paromalomena Rong, 1984

  • Type species: Platymena? polonica Temple, 1965; Dalmanitina Beds, Hirnantian (Upper Ordovician); Stawy, Holy Cross Mountains, Poland.

  • Paromalomena polonica (Temple, 1965)
    Fig. 4A, B, F-H.

  • 1965 Platymena? polonica n. sp.; Temple 1965: 407–410, pl. 15: 1–4; pl. 16: 1–5.

    2008 Paromalomena polonica (Temple); Cocks 2008: 63 (see synonyms therein).

  • Material.—Eight specimens embedded in rock. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); Latvia, Estonia. Drill core and depth (in meters) of sample with brachiopod. Depth of fragmentary specimens is marked by “?”. Collection GIT 542: Aispute-41, 1000.5; Ikla, 535.3; Mežmaļi-16, 911.8–912.5; Ruhnu-500, 616.2 (the lowermost scabra chitinozoan zone; Nõlvak 2003), 617.4; Vilcini-15, ?914.5; collection LDM G: Pliekalni-14, ?884.0–884.6.

  • Remarks.—Small semicircular shells up to 15 mm wide and 10.5 mm long with multicostellate ornament of fine and uniform costellae; the irregular growth lines, partly developed as low rugae, are most similar to those on the Polish specimens of P. polonica (Temple 1965: pl. 15: 2, 3). The Baltic specimens have obtuse to acute cardinal angles, hinge line 0.9 of valve width. Costellae appear at 2 to 3 mm growth stage; up to 9 costellae per 1 mm at 5 mm from apex, few costellae appear along the posterior margin. Ventral valve very weakly convex around the umbo; dorsal valve flat, anteriorly slightly concave with a low, anacline dorsal interarea with grooved chilidium (Fig. 4H1).

    The Baltic specimens differ somewhat from the Polish specimens in shell outline, the ratios of width to length are about 1.4 and 1.6, respectively (Temple 1965).

    The Baltic as well the Polish specimens differ from those from China (Rong 1984; Zhan et al. 2010) in having somewhat larger shells. The early growth stages, up to 3.5 mm generally lack radial ornament, similar to some specimens of the genus Proboscisambon.

  • Stratigraphic and geographic range.—This widespread species is mainly restricted to the Hirnantian Stage, Upper Ordovician, although similar forms occur in both the highest Katian and lowest Llandovery. It is common in deeper-water facies in Argentina, Burma, China, Thailand, and parts of Europe, for example, Austria, the Czech Republic, Poland, and England (see, e.g., Benedetto 1990; Rong and Harper 1988; Rong et al. 2002; Temple 1965). In Baltoscandia, it occurs in Sweden (Bergström 1968), Estonia, and Latvia (this paper).

  • Genus Proboscisambon Havlíček and Mergl, 1982

  • Type species: Strophomena quaesita Barrande, 1879; Králodvor Formation, Katian (Upper Ordovician); Jezerka, Bochemia, Czech Republic.

  • Proboscisambon? sp.
    Fig. 4D, E.

  • Material.—Seven specimens embedded in rock. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); southwestern Estonia and western Latvia. Drill core and depth (in meters) of sample intervals with brachiopods. Depth interval of fragmentary specimens is marked by “?”. Collection GIT 542: Aizpute-41, 997.7–1000.35; Stirnas-18, 908.2; Riekstini-15, 858.6; Ruhnu-500, 613.5; Vilcini-15, 909.5; ?Prabut Formation (Podhalańska 2009): Petrobaltic S-8, 2618.2–2618.28 (sample 47 from the interval 2614.4–2633 m).

  • Remarks.—Small, laterally elongated shells, up to 6.5 mm wide and 4.2 mm long with weakly developed ornament of concentric filae and few costae (nine on the largest valve) appearing at about 2.5 mm from the umbo reminiscent of the genus Proboscisambon first described from Bohemia (Havlíček and Mergl 1982). The Baltic specimens have obtuse cardinal angles and almost flat valves. The studied specimens are similar to those Paramalomena in having a fine radial ornament differing in lacking the growth lines characteristic of that genus. Depending on preservation, the small Glyptomeninae may be erroneously assigned to the genus Foliomena; see for example, one specimen (GIT 542-53) from the lowermost Porkuni Regional Stage in the Stirnas-18 core (Hints et al. 2010). This specimen, in fact, belongs to the genus Proboscisambon.

  • Family Leptostrophiidae Caster, 1939
    Genus Eostropheodonta Bancroft, 1949

  • Type species: Orthis hirnantensis M'Coy, 1851; Hirnant Formation, Hirnantian (Upper Ordovician); Aber Hirnant, near Bala, Gwynedd, Wales, UK.

  • Eostropheodonta hirnantensis hirnantensis (M'Coy, 1851)
    Figs. 5, 6A1.

  • 1851 Orthis Hirnantensis M'Coy, 1851: 395.

    2008 Eostropheodonta hirnantensis hirnantensis (M'Coy, 1851); Cocks 2008: 68 (see synonyms therein).

  • Material.—Variably preserved specimens in 129 samples with one or more specimens in each. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia, southern Estonia. Drill core and depth (in meters) of sample intervals with brachiopods. Depth of fragmentary preserved material is marked by “?”. Collection GIT 542: Adze-6, 840.9–844.2; Aispute-41, 987.7–1000.5; Engure, 884.3–884.4; Ikla, ?531.0–?535.7; Mežmaļi-16, 906.56–?913.4; Priekule-20, 1357.3–?1363.4; Riekstini-15, 846.15–?860.4; Ruhnu-500, ?616.3–617.4; Stirnas-18, 899.0, Taagepera, 413.7–413.9; Vilcini-19, 895.7–909.6; collections LDM G: Adze-6, ?838.1–?844.8, Dizrungi-17, ?893.8, Dreimaņi-11, ?953.6–953.75; Kandava-52, ?930.3–?931.5; Mežmali-16, ?917.8–921.35 (the revised depths are published by Gailite et al. (1989) and Hints et al. (2012) (the lower boundary of the Porkuni Stage is at a depth of 916 m); Mežvagari-13, ?871.2; Pliekalni-14, ?883.7; Priekule-20, ?1355.5–1363.4; Priekule-23, ?1392.1–1395.6; Remte-3, 958.0–958.8; Talsi-55, ?866.7–?867.1.

  • Description.—Plano- to concavo-convex shell, transversely subquadrate in outline, length-width ratio 0.6–0.8, maximum width commonly less than 20 mm at hinge line. Cardinal angles obtuse on smaller and acute on larger shells. Ventral valve slightly convex, maximum convexity in posterior part. Specimens less than 3 mm long have a small beak extending backward; small depression begins anterior of beak. Ventral interarea up to 0.5 mm high, delthyrium with small protegular apical deltidium. Dorsal valve commonly has small drop-shaped protegulal node covered by concentric filae, on some valves it continues anteriorly as median costae. Radial ornament parvicostellate in most specimens, becoming multicostellate with faint short rugae in postero-lateral parts of shell; accentuated costae divide the ornament into 7 or more sectors, 6–12, on average 9, costellae per 2 mm at 5 mm from beak. The interspaces between ribs are densely covered by concentric filae (Fig. 5B1).

    Ventral interior (Fig. 5D) has small teeth with minute crenulations on upper side; crural fossettes strong, dental plates short, divergent.

  • Remarks.—The material is insufficient for precise differentiation between the subspecies E. hirnantensis hirnantensis (M'Coy, 1851) and E. hirnantensis siluriana (Davidson, 1871), which are defined mainly by the ribbing. The latter subspecies (Hiller 1980) has coarser, more fascicostellate ribbing than E. h. hirnantensis. Ornament of a few specimens reminiscent of E. h. siluriana (Fig. 6A2) co-occurring in some samples with specimens more similar to H. h. hirnantensis (Fig. 6A1).

    The Baltic specimens are similar to E. h. hirnantensis from Poland and England (Temple 1965) in shape and size and in the radial ornament, showing a strong median costa on the ventral valve and in the arrangement of ribs into the sectors between the stronger ribs. In many samples, indeterminate strophomenoid brachiopods are represented by incomplete valves and fragments having an ornament similar to Eostropheodonta and, more specifically, to E. h. hirnantensis.

  • Stratigraphic and geographic range.—This key taxon occurs mainly in the Hirnantian Stage, Upper Ordovician. The species is a characteristic component of the globally distributed Hirnantia brachiopod fauna in Africa, Canada, China, the Czech Republic, England, and Ireland (Rong and Harper 1988). In Baltoscandia it occurs in Norway (Cocks 1982), Sweden (Bergström 1968), Estonia, Latvia (this paper), and Lithuania (Paškevičius 1997).

  • Fig. 5.

    Leptostrophiid brachiopod Eostropheodonta hirnantensis (M'Coy, 1851) from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia. A. Ventral valve, GIT 542-316, Riekstini-15, depth 850.3 m, ventral exterior view. B. Ventral valve, GIT 542-333, Riekstini-15, 854.0 m, fragment of valve with ornament (B1) and exterior view (B2). C. Juvenile specimen, GIT 542-319, Mežmali-16, 906.56 m, ventral exterior view. D. Ventral valve, GIT 542-385, Aispute-41, 1000.5 m, interior view. E. Bedding plane with numerous casts of valves, GIT 542-360, Vilcini-19, 906.5 m.

    f05_395.jpg

    Fig. 6.

    Leptostrophiid brachiopods from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia (A–E, G) and southwestern Estonia (F). A. Eostropheodonta hirnantensis (M'Coy, 1851), Taagepera, depth 413.9 m, moulds of ventral valves, GIT 542-337/1 (A1) and GIT 542-337/2 (A2), exterior views. B, E. Eostropheodonta cf. parvicostellata Rong, 1984. B. Shell, GIT 542-381, Vilcini-19, 906.4 m, exterior view of dorsal valve (B1) and view on interarea (B2). E. Mould of dorsal valve, LDM G328-12, Priekule-33, 1395.6 m, exterior view. C, D, F, G. Coolinia sp. C. Mould of ventral valve, LDM G328-26, Dižrungi-17, 984.5 m, exterior view. D. Ventral valve, LDM G328-26, Dizrungi-17, 984.5 m, exterior view. F. Fragment of dorsal valve, GIT 542-364, Ruhnu-500, depth 612.9 m, view of cardinalia (F1) and exterior view (F2). G. Incomplete ventral valve, GIT 542 48-2, Stirnas-18, 899.0 m, interior view. Scale bars 2 mm.

    f06_395.jpg

    Eostropheodonta cf. parvicostellata Rong, 1984
    Figs. 6B, E.

  • Material.—Six specimens embedded in rock. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia. Drill core and depth (in meters) or sample intervals with brachiopods. Collection GIT 542: Adze, 844.2; Aispute-41, 995.9; Riekstini-15, 850.0; Vilcini-19, 894.7–906.4; collection LDM G: Priekule-23, 1395.6.

  • Remarks.—Some specimens, amongst those assigned to the genus Eostropheodonta, are particularly distinctive, having a parvicostellate ornament of fine costae and costellae of nearly equal strengths, 9–11 costellae per 2 mm at 5 mm from umbo. The shells are small up to 13 mm wide at the hinge line and about 9 mm long. Cardinal extremities are acute. Ventral interarea is 0.3 mm high, delthyrium is open (Fig. 6B1). Dorsal valve is flat, interarea low, notothyrium covered by small, convex chilidium. The radial ornament of these Baltic specimens is most similar to E. parvicostellata from the Hirnantian of China (Rong 1984). However, our specimens are relatively small. The lack of interiors negates precise species level identification. E. parvicostellata differs from E. h. hirnantensis in having a median process between the cardinal process lobes (Rong 1984: fig. 15). However, the very high variability of radial ornament of the latter species (Rong and Cocks 1994) does not exclude possible assignment to E. h. hirnantensis.

  • Fig. 7.

    Leptostrophiid brachiopod Eostropheodonta cf. schmalenseei (Bergström, 1968) from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, southwestern Estonia (A) and western Latvia (B–D). A. Ventral valve, GIT 542-218, Ruhnu-500, depth 616.9 m, interior (A1), posterior (A2), and lateral (A3) views of the teeth. B. Incomplete ventral valve, LDM G328-10, Talsi-55, 866.7 m, interior view (B2) and view of interarea (B1). C. Dorsal valve, GIT 542-220, Mežmali-16, 913.0 m, view of posterior part with convex chilidium (C1) and view of surface with endopunctae on the middle part of the valve (C2). D. Incomplete ventral valve, LDM G 328-9, Talsi-55, 866.7 m, exterior view.

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    Eostropheodonta cf. schmalenseei (Bergström, 1968)
    Fig. 7.

  • Material.—11 variably preserved specimens, embedded in rock. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia, southwestern Estonia. Drill core and depth (in meters) of sample intervals with brachiopods: collection GIT 542: Engure, 882.9; Mežmaļi-16, 913.0, 913.4; Piltene-1, 1017.1; Ruhnu-500, 616.9; collections LDM G: Cicere (depth is unknown); Kronauce, 1055.1; Talsi-55, 866.7–867.1; Pāvi losta,1097.0–?1097.5; Remte-3, 965–966.

  • Description.—Large oval, thin-shelled specimens. Ventral valve is very weakly convex in posterior part, dorsal valve is almost flat with weak concavity in the middle. The largest specimen is 52.3 mm wide and over 37 mm long. Ornamentation parvicostellate, with about 50 ribs at the 10 mm growth stage along the valve margins, 28 of which are accentuated; 5 ribs occur per 2 mm at 20 mm from umbo; 2–3 stronger ribs with 1–2 finer costellae between them occur per 2 mm on the anterior margin. Concentric growth lines very fine with 8–10 lines per 1 mm.

    Ventral interarea is flat, 0.5 mm high. The denticulate teeth diverge at 110°. Dental plate with about 12 denticles capping the teeth and with crenulations on the antero-median faces. The denticles and crenulations continue along the edge of the delthyrium as small protuberances (Fig. 7A2, A3, B1). Dental plates very short. Muscle scars not visible. The external ornament is impressed on the interior valve surface; tubercles (pseudopunctae) are sporadically developed.

    Two dorsal valves have low interareas with notothyrium covered by convex, non-grooved chilidium (Fig. 7D1); interior is unknown.

  • Remarks.—Eostropheodonta cf. schmalenseei differs from E. h. hirnantensis by its larger size and less well-differentiated ornament. The ventral valves of E. cf. schmalenseei differ from E. h. hirnantensis in having teeth, developed as oblique plates with about 12 denticles on the anterior margin of the interarea. The latter species has antero-laterally directed teeth with 4–6 denticles (Temple 1965: pl. 17: 4, 5; Bergström 1968, pl. 6: 7; Rong and Cocks 1994). The lack of a median groove on the chilidium (Fig. 7D1) in the East Baltic species confirms the close relationship with E. schmalenseei from Sweden.

    Eostropheodonta cf. schmalenseei is similar to Eostropheodonta luna from the Boda Limestone (Cocks 2005: pl. 9: 11–15) and to the Estonian species Pirgumena (= Eostropheodonta by Cocks 2005: 269) martnai (Rõõmusoks 2004: pl. 15: 8–10; pl. 16: 1–5) by the shell size. However, E. cf. schmalenseei has a more uniform ornament with wider interspaces between costellae, especially on the postero-lateral parts of valves. E. luna and the Estonian species have ornaments bearing accentuated ribs. There are insufficient data on the interiors of related species to permit their clear comparison.

  • Fig. 8.

    Plectambonitoid brachiopods from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia. A. Leangella sp., shell, GIT 542-222, Riekstini-15, depth 859.3 m, ventral (A1), dorsal (A2), and lateral (A3) views. B. Eoplectodonta sp., incomplete ventral valve, GIT 542-21, Stirnas-18, 911.9 m, ventral view.

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    Superfamily Plectambonitoidea Jones, 1928
    Family Leptestiidae Öpik, 1933
    Genus Leangella Öpik, 1933

  • Type species: Plectambonites scissa var. triangularis Holtedahl, 1916; Solvik Formation, Lower Llandovery (Silurian); Asker, Norway.

  • Leangella (Leangella) cf. scissa (Davidson, 1871)
    Fig. 8A.

  • Material.—Two specimens. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia. Drill core and depth (in meters) of brachiopod samples: collection GIT 542: Riekstini-15, 859.3, 861.8–861.85.

  • Remarks.—Small, strongly concavo-convex shells, sub-triangular in outline with 5 primary ribs, similar to L. (L.) cf. scissa from the Dalmanitina Beds in Sweden (Bergström 1968) and also to L. (L.) scissa from the lowermost Silurian in the East Baltic (Rubel 2011). The larger of two specimens is 6.4 mm wide, 5.8 mm long and 2.9 mm deep, the smaller is only 1.5 mm wide. Cardinal angles rounded; anterior commissure slightly sulcate. Ventral interarea almost orthocline, slightly concave at the apex, 0.6 mm high. Delthyrium with small deltidial plates in apical part. Dorsal interarea flat, hypercline, 0.4 mm high. Notothyrium filled with trilobate cardinal process. Radial ornament of five primary and 4 additional ribs on ventral valve with very fine intercalated costellae. The Swedish species apparently differs from the Baltic material in lacking fine costellae between the primary costae; the Silurian L. (L.) scissa has a wider shell and more convex ventral valve.

    Although rare in the Hirnantia brachiopod fauna, Leangella is common in the Boda Limestone (Sheehan 1979; assigned to Diambonia, Jaanusson 1982). The species L. (L.) longae Cocks, 2005 from the lowermost Boda flank facies at Osmundsberget differs from the Hirnantian specimens in having more numerous (up to 14) primary costae.

    The Hirnantian material of Leangella from the central Oslo Region in Norway, identified as L. aff. cylindrica (Reed, 1917) by Cocks (1982), is insufficiently known for adequate comparison.

  • Family Sowerbyellidae Öpik, 1930
    Subfamily Sowebyellinae Öpik, 1930
    Genus Eoplectodonta Kozłowski, 1929

  • Type species: Sowerbyella precursor Jones, 1928; Upper Haverford Mudstone Formation, Lower Llandovery (Silurian); Haverfordwest, Dyfed, Wales, UK