Balanoglossites Ichnofabrics from the Middle Ordovician Volkhov ...

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Abstract—The limestone succession of the Middle Ordovician Volkhov Formation in the St. Petersburg region (Russia) exposes numerous horizons with firm and ...
ISSN 08695938, Stratigraphy and Geological Correlation, 2013, Vol. 21, No. 3, pp. 265–279. © Pleiades Publishing, Ltd., 2013. Published in Russian in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2013, Vol. 21, No. 3, pp. 22–38.

Balanoglossites Ichnofabrics from the Middle Ordovician Volkhov Formation (St. Petersburg Region, Russia)1 D. Knausta and A. Dronovb a

Statoil ASA, N4035 Stavanger, Norway Geological Institute of Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017 Russia email: [email protected]; [email protected]

b

Received December 24, 2012

Abstract—The limestone succession of the Middle Ordovician Volkhov Formation in the St. Petersburg region (Russia) exposes numerous horizons with firm and hardgrounds below omission surfaces, which con tain trace fossils attributable to the ichnogenus Balanoglossites Mägdefrau, 1932. Although known from the literature since 75 years, such trace fossils were previously only informally described as “Korrosionsgruben”, “Karandashi”, or ascribed to different ichnotaxa such as Trypanites, Arenicolites and Pseudopolydorites. Owing to their complexity and often high bioturbation density, the morphology of these trace fossils is diffi cult to capture. Ichnofabrics containing Balanoglossites triadicus were studied in detail from sawn rock faces, broken rock blocks and sectioned slabs, including those from historical buildings in St. Petersburg. Accord ingly, different tracefossil elements can be revealed in dependence on the original substrate consistency, reflecting various stages of lithification: Mineralstained and Trypanitesperforated hardground surfaces are bioeroded with long elongated grooves which are assigned to the ichnogenus Sulcichnus. Subtle openings lead into the partly lithified limestone where they branch into complex galleries of B. triadicus. They are charac terized by J, U and Yshaped shafts and multiply branched tunnels, which gradually continue into the underlying firmground. Other portions of the ichnofabric only exhibit biodeformational structures or the strongly compacted and branched burrow networks Labyrintichnus, which is due to the original soft sediment consistency. Balanoglossites ichnofabrics demarcate certain omission surfaces within the Dikari Limestone and can be traced for more than 300 km, supporting the regional lithostratigraphical correlation. The trace maker of B. triadicus and related trace fossils is interpreted to be a eunicid polychate with the ability to bioerode and bur row the sediment. The studied material from the Ordovician is similar to the Balanoglossites from the type area, the Triassic of Germany, in many respects. B. triadicus is a very common trace fossil in Ordovician and other Palaeozoic rocks of Baltoscandia and North America but so far has been seldom identified as such but instead is commonly confused with Thalassinoides, from which, however, it differs in several aspects. Keywords: Balanoglossites triadicus, Thalassinoides, ichnofabrics, borings, burrows, eunicid polychaetes, Middle Ordovician, Baltoscandia DOI: 10.1134/S0869593813030040

1. INTRODUCTION

The ichnogenus Balanoglossites was erected by Mägdefrau (1932) from Middle Triassic carbonates of the epicontinental Germanic Basin for “thick borings with irregular branches and several openings” (trans lated from Mägdefrau 1932, p. 153). On the basis of a restudy of the type specimens and additional substan tial material from the type area, Knaust (2008) revised Balanoglossites and reveals its nature as a complex trace fossil containing both burrow and boring compo nents. The revised diagnosis is “branched galleries with several openings and acorn, bulb or lance shaped side branches. Tunnels are elliptical or circular in crosssections, margin is unlined and locally stri ated; tunnel size varies in the order of several magni 1 The article is published in the original.

tudes within a single gallery system” (Knaust, 2008, p. 352). In addition to the type ichnospecies B. triadi cus with predominantly deep U or Yshaped tunnel elements, B. ramosus was introduced to comprise irregularly ramified galleries. Since its introduction, the ichnogenus Balanoglos sites has been well recognized in the Middle Triassic of the Germanic Basin, but it has only rarely been reported from elsewhere. This fact may partly be related to the complexity of the trace fossil as well as its similarity to the wellestablished ichnogenus Thalass inoides. However, as shown in this article, Balanoglos sites is a very common constituent of Palaeozoic firm and hardgrounds, from where it has been sporadically described, mentioned or figured (e.g. Goldring and Ka zmierczak, 1974; Chamberlain, 1977; Palmer, 1978; Ekdale et al., 2012). Moreover, it is likely that

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numerous records of this common ichnotaxon were misidentified and assigned to the similar but different ichnogenus Thalassinoides or left undetermined (e.g. Sheehan and Schiefelbein, 1984; Brett and Liddell, 1978; Jin et al., 2011). Ordovician deposits of the St. Petersburg region are rich in trace fossils. They are especially abundant in condensed sections of the uppermost Early Ordovi cian and the lowermost Middle Ordovician of the region (Leetse and Volkhov formations). Burrows and borings from these formations have been mentioned in the literature since the 1930s (Vishniakov and Hecker, 1937; Orviku, 1960; Männil, 1966a, 1966b; Dronov et al., 1993). They even have been used for identifica tion and tracking of some omission surfaces within the Volkhov Formation (Dronov et al., 1996, 2000; Knaust et al., 2012). During the last years, palaeoich nological investigations of the Volkhovian deposits have been undertaken in the region (Dronov et al., 2002, 2006; Mikuláš and Dronov, 2004a, 2004b, 2005; Dronov and Mikuláš, 2010; Knaust et al., 2012). Because of the complex character and complicated morphology of some of these trace fossils, their ichno taxonomic affinity has been under discussion for a long time (Dronov and Mikuláš, 2010). Given the nature of carbonates and their affinity of earlydiage netic cementation, a rigid discrimination between burrows (in soft sediment) and borings (in lithified deposits) is often impossible, and morphologically similar or identical trace fossils may originally been created in firm and in hard substrate. As a conse quence, those forms were assigned to different ichno taxonomic categories. Moreover, the complex mor phology and compound character of some of these trace fossils may hamper their identification when occurring as parts in isolated and small separated blocks. This article deals with omission surfaces in carbon ates of the Lower Volkhovian Formation in the vicinity of St. Petersburg and aims for a detailed description and interpretation of burrows and borings assigned to the ichnogenus Balanoglossites. In previous publica tions, these trace fossils were mentioned under the informal name of “Karandashi”, which means “pen cils” in the Russian language (Dronov et al., 1993; Dronov, 1998), and were partly assigned to ichnogen era such as ?Dolopichnus (Dronov et al., 2002), Try panites (Mikuláš and Dronov, 2005), Arenicolites and Pseudopolydorites (Dronov and Mikuláš, 2010). These trace fossils are especially widespread in the Dikari Limestone (lower subformation of the Volkhov For mation, lower substage of the Volkhov regional stage (BIIα), Dapingian Global Stage of the Middle Ordov ician Series). The Dikari Limestone has been the target of exten sive quarrying for building purposes since the founda tion of the town St. Petersburg in 1703. For that rea son, the Dikari Limestone is well exposed and easily

accessible for study in numerous old and still active quarries as well as in the natural outcrops along the BalticLadoga Klint line. Along some of the omission surfaces in the Dikari Limestone, Balanoglossites is extremely abundant and their densely arranged shafts give reason for the formation of characteristic ichno fabrics. Because of that, these omission surfaces are easy to recognize over a long distance and can be used for a bedtobed or surfacetosurface correlation of individual sections within this stratigraphic interval. 2. STRATIGRAPHY In northwestern Russia (St. Petersburg region), the Lower Palaeozoic strata are almost flatlaying with a slight dip (2.5–3.5 m/km) to the south. The main natural outcrops of the Lower and the lowermost Mid dle Ordovician are mainly concentrated along the river valleys crossing the natural geomorphologic escarp ment (BalticLadoga Klint), which separates the Ordovician plateau from the Neva lowlands (Fig. 1). The trace fossils under consideration are widespread in the Volkhovian Formation, which is also known under the old informal name of “Glauconite Lime stone” (Selivanova and Kofman, 1971). The Volkhov Formation consists of three subformations (lower, middle and upper) with their own traditional informal names “Dikari”, “Zheltiaki” and “Frizy”, respec tively (Dronov and Mikuláš, 2010). Balanoglossites mainly occurs in the lower subformation of the Volkhov Formation (Dikari Limestone), which corre sponds to the lower part of the Middle Ordovician Dapingian Stage and comprise a time span of about 1.5 Ma (Bergström et al., 2009). Since Schmidt (1858) the letter indices are also used for the regional stages as introduced on the basis of the local lithostratigraphic units (e.g. “BII” index for the “Glauconite Lime stone”). Accordingly, Lamansky (1905) introduced BIIα, BIIβ and BIIγ indices for the Dikari, Zheltiaki and Frizy units. The Dikari Limestone (BIIα) is represented by dense and thickbedded bioclastic limestone, which since the foundation of St. Petersburg was intensively quarried for building purposes. The basements and staircases of many of the buildings and palaces in the historical part of the city are made from this excellent building material. During the almost 300 years of per manent quarrying, the quarrymen worked out their own detailed stratigraphy and nomenclature for the individual beds in this stratigraphical interval. The Dikari Limestone consists of 15 individual beds with their own names (Fig. 2), which can easily be identi fied in the quarry sections due to their specific charac teristics, including tracefossil occurrences and ichno fabrics. These beds can be recognized even in the base ments of the buildings and can be traced in the natural outcrops along the BalticLadoga Klint line within the St. Petersburg region and northeastern Estonia (Dronov et al., 2000).

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In the 19th and the first part of the 20th century, the stratigraphic interval corresponding to the “Glauco nite Limestone” was assumed to correlate with the stratigraphic interval of the Regional Stage BII as introduced on its base, which was formally named the Volkhov Stage. However, after more detailed investiga tions it became obvious that the most distinct and sharp changes in faunal composition occurred within the Dikali Limestone and not at its base, which mainly reflects a lithological change. The change in faunal composition occurs within the 5th bed of the Dikari Limestone (counted from the base), which is called “Zeliony” (“Green” in Russian) due to its high con centration of scattered glauconite grains and glauco nitestained hardground surfaces. The most prominent hardground surface within this bed is called “Steklo” (“Glass” in Russian) because of its flatness and bright green colour resulting from glauconite impregnation. This surface is easily recognizable in natural outcrops and boreholes. It can be traced over a distance of more than 500 km along the Russian and Estonian segment of the Baltic Ladoga Klint (Dronov et al., 2000) and is interpreted as a sequence boundary (Dronov and Holmer, 1999). The drastic change in faunal complexes occurs just across the “Steklo” surface. All these aspects make this level more convenient for regional correlation than the base of the Dikari Limestone and for that reason; Männil (1966b) changed the original concept of Lamansky (1905) by shifting the base of the Volkhov Regional Stage from the base of the Dikari Limestone to the “Steklo” sur STRATIGRAPHY AND GEOLOGICAL CORRELATION

face within the Zeliony Bed. This boundary is coincid ing with the boundary between the Megistaspis estonica and the M. lata trilobite zones, as well as with the boundary between the Oepicodus evae and the Balto niodus triangularis conodont zones (Dronov et al., 2003). The base of the Volkhov Regional Stage at the “Steklo” level also coincides with the boundary between the Lower and the Middle Ordovician series and consequently with the boundary between the Floian and Dapingian Global Stages according to the new international stratigraphic classification for the Ordovician system (Bergström et al., 2009). 3. DEPOSITIONAL ENVIRONMENT The Volkhovian succession crops out in the St. Petersburg region in numerous natural outcrops and quarries along the BalticLadoga Klint line (Fig. 1). The thickness of the Volkhovian deposits ranges from 2.5 m in the West to 7 m in the East of the region. The deposits are enriched with glauconite and possess char acteristics of a condensed section (Loutit et al., 1988). The lower subformation of the Volkhovian Formation (Dikari Limestone) is represented by a hard, thick bedded bioclastic limestone (packstone and matu rated wackestone) with numerous omission surfaces highlighted by glauconitic and/or goethitic impregna tion. The glauconite grains are visible with the naked eye and are abundantly scattered within the rock. The middle and the upper subformations (“Zheltiaki” and “Frizy”, respectively) of the Volkhovian Formation are represented by intercalations of bioclastic lime Vol. 21

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stone layers (1.5–4.5 cm in thickness) and clay layers of comparable thickness. The Volkhovian deposits correspond to a single sedimentary (regressivetransgressiveregressive) cycle (depositional sequence) and are bounded at the base and the top by unconformities (Dronov and Holmer, 1999). They are represented by coolwater carbonates

that were formed on a stormdominated shallow water shelf (Dronov, 1998; Dronov and Rozhnov, 2007). The storms modified very characteristic sheet like skeletal sand beds of considerable lateral extent (300 km). Individual storm beds (3–4.5 cm thick) might have been deposited in a few days, whereas amalgamated composite beds (up to 20–30 cm thick)

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or bed packages (up to 1 m thick) reflect a time interval of 100000–400000 years. Carbonate beds consisting predominantly of coarsegrained shell debris have sharp erosional bases and most of them are distinctly graded. Brachiopods, ostracods, echinoderms, bryo zoans and trilobites are the main contributors. Cepha lopods and gastropods are also present (Dronov, 1998). A proximaldistal trend of the tempestites can be recognized. The Dikari Limestone is interpreted as proximal tempestites based on its coarsegrained com position, the tendency of amalgamation and the lack of clay layers. The Zheltiaki and Frizy limestones can be interpreted as distal tempestites because the car bonate beds are thinner and finergrained (Dronov, 1998). The carbonate tempestites were formed on a lowangle homoclinal ramp in relatively shallow water as part of the NorthEstonian Confacies belt of the Ordovician basin in Baltoscandia. The water depth in the basin increased from the Gotland Bank towards southeast, implying that the sections in the eastern segment of the BalticLadoga Klint reflect relative deeperwater deposits compared to the sections in the western segment. Due to condensation of the Volkhovian succession, trace fossils are very abundant but their diversity is rel atively low. Besides Balanoglossites, the ichnogenera Thalassinoides, Phycodes, Bergaueria, Palaeophycus, Trypanites and Gastrochaenolites were recognized in the Dikari Limestone. In the Zheltiaki and Frizy lime stone, Arachnostega and Chondrites were found together with the abovementioned trace fossils (Dronov and Mikuláš, 2010). 4. HISTORY OF PALAEOICHNOLOGICAL RESEARCH IN THE REGION Despite of the abundance of trace fossils in the Lower and Middle Ordovician of the study area, they have never been subject of systematic research. The biological nature of some sedimentary structures from the Volkhovian strata has been suggested by Vishnia kov and Hecker (1937) for the first time. Since then, and for a long time, the Trypanites borings (Vishniakov and Hecker, 1937) and “amphoralike burrows” according to the interpretation of these authors, or “amphoralike borings” according to Orviku (1940; 1960), were the only trace fossils described from these deposits. The “amphoralike structures” were later assigned to the newly established ichnogenus Amphorichnus Männil, 1966a. Together with Vishniakov and Hecker (1937), Männil (1966a) interpreted these trace fossils as burrows. On the basis of material from the island Öland (Sweden), Ekdale and Bromley (2001) demon strated that the “amphoralike” traces may result from both boring and burrowing activity, and assigned them to the ichnogenus Gastrochaenolites. It should be men STRATIGRAPHY AND GEOLOGICAL CORRELATION

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tioned, however, that the biological nature of these structures known as “Korrosionsgruben” in the Swed ish Ordovician have been recognized already by Andersson (1896). The presence of Balanoglossites in the shallowmarine facies of the Arenig in eastern Bal tic was only briefly mentioned in an abstract (Männil et al., 1984) and without illustrations, descriptions or exact data on the stratigraphic position of these trace fossils. Trace fossils assigned here to the ichnogenus Balanoglossites could be seen in the same samples which was collected by Vishniakov and Hecker (1937) and Hecker (1960) as examples of Trypanites and for “amphoralike burrows” from the “Steklo” surface in Putilovo and Kingissepp quarries (Figs. 3a, 3b, 3e) (in the collection of the Paleontological Institute of Rus sian Academy of Sciences, Moscow). A rich collection of samples with Balanoglossites from this and other levels of the Dikari Limestone exists also in the Museum of the Department of Historical Geology of the St. Petersburg State University in St. Petersburg. In recent years, investigations of the Ordovician trace fossils have been reinitiated in the region (Dronov et al., 2002; Mikuláš and Dronov, 2004a, 2004b, 2005; Dronov and Mikuláš, 2010; Knaust et al., 2012). Apart from Trypanites and Gastrochaeno lites in the Volkhovian Formation, ichnogenera such as Thalassinoides, Phycodes, Bergaueria, Palaeophycus, Planolites, Chondrites and Arachnostega could be iden tified (Dronov and Mikuláš, 2010). However, the ich notaxonomic affinity of numerous small subvertical shafts of boring and/or burrows (“Karandashi”), which are widespread in the Dikari Limestone, remained uncertain. Particularly the distinguishing between burrows and borings turned out to be prob lematic due to the fact of gradual transitions between firm and hard substrates. Thus, morphologically iden tical or similar traces were found as burrows in firm ground but also as borings penetrating hardground. Another problem has been the understanding of the real and entire morphology of the trace fossils as this turns out to be rather complex. In many instances along the vertical sections, only subvertical and slightly inclined shafts can be observed, while other sections reveal a J, U or Yshaped morphology of the traces. For these reasons, trace fossils which were known under the informal name of “Karandashi” (Dronov et al., 1993) can now be attributed to different ichno genera such as ?Dolopichnus (Dronov et al., 2002), Trypanites (Mikuláš and Dronov, 2005), Arenicolites and Pseudopolydorites (Dronov and Mikuláš, 2010). These trace fossils have been subject of a special inves tigation in the course of the preparation of the excur sion associated with the IV International Workshop on Ichnotaxonomy in June 2010. After a close exami nation of the “Karandashi” trace fossils followed by discussion during the excursion we came to the con clusion that these complex trace fossils can chiefly be Vol. 21

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assigned to the ichnogenus Balanoglossites Mägdefrau, 1932. 5. BALANOGLOSSITES FROM THE DIKARI LIMESTONE Balanoglossites is widespread in the lower part of the Volkhov Formation (Dikari Limestone, Fig. 4a), where it is most evident at levels with a high concen

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tration of closely spaced subvertical shafts. These lev els are known from the Konopliasty and the Krasny beds of the Dikari Limestone under the informal name of “Karandashi” (Dronov et al., 1993, 1996). In these beds, Balanoglossites is the dominant ichnotaxon and occurs almost monoichnospecifically (Figs. 2, 4f). Bal anoglossites is also present in the other beds of the Dikari Limestone, but there it does not create such a monoich nospecific tracefossil association and ichnofabric.

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Fig. 3. Ichnology of the hardground facies. (a) Upper surface of a hardground with pervasive penetration of its uppermost layer by short, cupshaped Trypanites isp. (T), which are crosscut by elongate grooves attributed to Sulcichnus isp. (S). Scale bars in millimetres. Paleontological Museum Moscow, Nr. 1747132 (original collected by Hecker, 1924). (b) Same specimen as in Fig. 3a, vertical section corresponding to the lower edge in Fig. 3a. The elongate grooves (Sulcichnus isp.) from the hardground surface integrate with a dense Balanoglossites triadicus ichnofabric (B) in the subsurface, which changes from sharply outlined burrows in the upper part (firmground) to weaklydefined and finally mottled fabric (m) in the lower part (softground). Scale bars in millimetres. (c) Hardground surface (ochre colour) with marlfilled Trypanites isp. (T), Sulcichnus isp. (S) and openings of B. triadicus (B). Top of the Butok Bed in the Babino Quarry. Scale bars in millimetres. (d) Same hardground surface as in Fig. 3b but with weatheredout marl and thus wellpronouncing Sulcichnus isp. (S) and Trypanites isp. on the ridges in between (partly crosscut by Sulcichnus isp.). The rounded edges indicate a long period of omission and probably result from frequent current and/or wave action. Top of the Butok Bed, Tosna River canyon. (e) Vertical section of a hardground (ochre colour) with B. tri adicus (B, preomission suite) and Gastrochaenolites oelandicus (G, omission suite), the latter filled with glauconiterich sediment from above. “Steklo” surface from Putilovo Quarry. Coll. Hecker, Paleontological Museum, Moscow. (f) Hardground surface with openings of B. triadicus (small openings) and G. oelandicus (large openings). Base of the Volkhov Regional stage (“Steklo” surface), green bed of the Dikari Limestone, Kingisepp Quarry. Scale bar in centimetres.

In the studied sections, Balanoglossites appears as complex trace fossils consisting of morphologically different elements in dependence on the penetrated substrate. If they would be found in isolation, these elements of the same complex trace fossil Balanoglos sites triadicus could give reason for their assignment to various ichnotaxa. In addition to its complexity, Bal anoglossites commonly interacts with other trace fos sils and thus complicates a definite characterization. The grade of complexity varies between relatively sim ple to very complex and gradually leads to an increase in bioturbation intensity. Following the vertical subdi vision of an ideallydeveloped Balanoglossites ichno fabric into tiers with a primary substrate consistency of hard, firm and soft (from the omission surface at the top downwards into the sediment), the following mor phological elements can be outlined (Figs. 3–6). 5.1. Hardground Elongate, rarely branched grooves occur along the surface in a partly irregularpatchy manner, and are typically associated with a dense accumulation of Try panites isp (Figs. 3a, 3d). Some of the grooves are crosscutting the Trypanites borings and bioclasts, and are interconnected with more branched subsurface galleries, the latter which are passively filled with marl from the layer above the hardground (Fig. 3b, 3c). Welldeveloped borings occur in the Konopliasty Bed (Fig. 4f) along the “Steklo” surface (Figs. 3e, 3f), and at the hardground surface within the Zhelty Bed. Within the “Steklo” hardground, the Balanoglossites borings (smaller openings) occur in association with Gastrochaenolites borings (larger openings) (Fig. 3f). On the hardground surface at the top of the Butok Bed, horizontal and occasionally branching and slightly curving grooves are developed (Fig. 3a, 3d). Some of these grooves connect with vertical Balano glossites shafts which penetrate the hardground surface (Fig. 3b). In addition to Balanoglossites, this surface is pitted with short and blunt Trypanites (Dronov et al., 2002). The horizontal grooves are typically much wider than the opening diameter of Balanoglossites and may reach a width of up to 1.5 cm (Fig. 3d). This STRATIGRAPHY AND GEOLOGICAL CORRELATION

can probably be explained by the compound nature of these tracefossil elements. They have probably been used more than once and by more than one animal being widened in the process of excavation. This situa tion is also typical for some subvertical shafts (Figs. 3e, 4b, 4d). Sometimes it can be observed that a wider (compound) segment of Balanoglossites passes into a narrow individual tunnel (Fig. 4d). The Yshaped seg ment of Balanoglossites can be seen on Figs. 4c, 5. It is interesting to note that horizontal grooves con necting subvertical components of Balanoglossites could be found not on every hardground surface in the Dikari Limestone. They were only observed at the top of the Butok Bed and for the hardground surfaces within the Zhelty Bed, but not on the “Steklo” hard ground surface. This seems to be related to the depth of erosion. On the “Steklo” hardground surface, all the “shallow” near surface layers are removed by ero sion and only remnants of subvertical components of Balanoglosites structure can be seen (Fig. 3f). Subver tical boring components reach their maximal density on hardground surfaces within the Konoplisty (Fig. 4f), Krasny and Krasnenky beds, where they form a char acteristic ichnofabric known as “Karandashi” (Dronov et al., 1993; Fig. 2). Vertical and horizontal crosscuts of this ichnofabric can be seen on Fig. 4e and Fig. 4f, respectively. 5.2. Firmground The firmground contains the most spectacular trace fossils due to their mineral staining, sharp boundaries and passive fill. They commonly have sub vertically orientated Ushaped morphologies con necting to the surface, but may also connect with lab yrinthlike burrow systems in the underlying soft ground (Fig. 5). On horizontal surfaces, Balanoglossites is repre sented by subcircular spots with an average diameter varying between 2 and 5 mm, rarely reaching 7 mm. These crosssections are easily visible because of their contrasting infill (mineralogy, grain size and colour) in comparison with the host rock. This contrast is usually enhanced by secondary mineralization such as dolo Vol. 21

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Fig. 4. Ichnology of the firmground facies. (a) Sawn section through the Dikari unit, Babino Quarry. Thickness = 1.1 m (scale bar = 20 cm). (b) About eight bedding surfaces emphasized by dissolution seams. These surfaces acted as colonization surfaces for the Balanoglossites producer and the beds contain multiple burrow elements with a preferred Ushaped morphology and greenish marly fill. Dikari Limestone, Pereplet Bed, Babino Quarry. Scale bars in millimetres. (c) Vertical section displaying two partly eroded colonization surfaces (arrows). An intermediate stage between original firm and hard substrate can be assumed because the U and Yshaped Balanoglossites triadicus burrows/borings crosscut a large bioclast (b). Interval between the Kras nenky and Melkotsvet beds, Putilovo Quarry. (d) Part of a Ushaped B. triadicus burrow system (in vertical section) with a cham berlike extended basal portion and multiple Yshaped openings. The floating lithoclasts in the fill are reworked from the host rock. Building stone, St. Petersburg. (e) Polished slab (plan view) showing numerous shafts of dolomitized B. triadicus in cross section. Miagonky Bed, Dikari Limestone, Babino Quarry. (f) Closely spaced and slightly curved subvertical B. triadicus shafts (“Karandashi”), enhanced by differential mineral staining. Konopliasty Bed, Dikari Limestone, Babino Quarry.

mitization. Some shafts with a similar diameter are arranged in pairs and seem to represent the crosssec tions of the upper part of a Ushaped trace fossil (Fig. 4e). In other cases, the relative dense occurrence of crosssections with a similar diameter does not allow to recognize pairs at all. The boundary of the trace fossils can be diffuse or sharp, reflecting the con sistency of the substrate at the time of penetration. In many instances, a firmground stage of the substrate is most likely, as indicated by bioclasts reaching from the surrounding matrix into the burrows, although an incipient hardground stage cannot be ruled out. In vertical sections, Balanoglossites typically appears as closely spaced, slightly curved subvertical shafts with a penetration depth of 2–4 cm (Figs. 3b, 4f). Superficially, these traces may resem ble the ichnogenus Trypanites (a boring); however, they differ from it by a more irregular outline and winding course. Furthermore, many specimens con tinue further down into the sediment in form of curved burrows which resemble J and Ushaped morphologies (Fig. 4b, 4c). The shape of the burrows varies between ideally developed, compressed and asymmetrical with inclined portions (Fig. 4c). In

twodimensional vertical sections, only the frag ments of the Ushaped traces are commonly seen (Fig. 4b, 4c, 4f), suggesting a more complex and curved morphology in threedimension (Fig. 5). 5.3. Softground Coinciding with a gradual increase of softness far ther down in the sediment, the burrow outlines in the softground become more diffuse. Weaklyconstrained burrow morphologies of various shapes (mainly U and Yshaped elements) are still recognizable along the vertical softground/flrmground interface, while a plan view reveals an irregular network with loosely winding portions, blindending terminations and the tendency to anastomosing networks. Other portions of the ichnofabric are dominated by biodeformational structures. The fill of the strongly compacted burrows is marly (probably passively introduced) (Figs. 6a–6d). The most complete threedimensional Balanoglos sites system could be studied on an isolated rock block from the uppermost bed of the Dikari Limestone (Butok Bed) in the Putilovo Quarry (Fig. 5). This block comprises an approx. 20 cm thick interval with a

Fig. 5. Isolated block revealing a Balanoglossites triadicus ichnofabric with burrow elements in firmground facies (pinkishgrey colour) as well as borings penetrating thin hardground layers (ochre colour). The layer thickness is 20 cm. Butok, Dikari Lime stone, Putilovo Quarry. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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(a)

(b)

(c)

(d)

1 cm

Fig. 6. Ichnology of the softground facies. (a) Bedding plane with a dense network of Balanoglossites triadicus, characterized by highly irregular shape and varying burrow size. Pereplet Bed, Putilovo Quarry. Scale bars in millimetres. (b) Bedding plane with a more regular burrow network of B. triadicus, which could equally be attributed to Labyrintichnus isp. Putilovo Quarry. Scale bars in millimetres. (c) Labyrintichnus isp. with weatheredout marl, locally showing faint scratches indicative of partly firm substrate. Butok Bed, Dikari, Putilovo Quarry. Scale bars in millimetres. (d) Labyrintichnus isp. network passively filled with marl. Butok Bed, Babino Quarry.

welldeveloped Balanoglossites ichnofabric. The litho facies mainly consists of a glauconiterich, bioclastic limestone (packstone to wackestone), which is inter rupted by two 2–5 cm thick, ochreco loured and goethitestained intervals with a sharp top surface and downwarddecreasing staining intensity. Based on the lithological, ichnological and mineralogical features, the background sediment is interpreted to represent a firmground during its origin, while the two thin hori zons represent hardground layers. The ichnofabric is monoichnospecific and only contains Balanoglossites triadicus, originating from at least two colonization surfaces (the hardground levels at the top and in the middle). Vertical trace components are conspicuous, probably as a result of the rapid stormevent deposi tion of the two calcareous beds and their separation by an omission surface on top of the middle hardground.

Some shafts from the upper firmground continue into the lower one and penetrate the hardground in between. This penetration results in a range of winding and narrowed shafts with funnelshaped openings. Other elements in the lower part reveal a complex, overall Ushaped morphology with multiple arms. 6. DISTRIBUTION AND SIGNIFICANCE OF BALANOGLOSSITES FOR REGIONAL CORRELATION Balanoglossites can be found throughout the Dikari Limestone but its stratigraphical distribution is not uniform. Its highest concentration is associated with the occurrence of hardgrounds, such as the Butok, Konopliasty, Nadzhelty, Zhelty, Krasny, Staritsky, Zeliony, Beloglaz, Krasnenky, Melkotsvet and

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Barkhat beds (Fig. 2). In the hardground at the top of the Butok Bed, Balanoglossites borings are associated with Trypanites (Fig. 3a, 3c, 3d), while in the “Steklo” hardground within the Zeliony Bed, Balanoglossites occurs together with Gastrochaenolites (Fig. 3e, 3f). Both hardground surfaces are interpreted as key sequencestratigraphic surfaces. The hardground sur face at the top of the Dikari Limestone is interpreted as a transgressive surface, while the “Steklo” hard ground surface is interpreted as a sequence boundary (Dronov and Holmer, 1999; Dronov et al., 2002). Although Balanoglossites triadicus is quite abundant at these two surfaces, they are accompanied by other trace fossils. The top of the Butok Bed is characterized by abundant Trypanites isp., while the “Steklo” sur face is dominated by the occurrence of Gastrochaeno lites oelandicus. On the other hand, surfaces with a maximal density of subvertical B. triadicus shafts (“Karandashi”) (Fig. 4f) have no significance with key sequencestratigraphic surfaces and the reason for the extremely dense development of B. triadicus borings on these surfaces must be related to particular palae oenvironmental conditions. Irrespective of this, these surfaces serve as markers for a regional correlation. The level with “Karan dashi” (Balanoglossites ichnofabric) inside of the Konopliasty Bed easily can be identified in all the out crops along the eastern segment of the BalticLadoga Klint line from the Syas’ River in the east to the Narva River and outcrops near Udria in northeastern Esto nia in the west (Dronov et al., 2000). Similar levels with Balanoglossites ichnofabrics within the Krasny Bed could be traced even further to the west till the Saka village in northeastern Estonia (Dronov et al., 2000). 7. THE TRACE MAKER OF BALANOGLOSSITES TRIADICUS The morphological features of the studied Balano glossites triadicus, their range of variation together with the variety of the hosting substrates and the palaeonto logical circumstances provide important evidence for the interpretation of the trace maker. Overall, the described material closely resembles Balanoglossites as known from its type area, the Middle Triassic Ger manic Basin, in many aspects (cf. Knaust, 2007, 2008; see also Knaust and Costamagna, 2012). Both, in the German Triassic and the Russian Ordovician, the morphological variation of B. triadicus is dependent on the substrate condition and comprises three major elements. Assuming an omission surface with a downward decreasing sediment consistency from a thin hard ground at the top via a firmground as major part in the middle to softground in the lower part, three main components of the complex trace fossil are evident. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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The hard surface contains elongate bioerosional grooves (Sulcichnus isp.) (Figs. 3a, 3d), and semicircu lar openings lead into a complex system with U and Yshaped elements (Fig. 4c). The gradual hard ground/firmground transition documents integration from borings in the upper part to burrows below, within one and the same trace fossil. In the deeper tier (soft ground) of the ichnofabric, the morphology of such burrow systems typically becomes blurred (mottled) or replaced by a netlike burrow system similar to Laby rintichnus isp. (Fig. 6b–6d). The simultaneous ability of the trace maker to bio erode and to burrow into substrate is rather uncom mon and affords a challenge for the ichnotaxonomist (see discussion in Bertling et al., 2006; Carmona et al., 2007), although similar cases are known from various burrowing organisms, including polychaetes, sipuncu lids, bivalves, crustaceans and others. As stressed by Knaust (2008), the ichnogenus Balanoglossites belongs into this category of boring and burrowing producers, and thus already restricts the range of its maker. Fol lowing the detailed argumentation by Knaust (2008), vermiform organisms with a proboscis are the pre ferred interpretation of the producer in order to explain the variation in morphology and size as well as the diagnostic blind tunnel terminations. By analogy, the ichnospecies Sulcichnus is inter preted to be the product of eunicid polychaetes (Mar tinell and Domènech, 2009). Not only does the mor phology of some Balanoglossites parts resemble the activity of modern polychaetes, but the Triassic mate rial from Germany also contains Eunicelike remains in exceptional preservation within their burrows (Knaust, 2008, fig. 9). Another indirect evidence for a polychaete interpretation of Balanoglossites is given by the abundant occurrence of jaw apparatuses of eunicid polychaetes (scolecodonts) both in the Triassic and the Ordovician material. Scolecodonts experienced a pro fuse diversification during the Ordovician and is a major group of fossils particularly in the Baltic regions and North America (Hints and Eriksson, 2010). We therefore argue that the plentiful distribution of Bal anoglossites triadicus in the Baltic Ordovician and their cooccurrence with abundant scolecodonts is related to the increased activity of eunicid polychaetes. 8. COMPARISON OF BALANOGLOSSITES WITH THALASSINOIDES, AND ITS FARREACHING DISTRIBUTION The ichnogenus Balanoglossites is commonly reported from its type area of original description, the Middle Triassic Muschelkalk group of the Germanic Basin (see Knaust, 2008, and references therein), and recently has been described from corresponding strata of the Tethys area (Knaust and Costamagna, 2012). The herein described Balanoglossites from the Middle Vol. 21

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1 cm (a)

(b) Fig. 7. Ichnofabric with Balanoglossites triadicus (vertical expression) in muddy and dolomitic limestone from the Devonian Pal liser Formation in Alberta, Canada. (a) Fresh rock surface with complex burrow systems filled with sucrose dolomite. Note the varying burrow shapes and sizes. (b) Slightly weathered rock face, enhancing the vertical and Ushaped tracefossil elements. Note the numerous bulges and blindending terminations, suggesting a proboscisbearing worm as producer.

(a)

5 cm

5 cm (b)

Fig. 8. Comparison of the ichnogenera Balanoglossites and Thalassinoides (see table for details). (a) Balanoglossites triadicus (pol ished surface in plain view) from a floor stone in the main building of the University of St. Petersburg, Russia. Middle Ordovician limestone. (b) Thalassinoides suevicus (preserved in positive hyporelief) from Friley Brigg/Carr Naze at the Yorkshire coast, UK. Upper Jurassic, Lower Coralline Oolite.

Ordovician of Baltoscandia adds another conspicuous occurrence to that ichnogenus. In fact, Balanoglossites seems to be even more common and much wider dis tributed in Ordovician than in Triassic deposits, because it also widely occurs in Cambrian, Ordovician and Devonian strata of North America (Kendall, 1977, p. 12, Fig. 2x; Chamberlain, 1977; Palmer, 1978; Gingras et al., 2002, 2004a, 2004b; Ekdale et al., 2012; Fig. 7) (Figs. 7a,7b). However, in many cases it has not been recognized as such but attributed to the ichnogenus Thalassinoides.

The common misinterpretation of Balanoglossites as Thalassinoides is not surprising as both ichnogenera have elements they share and overall resemble each other. Nevertheless, a closer analysis reveals important differences which not only allow for a certain assign ment of the examined specimens to one or the other ichnogenus, but furthermore implicates contrasting producers, palaeoecological conditions and sedimen tary environments. Table highlights major differences between both ichnogenera, whereas Fig. 8 illustrates the contrasting morphology based on two representa tive ichnospecies.

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Comparison of Balanoglossites and the similar ichnogenus Thalassinoides Characteristics

Balanoglossites

Thalassinoides

Geometry

Boxwork, complex trace fossil

Boxwork, netlike trace fossil

Overall morphology

U and Yshaped vertical elements

Horizontal polygons connecting to sur face by vertical shafts

Anastomosing

May occur, but not common

Diagnostic and abundant

Branching pattern

Very irregular

Relative regular, Y and Tshaped

Elements between branching points

Irregular and winding

Regular, mostly straight or curved

Enlarged portions

Common, extended chambers between branch Common, turning chambers at branch ing points ing points

Substrate

Firmground, but also soft and hardground

Firmground, softground

Density

Typically very high (up to 100% bioturbation)

Typically moderate (ca. 40–60% biotur bation)

Size variation

Considerably large (several orders of magnitude) Insignificant

Shape variation

High, very irregular trace fossils

Moderate to low, relative regular burrow systems

Supposed producer

Polychaetes

Arthropods (most common), vermiform animals

9. CONCLUSIONS (1) The highly bioturbated deposits of the Dikari Limestone (lower subformation of the Volkhov For mation, Middle Ordovician) of the St. Petersburg region have been studied with respect to their trace fossil content. Most trace fossils can be attributed to Balanoglossites triadicus. (2) In dependence on the degree of lithification, the trace maker was able to bioerode and to burrow the deposits and thus created a complex trace fossil com prising different elements: elongate grooved along the hardground (Sulcichnus isp.), variably branched bor ing and burrow systems (Balanoglossites triadicus) in the hard to firm substrate, biodeformational structures and Labyrintichnus within the soft sediment. (3) The ichnogenus Balanoglossites comprises J, U and Yshaped elements, multiple branching, and its elements are highly variable in size and shape. These, and other features, enables its differentiation from the similar ichnogenus Thalassinoides. (4) The simultaneous occurrence of boring and burrowing elements within one trace fossil provides an example where ichnotaxonomical discrimination between burrows and borings is not sharp and less suit able. (5) On the basis of the trace fossil’s diagnostic fea tures (“fingerprints”), constructional evidence, com parison with modern analogues and the cooccur rence of abundant scolecodonts, it can be assumed that eunicid polychaetes were the producers of Balan STRATIGRAPHY AND GEOLOGICAL CORRELATION

oglossites. This interpretation is also supported by the close affinity of the described material with Balanoglo ssites from the Triassic of Germany, where Eunicelike producers were found within the burrows. (6) Omission surfaces marked with Balanoglossites ichnofabrics can be used for stratigraphical subdivi sion and for bedtobed correlation on a regional scale. This is because these surfaces were extremely widespread within the relatively shallowwater envi ronment and now can be recognized and traced over a distance of more than 300 km. (7) A review of published data in conjunction with own examinations indicates that Balanoglossites ich nofabrics are a very common feature of Palaeozoic (particularly Ordovician) strata worldwide but so far were not identified as such. REFERENCES Andersson, J.G., Über cambrische und silurische phospho ritführende Gesteine aus Schweden, Bull. Geol. Inst. Upsala, 1896, vol. II, pp. 35–41. Bergström, S.M., Chen, Xu., GutiérrezMarco, J.C., and Dronov, A.V., The new chronostratigraphic classification of the Ordovician system and its relations to major series and stages and to δ13C chemostratigraphy, Lethaia, 2009, vol. 42, pp. 97–107. Bertling, M., Braddy, S.J., Bromley, R.G., et al., Names for trace fossils: a uniform approach, Lethaia, 2006, vol. 39, pp. 265–286. Vol. 21

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Brett, C.E. and Liddell, W.D., Preservation and paleoecol ogy of a Middle Ordovician hardground community, Paleo biology, 1978, vol. 4, pp. 329–348. Carmona, N.B., Mángano, M.G., Buatois, L.A., and Ponce, J.J., Bivalve trace fossils in an Early Miocene dis continuity surface in Patagonia, Argentina: burrowing behavior and implications for ichnotaxonomy at the Firm groundHardground divide, Palaeogeogr. Palaeoclimat. Palaeoecol., 2007, vol. 255, pp. 329–341. Chamberlain, C.K., Ordovician and Devonian trace fossils from Nevada, Nevada Bureau of Mines and Geology, Bull., 1977, vol. 90, pp. 1–24. Dronov, A.V., Storm sedimentation in the Lower Ordovi cian carbonateterrigenous deposits of the St. Petersburg Region, Byull. Mosk. Ova Ispyt. Prir., Otd. Geol., 1998, vol. 73, no. 2, pp. 43–51. Dronov, A.V. and Holmer, L.E., Depositional sequences in the Ordovician of Baltoscandia, in Quo vadis Ordovician? Short Papers 8th Int. Symp. on the Ordovician System. Acta Univ. Carolinae. Geol., Kraft, P., and Fatka, O., Eds., 1999, vol. 43, pp. 133–136. Dronov, A.V., Koren, T.N., Tolmacheva, T.J., et al., “Volkhovian” as a name for the third global stage of the Ordov ician system, in Ordovician from the Andes, Albanesi, G.L., Beresi, M.S., Peralta, S.H., Eds., INSUGEO. Ser. Correl. Geol., 2003, vol. 17, pp. 59–63. Dronov, A.V., Meidla, T., Ainsaar, L., and Tinn, O., The Billingen and Volkhov stages in the Northern East Baltic: detailed stratigraphy and lithofacies zonation, Proc. Esto nian Acad. Sci. Geol., 2000, vol. 49, pp. 3–16. Dronov, A.V. and Mikuláš, R., Paleozoic ichnology of St. Petersburg Region. Field Guide. 4th Int. Workshop on Ichnotaxonomy, Moscow, St. Petersburg, Trans. Geol. Inst., 2010, vol. 596, pp. 1–70. Dronov, A.V., Mikuláš, R., and Logvinova, M., Trace fossils and ichnofabrics across the Volkhov depositional sequence (Ordovician, Arenigian of St. Petersburg Region, Russia), J. Czech Geol. Soc., 2002, vol. 47, pp. 133–146. Dronov, A.V., Mikuláš, R., and Savitskaya, M., Gastro chaenolites oelandicus and similar borings and/or burrows in the Ordovician of Baltoscandia, in Abstr. Book: Workshop on Ichnotaxonomy–III, Prague and Moravia, Czech Republik, September 2006 Mikuláš, R. and Rindsberg, A., Eds., Pra gue: Inst. Geol., Acad. Sci. of the Czech Republic, 2006. Dronov, A.V. and Rozhnov, S., Climatic changes in the Bal toscandian basin during the Ordovician: sedimentological and palaeontological aspects, Acta Palaeontol. Sinica, 2007, vol. 46 (Suppl.), pp. 108–113. Dronov, A.V., Savitsky, J.V., Fedorov, P.V., and Tsyganova, E.A., Detailed lithostratigraphy of the Ordovician lower Volkhovian limestone along the Eastern part of the Baltic Ladoga glint, northwestern Russia, GFF, 1996, vol. 118, pp. 19–24. Dronov, A.V., Savitsky, J.V., and Tsyganova, E.A., Ordovician carbonates of the St. Petersburg region: stratigraphy of the Dikari unit, Vestn. SPbGU 1993, Ser. 7, no. 21, pp. 36–42. Ekdale, A. and Bromley, R., Bioerosional innovation for living in carbonate hardgrounds in the Early Ordovician of Sweden, Lethaia, 2001, vol. 34, pp. 1–12.

Ekdale, A.A., Bromley, R.G., and Knaust, D., The ichno fabric concept, in Trace Fossils as Indicators of Sedimentary Environments: Developments in Sedimentology, vol. 64, Knaust, D. and Bromley R.G., Eds., Amsterdam: Elsevier, 2012. Gingras, M.K., MacMillan B., Balcom B.J. Visualizing the internal physical characteristics of carbonate sediments with magnetic resonance imaging and petrography, Bull. Can. Petrol. Geol., 2002, vol. 50, pp. 363–369. Gingras, M.K., Mendoza, C.A., and Pemberton, S.G., Fossilized worm burrows influence the resource quality of porous media, AAPG Bull., 2004a, vol. 88, pp. 875–883. Gingras, M.K., Pemberton, S.G., Muelenbachs, K., and Machel, H., Conceptual models for burrowrelated, selective dolomitization with textural and isotopic evidence from the Tyndall Stone, Canada, Geobiol., 2004b, no. 2, pp. 21–30. Goldring, R. and Ka zmierczak, J., Ecological succession in intraformational hardground formation, Palaeontology, 1974, vol. 17, pp. 949–962. Hecker, R.F., Fossil facies of smooth, rocky seafloors, Tr. Inst. Geol. Estonskoi SSR, 1960, no. 5, pp. 199–227. Hints, O. and Eriksson, M.E., Ordovician Polychaeturid Polychaetes: taxonomy, distribution and palaeoecology, Acta Palaeontol. Polonica, 2010, vol. 55, pp. 309–320. Jin, J., Harper, D.A.T., Rasmussen, J.A., and Sheehan, P.M., Late Ordovician massivebedded thalassinoides ichnofacies along the palaeoequator of Laurentia, Palaeogeogr. Palaeo climat. Palaeoecol., 2011, vol. 367–368, pp. 73–88. Kendall, A.C., Origin of dolomite mottling in Ordovician limestones from Saskatchewan and Manitoba, Bull. Can. Petrol. Geol., 1977, vol. 25, pp. 480–504. Knaust, D., Invertebrate trace fossils and ichnodiversity in shallowmarine carbonates of the German Middle Triassic (Muschelkalk), in SedimentOrganism Interactions: a Multi faceted Ichnology. SEPM Spec. Publ., Bromley, R.G., Bua tois, L.A., Mangano, M.G., et al., Eds., 2007, vol. 88, pp. 221–238. Knaust, D., Balanoglossites Mägdefrau, 1932 from the Mid dle Triassic of Germany: part of a complex trace fossil prob ably produced by burrowing and boring polychaetes, Palä ontologische Zeitschrift., 2008, vol. 82, pp. 347–372. Knaust, D. and Costamagna, L.G., Ichnology and sedi mentology of the Triassic carbonates of NorthWest Sar dinia, Italy, Sedimentology, 2012, vol. 59, pp. 1190–1207. Knaust, D., Dronov, A.V., and Curran, H.A., Shallow marine carbonates, in Trace Fossils as Indicators of Sedimen tary Environments: Developments in Sedimentology, vol. 64, Knaust, D. and Bromley, R.G., Eds., Amsterdam: Elsevier, 2012. Lamansky, V.V., Drevneishie sloi siluriiskikh otlozhenii Rossii (Ancient layers of Silurian deposits of Russia), St. Petersburg, 1905, 203 p. [in Russian]. Loutit, T.S., Hardenbol, J., Vail, P.R., and Baum, G.R., Condensed sections: the key to age dating and correlation of continental margin sequences, in Sealevel Changes—an Integrated Approach. SEPM Spec. Publ., 1988, no. 42, pp. 183–216. Mägdefrau, K., Über einige Bohrgänge aus dem Unteren Muschelkalk von Jena, Paläontologische Zeitschrift, 1932, vol. 14, pp. 150–160. ‹

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