New Hirnantian orthide brachiopods from the type section of the Porkuni Stage ( Porkuni quarry , northeastern Estonia )

Four new Hirnantian species of orthide brachiopods, Sigmelasma peepi, Mendacella aerinensis, Drabovia? minuta and Tyronella siugensis are described from the type section of the Porkuni Regional Stage in the Porkuni quarry, northeastern Estonia. These species occur in the reef-related shallow-water bituminous limestone (Siuge Member of the Ärina Formation). Sigmelasma peepi and Tyronella siugensis represent the families Wangyuiidae and Tyronellidae, respectively, which were hitherto unknown in the Baltic Ordovician. Additionally, a harknessellid Reuschella sp. is described from the stromatoporoid-coral reefs (Tõrevere Member) and skeletal grainstone (Vohilaid Member) of the same locality and formation. Together these new finds extend our knowledge of the latest Ordovician brachiopod fauna in the Baltic region, showing higher than previously expected diversity of the Porkuni Stage and distinctness of reef-related brachiopods in the shallow shelf environment. The new species are not present in the Hirnantia brachiopod fauna in deeper-water environments of the Central East Baltic.


INTRODUCTION
During the last decade much attention has been paid to the correlation of different facies of the Porkuni Regional Stage in the East Baltic, where the Hirnantia brachiopod fauna is distributed in the Livonian Tongue of the Central Baltoscandian Facies Belt (Brenchley et al. 2003;Kaljo et al. 2008;Hints et al. 2010Hints et al. , 2012) ) and the Elsaella and Streptis associations (Hints 1996;faunas in Harper & Hints 2001) occur in the shallower environments of the Estonian facies belt (Fig. 1).The first association characterizes the dolomites of the Röa Member in the basal part of the Porkuni Stage (Rõõmusoks 1991;Hints et al. 2000) and the Streptis Association belongs to the reef-related skeletal grainstone (Vohilaid Member), bituminous limestone (Siuge Member) and stromatoporoid-coral reefs (Tõrevere Member).
Detailed data on the carbon isotope trend and occurrence of zonal and characteristic chitinozoans and conodonts in the uppermost Ordovician have enabled reliable correlation of sections across different facies belts (Kaljo et al. 2001(Kaljo et al. , 2008)).However, the knowledge of the taxonomic composition of faunas in separate parts of the Baltic Basin is uneven.The early studies of brachiopods from the Porkuni Stage in northern Estonia, beginning from Schmidt (1858), Öpik (1934), Oraspõld (1959), Hints (1975Hints ( , 1986) ) and Rõõmusoks (1991), are complemented with only a few recent studies (e.g.Rõõmusoks 2004).Several brachiopod taxa of about 20 listed by Rõõmusoks (1967) from the Porkuni Stage are mentioned under open nomenclature ('sp.' or 'sp.nov.') and they need formal description or revision.However, the Hirnantian cosmopolitan brachiopods of the Livonian Tongue, mainly from the Kuldiga Formation (Hints et al. 2010), have been described by different authors outside the East Baltic, as for example by Cocks (1982) in Norway, and Bergström (1968) and Jin (Jin & Bergström 2010) in Sweden.The immigration of the new Hirnantia fauna into the Baltic Basin increased brachiopod diversity.At the same time the habitable areas and faunal diversity decreased in the Estonian facies belt due sea level fall caused by the development of the Gondwana ice cap (Hints & Harper 2003;Kaljo et al. 2011).
The taxonomy of tiny (mainly less than 5 mm in size) brachiopods in the type section of the Porkuni Stage in the Porkuni quarry, northeastern Estonia, was the aim of this study.Four new species, Sigmelasma peepi, Mendacella aerinensis, Drabovia?minuta and Tyronella siugensis, were identified in the samples from the Porkuni quarry.The new species, together with Reuschella sp., enrich the data on brachiopod diversity of the Streptis Association and underline the specificity of the fauna in the reef complex.Sigmelasma peepi and Tyronella siugensis belong to the Wangyuiidae and Tyronellidae, respectively, being the first representatives of these families in the Ordovician of the Baltic region.The new species are identified only in the inter-reef facies of the Siuge Member, which is characterized by the low siliciclastic component (less than 10%), admixture of kerogen, and skeletal debris forming up to 25% of the rock (Hints et al. 2000).The harknessellid Reuschella sp.occurs in dolomitic skeletal limestones of the Vohilaid Member and stromatoporoid-coral limestones of the Tõrevere Member.The brachiopods described here are unknown in the Röa Member of the basal part of the Ärina Formation in northern and central Estonia and are missing also in the composition of the Hirnantia brachiopod fauna in southern Estonia and Latvia (Fig. 1; see also Hints et al. 2010).

MATERIAL AND METHODS
The textural and structural properties of the rocks of the reef complex complicate separation of fossils.This study is based on 11 samples (Fig. 1), which were collected by Peep Männik from the 5.5 m thick section of the Porkuni quarry and dissolved in acetic acid for extraction of conodonts.
The bulk sample size varied from 2 kg to more than 10 kg.Silicified or dolomitized shelly fauna, including the brachiopods described herein, was picked from the insoluble residues of these samples.The two lowermost samples from dolomites of the Röa Member comprised only poorly preserved stem ossicles of echinoderms.Three samples from the Vohilaid and Tõrevere members contained corals and bryozoans.The diverse shelly fauna of brachiopods, including the new species, bryozoans, tabulate and rugose corals, and skeletal fragments of echinoderms, was found in samples from the Siuge Member.The preservation of brachiopods is variable.Some structures are relatively well preserved due to silicification, whereas in other cases the shells have suffered from dissolution and silicification processes, which hampers identification of brachiopod shell structure, including the occurrence of punctae.
Most of the photos were taken with the Leica M205A motorized stereo microscope, equipped with a digital camera and Leica Multifocus z-stacking software.As the brachiopod specimens are very delicate, they were, with the exception of specimens of Reuschella, not white-coated before making photos because subsequent cleaning might damage them.The material studied is housed at the Institute of Geology at Tallinn University of Technology (institutional abbreviation GIT) and Museum of Geology, University of Tartu (TUG).Full data on individual collection specimens are accessible online in the Estonian geocollections database (http://geokogud.info).

SYSTEMATIC PALAEONTOLOGY
Order ORTHIDA Schuchert & Cooper, 1932Suborder ORTHIDINA Schuchert & Cooper, 1932 Superfamily PLECTORTHOIDEA Schuchert & LeVene, 1929Family WANGYUIIDAE Zhang, 1989 Genus Sigmelasma Potter, 1990b Sigmelasma peepi sp.nov.Figures 2, 3; Description.The shell is small suboval with the lengthwidth ratio about 0.7 in smaller and up to 0.9 in larger specimens, maximum width located near mid-length.The cardinal angles are obtuse and the anterior commissure is rectimarginate to weakly sulcate.The hinge line is 80-90% of shell width.The ventral valve has the highest convexity in the middle or in the posterior part.The interarea is apsacline up to 1.2 mm high, flat, weakly concave below the umbo.The delthyrium is twice as high as wide on the hinge line.The pedicle callist on top of the delthyrium occurs in the form of arched thickening merging with thickening along the margins of the delthyrium (Fig. 2C5).The dorsal valve is less convex than the ventral valve, with the highest point in the posterior half.The sulcus is low, weakly developed.The dorsal interarea is anacline, about one-quarter height of the ventral interarea.The notothyrium is open.The radial ornament is ramicostellate, consisting of about 15 costae appearing at the umbo; costellae appear by bifurcation.The total number of radial ribs along the shell margins is about 45, in 2 mm 7-12, most commonly 10.Rare aditicules occur on the crests of ribs.
The ventral valve has small teeth joining with callosities along the interior sides of the delthyrium; in the posterior part of the delthyrium the teeth are supported by short subparallel dental plates which merge anteriorly with septa bounding laterally the short muscle field (Fig. 2C3).The length of the muscle field is about 30% of valve length.
The dorsal valve has a bilobed cardinal process; the myophore is crenulated; the shaft cannot be differentiated.The cardinal process fills most part of the notothyrium, but does not extend above the interarea.Brachiophores are developed as thin vertical plates, S-shaped in cross section, extending to the mid-length of the valve.Brachiophores stick out from the walls of the notothyrial chamber and anteriorly are suspended over the valve floor.Dental sockets are antero-laterally supported by fulcral plates.The median ridge is not developed, muscle field is not expressed.
Comparison.Sigmelasma peepi sp.nov.resembles brachiopods of both genera Sigmelasma Potter, 1990b andBowanorthis Percival, 1991 of the family Wangyuiidae.Bowanorthis differs from Sigmelasma mainly in having a prominent median ridge.The Estonian species is assigned to Sigmelasma due to the absence of the median ridge, its smaller size, less dense ribbing, and weakly developed sinus and sulcus.
The new species is most similar to Sigmelasma sp. 1, described by Potter (1990b) from the Ashgillian in northern California, USA.The Estonian species has less transverse valves, with length-width ratios of ventral and dorsal valves about 0.8, compared to 0.72 and 0.67 in Sigmelasma sp. 1.Details of interiors are poorly known in the American specimens, but apparent absence of the cardinal process can be a feature which differentiates the American and Estonian specimens.The new species differs from the type species S. pantherae Potter, 1990b in having a less transverse outline, more numerous costae and costellae and supposedly a shorter ventral muscle field.Silurian (Llandovery to Wenlock) brachiopods of the Wangyuiidae in Estonia, such as Wangyuia sp.(Rubel 2011), differ from the new species in stronger ribbing and the presence of the shaft of the cardinal process.
Sigmelasma peepi from the topmost Ordovician Porkuni Stage is exteriorly rather similar to Kinnella kielanae (Temple 1965), a characteristic brachiopod of the Hirnantia brachiopod Fauna (Lesperance & Sheehan 1976;Rong 1984;Stott & Jin 2007).Kinnella also comprises specimens of small size, with a high ventral interarea and narrow delthyrium with pedicle callist.An essential difference is the punctate shell structure of Kinnella compared to the impunctate structure of Sigmelasma.
Diagnosis.Small subcircular shell, hinge line less than half shell width.Ventral beak incurved towards dorsal beak, delthyrium about twice as wide as high.Cardinal process with high middle part (trilobed) extending over dorsal interarea, brachiophores with ventro-laterally directed prolonged extensions.Ornament ramicostellate, with two median costae on ventral and one on dorsal valve.
Description.The shell is small, ventri-biconvex in profile, outline subcircular to transversely oval, with the lengthwidth ratio from 0.8 to 1.0.Maximum width located near mid-length or slightly anterior of that.Cardinal angles are obtuse, the anterior commissure is weakly sulcate or rectimarginate.The hinge line attains 40% to 50% of shell width.The ornament is ramicostellate with two subparallel median costae on ventral and one costa on dorsal valve; 9 and 10 costae appear around the umbo on the dorsal and the ventral valve, respectively.Around the shell margins there occur up to 36 and in 2 mm up to 8 costae and costellae, which appear by bifurcation.
The ventral valve is almost twice as high as the dorsal valve, with the highest point in the posterior half.The interarea is short, apsacline, up to 1.5 mm high and concave below the umbo.The beak is incurved, lying close to the dorsal beak.The delthyrium is right-angled, about a half as high as wide on the hinge line.The teeth are stout, extending above the hinge line; crural fossettes are small.Short dental plates merge with low septa delineating laterally the weakly impressed elongate oval muscle field, whose length is 40% of valve length; the adductors occupy the middle third of the muscle field, enclosed laterally by narrow elongate diductors.
The dorsal valve is moderately convex, with faint sulcus comprising a sector with three median costae and their second-and third-order costellae.The dorsal interarea is very low, nearly orthocline.Cardinalia occupies less than one fifth of valve length.Ventrally directed brachiophores with relatively long extensions (crura) are supported laterally in proximal parts by secondary shell material.Fulcral plates are indistinguishable from the callosities bounding the dental sockets antero-laterally.On smaller specimens the sockets lie on the valve floor between the brachiophores and anterior edge of the interarea (Fig. 4F).Large specimens have the brachiophores with small plates on the lateral side, which extend ventrally higher above brachiophores and merge with thickened shell deposits around the dental sockets (Fig. 4C1, 2).These plates resemble the inner socket plates.The cardinal process occupies most of the notothyrium and extends over the interarea.It has a high ________________________________________________________________________________________________________ Fig. 4. A-G, Mendacella aerinensis sp.nov.Porkuni quarry, Porkuni Stage, Ärina Formation, Siuge Member.A1-5, holotype, conjoined valves GIT 626-3, ventral, dorsal, anterior, lateral and posterior views.B1-3, dorsal valve GIT 626-37, exterior and interior views.C1, 2, incomplete dorsal valve GIT 626-5, views of cardinalia.D1, 2, incomplete dorsal valve GIT 626-4, exterior and interior views.E1, 2, ventral valve GIT 626-2, exterior and interior views.F1, 2, fragment of dorsal valve GIT 626-152.G, view of the cardinal process in fragment of dorsal valve GIT 626-7.median lobe (Fig. 4C2), formed by merging of inner elevated parts of the bilobed process (Fig. 4G).The median ridge is low and wide, reaching the first half of the valve.The muscle field is weakly expressed, posterior adductors are shorter than anterior adductors.External ornamentation is reflected on the inner surface by low furrows reaching up to the middle on smaller valves.The penetrating punctae are not visible because of silicification of shell substance.One row of aditicules on rib crests is observed at the anterior margin of the shell (Fig. 4A3).
Comparison.The small size and absence of fulcral plates can rise a question about the generic affiliation of the described Estonian species.It has three to four times smaller shells than the type species Mendacella uberis (Billings) and other species of the genus in the Upper Ordovician Ellis Bay Formation in Anticosti Island (Jin & Zhan 2008).An exception is Mendacella sp. from the two uppermost units of that formation (Jin & Zhan 2008), which by the small shell and ornamentation resemble the Estonian species.Most specimens of M. aerinensis differ from Anticosti species by less strongly impressed muscle fields and more delicate cardinalia.However, at least one dorsal valve (GIT 626-152; Fig. 4F) has the cardinalia with a raised notothyrial platform with a prominent cardinal process and strong median septum, as characteristic of species of Mendacella.The occurrence or absence of the fulcral plates and the diagnostic value of that feature for brachiopods of related genera Mendacella and Dalejina have been discussed by several authors (Williams & Wright 1963;Boucot et al. 1965;Harper 2000;Jin & Zhan 2008).The occurrence of those plates on specimens of Mendacella has been debated by Jin & Zhan (2008), according to whom these plates could have been developed very differently.On small valves of M. aeriensis the thickened shell deposit probably provides necessary support for teeth without fulcral plates.Both mentioned differences from the typical representatives of Mendacella are considered here as intraspecific.The shell form, the ornamentation with two subparallel costae on ventral valve and aditicules in one row on the crests of ribs support also the opinion that the species described above belongs into the genus Mendacella.
Up to now, two species, Mendacella borrbyensis Hints from the Vormsi and Pirgu stages (uppermost Katian) and Mendacella circularis Rubel from the Juuru and Raikküla stages (lower Llandovery), have been identified in the Ordovician-Silurian transition in Estonia.Mendacella aeriensis differs from the former species by a biconvex shell with a more convex ventral valve and transversely oval shell outline.Mendacella borrbyensis has a relatively larger equally convex or even dorsibiconvex shell of subtriangular outline.Mendacella aerinensis resembles M. circularis in general shell outline but the Silurian species differs in possessing fulcral plates and in the anteriorly extending shaft of the cardinal process.
Family TYRONELLIDAE Mitchell, 1977Tyronella Mitchell, 1977 Tyronella siugensis sp.nov. Figure 5 Derivation of name.From the locality and member name Siuge in Estonia.
Holotype.Conjoined valves GIT 626-57 (Fig. 5A1-4 Comparison.The Estonian specimens are most similar to Tyronella killeyensis from the Killey Bridge Formation in Northern Ireland (Mitchell 1977) by the punctate shell substance, minute shell size and unusual for dalmanellid brachiopods transversely oval outline.They are distinguished from the Irish specimens by the absence of the cardinal process and very robust ornamentation consisting of only 5 and 6 costae, instead of 10-12 on the first specimens.The similarity of Tyronella to impunctate orthide brachiopods such as Toxorthis (Temple 1968) has been discussed by Rong (1984).
Superfamily ENTELETOIDEA Waagen, 1884 Family DRABOVIIDAE Havliček, 1950Subfamily DRABOVIINAE Havliček, 1950Drabovia Havliček, 1950 Drabovia?minuta sp.nov. Figure 6; Table 3 Remarks.The generic position of the new species is not clear as it shares common features with different brachiopods among draboviniins (see below).It is tentatively assigned to Drabovia due to its relatively short hinge line and characteristics of the dorsal interior suggesting similarity with Drabovia westrogothica in Sweden (Bergström 1968) and Drabovia sp. in Norway (Cocks 1982).The Estonian species differs from the Swedish species in a smaller size and fascicostellate ornamentation.Moulds of Drabovia sp. from Norway are insufficiently known for more detailed comparison.
Diagnosis.Small biconvex fascicostellate shell, fine spine-like brachiophores directed antero-ventrally, in posterior part supported by subparallel brachiophore plates.Notothyrial platform narrows anteriorly and merges with the median septum on elevated part of the valve floor; cardinal process granulated; dental plates divergent, ventral muscle field large, bilobate anteriorly.
Description.Small ventri-biconvex shell, up to 6 mm wide, suboval to circular in outline with rounded cardinal angles.The hinge line is 40-60% of shell width.The ventral interarea is apsacline, weakly concave below the umbo; the delthyrium has the pedicle callist in the apex.The dorsal interarea is orthocline, half as high as the ventral interarea, the notothyrium is open.The ornament is fascicostellate with high angular costae increasing in number by bifurcation.Around the umbo 8 and 9 costae occur on the dorsal and ventral valve, respectively; up to 33 costae and costellae are counted along the shell margin and 5-7 in 2 mm on the anterior margin.The sulcus on the dorsal valve is bounded laterally by two median costae with up to three costellae.The shell surface is stepped due to strong growth lines.
The ventral valve has small triangular teeth and thin laterally divergent dental plates, which merge with the laterally arched or weakly wavy septa bordering the oval to cordate muscle field.The adductor field seems to be shorter than the diductor fields.The dorsal valve has antero-ventrally directed long and slender brachiophores supported with subparallel plates which limit the notothyrial platform.The platform narrows anteriorly and merges with the elevated part of the valve interior corresponding to the sulcus on the exterior surface.The cardinal process has a notched myophore and an anteriorly extending shaft.Shell substance is porous.
Comparison.The cardinalia of Drabovia?minuta sp.nov. is similar to that of draboviniid species in having a crenulated cardinal process and thin brachiophore bases merging with the notothyrial platform.In its small size the new species is similar to Diorthelasma in the Caradocian (Sandbian to Katian) of North America (Cooper 1956) but differs from the latter in more circular outline and a short hinge line, and in a narrow anteriorly elongate notothyrial platform.The other North American draboviniins Fascifera subcarinata and F. stonensis (Cooper 1956) resemble the described Estonian specimens in their general shell outline and characteristics of the cardinalia.However, the American specimens are much larger, have a less convex dorsal valve and fine radial ornamentation.
Family HARKNESSELLIDAE Bancroft, 1928 Genus Reuschella Bancroft, 1928 Reuschella sp. Figure 7 Material.The collection consists of four ventral and two incomplete dorsal valves.Five of these specimens occur in strata that most likely correspond to the Tõrevere Member, and one specimen comes presumably from the grainstone of the Vohilaid Member.The seventh specimen from the Siuge?Member (TUG 101-78), although labelled as Reuschella sp., obviously does not belong to that genus due to its fibrous shell structure and absence of punctae.This specimen resembles shells of Cliftonia, but is too fragmentary for taxonomic identification.
Description.Comparison.In spite of insufficient preservation, the outline, the shell shape and ornamentation of the described valves show the greatest similarity with the harknessellid brachiopods.The moderately convex valves of Reuschella sp. are similar to specimens of the Reuschella horderleyensis group (Bancroft 1945).The representatives of the R. bilobata group have a strongly convex dorsal valve as, for example, R. magna from the mid-Caradoc (Oandu Stage) of the East Baltic (Hints 1975).The described specimens differ from the Ashgillian Reuschella sp.(Sheehan, 1987) in Belgium and those in the Portrane Limestone (Wright 1964) in a smaller size and more robust ornamentation.The Ashgillian Reuschella sp. from northern Wales (Hiller 1980) is too poorly known for comparison with the Estonian material.
However, in the shell size and outline the described Reuschella sp. is similar to Harknessella subplicata (Bancroft 1945), from which the Estonian specimens differ in having more robust ornamentation.
Distribution.Northern Estonia, Porkuni Stage, Tõrevere Member in the Tõrevere and Porkuni old quarries, erratic boulder in the village of Kännu; Vohilaid Member in the Röa-Jakobi old quarry.

DISCUSSION
The described new species Sigmelasma peepi, Mendacella aerinensis, Drabovia?minuta and Tyronella siugensis together with Reuschella sp.enrich the latest Ordovician brachiopod diversity in northern Estonia with specific taxa representing different families.This is the first time that the families Wangyuiidae and Tyronellidae are recognized in the Ordovician of the Baltic Basin.Sigmelasma peepi, of the former family, has most similar species in North America (Potter 1990b) and Australia (Percival 1991).In both regions the species of Sigmelasma and of its closely related Bowanorthis are represented by silicified specimens (Percival 1991), as are those of S. peepi in Estonia.It is likely that the silicification has contributed to the preservation of these small and delicate brachiopods in rocks of different ages and regions.The new species of both newly established families in the Ordovician of the Baltic region occur in a relatively shallow-water inter-reef environment.The brachiopods of Sigmelasma in North America (northern California) (Potter 1990a(Potter , 1990b) ) and of Tyronella in northern Ireland (Mitchell 1977;Candela 2005) and the Percé Region of Quebec (Sheehan & Lespérance 1979) belong to relatively offshore faunas (Benthic Assemblages 4 and 5 of Boucot 1975).The appearance of these brachiopods in Estonia in the shallow-water environment (Benthic Assemblage 2) can be explained by the movement of deeper-water faunas into shallower environments during the eustatic seal level lowering at the time of the development of the Gondwana ice cap (Sheehan & Lespérance 1979).
Mendacella, Drabovia and Reuschella, or genera closely related to them (for example, Dalejina among Rhipidomellidae and Horderleyella among Harknessellidae), are common elements of the Hirnantia brachiopod fauna in different parts of the world.However, their role is much smaller than that of the species commonly listed as typical of that fauna.Several species of Mendacella occur in the Ellis Bay Formation of the Hirnantian Stage on Anticosti Island (Jin & Copper 2008;Jin & Zhan 2008;), Dalejina is known in Morocco (Rong & Harper 1988), harknessellids are found in Sweden and China (Bergström 1968;Rong 1984) and Drabovia has been recognized in Bohemia, Sweden and Norway (Bergström 1968;Cocks & Price 1975;Cocks 1982).

Fig. 1 .
Fig. 1.Locality map with facies belts, log of the Porkuni old quarry section (modified from Hints et al. 2000) and distribution of the studied brachiopods.The levels of the studied samples processed by P. Männik for conodonts (Prk1-7 and M656, M334, M658, M335) are shown on the right side of the log.The rectangle marked by the dashed line denotes the distribution of the brachiopod on the level of the member.Legend for the log: 1, stromatoporoid-coral reef limestone; 2, bituminous limestone; 3, skeletal grainstone; 4, dolomite with crinoid ossicles; 5, argillaceous dolomite with discontinuity surface.F I c -Pirgu Stage.
convex, with the sulcus developed between the median costae.Two sulcus-like radial furrows on lateral parts of the valve appear as a median sulcus close to the beak.The maximum width seemingly coincides with the width of the hinge line.The radial ornament is fascicostellate with up to 8 costae appearing at the beak of the dorsal valve.The costae increase in number by bifurcation; up to 32 ribs occur along the shell margin.The median sulcus comprises at least four costellae which are much thinner than the costae bounding the sulcus.Two to three ribs occur on 2 mm on the anterior margin.Interiors are unknown.Tiny punctae are preserved on some specimens.