The Ordovician brachiopod genus Cyrtonotella: taxonomy and distribution in the Baltic Basin

The taxonomy of the Baltic Ordovician orthoidean brachiopods of the genus Cyrtonotella Schuchert & Cooper, 1931 is discussed, two neotypes are suggested and emended descriptions of species and subspecies are presented. This taxonomic revision of Cyrtonotella-like brachiopods highlights their differences from the family Nanorthidae Havlíček, 1977 in which they have been included up to now. The well-preserved Baltic representatives of the genus Cyrtonotella display several morphological features, which are unique to this group of brachiopods and may have a family-level importance. The studied brachiopods belong to the shallow-water faunal associations of the Baltic Basin, including its easternmost parts (the Moscow Basin). These brachiopods disappear during the faunal crisis at the Sandbian–Katian transition. The global distribution of Cyrtonotella is poorly known due to taxonomic problems. New data on the Baltic species, which specify their stratigraphic and spatial distribution in the Baltic Basin, is a step towards reducing the taxonomic confusion among Cyrtonotella species.


INTRODUCTION
The Ordovician brachiopods of the genus Cyrtonotella occur in the shallow-water environments of the Baltic Basin, especially in areas of the distribution of kukersite kerogen. They are also found in more or less argillaceous deposits of its easternmost parts, known as the Moscow Basin (Alikhova 1969). The specimens included in Cyrtonotella are well recognizable by their unique cardinal process (Hints 2004). Presently, the systematics of Cyrtonotella requires revision in order to assess the biogeographic and stratigraphic distribution of this genus. This paper presents the first modern systematic revision of Cyrtonotella species from Baltica, which enables better understanding of their taxonomy on the genus and family level in future studies.
The taxonomic study of the Ordovician orthoidean brachiopods presently assigned to the genus Cyrtonotella Schuchert & Cooper, 1931 dates back to the classical papers of Eichwald (1829Eichwald ( , 1861. Eichwald (1829) described the species Terebratula semicircularis, which was selected by Schuchert & Cooper (1932) as a type species of their new genus Cyrtonotella Schuchert & Cooper 1931. Eichwald (1861 Alikhova 1951, p. 26) and should be considered as nomen nudum. Some small Middle Ordovician orthides described by Rubel (1961) were excluded from the genus Cyrtonotella by Hints (2014). The descriptions of species of the genus Cyrtonotella in the Baltic region have been provided by Öpik (1930a, 1930b, 1934) and Alikhova (1951Alikhova ( , 1953Alikhova ( , 1969. Öpik (1930a, 1934) mentioned six taxa related to O. kuckersiana Wysogórski and O. frechi Wysogórski, which occur mainly in the lowermost Upper Ordovician Kukruse Regional Stage (RS) and Haljala RS. The species Orthis bekkeri (Öpik 1930a, pl. III, fig. 25) was excluded from the present study because it is more similar to members of the genus Sivorthis Jaanusson & Bassett, 1993 than other Cyrtonotella species.
The present study is the first step towards the taxonomic stability of the Baltic species of Cyrtonotella. For that, two neotypes are selected and detailed descriptions of two subspecies and two species are presented. The description of the type species Cyrtonotella semicircularis (Eichwald) will be presented in another paper which deals with the genus-and family-level taxonomy of cyrtonellid brachiopods.
The specimens used in this study (in total about 400 specimens) are housed in Estonia at the Department of Geology, Tallinn University of Technology (institutional abbreviation GIT), Natural History Museum, University of Tartu (TUG, including the type material of A. Öpik) and the Estonian Museum of Natural History in Tallinn (ELM, including the specimens collected in the 19th century by F. Schmidt and A. Mickwitz). Some specimens are kept in the Peabody Museum (YPM, collections of C. Schuchert and G. A. Cooper), Swedish Museum of Natural History (RM, about 30 specimens collected from Estonia) and F. N. Chernyshev Central Geological Scientific Research and Exploration Museum (CNIGR) in St Petersburg (material described by T. N. Alikhova).
Discussion. The orthoidean genus Cyrtonotella is included in the family Nanorthidae Havlíček, 1977(Williams & Harper 2000. However, the brachiopods described below, which are assigned to that genus, differ essentially from Nanorthidae in the much larger shell size and interior of the dorsal valve, especially in the unique cardinal process (Hints 2004), and supposedly belong to a distinct new family-level group. The identification of a new family and revision of the type species of the genus Cyrtonotella semicircularis (Eichwald) are addressed in another paper in progress and thus the family-level status is not defined here.
Discussion and comparison. In literature, two different spellings of the species name have been used. Wysogórski (1900) derived the name kuckersiana from Kuckers, an old German name for a locality in northeastern Estonia. Based on the Estonian spelling of the locality, Öpik (1930a), used kukersiana instead of the original name, which still has priority (Article 32.1 in ICZN 1999). Cyrtonotella kuckersiana and related Ordovician taxa described below differ from the type species of the genus C. semicircularis in the costate ornament, consisting of ribs bifurcating very close to the umbo, and in a smaller number of costae along the shell margin (see Hints 2014, fig. 1). The cardinal process on the type species is a delicate folded ridge instead of the wedgelike strong process in Upper Ordovician species.
Following Alikhova (1953), Rõõmusoks (1970) accepted two subspecies, Cyrtonotella kuckersiana kuckersiana and C. kuckersiana frechi, which are distributed in the Kukruse and Haljala RSs and in the lowermost Keila RS. The oldest and youngest specimens of C. kuckersiana from the Kukruse RS and Jõhvi Substage of the Haljala RS, respectively, differ clearly from each other ( Fig. 1A1-A5, B1-B5). They could be treated as separate species by analogy with two other species, C. laine and C. barbara. However, it is complicated in practise. Supposing the evolutionary trend in changes of shell morphology from large, planoconvex shells to smaller, weakly concavo-convex shells, there occur specimens with transitional characteristics. For this reason many species have been identified not on the subspecies, but on the species level. In the present study, the descriptions of the subspecies characterize the two groups of related brachiopods belonging to C. kuckersiana.
Distribution. The C. kuckersiana kuckersiana-type shells occur mainly in the Kukruse RS. The distribution of that species in different localities of the Kukruse RS is shown already by Bekker (1921). The C. kuckersiana frechi-type shells are most common in the Haljala RS. In the Baltic region, the type of the genus C. semicircularis is the oldest, occurring in the Aseri RS of northwestern Russia (St Petersburg region). In the studied collections one C. kuckersiana kuckersiana-type brachiopod shell (ELM G1:4948) is labelled as a specimen from the stage C 1 in Tallinn (Fig. 1G). This is the oldest occurrence of Cyrtonotella in Estonia. The occurrence of C. kuckersiana in the Middle Ordovician Uhaku RS (C 1 c) is shown also by Öpik (1930a, p. 237, in C 2 α; = C 1 c by Rõõmusoks 1970). The total stratigraphic range of brachiopods of the genus Cyrtonotella in Baltica comprises the interval from the Aseri RS to Keila RS, which corresponds to about 11 Ma (Kaljo et al. 1996). An exception is the Lasnamägi RS, where the occurrence of Cyrtonotella is not identified up to now.

Fig. 2. A, Cyrtonotella kuckersiana kuckersiana
Description. Large, up to about 50 mm wide and over 30 mm long suboval shell (Fig. 3) with a moderately convex ventral valve, whose length corresponds to 0.73-0.81 width and to 0.3-0.4 thickness; the highest convexity is in the midline or posterior half; dorsal valve flat or very weakly concave at the umbo. Cardinal angles sharp to obtusely rounded on mature shells; hinge line corresponds to 0.9 shell width or is equal to it. Anterior commissure rectimarginate or very weakly sulcate. Ventral umbo rounded, interarea concave, apsacline, 1.7-2.5 mm high; delthyrium open, delthyrial angle about 90°. Dorsal umbo flattened; interarea catacline, up to 2 mm high; notothyrium open, partly occupied by the cardinal process.
Ornament costate, on dorsal valve 12-14 costae appear at the umbo. The number of costae increases by bifurcation at a distance of 2-3 mm from the umbo, additional new costae appear along the posterior margin during the growth of the shell. On average 38 costae (variation 33-47) are counted along the lateral and anterior margins. Capillae are visible on the lateral parts of some mature shells, mainly on the interspaces. Fine filae (lamellae), 7-11 in 2 mm, occur on costae and interspaces (Fig. 5A1, A2); anterior one third of the valve length may be formed during hemiperipheral growth (Fig. 5A1); stronger growth lines mark changes in shell outline from suboval with acute cardinal angles to subrectangular with obtuse angles.
Ventral interior ( Fig. 2A) with small triangular teeth; crural fossettes form straight notches on inner sides of teeth; dental plates slightly arched laterally, convergent on the valve floor and merging with low ridges surrounding antero-laterally the muscle field which occupies about 0.2 valve length; diductor scars elongately oval, occupying partly interior sides of the dental plates; adductor scar suboval, slightly shorter than diductor scars. A pair of parallel vessel-like septa marking the vascula media arise in front of diductors and curves smoothly laterally at a distance of 2/3 valve length from the umbo. A thin septum occurs between the vascula media.
Mature dorsal valves have expressive cardinalia with an elevated notothyrial platform parallel to the interarea. Cardinal process is wedge-shaped in posterior view, with up to 10 closely spaced folds extending above the interarea and fills about one third to a quarter of the width of the notothyrial cavity. Details of the cardinal process have been described earlier (Hints 2004). In ventral view brachiophores are arranged at about 90° from each other, on basal parts at about 120°; sockets occur as small triangular depressions between the brachiophores and posterior edge of the valve. Small triangular tooth-like structures above the sockets on the anterior edge of interarea provide a secure lock of the shell. The notothyrial platform continues anteriorly as a short and wide myophragm; lateral septa separate the anterior and posterior adductors. The adductor muscle field extends about to the middle of the valve; its width corresponds to 0.3 valve width and it is postero-laterally bordered by arched ridges. The crenulation on the inner peripheral margin occupies about 0.2 valve length or is practically missing on postero-lateral corners close to vascula cardinalia.
Discussion and comparison. Some specimens from the Jõhvi Substage of the Haljala RS (Fig. 1E, F) are included here within C. kuckersiana kuckersiana mainly because of large size (shell width up to 35 mm) and costate ornamentation. Weak concavity of the dorsal valve and somewhat more strongly developed filae supposedly indicate intraspecific variability, although marking some similarity with C. kuckersiana frechi.
The comparison with C. kuckersiana frechi and other species is presented below.
Description. Medium-sized for the genus plano-or slightly concavo-convex shell; ventral valve length corresponds to about 0.6-0.8 and thickness to 0.4-0.5 valve width. Cardinal angles acute to obtuse, length of hinge line corresponds to shell width. Ventral valve moderately convex; dorsal valve weakly concave in the middle; anterior commissure sulcate. Ornament costate, with second-order costae appearing within 2-5 mm from the umbo and along the posterior margin; on average 38 ribs are counted along the shell margin. Filae fine, about 7-12 filae in 2 mm (Fig. 5B, C). Ventral umbo is curved up to the hinge line; interarea apsacline to orthocline, concave, 1.5-2 mm high; dorsal interarea anacline, 1-1.3 mm high; delthyrium and notothyrium wide. Ventral interior is unknown. Brachiophores are short, tilted postero-laterally, disposed in ventral view at about 110°, basally on the valve floor at about 140° from each other. Sockets triangular between the brachiopore and posterior edge of the valve. Cardinal process with a folded myophore directed backwards and fills about 1/3 of the nothothyrium. Muscle scars weakly developed with anteriorly developed vascula antemyaria. The peripheral ribbing occupies up to 0.3 valve length, becoming shorter laterally. Dorsal muscle field weakly expressed.
Comparison and discussion. Cyrtonotella kuckersiana kuckersiana and C. kuckersiana frechi differ in shell size (Fig. 3), outline and details of ornament. The oval shell of C. kuckersiana kuckersiana is up to 50 mm wide, which corresponds to 0.7-0.8 shell length and 0.3-0.4 shell thickness. The medium-sized, suboval to subtriangular shell of C. kuckersiana frechi has shells whose length corresponds to 0.6-0.7 width and about 0.4 thickness.
The hemiperipheral growth of the shells of C. kuckersiana kuckersiana shows that younger specimens have shell outline similar to that of C. kuckersiana frechi, which does not reach the size of the first subspecies. This could be related to changes in environments -the larger specimen (C. kuckersiana kuckersiana) originates from the kukerite kerogen-rich facies and the smaller one (C. kuckersiana frechi) from the siliciclastic carbonate facies with a low content of kerogen. The subspecies presumably represent facies-dependent groups of the species C. kuckersiana.
Cyrtonotella kuckersiana kuckersiana has an almost flat dorsal valve while the shell of C. kuckersiana frechi has an anteriorly widening low sulcus and weakly sulcate anterior commissure. The differences in shell outline and cardinal angles are age-dependent, especially in C. kuckersiana kuckersiana, whose younger specimens have a suboval outline with acute cardinal angles resembling the specimens of C. kuckersiana frechi. Both subspecies exhibit costate ornament with 13-14 costae appearing around the umbo, and some ribs appear during the growth of the shell on the posterior edge. The branching of ribs occurs close to the umbo, within 2-4 mm (C. kuckersiana kuckersiana) or up to 5 mm from the umbo (C. kuckersiana frechi). The total number of costae varies from 35 to 43, reaching about 50 on the largest specimens.
The shell surface is covered by fine filae, whose number in 2 mm varies from 7 to 10, being somewhat denser on C. kuckersiana kuckersiana than on C. kuckersiana frechi. Capillae occur on some mature shells of C. kuckersiana kuckersiana but not on other taxa. Öpik (1930a) attributed taxonomic value to the vascular pattern. Cyrtonotella kuckersiana kuckersiana has vascula media curving smoothly laterally close to the midline. One specimen, identified by Öpik as O. cf. frechi, is included here in the species C. laine by the similarity in long subparallel vascula media observed on several specimens of the latter species. Most specimens of C. laine differ clearly from C. kuckersiana in a smaller size, strongly convex ventral valve and finer filae on the shell surface (Figs 1D1-D5, 5E). Both subspecies of C. kuckersiana have plate-like tilted and laterally divergent brachiophores, whereas brachiophores in C. kuckersiana frechi are more widely tilted than in C. kuckersiana kuckersiana.
Occurrence and material. Haljala and Keila RSs. Cyrtonotella kuckersiana frechi (Wysogórski) is represented in the collections of GIT, TUG and ELM by 163 specimens.
Cyrtonotella laine (Öpik, 1930a Öpik (1930a, pl . 3, fig. 19); in this paper Diagnosis. Small to medium-sized suboval shell with strongly convex ventral valve. Ventral interarea orthocline, dorsal interarea catacline. Ribs appear at 2 mm distance around umbo and along posterior edge, 35-39 ribs occur on valve margins. Ventral muscle field with elongated oval diductor scars impressed on valve floor. Vascula media expressed as two parallel septa originating before muscle scars and reaching up to the first half of valve. A thin thread-like septa separates the branches of vascula media.
Description. Suboval shell, small to medium in size, commonly less than 25 mm wide; ventral length corresponds to 0.8 shell width or is equal to it. Ventral valve strongly convex; valve thickness corresponds to 0.5-0.6 shell length; cardinal angles rounded to sharp. Dorsal valve weakly concave; anterior commissure weakly sulcate. Radial ornamentation consists of 35-39 ribs, 13 or more appear at 2 mm distance around the umbo and 5-7 along the posterior edge on both sides of the umbo.
Filae fine, about 15 in 2 mm. Growth lines mark the change in shell outline from laterally elongated with sharp cardinal angles to suboval in mature shells (Fig. 5E). Ventral interarea orthocline, concave, 1.5-2 mm high; dorsal area catacline, flat, 1.5-1.7 mm high. Interarea as wide as shell width or forms 0.9 of it; cardinal process reaches over the hinge line and fills partly the notothyrium and delthyrium.
Ventral umbo is curved over the hinge line; delthyrium right-angled; teeth small; dental plates subparallel, joining on valve floor with low ridges bounding the oval diductor scars; crural fossettes developed as small depressions in the middle of dental plates. Two parallel, low ridges before diductor scars reaching anterior half of the valve mark the vascula media; on six of seven specimens these ridges have asymmetrical position. Peripheral ribbing on the inner surface of valves occupies about 0.3 valve length in the middle of the valve.
Cardinal process on the raised notothyrial platform is wedge-shaped, with lobated myophore which reaches over the hinge line and fills partly the notothyrium and delthyrium. Brachiophores are short, divergent, connected with the raised notothyrial platform. Muscle field extends to the middle of the valve; posterior scars are shorter than anterior scars. Vascular system weakly developed.
Discussion and comparison. The described species differs from the C. kuckersiana-type brachiopods in having a relatively small size, high and strongly convex ventral valve and parallel branches of vascula media, which tend to have an asymmetrical position in relation to valve midline. One ventral valve (GIT 400-19; Fig. 2B1, B2) differs from other specimens of C. laine in the large size. This specimen, like a specimen identified by Öpik (1930a, pl. 3, fig. 21) as C. cf. kukersiana, is included in C. laine having parallel and asymmetrical vascula media.
Cyrtonotella laine has some similarity with Wysogórski's (1900) figure of Orthis concava Schmidt mscr. The description of Lithuanian specimens with the name O. concava by Alikhova (1953) should be treated as nomen nudum as the species has not been described and the original material is lost. Alikhova (1951) considered the specimen figured by Wysogórski as the lectotype of the species Cyrtonotella concava (Schmidt). She described it on the basis of Wysogórski's figure and few specimens from Leningrad District. However, the Leningrad specimens differ from the figured one in smaller convexity and ribbing of the ventral valve, and presumably belong to the C. kuckersiana kuckersianagroup species.
Occurrence and material. This specimen is relatively rare in the Kukruse and Haljala RSs (Sandbian) in northern Estonia; 28 specimens are represented in the GIT, TUG and ELM collections.
Diagnosis. Medium-sized subcircular shell up to about 30 mm wide (Fig. 3). Flat to weakly concave dorsal and moderately convex ventral valve. Ornament costate with up to 40 rounded-triangular ribs on shell margins, few second-order costellae appear in anterior half of valves; filae thin, weak; ventral umbo curved over hinge line; cardinal process on notothyrial platform has thickened basal part with ridge-like folded myophore reaching backwards over interarea.
Description and discussion. Medium-sized subcircular shell; hinge line corresponds to 0.7-0.8 shell width; ventral valve convex, its length corresponds to 0.8 and thickness to 0.4 shell width; dorsal valve weakly concave in the middle; anterior commissure rectimarginate. Radial ornamentation consists of 40 low rounded costae, few second-order ribs appear on the anterior half of valves; interspaces between costae are wider than ribs. About 30 ribs appear around the umbo at a distance of 2 mm; 3-4 ribs and 3 ribs occur in 5 mm on the anterior and lateral margins, respectively. Filae, 6-8 in 2 mm, are thin and weakly developed (Fig. 5D). Ventral umbo extends over the hinge line; interarea strongly apsacline, up to 2 mm high, dorsal interarea flat, anacline; delthyrium wide, open, notothyrium partly filled with a narrow, weakly folded cardinal process; myophragm short, separates posterior adductor scars (Fig. 4D-E).
Ventral interior is unknown.
The specimen of C. barbara differs from the C. kukersiana-type specimens and from C. laine in having subcircular outline, a shorter hinge line and low and rounded ribs with few second-order ribs. The taxonomically unclear specimen figured by Wysogórski (1900) as Orthis concava Schmidt mscr. differs from C. barbara in the more oval outline and seemingly in a higher number of ribs.
Distribution and material. Kukruse and Haljala RSs; 13 specimens in GIT, TUG and ELM collections.

DISTRIBUTION OF CYRTONOTELLA IN BALTICA
The earliest occurrence of Cyrtonotella in Baltica is C. semicircularis in the Aseri RS (Alikhova 1953).
However, most Baltic Cyrtonotella records come from the interval from the Uhaku RS to the Keila RS (Darriwilian to Sandbian stages; Fig. 6). The occurrence of cyrtonellids (Cyrtonotella sp. nov. 1 and Cyrtonotella sp. A, in Ropot & Pushkin 1987) in the Oandu and Vormsi stages of Belarus is not clear because neither descriptions nor figures are available. The Baltic brachiopods of the genus Cyrtonotella became extinct during the mid-Caradoc (Keila-Oandu) faunal turnover in the Baltic Basin (Meidla et al. 1999;Ainsaar et al. 2004).
The brachiopods of the genus Cyrtonotella have been identified in the Moscow Basin in the Kukruse RS (Alikhova 1969) and Haljala RS (Dmitrovskaya 1977) in relatively shallow-water environments as in the East Baltic. The marls and limestones enriched in kukersite kerogen occur in the Kukruse RS up to the middle part of the Moscow Basin (Krestsy, Valdai and Porkhov drill cores, Alikhova 1969; Fig. 6II). The easternmost finds of Cyrtonotella in the southeast of the basin (Rostov R-1 drill core; specimens GIT 154-314-1, 154-478) come from the Kukruse and Haljala RSs. The transition from the Kukruse RS to the Haljala RS (lower half of the Sandbian) in this core coincides with a change in brachiopod composition. There appear several new brachiopods (Eorhipidomella ovalis, Hints 1971, Multicostella rostovensis nom. nud. in the collection GIT 154) unknown in the East Baltic. The latter species, however, could be related to the Baltic plaesiomyd genus Madiorthis (Zuykov et al. 2008) from the Haljala RS of the same interval.
The cluster analysis of the dynamics of the rhynchonelliformean brachiopods in the Baltic Basin (Hints et al. 2018) reveals the same trend of faunal diversity changes during the transition of the Kukruse and Haljala times. In the Moscow Basin, the brachiopods common with the Baltic Basin (such as Leptelloidea, Estlandia, Clitambonites; Alikhova 1969) dominated up to that time. Subsequent development of faunas in the Moscow Basin is poorly known due to the development of facies (dolomites, sandstones) unsuitable for many groups of benthic organisms (Alikhova 1969;Kheraskova et al. 2005). Besides, on different stratigraphic levels there occur erosional gaps and the Devonian deposits overlie the Ordovician deposits of different age (Alikhova 1969).
The distribution of Cyrtonotella and associated brachiopods (Hints et al. 2018) in the Baltic and Moscow basins highlights their close palaeobiogeographical relationships through the early Sandbian Stage.

CONCLUSIONS
The cyrtonellid brachiopods are represented by four species (one of these with two subspecies) in the shallow sea environments of the Middle and Upper Ordovician of the Baltic Basin.
The type specimens have been selected for two subspecies, and the emended descriptions of all species and subspecies are presented. The cyrtonellid brachiopods are easily identifiable by their cardinal process, whose folded myophore reaches backwards from the notothyrium.
The stratigraphic range of the Baltic Ordovician cyrtonellids, together with the type species C. semicircularis from the Middle Darriwilian, covers about 11 Ma, which corresponds to the interval from the Aseri RS to Keila RS (mid-Darriwilian-Katian). The cyrtonellid brachiopods became extinct in the Baltic Basin during the Sandbian faunal turnover at the transition from the Keila RS to the Oandu RS.
In the easternmost parts of the Moscow Basin Cyrtonotella disappeared already during the Kukruse Age. Up to that time, the faunas of the Baltic and Moscow basins had a close relationship. The global distribution of the representatives of the genus Cyrtonotella is still insufficiently known due to the taxonomic problems on the species and genus levels.