Revision of the concept of the orthide brachiopod Cyrtonotella in the Middle Ordovician of the East Baltic

The two lowermost Darriwilian (Middle Ordovician) brachiopod species that were previously included in the genus Cyrtonotella are re-studied and compared with the type species Cyrtonotella semicircularis (Eichwald) from the St Petersburg Region. Based on the new data the classification of the important group of orthids, widely distributed in the Sandbian Kukruse to Keila regional stages in the East Baltic, is improved. The lower Darriwilian specimens, previously considered to be Cyrtonotella semicircularis, are ascribed to the genus Leoniorthis as the new species Leoniorthis rubeli, which occurs in the St Petersburg Region and in northern Estonia. The other species Cyrtonotella pakriensis Rubel is included in the new genus Rogorthis, which is restricted to the sandy limestones of the Pakri Formation (Kunda Stage) in northern Estonia. The type species of the genus Cyrtonotella, C. semicircularis (Eichwald), probably occurs in the upper Darriwilian Aseri Regional Stage in the St Petersburg Region, and is the first appearance of that genus.


INTRODUCTION
described the evolutionary trends of the Orthis calligramma group and differentiated the concavo-convex brachiopods as a separate linage with the species Orthis moneta Eichwald, 1861, O. moneta mut. Wysogórski, 1900, O. actonia Sowerby 1839, typ. and O. actonia var. Wysogórski, 1900.These species were later included in the genus Nicolella (Öpik 1930;Alikhova 1953;Williams in Whittington & Williams 1955;Oraspõld 1959).This linage has a lateral branch with two species Orthis semicircularis (Eichwald, 1829) and an informally identified transitional species O. semicircularis mut.Both species differ from O. moneta in their fascicostellate ornamentation.The shells of O. semicircularis mut.Wysogórski are much smaller than the specimens of O. semicircularis typ.(Terebratula semicircularis of Eichwald 1829), which occurs stratigraphically at a somewhat higher level.Schuchert & Cooper (1931) formally introduced the genus Cyrtonotella and described it in detail in 1932.This genus contains the Ordovician brachiopods from the East Baltic, which differ from Orthis in 'the character of the ornamentation and the peculiar profile of the shell' (Schuchert & Cooper 1932, p. 77).Terebratula semicircularis Eichwald, 1829 was identified as the type species of the new genus.In addition to the type species, Schuchert & Cooper (1932) included in Cyrtonotella only Orthis concava Schmidt (Wysogórski 1900, pl. 8;Alikhova et al. 1959) from the Upper Ordovician Kukruse Stage.However, in plate 2, fig.14 they figured also a specimen named as Cyrtonotella sp.aff.C. frechi (Wysogórski).The interpretation of this specimen from the Echinosphaerites-limestone (Aseri Stage; Middle Ordovician), is unclear.Cyrtonotella frechi itself occurs in much younger strata (Wysogórski 1900;Öpik 1930).Schuchert & Cooper (1932) did not mention any other species described by Öpik (1930) and included by him later (1934) into the genus Cyrtonotella, although they were familiar with Öpik's study since they accepted the genus Glossorthis established in his former publication.
In the monographic study on the Early and Middle Ordovician brachiopods of the Baltic region Rubel (1961) selected the types (holo-or neotypes) for all species described by him.He designated a neotype also for the species Cyrtonotella semicircularis (Eichwald).That is a small orthide specimen (GIT 125-93 in Rubel 1961, pl. 17, figs 1-5) from the Kunda Regional Stage of the Putilovo locality (Popov et al. 2005) in the St Petersburg Region.Unfortunately the type and other Darriwilian specimens of Cyrtonotella semicircularis noted by Rubel differ essentially in their small size and external ornamentation from Eichwald's Terebratula semicircularis described also by Murchison et al. (1845).The neotype, which does not correspond to the original description and published image of Cyrtonotella semicircularis, must be regarded as invalid (see ICZN 1999, Article 75.3.5).
The short description of Terebratula semicircularis by Eichwald (1829) includes measurements of the shell: width 8.5 lin.(the old measure 'line' or 'liniya' corresponds to 1/10 or 1/12 of the inch), length 7 lin., thickness 3 lin.According to these measurements, the specimens of C. semicircularis are rather large and more than 20 mm wide, about 16 mm long and 7 mm thick.A similar size for C. semicircularis was mentioned by Murchison et al. (1845, p. 212).
One reason for the later misidentification of Cyrtonotella semicircularis is the erroneous magnification (5 ×) of the specimen figured by Schuchert & Cooper (1932, pl. 4, figs 1, 4, 5 and 11) and later by Williams & Harper (2000, fig. 537, 4a-c).The figured specimen is less than 6 mm wide when calculating the shell size using this magnification.However, the specimen figured by Schuchert & Cooper (1931; YPM IP 000169A, named as hypotype) is 18.8 mm wide, the ventral valve is 16.9 mm long and shell thickness is 7.5 mm (Fig. 1A).One other specimen in the same collection is even 24.9mm wide.Of similar size are the specimens from Popovka in Naturhistoriska Riksmuseet, Stockholm (RM Br. 131691, Br. 74880, Br. 74374, Br. 74375) (Fig. 1B).Popovka is a locality known to Eichwald (1861) where he recorded several brachiopod species, including C. semicircularis.The taxonomy of the genus Cyrtonotella requires future clarification and the type species would benefit from an improved description based on specimens housed in different museums.The aim of the present study is to explain the problem with the unsuccessful designation of the neotype for the type species of the genus Cyrtonotella and to describe the species which clearly do not belong to that genus.The early Darriwilian specimens of 'semicircularis' with a shell width of 10 to 16 mm described by Rubel (1961) are most similar to Wysogórski's species O. semicircularis mut.Wysogórski (1900, fig. 8).Alikhova (1969, p. 22) noted that the brachiopods, described by Rubel (1961) from the Kunda Stage (lowermost Darriwilian) within the genus Cyrtonotella, should be excluded from that genus because they essentially differ from the type and all other species of that genus (Öpik 1930;Alikhova 1953) Diagnosis.Small orthides with suboval shell, 0.8-0.9 as long as wide, with plano-to weakly concave dorsal valve.Radial ornamentation fascicostellate, with two median primary costae on dorsal valve having up to eight second-and third-order costellae.Brachiophores widely divergent, cardinal process stout.
Description.Suboval plano-to concavo-convex, slightly sulcate shell with maximum width at the hinge line at earlier growth stages and at about mid-length in adult specimens.Cardinal extremities acute to obtuse; anterior commissure rectimarginate or weakly sulcate.Radial ornament fascicostellate, with 10-12 primary costae, increasing in number by bifurcation up to 25.The secondand third-order costellae form bundles, the two median costae on the dorsal valve have up to eight costellae, two of which bifurcate close to the umbo.Lateral bundles consist of 3-4 costae and costellae.Growth lines mark the allometric growth of the shell at later growth stages; filae, 7-10 in one mm, are well developed.Ventral valve 0.8-0.9 as long as wide, moderately convex, with the maximum height at the mid-length.Interarea concave, 0.81-0.94as wide as maximum shell width.Delthyrium triangular, open.Teeth small, with parallel dental plates.Muscle field is not defined anteriorly.
Dorsal valve almost flat, with weakly convex posterior and weakly sulcate anterior half.Interarea flat, anacline; notothyrium open, partly filled by the cardinal process.Brachiophores stout, divergent laterally and thickened by secondary shell material forming oblique dental sockets.Cardinal process stout or wedged, narrowing towards the umbo; notothyrial platform continues anteriorly as wide median ridge up to the thickened and crenulated anterior margin.Dorsal adductor muscle field weakly impressed, situated at the mid-valve.
Discussion.In size and external ornamentation the described species has some similarity to the specimen figured by Wysogórski (1900, table 8) as the variety Orthis semicircularis mut.This similarity may have been a reason why initially (Rubel 1961) the brachiopods, described in this paper, were included in the genus Cyrtonotella.Wysogórski's Orthis semicircularis mut. is probably conspecific with the described species Leoniorthis rubeli.
The new species Leoniorthis rubeli differs from Cyrtonotella semicircularis and from all other Upper Ordovician species assigned to the genus Cyrtonotella in its essentially smaller size, fascicostellate radial ornament and the features of dorsal interior (the brachiophores are less divergent, the cardinal process is stout or wedged).
The species Leoniorthis rubeli differs from Leoniorthis robusta Egerquist, the type species of the genus, in being double the size (see Egerquist 2003, p. 34), a more transverse shell outline (ratios of ventral valve length to shell width up to 0.88 and 0.72, respectively), stronger convexity of the ventral valve and a welldeveloped notothyrial platform with the wedge-like cardinal process.Besides the differences in shell morphology, L. rubeli and L. robusta apparently occur in the rocks laid down in different environments.The majority of specimens of L. robusta are from the clayey mud mounds (Egerquist 2003;Tolmacheva et al. 2003).Leoniorthis rubeli occurs in lithologically different more or less glauconitic carbonates with marl or clay interlayers (Meidla 1997).However, some exceptional variation in shell form and ornamentation of L. robusta is considered as intraspecific (Egerquist 2003).
The genus Leoniorthis differs from the Early and Middle Ordovician orthid brachiopods Paralenorthis, Orthambonites, Sulevorthis and Sivorthis (see Egerquist 2003) in the characters of radial ornament.The species Diagnosis.Medium-sized orthide with planoconvex shell; ornament fascicostellate, with 12-14 coarse primary costae branching on anterior half of valves.Cardinal process stout on elevated notothyrial platform.Subparallel vascula media divergent anteriorly; thickened rim along interior margins of the valves.
Comparison.Brachiopods of the new genus show some similarity with the East Baltic Middle Ordovician genera Orthambonites, Paralenorthis and Orthis in shell outline, general features of radial ornamentation (coarse costae, occurrence of filae and capillae) and triangular anterior outline of the ventral muscle field.Rogorthis differs from the first two genera in a flat dorsal valve, rib bifurcation, stout cardinal process and the position of the vascula media.In Orthambonites the vascula media are parallel, whereas in Paralenorthis they are divergent immediately in front of the muscle field (Jaanusson & Bassett 1993).The new genus is most similar to Orthis in its plano-convex shell and the vascula media, which are parallel in front of the muscle field and then diverging anteriorly.Rogorthis differs from Orthis in the small size of the shell, stout cardinal process and bifurcating ribs.
Occurrence.The middle and upper parts of the Kunda Stage, northern Estonia (lowermost Darriwilian).

Diagnosis. As for genus.
Description.Planoconvex suboval shell, 0.7-0.9 as long as wide.Cardinal extremities obtuse.Interarea 0.79 and 0.81 as long as shell width in two specimens.Ornament with 12-14 rounded first-order coarse costae bifurcating (or branching) in the anterior half of the valve into two or three; interspaces between costae are narrow.Filae weakly developed, occur along the crests of costae.Capillae very fine, up to 10 in 1 mm.Ventral valve lateral profile evenly convex, with the highest point in the middle; interarea short, orthocline; delthyrium open, forms together with semicircular foramen? a wide subtriangular opening in the apex of the valve.Teeth small, triangular, dental plates convergent, bordering the suboval muscle field laterally; crural fossettes small.Adductor and diductor muscle scars weakly differentiated.Vascula media subparallel posteriorly, divergent anteriorly.Dorsal valve flat, interarea 0.5 mm high, anacline; notothyrium open, triangular, almost completely filled by the cardinal process.Notothyrial platform with a thickened floor and stout cardinal process, triangular in posterior view; together with a short median septum brachiophores and cardinal process form a thick crest-like structure in the posterior part  (Rubel, 1961) of the dorsal valve.The edge of valve interior is marked by a narrow two-step peripheral rim of short grooves.
Comparison.Rogorthis pakriensis is most similar to Orthis callactis, however, it differs from that species in its smaller shell size, stout cardinal process and bifurcating costae.The ribbing on the rim of the interior valve margins of R. pakriensis is similar to Sulevorthis lyckholmiensis (Wysogórski) (Jaanusson & Bassett 1993).
A specimen (GIT 125-3) from the oolitic limestones of the Napa Formation of the Kunda Stage, which Rubel (1961) included with some doubt in the genus Apheoorthina, is considered here as conspecific with Rogorthis pakriensis.
It has a deformed ventral valve with coarse costae branching in the anterior part.This specimen presumably has filae.Filae are not seen on the badly preserved specimens from the Pakri sandstones.All described specimens have thick valves, presumably due to adaptation to high-energy environments or grainy deposits (calcareous sandstone or limestone with iron oolites) and high water energy.
Material.Seven specimens; see the Estonian geocollection database, collection GIT 125 (http://sarv.gi.ee/).Occurrence.Lowermost Darriwilian; calcareous sandstone of the Pakri Formation and the limestone with iron ooids of the Napa Member of the Kandle Formation in the middle and upper parts of the Kunda Stage.
The early Darriwilian new species Leoniorthis rubeli from the Kunda Regional Stage extends the stratigraphic range of the genus Leoniorthis, identified earlier as from the uppermost Floian and Dapingian stages (Billingen to Volkhov stages; Egerquist 2003).
The total stratigraphical range of Leoniorthis from the Billingen to Kunda stages embraces carbonate strata with varying contents of glauconite grains and faunas rich in Baltic endemics (Jaanusson 1979;Hints & Harper 2003;Tolmacheva et al. 2003).The species Rogorthis pakriensis (Rubel) seems to be distributed mainly in shallow-water carbonate sandstones and oolitic limestones of the Kunda Stage (early Darriwilian), emphasizing the endemicity of brachiopod fauna in those rocks.
Acknowledgements.The author is grateful to M. Bassett (National Museum of Wales) for useful comments during the study of Cyrtonotella-group brachiopods and for the opportunity to examine the original specimens of Cyrtonotella semicircularis, which he had borrowed from the Yale Peabody Museum.Naturhistoriska Riksmuseet in Stockholm is thanked for providing access to the brachiopod specimens from the Popovka locality.R. Cocks from the Natural History Museum in London and L. Popov from the National Museum of Wales, Cardiff are thanked for useful critical comments and improvements in the manuscript.The author thanks G. Baranov (Institute of Geology at Tallinn University of Technology) for preparing the photos of the brachiopods.The study was financially supported by the Estonian Ministry of Education and Research (project SF0140020s08 and grant ETF8182).This paper is a contribution to IGCP Project 591.
. In the current study the specimens of C. semicircularis (Wysogórski) sensu Rubel are included in the genus Leoniorthis as a new species L. rubeli and the other species Cyrtonotella pakriensis, together with Apheoorthis?sp., to a new genus Rogorthis.The studied brachiopods belong to the collections, housed at the Institute of Geology at Tallinn University of Technology (institutional abbreviation GIT), Estonia, Yale Peabody Museum of Natural History, U.S.A. (YPM) and Naturhistoriska Riksmuseet, Stockholm, Sweden (RM).Cyrtonotella Schuchert & Cooper, 1931; Rubel, p. 180.2003 Leoniorthis gen.nov.; Egerquist, pp.35, 36.Type species.Leoniorthis robusta Egerquist, 2003, St Petersburg Region and northern Estonia, Billingen and Volkhov stages, uppermost Lower and lower Middle Ordovician.

Table 1 .
Dimensions (in millimetres) of Leoniorthis rubeli sp.nov.Ratios of measurements in brackets Derivation of name.From the historical Swedish name Rogö for the Island of Pakri in northwestern Estonia.