A new glyptorthid species (Brachiopoda: Orthida) from the Upper Ordovician of Estonia

A new glyptorthid brachiopod species Bassettella alata with surface pits and lamellose frills is described from the Upper Ordovician Haljala and Keila stages of northern Estonia. The new species appears in the Jõhvi Substage of the Haljala Stage (mid-Sandbian) and is somewhat younger than the type species B. gracilis appearing in the lower Idavere Substage of the same stage in NW Russia. In the uppermost Keila Stage (lowermost Katian), B. alata belongs to a mixed faunal association of the marginal facies of the Vasalemma Formation comprising the oldest Ordovician reefs in Estonia. The distribution pattern of brachiopods of the genus Bassettella shows a westward shift in time and changes in shell morphology from subrectangular to subtriangular with alate cardinal extremities.


INTRODUCTION
Recently Zuykov & Butts (2008) discussed the taxonomy of the family Glyptorthidae Schuchert & Cooper, 1931.They also described a new genus Bassettella, which is characterized by enigmatic surface pits and lamellose frills.The type species of the genus, B. gracilis, occurs in the lower part of the Haljala Stage (Idavere Substage) in NW Russia and in the upper part of the same stage (Jõhvi Substage) in northern Estonia (Zuykov & Butts 2008).
Brachiopods with the external sculpture similar to that of Bassettella were identified in the collections from northern Estonia.The differences in shell morphology, however, suggest that these specimens are not conspecific with the type species and thus a new species Bassettella alata is erected.The new species is most common in the interlayers of argillaceous skeletal limestone in the lower part of the Vasalemma Formation (Põlma et al. 1988;Hints 1990) containing a mixed assemblage of fossils.The brachiopods characteristic of the Keila Stage, such as Sowerbyella (S.) tenera Rõõmusoks, Estlandia pyron silicificata Öpik and Horderleyella kegelensis (Alichova) (see Hints & Meidla 1997), co-occur in these interlayers with remains of various echinoderms (cystoids, paracrinoids and crinoids), which are the main constituents of grainstones surrounding the reefs of the Vasalemma Formation.The new species B. alata is, however, known only in the lowermost part of this formation.
The studied material comprises 20 specimens, most of which have been collected from shallow cores drilled in NW Estonia in the 1970s.This area is now part of a large limestone quarry in Vasalemma (Fig. 1).The specimens described are housed at the Institute of Geology at Tallinn University of Technology (institutional abbreviation for the collection is GIT).The two specimens of Glyptorthis gracilis mentioned in Zuykov & Butts (2008) are housed at the Geological Museum, University of Tartu (institutional abbreviation TUG).
Diagnosis.Shell ventribiconvex with alate cardinal extremities, ramicostellate with 5-7 primary costae on the ventral valve.Lamellose frills strong, with intervals up to 0.65 mm.Superficial pits round to oval, increasing in size anteriorly and on every frill.Delthyrium narrow, up to twice as high as wide, cardinal process wedgeshaped, thickening anteriorly; ventral muscle field with elongate, anteriorly elevated adductor tracks supported by very short median septum.
Description.The subequally biconvex shell with alate cardinal extremities is about 0.6 as long as wide (Fig. 2A, B).The dorsal valve is weakly convex with a shallow sulcus between the middle primary costae, ventral valve slightly carinate (Fig. 3A-E).The anterior commissure is rectimarginate to slightly sulcate.Ornamentation is ramicostellate with 5-7 costae on the ventral and 4-6 on the dorsal valve.The first-order secondary costellae appear close to the umbo; one or two postero-lateral costellae originate on the posterior margin of the valve.The total number of costae and costellae reaches up to 29 (Fig. 2C).In the middle sector the costae are symmetrically triangular in cross section,  in lateral parts become asymmetrical with a shorter flank towards the midline of the shell.The lamellose frills in the posterior part of the shell are weakly developed and occur at intervals of 0.25 mm, anteriorly the intervals between frills increase up to 0.65 mm.The superficial pits occur unevenly, most densely on the anterior parts of the lamellose frills.The size of the pits increases anteriorly from 0.06 to 0.09 mm; they are rare or missing on the posterior parts of the valves.The pits penetrate the anterior parts of the lamellose frills lifted from the valve surface.The edges of the frills can be notched (Fig. 3K, L) due to incompletely formed pits or mechanical damages.
The ventral area is apsacline, slightly concave.The delthyrium is triangular open, up to twice as high as wide or about as wide as high in smaller specimens.The dorsal area is low, almost anacline.The notothyrium is open.
The ventral interior shows small hinge teeth with weakly developed dental plates, which bend towards the delthyrial cavity and delineate laterally the muscle field.Diductor scars are elongate, about as long the adductor field in the form of an anteriorly elevated tongue-shaped platform with subparallel lateral edges.On the older specimen the anterior part of the platform is sub-vertical in relation to the valve surface and two small cavities occur on both sides of the short septum-like continuation of the platform (Fig. 4B, E).
The dorsal interior is known by two valves (Figs 3N, 4A).The notothyrial platform is elevated, subtriangular, formed laterally and anteriorly of convergent brachiophore plates.The cardinal process is wedge-like with a sharp top; brachiophores are divergent, bounding the sockets laterally.The muscle scars are not expressed.The external ribbing pattern is developed on the interior surface of the valves as radial grooves and wide roundedtop ridges.
The peripheral rim is knee-like (Fig. 3N, O).The pitted ribbing along the anterior edge is delimited from the interior surface of the valve by tiny growth-line-like concentric filamentous sculpture on top of the knee (Fig. 3O).
For measurements of specimens see Fig. 2A.
Discussion.The small surface pits of the brachiopod Bassettella can be regarded as exopunctae because they do not penetrate to the internal surface (Williams & Brunton 1997).However, their uneven distribution and size relation with distinct growth stages differentiates Bassettella from several other brachiopods with evenly distributed and densely spaced exopunctae (Jin et al. 2007).It seems possible that in case of Bassettella Wright's (1981) opinion about the development of surface pits is applicable.The surface pits penetrating the margin of the valve supposedly housed temporary caeca, whose development depends on the growth of the valve.The subperipheral rim on the internal margin with densely spaced fine growth lines (Fig. 3O) delimits the pitted margin of the valve which probably forms a new frill during the further growth of the shell.
The pitted external sculpture and strongly impressed ventral muscle field with anteriorly elevated adductor tracks differentiate both species of Bassettella from the typical species of Glyptorthis from North America (Cooper 1956) and Europe (Williams 1962;Hansen 2008).Another atypical Ordovician glyptorthid with minute tubercles on the shell surface has been described from the Darriwilian of China (Zhan & Jin 2005).The Chinese species Glyptorthis sarcina Zhan & Jin is among the oldest species of the genus Glyptorthis.It is similar to the Estonian species in having a relatively small number of ribs (9-11).However, the Estonian species lacks tubercles, has a shorter notothyrial platform and possibly lacks the median ridge.
The new species differs from the Russian specimens of Bassettella gracilis from the Gryazno Formation (Fig. 5) in its alate outline (Fig. 2B), less convex dorsal valve, more robust radial ornamentation (Fig. 2C), higher and almost flat ventral area, and more sparsely spaced lamellose frills.However, somewhat younger Estonian specimens of B. gracilis from the Jõhvi Substage of the Alliku and Kiikla localities (Fig. 4I-L) have acute cardinal angles as in the new species B. alata, but denser lamellose frills and finer surface pits.Bassettella gracilis and B. alata supposedly occur in different parts of the Jõhvi Substage (Fig. 5).At least the specimen of B. gracilis from Alliku (Zuykov & Butts 2008) is older than B. alata from the Keila locality, because in these localities, respectively, the lower and the upper part of the Jõhvi Substage are exposed (Männil 1950).
The distribution pattern of Bassettella displays a westward shift in time from early Haljala (mid-Sandbian) time in northwestern Russia to late Keila (early Katian) time in northern Estonia (Fig. 5).During that time interval the shell morphology changes from subrectangular to subtriangular with alate cardinal extremities.

Fig. 2 .
Fig. 2. Dimensions of the studied specimens in millimetres (A), valve length/width (B) and number of ribs/valve width (C) ratios.

Fig. 1 .
Fig. 1.Sketch map of localities in NW Estonia (A), Vasalemma settlement (in grey) and quarry (B) and sites of the studied drill cores in the Vasalemma quarry (C).