Glyptomena kaskolica, F.Nikitin & E.Popov & G.Bassett, 2003

F. Nikitin, Igor, E. Popov, Leonid & G. Bassett, Michael, 2003, Late Ordovician brachiopods from the Selety river basin, north Central Kazakhstan, Acta Palaeontologica Polonica 48 (1), pp. 39-54 : 44-46

Glyptomena kaskolica sp. nov.

Figs. 3I–M, 5H ; Tables 3, 4.

Derivation of name: After lake Kaskol near the type locality. Holotype: NMW 98.30 G.12, ventral internal mould.

Type locality: Sample 550a, Bolshoi Kaskol lake, Selety river basin, Kazakhstan.

Type horizon: Ordovician, Tauken Formation.

Paratypes.—17 ventral and 11 dorsal valves.

Diagnosis.—Shell strongly concavoconvex, about 85% as long as wide; ventral valve strongly and evenly convex, about 40% as thick as long with low apsacline interarea; dorsal valve evenly concave, rarely slightly geniculated in large specimens; radial ornament inequally parvicostellate with 3 to 4 parvicostellae between accentuated ribs and 10 to 12 ribs per 3 mm along the anterior margin of adult specimens; weak, oblique rugellae developed along the posterior margin.

Description.—Shell strongly concavoconvex, transversely semioval in outline with maximum width along the hinge line.Cardinal extremities slightly alate, acute to right angled.Ventral valve strongly and evenly convex in lateral profile.Interarea low triangular, plane, apsacline with a broad, triangular delthyrium which has an apical convex pseudodeltidium.Dorsal valve evenly concave in lateral profile, or slightly geniculated anteriorly.Dorsal interarea linear, plane, anacline.Notothyrium broad, triangular, with discrete lateral chilidial plates.Radial ornament finely and unequally parvicostellate with accentuated ribs separated by 3 to 4 finer parvicostellae and with 5 to 6 ribs per 1 mm along the anterior margin of adult specimens about 15 mm long.Fine, evenly spaced concentric fila cross the costellae. Weak, obliquely concentric rugellae are developed along the posterior margin. Ventral interior with strong, obliquely directed teeth bearing 5 to 6 crenulations on the postero−dorsal surface (see also

Rong and Cocks 1994: fig.8).Dental plates short, divergent, strongly thickened, with ridge−like extensions laterally bounding a subcircular muscle field, about 25% as long as the valve.Adductor scars linear, slightly raised, separating large subcircular diductor scars.Mantle canals with thin, di − vergent vascula media.Dorsal interior observed only in a sin − gle poorly preserved specimen.It has a double cardinal pro − cess and straight, widely divergent socket plates.

Discussion.— Glyptomena kaskolica differs from the type species Glyptomena sculpturata Cooper, 1956 from the Chatham Hill Formation (Caradoc) of Virginia in having a strongly convex ventral lateral profile with the maximum thickness at mid−valve length, and in the weakly developed to absent geniculation of the dorsal valve.The number of fine intercostal parvicostellae in the Kazakhstanian species does not exceed 4, whilst in the type species the interspaces between accentuated ribs are somewhat wider and include up to 7 parvicostellae.In convexity and lateral shell profile G. koskolica resembles Glyptomena parvula Cooper, 1956 , from the Effna and Rich Valley formations (Caradoc) of Virginia, but differs in its pattern of more closely spaced accentuated costellae, the relatively small ventral muscle field which occupies no more than one quarter of the maximum valve length, and in having oblique concentric rugellae along the cardinal margin.

This new species also somewhat resembles Glyptomena subcraignsis Severgina (in Kulkov and Severgina, 1989: figs.p.132, pl.17: 6–10), but differs in having a larger shell of semioval outline with maximum width at the hinge line, maximum height at about mid−length, slightly alate ventral cardinal extremities and weakly developed to absent geniculation in the dorsal valve, which occurs rarely and only in gerontic specimens, plus finer radial ornament with weak oblique rugellae along the hinge line.

Occurrence.—Localities 550a, 780 and 2523.

Figures

Fig.5. A–F. Rhynchotrema seletensis sp.nov. A, C, F, sample 2523; B, D, E, sample 550a. A. NMW 98.30G.40, ventral (A1), dorsal (A2), anterior (A3), and lateral (A4) views of conjoined valves, × 2.25. B. NMW 98.30G.43, ventral internal mould (B1) and latex cast (B2), × 3. C. NMW 98.30G.41, holotype, ventral (C1), dorsal (C2), lateral (C3), and anterior (C4) views of conjoined valves, × 2.25. D. NMW 98.30G.45, latex cast of dorsal interior, × 2.5. E.NMW 98.30G.42, latex cast of dorsal valve interior (E1) and dorsal internal mould (E2), × 2.25. F. NMW 98.30G.44, anterior (F1), ventral (F2), dorsal (F3), and lateral (F4) views of conjoined valves, × 3. G. Dinorthis taukensis sp. nov.; NMW 98.30G.37, holotype, sample 2523; dorsal (G1), ventral (G2), lateral (G3), anterior (G4), and posterior (G5) views of conjoined valves, × 2.25. H. Glyptomena kaskolica sp. nov.; NMW 98.30G.12, holotype, sample 550a; latex cast of ventral interior showing muscle field, × 2.25.