Dinorthis taukensis, 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 : 47-48

Genus Dinorthis Hall and Clarke, 1892 Dinorthis taukensis sp. nov.

Figs. 4I–Q, 5G ; Tables 7, 8.

Derivation name: After the Tauken Mountains near the type locality. Holotype: NMW 98.30 G.37, complete shell.

Type locality: Sample 2523, Shollakkarasu river, Selety river basin, Kazakhstan.

Type horizon: Ordovician, Tauken Formation.

Paratypes.—265 complete shells, 42 ventral and 63 dorsal valves.

Diagnosis.—Dorsibiconvex, about 80% as long as wide and 55% as high as long; evenly convex dorsal lateral profile with a shallow ventral sulcus originating near mid−length; radial ornament costellate with 20 to 30 rounded ribs.

Description.—Shell dorsibiconvex in mature specimens, transversely suboval in outline with maximum width at the mid−length.Cardinal extremities rounded.Anterior commissure broadly uniplicate.Ventral valve moderately convex with maximum height slightly anterior to the umbo at or about a mid−point between the umbo and mid−valve.Inter − area subtriangular, apsacline, weakly concave.Delthyrium open, triangular.Beak pointed and slightly erect posteriorly. Sulcus broad and shallow, originating somewhat posterior to mid−valve.Dorsal valve about 80% as long as wide, moder − ately convex in lateral profile with maximum thickness at about mid−length.Dorsal interarea low, planar, anacline.An indistinct dorsal median fold occurs near the anterior margin in some specimens.Radial ornament costate with 20 to 30 rounded ribs separated by interspaces of about equal width. Concentric ornament of fine, evenly spaced and slightly elevated growth lines.

Ventral interior with strong teeth and short divergent dental plates.Muscle field large, slightly elongated, rounded subpentagonal in outline, about 45% as long as the valve. Large diductor scars completely surround small elongate adductor scar in the posterior half of the ventral muscle field. A low median ridge bisects the ventral muscle field longitudinally anterior to the diductor scars.Mantle canals saccate with short, straight, slightly divergent vascula media.Dorsal interior with ridge−like cardinal process crenulated posteriorly.Brachiophores blade−like with widely divergent bases. Adductor scars weakly impressed, bisected longitudinally by a low median ridge.

Variability.—There is fairly considerable variation in the relative convexity and transverse profile of both valves in the available samples.Dorsibiconvex shells are predominant, but subequally biconvex and slightly ventribiconvex shells comprise about 20% of specimens in sample 2523 and up to 30% in sample 5190.Shells with 22 to 26 ribs are the most abundant and represent 83% of the total number of specimens in both samples.

Discussion.—This species is similar to Dinorthis ortonurensis Misius (in Andreeva and Misius 1977: 106, pl.25: 12–16; Misius 1986) from the Ichkebash Formation (upper Caradoc) of North Kyrgyzstan and to Dinorthis kassini Rukavishnikova (1956: 126 , pl.1: 12–16) from the Dulankara Formation (upper Caradoc) of the southern Chu−Ili Range, but differs from both these species in having a less convex dorsal lateral profile and in the absence of an umbonal dorsal sulcus. D. taukensis sp.nov. is about twice as small and has not more than 30 ribs by comparison with the 40 to 48 ribs in the Kyrgyzstanian species.

D. taukensis is also somewhat comparable with Dinorthis westfieldensis Laurie (1991: 34 , fig.30A) from the Benjamin Limestone (upper Caradoc) of Tasmania in its general shape and radial ornament, but differs in having an evenly convex dorsal lateral profile and a shallow, but well−defined sulcus in the anterior half of the ventral valve.

Occurrence.—Localities 397a, 550a, 780, 2523, 5190.

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.