Tetraphalerella bestiubensis, 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 : 42-44

Tetraphalerella bestiubensis sp. nov.

Figs. 3A–H, 4A, B; Tables 1, 2.

Derivation of name: After the town of Bestiube near the type locality.

Holotype: NMW 98.30 G.5, dorsal internal mould.

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

Type horizon: Ordovician, Tauken Formation.

Paratypes.—Six conjoined valves, 60 ventral and 56 dorsal valves.

Diagnosis.—Convexiconcave to convexiplane, transverse, suboval shell about 75% as long as wide with flattened lateral ventral profile; dorsal valve moderately and evenly convex, about 20% as high as long with very weak umbonal sulcus fading anteriorly; radial ornament unequally parvicostellate with 13–15 ribs per 3 mm along the anterior margin of mature specimens and with up to 5 parvicostellae in interspaces between accentuated ribs.

Description.—Shell slightly convexiconcave to convexiplane, transverse, suboval in outline with maximum width at the hinge line.Cardinal extremities slightly acute to right angled. Anterior commissure rectimarginate.Ventral valve flattened with lateral profile slightly convex in the umbonal area, very gently and evenly concave anteriorly.Ventral interarea plane, apsacline with broad, convex pseudodeltidium perforated apically by a minute, rounded foramen.Dorsal valve moderately and evenly convex in lateral profile.Dorsal interarea apsacline with a convex chilidium.Dorsal umbonal area with a very shallow, weakly defined sulcus in some specimens.Radial or − nament unequally parvicostellate with accentuated costellae of three or four generations originating in the umbonal area, at about mid−valve length, and near the anterior and lateral margins in adult specimens.Number of costellae varying from 13 to 15 per 3 mm along the anterior margin of mature specimens.

Ventral valve interior with strong oblique teeth; short, widely diverging dental plates continue anteriorly into ridge−like extensions that laterally bound a large, rounded subpentagonal muscle field about 40% as long as the valve.Ven − tral adductor muscle scars narrow, strongly elongated, suboval, occupying the median part of the muscle field, somewhat shorter than large, rounded, subtriangular diductor scars.Dor − sal interior with small, bilobed cardinal process; a low ridge bisects the area between the lobes.Socket ridges widely diver − gent, slightly recurved towards the hinge line.A low, broad median ridge divides the entire dorsal adductor muscle field.

Discussion.—This is the first record of Tetraphalerella in Kazakhstan.It resembles Tetraphalerella cooperi Wang (1949: 29 , pl.8: 1–6) from the Maquoketa Shale (Ashgill) of Iowa in its semioval shell outline and lateral profile of both valves, but it is distinguished in having shells that are about half the size of the American species, a flattened ventral valve, and an evenly convex dorsal valve which is not flattened in the umbonal area.The radial ornament in T. bestiubensis is characterized by the presence of 2 to 5 finer ribs in the wide interspaces between accentuated costellae, whereas in T. cooperi these interspaces contain usually not more then 1 to 2 finer ribs.

Among Kazakhstanian strophomenids T. bestiubensis can be compared only with Strophomena dzhumgalica Misius (1988: 160, pl.18: 1–18, pl.19: 1–6) from the Tabylgaty Formation (upper Caradoc) of North Kyrgyzstan, but differs in having a much smaller average shell size, a strongly convex dorsal lateral profile, more clearly differentiated radial ornament, and in the absence of dorsal side ridges and a subperipheral rim in the ventral valve.

Occurrence.—Localities 550a and 2523.