Tunisiglossa almalensis Popov and Mambetov, 2007

Popov, Leonid E., Ebbestad, Jan Ove R., Mambetov, Amanbek & Apayarov, Farid Kh., 2007, A low diversity shallow water lingulid brachiopod-gastropod association from the Upper Ordovician of Kyrgyz Range, Acta Palaeontologica Polonica 52 (1), pp. 27-40 : 31-32

Tunisiglossa almalensis Popov and Mambetov , sp. nov.

Figs. 3 , 4 .

Derivation of the name: After the type locality near the Almalysai River .

Holotype: NMW 98.66 G.870, Fig.3G , dorsal internal mould (L = 9.8 mm; W = 8.4 mm; Vl = 6.2 mm).

Type locality: Locality 509, northern side of Almalysai River, Kyrgyz Range, Kazakhstan.

Type horizon: Upper Member of Almaly Formation, Caradocian (Upper Ordovician).

Material.— The holotype and paratypes: 36 ventral and 42 dorsal valves from sample 509 (specimens NMW 98.66 G.860–869; 871–945); 6 ventral and 4 dorsal valves from sample 510 (specimens NMW 98.66 G.960–969) .

Diagnosis.—A species of Tunisiglossa with a shell elongate oval in outline, about 140% as long as wide and a small ventral interarea less then half as wide as the valve; ventral interior with visceral area about half valve length bordered anteriorly by a low rim; ventral umbonal muscle scars paired, situated on the ends of slightly raised muscle scars; dorsal interior with long, slightly divergent vascular media ; differs from Tunisiglossa tripolitanea Massa, Havlíček, and Bonnefous, 1977 in having a less elongated oval, not rectangular shell outline, well defined and slightly raised ventral umbonal muscle scars and a low rim bounding anteriorly a ventral visceral area.

Description.—Shell elongate oval to sub−rectangular, about 140% as long as wide with maximum width about midlength. Ventral valve gently and evenly convex with the apical angle about 90–100 °. Ventral pseudointerarea narrow, triangular, slightly less than half as wide as the valve, bisected by a narrow, deep pedicle groove. Ventral propareas raised above the valve floor, bearing well defined flexure lines. Dorsal valve evenly convex with a thickened posterior margin lacking pseudointerarea, slightly shorter than ventral valve, about 130% as long as wide. Larval and postlarval shell smooth, ornamented with fine growth lines.

Ventral valve interior with subtriangular visceral area about half valve length, bounded anteriorly by indistinct rim slightly raised above the valve floor and bisected by a pair of slightly divergent furrows representing a pedicle nerve impression ( Fig. 3I ). Umbonal muscle scars paired, forming elongate, slightly divergent muscle trucks on both sides of the pedicle nerve impression ( Fig. 3H 1 , H 2). Other muscle scars and mantle canals not impressed. Dorsal valve interior with visceral area about 55% as long as the valve. Scars of outside lateral, anterior lateral and middle lateral muscles situated on bands of slightly thickened shell along posterolateral margins of the valve ( Fig. 3G ). Central muscle scars elongate suboval, situated on anterior terminations of fine, elongate subtriangular muscle tracks divergent from the apex. Small paired scars if anterior lateral muscles occupying a short anterior projection of dorsal visceral area. Vascula media long, slightly divergent.

Measurements (in mm).—Ventral valves: Lv = 11.3; S = 1.42; min = 8.5; max = 13.4; N = 32; W = 8.3; S = 1.5; min = 6.2; max = 10.2; N = 32; Lv/W = 140%; S = 15.2; min = 115%; max = 181%. Dorsal valves: Ld = 10.1; S = 1.25; min = 8.4; max = 13.8; N = 38; W = 8.5; S = 0.87; min = 6.7; max = 10.8; N = 38; Ld/W = 127%; S = 9.0; min = 112%; max = 148%; Vl/L = 54%; S = 8.1; min = 43.5%; max = 62%, N = 5.

Remarks.—The lingulid shells from the Almaly Formation have paired umbonal muscle scars, symmetrically arranged muscle scars in both valves and paired dorsal vascula media proving their assignation to the family Obolidae ( Fig. 4 ). They are also characterised by the completely reduced dorsal pseudointerarea typical of Glossellinae . Among genera presently assigned to that subfamily only Tunisiglossa Massa, Havlíček, and Bonnefous, 1977 is characterised by a smooth shell lacking fine granulation on the shell surface, radial or pitted ornament, has very small ventral pseudointerarea and no dorsal median ridge (for details, see Holmer and Popov 2000). The only other known species of the genus is its type species Tunisiglossa tripolitanea Massa, Havlíček, and Bonnefous, 1977 from the Lower Ordovician, Tremadoc Sanrhar Formation of Libya. Tunisiglossa almalensis differs from it in having a less elongated oval, rather than rectangular shell outline, well defined and slightly raised ventral umbonal muscle scars and a low rim bounding anteriorly a ventral visceral area. It is likely that a simple shell morphology that characterises Tunisiglossa is more widely spread among the Ordovician obolides, and some poorly known taxa usually referred to Ectenoglossa , Lingulella , or Pseudolingula may belong to Tunisiglossa .

Tunisiglossa almalensis has some external similarity to Ectenoglossa sorbulakensis Popov, 1980 (see also Popov et al. 2002), that is widespread in the lower to middle Caradocian Anderken Formation of Chu−Ili Range in Southern Kazakhstan, however, the later species has more elongated shell and a fine dorsal median ridge.

Figures

Fig. 3. The lingulid brachiopod Tunisiglossa almalensis Popov and Mambetov, sp. nov. from the Almaly Formation, Caradocian (Upper Ordovician) of Kyrgyz Range. A. NMW 98.66G.869, ventral internal mould. B. NMW 98.66G.871, ventral valve, exterior. C. NMW 98.66G.865, dorsal valve exterior. D. NMW 98.66G.866, dorsal valve, exterior. E. NMW 98.66G.860–862, cluster of three disarticulated valves on a bedding surface. F. NMW 98.66G.872, dorsal valve, interior, latex cast. G. NMW 98.66G.870, holotype, dorsal valve, interior, latex cast showing position of muscle scars (tm., transmedian; o.l., outside latreral; a.l., middle lateral; c.m., central; anterior lateral) and proximal parts of dorsal vascular media (v.m.). H. NMW 98.66G.873, ventral valve, exterior, latex cast (H1) and ventral internal mould showing umbonal muscle scars (u.m.) (H2). I. NMW 98.66G.867, ventral valve, interiors showing position of pedicle nerve impression (p.n.) and NMW 98.66G.868, dorsal valve interior. Scale bars 2.5 mm.

Fig. 4. The lingulid brachiopod Tunisiglossa almalensis Popov and Mambetov, sp. nov. from the Almaly Formation, Caradocian (Upper Ordovician) of Kyrgystan. Schematic representation of ventral (A) and dorsal (B) valve interior showing position of muscle scars, mantle canals and a pedicle nerve impression.

Fig. 1. Geological map of the western part of Kyrgyz Range (after Farid Kh. Apayarov and Matvei M. Gutermakher, unpublished) showing position of the measured sections and fossil localities. This mountainous area is a central part of the North Tien Shan mountain system, which is a series of parallel south−southwest to east−northeast ranges stretching for about 2500 km from Turkestan in the west to northern China in the east. Black arrows indicate direction of water flow in tributaries and rivers; 509 and 510 are fossil localities.