Oglu senilis Havlíček, 1987

Baliński, Andrzej, 2012, The brachiopod succession through the Silurian-Devonian boundary beds at Dnistrove, Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 897-924 : 914-915

Oglu senilis Havlíček, 1987

Figs. 13 , 14H–J, M.

1987 Oglu senilis sp. n.; Havlíček 1987: 83–84, pls. 2: 7, 4: 1.

Material.—51 complete and 37 damaged shells. The species is quite common in beds 47–48 (5.5 m above the S–D boundary); a single specimen was also collected at 1 m above the boundary (bed 2), and another was found in a loose block from the uppermost part of the section (lower part of the Khudykivtsi Beds).

Description.—Shell rather small, usually up to 15 mm in length, slightly elongate to transverse, circular to shield−shape in outline, dorsibiconvex; hinge line short, nearly straight to slightly angular; maximum width posterior to midlength; anterior commissure rectimarginate to very weakly and broadly arched distally.

Ventral valve convex posteriorly, laterally and anteriorly planar to weakly concave; beak minute, incurved, interarea poorly defined, low, anacline; sulcus absent or very weak, almost imperceptible. Dorsal valve convex to strongly convex, greatest convexity slightly posterior to midvalve, fold not developed.

Ribs relatively coarse, undulose, especially posteriorly; 8 and 7 ribs near hinge margin on ventral and dorsal valves, respectively; ribs increasing usually by intercalation on dorsal and bifurcation on ventral valve; at anterior margin the total number of ribs ranges 17–20, exceptionally up to 25; ventral midrib pair slightly raised; growth lamellae undulating, extending into short trails and spines, the latter developed on both valves reaching up to 3.5 mm in length.

Interior of ventral valve with recognisable dental nuclei, thick pedicle callist developed into a collar in one of the sectioned shells ( Fig. 13B ); teeth stubby, subdorsally directed, with well−developed lateral lobes. Dorsal valve with distinct subtriangular in section median ridge; hinge plate umbonally lined with comb−like layer.

Remarks.—This rather small−sized atrypid is characterised by its shield shaped, dorsibiconvex shell ornamented by thick ribs, weakly undulating growth lamellae, and numerous short to quite long spines on both valves ( Fig. 14M). The studied specimens externally resemble Spinatrypa Stainbrook, 1951 , in shell shape, convexity, and spinose ornamentation, but they differ in their less undulose ribs, absence of dental plates, and by their occurrence much below the known stratigraphic range of the latter genus. Externally, the Podolian specimens also resemble Isospinatrypa Struve, 1966 , in many respects, differing by having an evident dorsibiconvex shell instead of a biconvex to weakly dorsibiconvex profile as in the latter genus. They are also very similar externally and internally to inadequately known genus Oglu Havlíček, 1987 (see remarks in Copper 2002: 1412). It seems that Podolian species is conspecific with O. senilis Havlíček, 1987 , which was described from a corresponding time interval (lowermost part of the Lochkov Formation) in central Bohemia ( Havlíček 1987). The differences between them appear rather minor and are expressed in the presence of spinosity on both valves and slightly larger shell of the specimens from Dnistrove, which are up to 16 mm in width (in comparison to 12.5 mm in the Bohemian species; Havlíček 1987: 83). According to Havlíček (1987) the spinose ornament in Oglu occurs on dorsal valves only, but it seems probable that the absence of spines on the ventral valves may be related to the preservation. Because these differences seem rather minor, the specimens here described are regarded as conspecific with the Bohemian Oglu senilis Havlíček, 1987 .

Stratigraphic and geographic range.—This species was not described from Podolia either by Kozłowski (1929) nor Nikiforova (1954). Originally the species was described by Havlíček (1987) from the lowermost part of the Lochkov Formation (Kotýs Limestone, Lochkov) in central Bohemia.

Figures

Fig. 13. Transverse serial sections through shell of Oglu senilis Havlíček, 1987. A. ZPAL Bp. 56/88. B. ZPAL Bp. 56/89. Numbers refer to distance in mm from ventral umbo. Dnistrove, beds 47–48.