Heterorthina, Bancroft, 1928

Hints, Linda & Harper, David A. T., 2015, The Hirnantian (Late Ordovician) brachiopod fauna of the East Baltic: Taxonomy of the key species, Acta Palaeontologica Polonica 60 (2), pp. 395-420 : 411

Heterorthina ? sp.

Fig. 12 .

Material.— One specimen, the single shell DML G 328-75 from the Kuldiga Formation , Hirnantian (Upper Ordovician); western Latvia. The Blīdene-5 drill core (depth 819.3 m) .

Remarks.—Specimen has, on the basis of outline, convexity of valves, its rectimarginate commissure and occurrence of aditicules similarities to the rhipidomellinae genera Mendacella and Dalejina . The external ornament with a median interspace on the dorsal valve, however, is similar to the ribbing pattern on Heterorthina . It differs from the first two genera ( Jin and Zhan 2008; Hints 1975, 2012; Rubel 2011) in having the interspace between the ribs on dorsal valve, a characteristic of Heterorthina . The relatively short hingeline and lack of a fold and sulcus, however, introduces some uncertainty with that assignment Heterorthina .

This species is medium-sized, transversely oval, moderately biconvex, 13.2 mm wide at mid valve length, 11.7 mm long, and 5.3 mm thick. Hinge line forms about 60% of the shell width.

Ventral valve convex, maximum convexity in posterior half. Interarea, low apsacline. Dorsal valve weakly sulcate in posterior part, anteriorly evenly convex, anterior commissure essentially rectimarginate. Dorsal interarea low (0.7 mm) anacline, notothyrium open, partly filled by bi(?)-lobate myophore.

Radial ornament fascicostellate with median costa on ventral valve and corresponding interspace on dorsal valve; posterior costellae curve towards interarea; ribs are triangular in cross-section. Along the commissure 65 costae and costellae; of which 16 primary, at 5 mm from umbo, 6 ribs per 2 mm. Growth lines strong on the anterior half. Aditicules on the rib crests are developed close to the growth lines. The shell external surface is covered by a dense, minute concentric ornament best visible in interspaces. The endopunctae are not visible.

Figures

Fig. 12. Heterorthid brachiopod Heterorthina? sp. from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia. Shell, LDM 328-75, Blīdene-5, depth 819.3 m, ventral (A), anterior (B), posterior (C), dorsal (D), and lateral (F) views, and detail of external ornament (E) of the ventral valve showing the growth lines and aditicules. Scale bars 2 mm.