Nothorthis subpyramidalis, Hansen & Holmer, 2011

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 96-97

Nothorthis subpyramidalis sp. nov.

Pl. 25, Figs. 7–14; Table 33

Derivation of name. Latin ‘ sub ’ and ‘ pyramidis ’, nearly pyramidal; refers to the shape of the ventral valve.

Holotype. Pl. 25, Fig. 9; TSGF16888 , ventral valve; 67 m above base of Profilbekken Member, Valhallfonna Formation, sample JH-95; Profilbekken meltwater stream, Basissletta, Ny Friesland, Spitsbergen.

Material. One whole specimen, 14 dorsal and 13 ventral valves, and nine fragments from the samples containing the trilobite A84285 and from samples F4679, F4721, F4782, JH-83, JH-95 and JH-161. The paratypes are TSGF16886 , TSGF16887 , TSGF16889 and TSGF16890 .

Diagnosis. Nothorthis with subpyramidal ventral valve and maximum width at cardinal extremities; steeply apsacline to catacline ventral interarea; rudimentary notothyrial platform highly raised above median ridge and level with brachiophores.

Description. Shell slightly ventribiconvex to planoconvex and transversely semi-oval to subangular or slightly subtriangular. Maximum width located at hinge axis. L/W ratio 0.59–0.77. Largest specimen 8.1 mm long and about 12.5 mm wide. Cardinal extremities short and generally changing from acute (about 70°) in early growth stages to obtuse in largest specimens. Anterior commissure moderately to strongly unisulcate. Ornamentation ramicostellate to strongly fascicostellate with capillae present distally; 4–7 high, angular costellae per mm at 1-mm valve length. Weak, fine filae present with broad interspaces. Costellae impressed on distal 20–27% of valve floor.

Dorsal valve gently to moderately convex, with small umbo and angular sulcus. Sulcus becoming wider and more rounded distally. Interarea nearly catacline, planar and 8–9% as long as valve. Brachiophores strongly divergent, short and rod-like. Fulcral plates present. Rudimentary notothyrial platform with posteriorly pointed, short and simple cardinal process. Notothyrial platform highly raised above median ridge and level with brachiophores. Median ridge merely an impression of sulcus.

Ventral valve subpyramidal with low but distinct crest and small umbo. Apex located at umbo. Interarea steeply apsacline or catacline and planar or slightly concave, its height equal to 25–42% of valve length. Delthyrium V-shaped, open, widening at about 40°. Teeth supported by recessive dental plates. Muscle field generally slightly raised above valve floor, extending to 31–36% of valve length, with prominent pedicle callist. Vascula media moderately divergent, with bases at anterolateral corners of muscle field.

Remarks. Nothorthis subpyramidalis sp. nov. is distinguished from most known species in that the maximum width is located at the hinge axis, the ventral valve is subpyramidal with its apex at the umbo, and the ventral interarea is steeply apsacline to catacline. This species most closely resembles the type species, N. delicatula Ulrich & Cooper, 1938 ; N. fascicostata Xu & Liu, 1984 ; and N. delicatula and N. fascicostata are distinguished by the notothyrial platform, which is not level with the brachiophores in the latter species, and by the rectangular to obtuse cardinal extremities of the latter species.

Occurrence. 67–70, 80, 83, 85 and 98 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.