Hesperonomia sp.

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 94

Hesperonomia sp.

Pl. 24, Fig. 13; Pl. 29, Figs. 10–11; Table 31

Material. Six dorsal and three ventral valves in samples F4600 and F 4610 in the collections at the Natural History Museum of London. Three ventral valves and two fragments from samples JH-64 and JH-71. The figured specimens are F4600, F4610 and TSGF 17085.

Description. Shell slightly planoconvex or ventribiconvex and semicircular to subangular, with rectangular or slightly pointed cardinal extremities. L/W ratio 0.58–0.63. Maximum width located at hinge axis. Largest measurable valve 9.8 mm long and about 15.5 mm wide. Commissure rectimarginate or slightly unisulcate. Ornamentation subequally to unequally parvicostellate and with filae; 8–9 low, subangular costellae per mm at 1-mm growth stage and 14 costellae per 2.5 mm at 2.5-mm growth state. Costellae impressed at least on distal part of valve floor.

Dorsal sulcus weak or absent. Valve deepest at 17% of valve length. Dorsal interarea slightly anacline. Ventral interarea concave, apsacline, 22% as long as valve.

Remarks. The poor preservation of the available specimens prohibits an exhaustive comparison with the known species of this genus. However, it is distinguished from most species by its very low L/W ratio. Among the species with low L/W ratios, Hesperonomia crassa Ulrich & Cooper, 1936 has rather strong costellae, a distinct sulcus, and an L/W-ratio between 0.70–0.87. H. dinorthoides Ulrich & Cooper, 1938 is concavoconvex and distinctly subangular. H. louisensis Ulrich & Cooper, 1936 has an L/W ratio of 0.67–0.79 and a stronger dorsal sulcus. H. nemea ( Hall & Whitfield, 1877) has a concave dorsal valve but is otherwise too poorly known for comparison. The specimens from Spitsbergen appear most similar to the Floian H. plana Ulrich & Cooper, 1938 based on their external features. However, according to the description of Ulrich & Cooper (1938), H. plana has a higher L/W ratio (0.67–0.74) and a shorter ventral interarea. H. subtransversa Ulrich & Cooper, 1938 has more prominent costellae and a distinctly concave dorsal valve. The Darriwilian H.? uzunbulakensis Nikitina et al., 2006 from Kazakhstan has an L/W ratio of 0.60–0.96 and a strongly arched ventral valve. The Middle Ordovician Hesperonomia sp. ( Williams 1974) from Shropshire in England is much more unequally costellate, has a concave dorsal valve, and has about 7 costellae per mm at 1-mm valve length.

Occurrence. Lower and middle parts of Nordporten Member, Kirtonryggen Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.