Trondorthis, Neuman, 1974

Hansen, Jesper & Holmer, Lars E., 2011, Taxonomy and biostratigraphy of Ordovician brachiopods from northeastern Ny Friesland, Spitsbergen 3076, Zootaxa 3076 (1), pp. 1-122 : 92

Trondorthis ? sp.

Pl. 24, Figs. 4 –7

Material. Four ventral valves from samples F4743/F4750 and F 4855 in the collections at the Natural History Museum of London. The figured specimens are F4743/F4750a–c.

Description of Spitsbergen Material. Moderately convex and subcircular or subangular with rectangular or slightly acute cardinal extremities. Largest measured specimen more than 14.5 mm long and 16.9 mm wide. Anterior commissure rectimarginate to weakly unisulcate. Ramicostellate tending toward fascicostellate, with 3–4 subangular costellae per mm at 1-mm growth stage. About 20 costae on umbo. Midline of ventral valve defined by a costella. Costellae impressed on distal 1/3 part of valve floor.

Ventral interarea rather long, apsacline to nearly orthocline, moderately to strongly concave. Open delthyrium moderately wide (~50˚), V-shaped. Teeth triangular, developing strong crural fossettes in larger specimens. Teeth supported by recessive, slightly converging dental plates. Pedicle callist apically located in delthyrial cavity. Muscle field broad and often slightly raised above valve floor. Diductor scars subcordate, slightly diverging, separated by a median field of about double width. Adductor scars weakly impressed, slightly longer than diductor scars, reaching about 30% of valve length. Ventral mantle canal system saccate. Vascula media diverging increasingly on valve floor from anterior points of diductor scars.

Remarks. No dorsal valves are available, why a precise taxonomic affiliation is prohibited. But for being slightly deeper, the ventral valves of the Svalbard material show strong resemblance to the type species Trondorthis bifurcatus .

Occurrence. 80 m above base of Olenidsletta Member and 70 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.