Pseudogruenewaldtia tschernyschewi Rzhonsnitskaya 1960

Pseudogruenewaldtia tschernyschewi Rzhonsnitskaya, 1960

(Figs 5; 6)

Pseudogruenewaldtia tschernyschewi Rzhonsnitskaya, 1960: 46, pl. 1: 7, pl. 2: 4; 1964: 107, pl. 2: 6, pl. 5: 4. — Rzhonsnitskaya et al. 1998: 328, figs 19, 20.

Atrypa zonata – Dames 1868: 497, pl. 11a-c.

Grunnewaldtia latilinguis – Ljaschenko 1959: 175, pl. 51: 5-7 [lapsus calami pro Gruenewaldtia].

Atrypa ex gr. reticularis – Grocholski 1969: 246, pl. 5: 2 (e.p.).

MATERIAL EXAMINED. — 21 articulated shells from Mokrzeszów, MB.B.297.1-10, 2419.1-2, 2420.1-2; MGUWr 5358s.1- 6, 5360s.

DESCRIPTION

Shell up to 49.4 mm wide (mean value in the sample 29.8 mm), more often wider than long (width to length ratio from 0.88 to 1.44; mean value 1.07, N=10), rounded in outline, dorsibiconvex in young specimens, ventribiconvex (e.g., Fig. 5 Q-U) to dorsibiconvex (e.g., Fig. 5 AA-EE) in adults. Maximal width somewhat anteriorly to midlength (½ to ³⁄₅ of the shell length), maximal thickness posteriorly to midlength (⅓ to ½ of the shell length). Anterior commissure uniplicate, its deflexion rounded in outline, relatively low, occupying ⅓ to ³⁄₅ of the shell width. Ventral interarea concave, apsacline in young specimens; beak (foramen not observed) strongly incurved in older specimens, the interarea becoming thus anacline. Dorsal interarea not visible. A shallow flat-bottomed ventral sulcus is present in the largest specimens, its width at anterior commissure being 1⁄₆ of that of the shell. Ribs undulose, fine, 8-10 per 5 mm at anterior commissure, new ones appearing (especially in posterior part) by bifurcation, seldom by intercalation. Growth lamellae (preserved only exceptionally) imbricate. Interior not studied.

DISCUSSION

The agreement of the described brachiopods from Mokrzeszów in external form (general shape and presence of a similar ventral furrow in the largest specimens), character and density of ornamenta- tion with the type material of Pseudogruenewaldtia tschernyschewi from Timan (northern Urals, Russia) make strong case for the conspecificity of both samples even if interior of the described material could not be studied because of scarcity of the collection. Pseudogruenewaldtia elongata Alekseeva, 1996 from north-eastern Siberia, has much more elongate shells.

For discussion of differences with Kyrtatrypa barnimi n. sp. co-occurring in the same outcrop, see above. The stratigraphic implications of the presence of Pseudogruenewaldtia tschernyschewi in the studied material are discussed below.

Reworked representatives of Pseudogruenewaldtia occur in Famennian volcanoclastics (Langenaubacher Tuffbrekzie) in the Dill Syncline (Rhenish Massif, Hessia, Germany) (Copper, pers. comm. 2011).

The peculiar palaeogeographic distribution of Pseudogruenewaldtia tschernyschewi (Timan, Bohemia, possibly Rhenish Massif; absent either in the Central Devonian Field or in the Holy Cross Mountains) would have been explained by a migration route along the southern margin of Fennoscandia through the Latvian Isthmus had it not been for brackish conditions in these regions in the Frasnian (Lebedev et al. 2010). A straighter dispersion route is to be invoked and the absence of the discussed species in the abovementioned regions is more probably related to the patchiness of the record.