Cyrtospirifer mylaensis, Sokiran, 2006

Sokiran, Elena V., 2006, Early-Middle Frasnian cyrtospiriferid brachiopods from the East European Platform, Acta Palaeontologica Polonica 51 (4), pp. 759-772 : 765-766

Cyrtospirifer mylaensis sp. nov.

Figs. 8 , 9 .

Holotype: CNIGR 8 /13157, complete shell; Fig. 8A .

Type locality: Myla river, Middle Timan.

Type horizon: Kraipol Formation.

Derivation of the name: from Myla river, Middle Timan.

Diagnosis.—Medium sized subtrapezoidal in outline Cyrtospirifer with extended cardinal extremities, moderately high apsacline ventral interarea and well defined ventral sulcus. Differs from close C. schelonicus in having weakly marked to almost absent dorsal median fold sometimes with shallow medial depression.

Material.—More than 100 well preserved specimens collected by Feodosij N. Chernyshev (Myla and Pizhma Pechorskaya rivers, Middle Timan); in addition six well preserved complete and six exfoliated shells recently collected from Pizhma Pechorskaya river.

Description.—Shell ventribiconvex, medium sized (up to 35 mm wide) transverse subtrapezoidal in outline, with maximum width at hinge line; cardinal extremities acute and alate; anterior commissure uniplicate. Ventral valve moderately to strongly convex with a small slightly curved beak; ventral interarea high, up to 8.4 mm in adult specimens, strongly apsacline; delthyrium triangular attaining 4.9–7.9 mm in width. Sulcus well defined, originates at the umbo, terminates with a low and rounded tongue. Dorsal valve moderately convex, slightly shallower than ventral valve. Dorsal median fold weakly developed to almost absent, sometimes with a shallow medial depression. Radial ornament with up to 24 simple low ribs separated by narrow interspaces on each flank. Ventral sulcus and dorsal median fold with up to 13 bifurcating ribs along the anterior margin. Microornament of fine radial capillae and fine concentric filae.

Ventral interior with a delthyrial plate, simple strong teeth, and long, divergent extrasinal dental plates. Dorsal interior with a ctenophoridium; a low median ridge present.

Remarks.—The main diagnostic future of Cyrtospirifer mylaensis is the absence or weak development of dorsal fold with shallow medial depression. However, the low dorsal fold is also characteristic for some specimens of C. “ disjunctus ” from Russia (sensu Verneuil 1845) and C. tenticulum ( Verneuil, 1845) . C. mylaensis differs from C. “ disjunctus ” in having more trapezoidal shell outline and higher, triangular ventral interarea which in the latter has its lateral edges nearly parallel to the hinge line. From C. tenticulum the new species differs by its larger shell and lower, not pyramidal ventral interarea. The median depression or longitudinal groove on the dorsal fold occurs also in C. bisellatus ( Gürich, 1903) and C. bisinus ( Le Hon, 1870) , but this is more distinct than that in C. mylaensis . In addition C. mylaensis differs from C. bisinus by having higher, more triangular ventral interarea. From C. bisellatus the new species differs in its more trapezoidal, not pentagonal shell outline and in having greatest shell width at its hinge line whereas the former is widest usually at the shell midlength.

Occurrence.—Middle Frasnian, Palmatolepis punctata – Pa. jamieae zones, Kraipol Formation, Middle Timan.

Figures

Fig. 8. Cyrtospirifer mylaensis sp. nov., Middle Frasnian, Kraipol Formation, Middle Timan. A. Holotype, CNIGR 8/13157, ventral (A1), dorsal (A2), posterior (A3), anterior (A4), and lateral (A5) views. B. CNIGR 9/13157, ventral (B1), dorsal (B2), posterior (B3), anterior (B4), and lateral (B5) views.

Fig. 9. Transverse serial sections of complete shell of Cyrtospirifer mylaensis sp. nov., from Middle Frasnian, Kraipol Formation, Middle Timan. CNIGR 10/13157. Distance in mm is measured from the posterior tip of ventral beak.