Cyrtospirifer sp. A

Sokiran, Elena V., 2006, Early-Middle Frasnian cyrtospiriferid brachiopods from the East European Platform, Acta Palaeontologica Polonica 51 (4), pp. 759-772 : 769

Cyrtospirifer sp. A

Fig. 10F .

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Material.—Two complete shells and ten incomplete single valves collected from Schelon' river basin.

Description.—Shell small sized (up to 20 mm in width) with maximum width at hinge line; cardinal extremities acute and short; anterior commissure uniplicate. Pyramidal ventral valve weakly convex with a small slightly curved beak; ventral interarea catacline to apsacline. Sulcus shallow originates at the umbo. Dorsal valve moderately convex with weakly developed fold. Radial ornament with up to 18 simple low ribs separated by narrow interspaces on each flank. Ventral sulcus and dorsal fold with up to 9 bifurcating ribs along the anterior margin.

Ventral interior with a delthyrial plate and divergent extrasinal dental plates. Dorsal interior with a ctenophoridium.

Remarks.—This is small−sized Cyrtospirifer with pyramidal ventral valve similar to Cyrtospirifer tenticulum . It differs from the latter by more rounded lateral margins and less convex ventral valve.

Cyrtospirifer sp. A is similar to Tenticospirifer lagovensis ( Gürich, 1896) from the Late Givetian of the Holy Cross Mountains, Poland, but differs in having thinner ribs, narrower and less sharply bounded sulcus and fold. C. sp. A is somewhat suggestive of the Late Frasnian North American Cyrtospirifer pyramidalis Cooper and Dutro, 1982 but it differs in generally weakly convex ventral valve and finer ribs. Sorokin (1978) listed this form from Porkhov Beds of the Schelon river basin as Cyrtospirifer schelonicus var. tenticula .

Occurrence.—Middle Frasnian, Palmatolepis punctata Zone, Porkhov Beds, Main Devonian Field.

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Figures

Fig. 10. Cyrtospiriferids from Middle Frasnian of the Main and Central Devonian Fields. A–C. Cyrtospirifer tenticulum (Verneuil, 1845). A. Buregi Beds, Main Devonian Field. CNIGR 93/6993, neotype, ventral (A1), dorsal (A2), posterior (A3), and lateral (A4) views. B, C Semiluki Horizon, Central Devonian Field. B. CNIGR 3535/2858, ventral (B1), dorsal (B2), posterior (B3), anterior (B4), and lateral (B5) views, and shell surface showing microornament (B6). C. CNIGR 3534/2858, ventral (C1), dorsal (C2), posterior (C3), anterior (C4), and lateral (C5) views. D, E. Cyrtospirifer lictor Nalivkin, 1930, Semiluki Horizon, Don river, Central Devonian Field. D. CNIGR 3530/2858, ventral (D1), dorsal (D2), posterior (D3), anterior (D4), and lateral (D5) views. E. CNIGR 3531/2858, ventral (E1), posterior (E2), anterior (E3), and lateral (E4) views. F. Cyrtospirifer sp. A, Porkhov Beds, Main Devonian Field. CNIGR 11/13157, ventral (F1), posterior (F2), anterior (F3), and lateral (F4) views. G, H. Cyrtospirifer schelonicus (Nalivkin, 1941), Svinord Beds, Main Devonian Field. G. CNIGR 88/6993, ventral (G1), dorsal (G2), posterior (G3), and lateral (G4) views. H. CNIGR 87/6993, ventral (H1), dorsal (H2), posterior (H3), and lateral (H4) views. I. Cyrtospirifer tschudovi Nalivkin, 1941, Chudovo Beds, Main Devonian Field, CNIGR 1/6993, ventral (I1) and posterior (I2) views.