Sibiritoechia convexa Alekseeva, 1966

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 88-89

Sibiritoechia convexa Alekseeva, 1966 ( Figs 17 ; 18 )

Sibiritoechia convexa Alekseeva, 1966: 1148 , fig. 1; 1967: 60, pl. VIII, fig. 7. — Alekseeva et al. 1970: 89, pl. IX, figs 6-8.

Sibiritoechia lata Alekseeva, 1966: 1149 , figs 1, 2; 1967: 62, pl. VIII, fig. 8. — Alekseeva et al. 1996: 108, pl. XII, fig. 5.

Sibiritoechia oblonga Alekseeva, 1966: 1149 , figs 1, 3; 1967: 63, pl. IX, figs 8-11. —? Alekseeva et al. 1970: 87, pl. IX, figs 4, 5.

MATERIAL EXAMINED. — Specimens are embedded in shelly limestone from which 50 complete shells of different sizes have been prepared. Southeastern Komsomolets Island, southern bank of Krasnaya Bay (loc. 21581) ( Männik et al. 2002: fig. 1); Severnaya Zemlya or Pod”emnaya formations.

A B C

REMARKS

There is a wide variation in the outline (from longitudinally oval to isometrical), and in convexity (equally biconvex or ventribiconvex) of the shell. The ribs arise at different distances from the beak, and their number varies from 9 to 11. One rib on the ventral sinus and two on the dorsal sulcus are constant but they are developed differently depending upon the outline and convexity of the shells. The internal structure is characterized by thin, short dental plates, divided cardinal plates, and obliquely inclined crura ( Fig. 18 ). The valve outline, and the other external features of these brachiopods of the same population, do not permit detailed systematic comparison. They can be assigned to species “ convexa ”, “ lata ”, and “ oblonga ”, studied by Alekseeva (1966: 147). V. V. Baranov concluded that “ lata ” and “ oblonga ” are, in reality, the same taxon ( Alekseeva et al. 1970). In fact, it is difficult to find distinctions between “ convexa ” and “ lata ”, except in the convexity of the ventral valve. Since these more convex valves are often seen in only a few members of a population, “ convexa ” is not examined here separately. S ibiritoechia is known from the Lochkovian and Pragian of Salair (Tom’chumysh Beds), and North East Russia (Sette-Daban Ridge, Sette- Daban Formation; Selennjakh Ridge, Sagyr Formation; Tas-Khayakhtakh Ridge, Datnin Formation).

Order ATRYPIDA Rzhonsnitskaya, 1960

Figures

FIG. 17. — Sibiritoechia convexa Alekseeva, 1966, complete shells (NN 44/12991, 45/12991), loc. 21581, Severnaya Zemlya or Pod’’emnaya formations; A, E, ventral view; B, F, dorsal view; C, G, lateral view; D, H, anterior view. Scale bar: 5 mm.

FIG. 18. — Selected transverse serial sections, illustrating the development of internal structures in Sibiritoechia convexa Alekseeva, 1966 (N 46/12991). Measurements: length 6.4 mm, width 6.6 mm, thickness 4.1 mm; loc. 21581, Severnaya or Pod’’emnaya formations. Numerical designations are the same as on Fig. 11. Scale bar: 1.9 mm.