Undispirifer, Havlicek, 1957

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 95-96

Undispirifer ? obsoletiplicatus n. sp. ( Figs 26 ; 27 )

Non Howellella pauciplicata Waite, 1956: 17 , pl. XII, fig. 1.

Howellella pauciplicata – Alekseeva 1967: 78, pl. XI, figs 7-12, pl. XII, fig. 1.

HOLOTYPE. — N 68/12991, CNIGR Museum , St. Petersburg ( Fig. 26B ), ventral valve; L = 14.2 mm, W = 16.7 mm.

ETYMOLOGY. — From obsoletiplicatus = obsoletus (Latin): decayed, and plicatus: folded.

TYPE LOCALITY. — Loc. 21581, southern bank of Krasnaya Bay, Komsomolets Island.

FORMATION. — Spokojnaya or Pod”emnaya formations.

AGE. — Lower Devonian.

MATERIAL EXAMINED. — 10 disarticulated shells and their external moulds from loc. 21581, Komsomolets Island ( Männik et al. 2002: fig. 1); Spokojnaya and/or Pod”emnaya formations.

MEASUREMENTS (IN MM). — Ventral valve: L = 14.6, W = 18.0-20.0?; dorsal valve: L = 13.7-14.7, W = 16.5-19.2.

DIAGNOSIS. — Shells moderately large, pentagonal in outline with inconspicuous plications; ventral sinus weakly developed, dorsal fold sharply limited anteriorly.

DESCRIPTION

Moderate size, biconvex shells of pentagonal outline with inconspicuous plications, gentle sinus and fold. Isometrical ventral valve with suberect beak. Maximum width of valve is located close to its midlength. Interarea trigonal, weakly concave, apsacline, low. Delthyrium open with narrow deltidial plates. The sinus on ventral valve is shallow. It originates at umbo and widens gently towards the anterior margin of the valve. Here, the margins of the sinus are well defined and it forms a deep tongue. The width of the dorsal valve is greater than its length, and it bears a distinctly developed fold which widens towards the anterior margin Here, a median furrow is sometimes observed. Surface is smooth with hardly visible plicae on the lateral flanks anteriorly. Microornament is not visible.

In the ventral interior, short dental plates are thickened by callus, which fills the beak. Muscle field is longitudinally oval, sharply outlined, well impressed. Adductors are indistinctly limited, diductors are radially striated. There are ovarian impressions lateral to the muscle field. In the dorsal interior crural plates do not reach the valve walls. Jugum and spiralia are not observed ( Fig. 27 ).

REMARKS

The species of Undispirifer are in need of revision. As was noted by Johnson (1975), this genus includes Lower Devonian spiriferids (among which there are many poorly plicated forms) with morphologies intermediate between Howellella and Undispirifer . As a result, laeviplicata resembles both genera ( Kozlowski 1929: 195; Smith 1980: 180; Lenz & Johnson 1985: 92). The specimens from Severnaya Zemlya are assigned here to Undispirifer on the basis of clearly developed ventral muscle field and the callus-filled beak. The last feature is missing on Howellella . The ventral valve of U.? obsoletiplicatus n. sp. differs from that of U. laeviplicatus ( Kozlowski, 1929) (Lochkovian of Prince of Wales Island, Canadian Arctic) by its smooth lateral plicae and weakly developed sinus ( Smith 1980: 71, pl. 33, figs 6- 36). The similarity to Howellella pauciplicata , found from the Settedaban Formation ( Alekseeva 1967: 78, pl. XI, figs 7-12, pl. XII, fig. 1), suggests that the Severnaya Zemlya and North East specimens are conspecific. This species differs from Howellella prima Alekseeva, 1967 in its sharply limited, anteriorly widening dorsal fold, high, rounded dorsal tongue, and weak outlines of the dorsal muscle field ( Alekseeva 1967: 82, pl. XII, figs 2-11).

Figures

FIG. 26. — Undispirifer? obsoletiplicatus n. sp.; A-C, ventral view of different sizes pedicle valves; A, N 69/12991; B, holotype (N 68/12991); C, N 70/12991; D, E, dorsal view of a brachial valve (NN 71/12991, 72/12991); F, posterior view of a pedicle valve with interarea and open delthyrium (N 73/12991); G, H, external moulds of pedicle valves, showing a well developed muscle field and gonad impressions (NN 74/12991, 75/12991), loc. 215581, Severnaya Zemlya or Pod’’emnaya formations. Scale bars: A-E, 6 mm; F-H, 5 mm.

FIG. 27. — Selected transverse serial sections, illustrating the development of internal structures in Undispirifer? obsoletiplicatus n. sp.; A, ventral valve (N 76/12991), length 17.3 mm, width 12.0? mm; B, dorsal valve (N 77/12991), length 14.7 mm, width 19.9 mm; loc. 21581, Severnaya Zemlya or Pod’’emnaya formations. Numerical designations are the same as on Fig. 11. Scale bar: 2.3 mm.