Borealis cristiformis (T. Modzalevskaya, 1985 )

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 82-83

Borealis cristiformis (T. Modzalevskaya, 1985) ( Figs 10 ; 11 )

Pentamerus ? cristiformis T. Modzalevskaya, 1985: 64 , pl. II, figs 4, 5, pl. III, fig. 1.

MATERIAL EXAMINED. — Two specimens from loc. 14, bed 10v, Matusevich River , and six specimens from loc. 51, sample 51-13f, Strojnaya River, October Revolution Island ( Männik et al. 2002: figs 2, 5, 6) ; Vodopad Formation.

REMARKS

These are the oldest pentamerids found in the Circum-Arctic region, and were described from the lowermost Persej Formation of Novaya Zemlya for the first time. The diagnostic features (the medium size, the ventral acute ridge – like elevation –, and the subtriangular outline) occurring on the shells of pentamerids on Severnaya Zemlya indicate that they belong to B. cristiformis . The long spondylium (supported by a short septum); separated low brachial plates slightly inclined to each other by their internal ends, and sharply curved inner brachial plates, allow to assigne this species to genus Borealis ( Fig. 11 ).

Figures

FIG. 10. — Borealis cristiformis (T. Modzalevskaya, 1985); A, dorsal view of a brachial valve (N 28/12991); B, internal longitudinal ventral septum (N 30/12991); C, D, complete slightly damaged shell (N 29/12991), loc. 14, bed 10b, Vodopad Formation; C, dorsal view; D, lateral view. Scale bars: A, B, 6 mm; C, D, 10 mm.

FIG. 11. — Selected transverse serial sections, illustrating the development of internal structures in Borealis cristiformis (T. Modzalevskaya, 1985) (N 31/12991). Measurements: length 29.1 mm, width 24.0 mm, thickness 22.2 mm; loc. 14, bed 10v, Vodopad Formation. Numbers on selected transverse sections indicate the distances (in mm) from ventral beak. Scale bar: 8 mm.