Borealis samojedicus ( Keyserling, 1846 )

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 83-85

Borealis samojedicus ( Keyserling, 1846) ( Fig. 12)

Pentamerus samojedicus Keyserling, 1846: 235 , pl. 9. — Lebedev 1892: 20.

Pentamerus oblongus Sowerby in Murchison, 1839 – Sapel’nikov & Beznosova 1980: 10, pl. V, figs 1-5. — Markovskij & Smirnova 1982: 40, 41. — Sapel’nikov 1985: 59, pl. XIX, figs 3, 4.

Borealis samojedicus – Modzalevskaya & Rubel 2000: 149, pl. I, figs 1-9, pl. II, figs 1-7.

MATERIAL EXAMINED. — Three specimens from loc. 18002-6, two specimens from loc. 18088-6, three specimens from loc. 18008-11, several specimens embedded in rock from loc. 18075-8, Komsomolets Island ( Männik et al. 2002: fig. 1) ; Vodopad ( lower Snezhinka ) Formation .

REMARKS

Large smooth shells (L = 72.4 mm, W = 49.5 mm, T = 35.0 mm) have inconspicuous radial furrows in the anterior part, and frequently developed concentric growth lines. The outline of the shell varies from rounded to subtriangular. Development of the median fold is variable, especially on the ventral valve. The specimens from coquinas, with high population frequency, have more laterally flattened shells and narrow spondylium. B. samojedicus has the same interior as B. borealis (Eichwald, 1842) ( Rubel 1970) . The narrow longitudinal shells, with distinctly developed ventral sulcus and strongly posteriorly

A F

B

G D A-E F, G C E

widened septum, are very similar to B. schmidti A

( Lebedev, 1892) from the Llandovery of

Nizhnyaya Tunguska (East Siberia), and could be assigned to this species ( Lebedev 1892: 22).

However, these specimens were found together with some subtriangular shells which possess a flattened dorsal valve, undulating surface, and sharp concentric lines. These features are typical of B. samojedicus ( Keyserling 1846: 235, pl. IX,

fig. 2) from the Chernaya Rechka Formation

(Llandovery) of the Timan Ridge. But the clearly developed lobate external shape, and the narrow-

ing of the shell anteriorly and posteriorly, make them similar to the Norwegian species B. borealis osloensis MØrk, 1981 , which is characteristic of bed 7a/b (Llandovery) in the Oslo region ( MØrk

1981: 544, pl. 84, figs 9-11).