Brachyprion ruminatus, Modzalevskaya, 2003

Modzalevskaya, Tatiana L., 2003, Silurian and Devonian brachiopods from Severnaya Zemlya (Russian Arctic), Geodiversitas 25 (1), pp. 73-107 : 78-79

Brachyprion ruminatus n. sp. ( Fig. 5 )

HOLOTYPE. — N 9/12991, CNIGR Museum , St. Petersburg ( Fig. 5A, B ), ventral valve; L = 20.1 mm, W = 26.1 mm.

ETYMOLOGY. — From ruminatus (Latin) : chewed.

TYPE LOCALITY. — Loc. 11, Matusevich River, October Revolution Island.

FORMATION. — Samojlovich Formation.

AGE. —?Llandovery,?Telychian.

MATERIAL EXAMINED. — Seven ventral valves partly buried in matrix with only their disc exposed, and dorsal valve impressions from loc. 11, bed 14 (1-A), and three specimens from loc. 12, bed 9, Matusevich River, October Revolution Island ( Männik et al. 2002: figs 2, 5).

MEASUREMENTS (IN MM). — L = 16.0-20.3, W = 17.4- 26.1.

DIAGNOSIS. — Small concavoconvex stropheodontids with subquadrate outline, unequally parvicostellate with strongly incurved rugae; cardinal extremities extended into acute ears.

DESCRIPTION

Shells small (less than 2 cm in length), concavoconvex of subquadrate outline, finely parvicostellate. Hinge line straight, cardinal extremities extended into acute ears. Ventral valve with sharply defined geniculation in midlength. Beak small, umbo convex. Ventral interarea narrow, long and orthocline. The hinge line denticulation occupies the middle of interarea. Diagnostic features include four to five strongly incurved rugae oriented parallel to each other. Radial ornament comprises unequally wavy parvicostellae, between there are two to seven fine, unequal striae. Thickened and more distinguished ribs are present anteriorly. A very weak fold is developed near the anterior border. Ventral muscle field is weakly impressed and divided by narrow median ridge. The cardinal process lobes on the inner side of dorsal valve are not joined at their base.

VARIATION

Young specimens have relatively flat ventral valves, but the rugae are clearly shown on the pedicle ring, which make them similar to leptaenids. The rugae of the adult specimens are smoother, and the shells are characterized by well distinguished geniculation.

COMPARISON

Megastrophia (Protomegastrophia) walmstedti (Lindström, 1861) , described by Bassett &

Cocks (1974: 16, pl. 3, figs 5-9) from the lower Visby Beds (Telychian) of Gotland, is the closest species. Its specimens differ from those of Brachyprion ruminatus n. sp. in having larger size and well developed rugae on the v e n t r a l v a l v e. Brachyprion omnutakensis , described by Lopushinskaya (1967: 90, pl. XX, figs 7-10) from the Omnutakh Formation, lacks rugae but possesses acute ears and bears unequal wavy striae of two orders. The pattern of ornamentation, including the form and distribution of rugae, on the ventral disc are similar to those of Brachyprion polaris Andreeva in Nikiforova & Andreeva, 1961 ( Nikiforova & Andreeva 1961: 189, pl. XL, figs 6-10), but the new species is distinguished from the Siberian one by its sharp concentric rugae and cardinal ears.

Family CHILIDIOPSIDAE Boucot, 1959

Figures

FIG. 5. — Brachyprion ruminatus n. sp.; A, B, pedicle valve (holotype N 9/12991), loc. 11, bed 14A, Samojlovich Formation; A, ventral view; B, lateral view; C, ventral view of an internal mould of a pedicle valve (N 10/12991), loc. 11, bed 14, Samojlovich Formation; D, ventral view of an internal mould of a pedicle valve with well developed impressions of concentric rugae (N 11/12991); E, note the radially arranged pseudopunctae at anterior margin (N 12/12991), loc. 11, bed 20, Samojlovich Formation. Scale bars: A, B, D, 6 mm; C, 5 mm; E, 2 mm.