Morinorhynchus rubeli, Musteikis & Cocks, 2004

Musteikis, Petras & Cocks, L. Robin M., 2004, Strophomenide and orthotetide Silurian brachiopods from the Baltic region, with particular reference to Lithuanian boreholes, Acta Palaeontologica Polonica 49 (3), pp. 455-482 : 474-475

Morinorhynchus rubeli sp. nov.

Figs. 6R–T, 10.

Fardenia cf. attenuata Amsden ; Nikiforova 1970: 99, pl. 1: 5–12 (non Amsden 1951).

Holotype: VU B20498 ( Fig. 10B ) from the Upper Ludlow Mituva Formation , at 881.5 m depth in the Virbalis−5 borehole, Lithuania.

Derivation of name: After Madis Rubel, in honour of his work on Baltic brachiopods.

Distribution.—Neris (Ludfordian), Vievis and Lapės (Pridoli) formations in east Lithuania; Ventspils (Ludfordian), Minija and Jūra (Pridoli) formations in central Lithuania; Nova Beds of Dubysa Formation and upper part of Rusnė Formation (Gorstian–Lower Ludfordian), Mituva, Ventspils, Pagėgiai (Ludfordian), Minija and Jūra (Pridoli) formations in west Lithuania (Appendix).

Diagnosis.— Morinorhynchus with plastic shell shape and outline; originally biconvex but adult parts of ventral valve varying from convex to concave; relatively strong socked plates with relatively small angle of separation; variable ornament.

Description.—Exterior: profile biconvex in juveniles but ranging in adults from biconvex to a largely convex dorsal valve with a convex, irregularly planar or in part even concave ventral valve: outline subcircular with weakly developed alae, maximum width on average at half to two−thirds valve length. Characteristic pseudodeltidium strongly developed, very small low chilidial plates are present, but are scarcely visible in many specimens. Ornament of subequal parvicostellae with new costellae arising by intercalation: variably−developed growth−lines sometimes accentuated to form fine concentric filae. Ventral interior with flaring teeth: muscle field very weakly impressed and merging anteriorly with valve floor: mantle canals not known. Dorsal interior with substantial upstanding cardinal process partly projecting over the valve posterior margin and conjoined laterally to substantial recurved socket plates; muscle field poorly impressed, but with suboval adductor scars present in some specimens: mantle canals not known.

Discussion.—A key feature, which distinguishes the new species from all others in the genus is the plastic shape of the shell. Whilst originally biconvex in smaller specimens, like the type species M. dalmanelliformis , and also M. adnatus , M. crispus , M. orbignyi , and M.wieniukowi , in adult specimens of M. rubeli the pedicle valve can vary from convex to concave, but through all combinations of the two in different parts of the same individual shell. We presume from these inconstant shapes that the adult shell lay subparallel to, and was influenced in form by, the ocean floor; rather than, as in the other species, being more erect and therefore more independent of the ocean floor topography. In addition, the socket plates in M. rubeli are more strongly developed, and at a less acute angle of separation, in contrast to the more antero−laterally flaring socket plates of M. adnatus , M. crispus , M. dalmanelliformis , and M. orbignyi : the dorsal interior of M. wieniukowi is not known. Various authors have used the “distinctive” concentric radial ornamentation to differentiate between different species of Morinorhynchus ; however, we have found (following examination of many specimens of the genus from the Welsh Borderland and Gotland, as well as the Lithuanian boreholes) that there is considerable variation of this attribute within populations and cannot use it to characterise the various species. Morinorhynchus miniparvicostellus from the Wenlock of Arctic Canada ( Zhang 1989; Jin and Chatterton 1997) is relatively wider than the new species and has a higher degree of parvicostellation in the ornament. M. subcarinatus from the Ludlow of Nevada is also more parvicostellate than M. rubeli and has finer ornament: also the socket plates diverge at a wider angle ( Johnson, Boucot, and Murphy 1976). M. variatus from the Pridoli of Urals (Brevel and Breivel 1988) differs from the new species in convex brachial valve and wider angle of separation of socket plates.

Many Morinorhynchus from northern and eastern parts of Baltica have been attributed as being related to “ M. attenuata Amsden ”. These records include the M. cf. attenuata of Sapelnikov and Mizens (1991: 93, pl. 9: 13, 14), Modzalevskaya, (1980: 82, pl. 1: 1, 2; 1997: 40, pl. 7: 3, 4), from the Wenlock to Pridoli beds of Vaigach Island, the Polar Urals west slope, Chernov Range (Pechora), and Dolgij island. Nikiforova (1970: 99, pl. 1: 5–12) identified Fardenia cf. attenuata from the Ludlow and Pridoli of Vaigach Island. However, Amsden’s species M. attenuata , whose type is from the Wenlock Henryhouse Formation of Oklahoma, U.S.A. ( Amsden 1951) is not an orthotetoid and is actually attributable to the strophomenoid Mesopholidostrophia . The Vaigach form identified by Nikiforova and Modzalavskaya is undoubtedly Morinorhynchus , as can be seen from the published illustrations of the interareas: we reassign the Vaigach material here to M. rubeli sp. nov., and suspect that the other quoted records from north−east Baltica are also that species.

Figures

Fig. 10. Morinorhynchus rubeli sp. nov. A. VU B20499, ventral exterior (A1), and interior (A2), Nova Beds of Dubysa Formation (Gorstian–Ludfordian), Šiupyliai−69, 879.4 m, × 6. B. VU B20498, holotype, ventral exterior (B1), and interior (B2), Mituva Formation (Ludfordian), Virbalis−5, 881.5 m, × 4. C. VU B20501, dorsal interior, Ventspils Formation (Ludfordian), Kurtuvėnai−162, 1097.2 m, × 2. D. VU B20500, dorsal interior, Vievis Formation (Pridoli), Grauţai−105, 580.9 m, × 5. E. VU B10146, dorsal exterior (E1), and interior (E2), Mituva Formation (Ludfordian), Virbalis−5, 887.45 m, × 5.