Eoplectodonta (Ygerodiscus) bella, 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 : 473

Eoplectodonta (Ygerodiscus) bella sp. nov.

Fig. 8M–Q

Holotype: VU B10040, a dorsal valve interior ( Fig. 8M), from the Wenlock Riga Formation at 890.6m in the Gėluva−99 borehole, Lithuania.

Derivation of the name: From Latin bellus, beautiful.

Distribution.—Vilkija Beds of Paprieniai Formation (Sheinwoodian) and Jonava Beds of Birštonas Formation (Sheinwoodian) in east Lithuania; Riga and Gėluva formations (Sheinwoodian and Homerian) in central Lithuania; Siesartis (Homerian), Dubysa and Rusnė (Gorstian) formations in west Lithuania ( Figs. 2 , 3 , 9 ). For localities see Appendix,

Diagnosis.— Ygerodiscus of small size with relatively high length/width ratio, sharp ridge crests, some of which are variably enhanced: erect and open delthyrium.

Description.—Exterior with evenly convex ventral valve with enhanced umbo, and strongly concave dorsal valve: semicircular outline with small alae slightly extending laterally: small pair of chilidial plates, but with delthyrium otherwise open to reveal cardinal process. Unequally parvicostellate ornament, with enhanced central costella and between three and six equally enhanced costella to each side of it; the enhanced costellae are at the crest of undulations which originate near the umbo and continue to the anterior: in larger specimens smaller subsidiary undulations and slightly enhanced costellae are only present anteriorly. Ventral interior with denticulate hinge line, weak divergent dental plates: well−impressed diductor muscle field enclosing less well−impressed lanceolate adductor scars separated by a variably developed very thin myophragm. Dorsal interior with fossettes on hinge line; prominent trifid undercut cardinal process between well−developed flaring socket plates which antero−laterally extend into the lateral edges of the variably−developed bema which bears two pair of side septa, with the central pair longer than the lateral pair and extending to two−thirds valve length: the exterior undulations can be seen internally: mantle canal system not known.

Discussion.—The Wenlock Lithuanian material is of smaller size than E. (Y.) undulata and also the contemporary E. (Y.) novemplicata ( Havliček, 1967) from Perunica, which also has a rather different ribbing style, and we differentiate the East Baltic material here as E. (Y.) bella sp. nov. The average width is 9.1 mm and the maximum (estimated) width is 11 mm, in contrast to the 16 mm ( Cocks 1970) achieved by Y. undulata and 15 mm by E. (Y.) novemcostata . E. (Y.) undulata also has much more prominently developed lateral alae than E. (Y.) bella , leading to a smaller length/width ratio. The only formally erected species of E. (Ygerodiscus) from rocks younger than the Llandovery is E. (Y.) cornuta ( Davidson, 1883) from the Wenlock of England, which was revised by Bassett (1974: 90); however, that species has a very distinctive pair of shell projections anteriorly, and thus that name cannot be used for the bulk of the Lithuanian borehole specimens, although we record true E. (Y.) cornuta rarely from the Riga Formation (Lower Homerian) in boreholes Gėluva−99 (890.6 m) and Pilviškiai−141 (908.0 and 866.5 m).

Figures

Fig. 2. Correlation of the Silurian of west, central and east Lithuania with the international Silurian stages, the East Baltic regional stages and the graptolite and conodont zones.

Fig. 3. Ranges of Silurian plectambonitoidean, strophomenoidean, and orthotetoidean brachiopods within the Lithuanian boreholes (see Appendix, Table 1 for details).

Fig. 9. Diagram showing the relative dispositions of E. (Eoplectodonta) and E. (Ygerodiscus) from west to east in the East Baltic platform.