Eoplectodonta (Eoplectodonta)

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 : 470-472

Subgenus Eoplectodonta (Eoplectodonta)

Kozłowski, 1929

Remarks.— Kozłowski (1929) erected Eoplectodonta with the type species Sowerbyella praecursor Jones , which was later placed into the synonymy of Orthis duplicata J. de C. Sowerby (1839) by Cocks (1970); both species are from the early Llandovery of the Llandovery type area, Wales. From Baltica, the following species have been recorded: E. (E.) duplicata (J. de C. Sowerby, 1839) , E. (E.) duvalii ( Davidson, 1847) , E. (E.) exceptionis ( Rybnikova, 1967) , E. (E.) penkillensis ( Reed, 1917) , E. (E.) transversalis ( Wahlenberg, 1818) , and E. (E.) jongensis Baarli, 1995 , which will now be considered in turn.

We have recognised Eoplectodonta (Eoplectodonta) penkillensis ( Reed, 1917) as common in the East Baltic ( Fig. 7 ); this species was originally described from the Penkill Formation of Girvan, Scotland and revised by Cocks (1970). Most of the Lithuanian specimens are small, with an average width of less than 6 mm. They occur in the strata near the middle of the Llandovery and the bottom of the Wenlock: Dobele, Jūrmala and Riga formations in the central Lithuania and Rasytė Formation in the west Lithuania ( Figs. 7 , 8G; occurrences see in Appendix). Cocks and Baarli (1982) have also determined the species from the Telychian Vik and Bruflat formations of the Oslo region, Norway.

E. (E.) transversalis is known only from Telychian rocks in Gotland—the differences between it and E. (E.) penkillensis were discussed by Cocks (1970). Despite a record of E. (E.) transversalis by Modzalevskaya and Pushkin (1989) from the Llandovery of the Brest Depression of Belarus, this was subsequently revised ( Pushkin et al. 1991) to E. (E.) penkillensis , and thus E. (E.) transversalis is not known from the East Baltic. Baarli (1995) erected the subspecies E. transversalis jongensis from the Aeronian Rytteråker Formation of the Oslo region, Norway: however, her statistics only compared that taxon with E. duplicata . The length/width ratios of E. jongensis , as seen in her figured specimens, are too wide to be included within E. (E.) transversalis , and E. jongensis therefore needs further detailed consideration in relation to the other Aeronian species of Eoplectodonta , such as those listed by Cocks (1970). Not many Aeronian specimens of E. ( Eoplectodonta ) have been recorded from Lithuania, and those that we have appear similar to E. (E.) penkillensis . E. (E.) exceptionis is known only from late Rhuddanian Coronograptus cyphus and early Aeronian C. gregarius zones calcareous nodular limestones in the Choldre borehole of Latvia ( Rybnikova 1967: 189), and Rybnikova’s types are reillustrated here ( Fig. 8H, I). This interval is represented only by graptolitic shales in Lithuania, and thus E. (E.) exceptionis is not recorded there .

At higher levels in various boreholes, small specimens of Eoplectodonta are found abundantly in the Wenlock; however the preservation is not good enough to identify most of them more narrowly than as Eoplectodonta (E.) sp., although a few larger specimens ( Fig. 8A–F), e.g., from 926–930 m in the Pilviškiai−141 borehole, we identify as E. (E.) duvalii . In slightly younger beds in the same sequence, some specimens show incipient undulations, although these are not strong enough to warrant attribution to Ygerodiscus. In general, the size of the specimens increases towards the shoreline and in time. The diachronous sequences of E. (E.) penkillensis , E. (E.) duvalii and E. (Ygerodiscus) bella sp. nov. (see below) suggest their distribution to be rather environmentally than evolutionary controlled and show the overall basin shallowing in time, especially in the eastern and central Lithuania ( Fig. 9 ).

Nikiforova (1954: 76) identified “ Sowerbyella transversalis var. lata Jones ” from the Wenlock and Ludlow of Podolia; however, we include her material within E. (E.) duvalii . Alichova et al. (1954: 36, pl. 22: 6, 7) described “ Sowerbyella transversalis (Wahlenberg) var. lata Jones, 1928 " from the Wenlock of southern Lithuania: judging from the figures this species should also be attributed to E. duvalii . From Lithuania we have identified Eoplectodonta (E.) duvalii from the Švenčionys (Telychian), Paprieniai and Birštonas (Sheinwoodian) formations in the east Lithuania; Jūrmala (Telychian), Riga and Gėluva (Sheinwoodian–Homerian) formations and in the Šešupė Beds of Dubysa Formation in the central Lithuania; Ragainė, Siesartis (Sheinwoodian–Homerian) and Dubysa (Gorstian) formations in the west Lithuania (Appendix).

Figures

Fig. 7. Eoplectodonta (E.) penkillensis (Reed, 1917), B20497, Riga Formation (Sheinwoodian, M. riccartonensis Zone), eroded bedding plane with the Clorinda sp., Vilkaviškis−129, 837.1 m, × 3.4.

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