Leptaena Dalman, 1828

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 : 460-462

Genus Leptaena Dalman, 1828

Remarks.—The type species of the genus is Leptaena rugosa Dalman, 1828 , from the Ashgill of Sweden. There is a great number of species of Leptaena from the Baltic area and worldwide—the variability of both the genus and the individual species within it are very substantial and a comprehensive review of this genus would be desirable, although Rong and Cocks (1994: 678) discussed the many generic names within its synonomy. Those species recorded from the Baltic Silurian are: Leptaena altera Rybnikova, 1966 , Leptaena depressa (J. de C. Sowerby, 1825) , Leptaena haverfordensis Bancroft, 1949 , Leptaena purpurea Cocks, 1968 , Leptaena rhomboidalis ( Wahlenberg, 1818) , Leptaena valentia Cocks, 1968 , Leptaena valida Bancroft, 1949 , and Leptaena venzavensis ( Rybnikova, 1966) . Five species are known from the Llandovery of the Oslo Region ( Baarli 1995; Cocks and Baarli 1982), but are not yet recorded from elsewhere in Baltica. One of these species, Leptaena dejecta, Baarli (1995: 34 , was assigned originally to the Ordovician genus Dactylogonia ), but the side septa and other structures characteristic of that genus are not seen, and we reassign the species here to Leptaena . L. haverfordensis , L. purpurea , and L. valida , also recorded by Baarli (1995), all have their type localities in the Llandovery of Wales and the Welsh Borderland, whilst the type locality of L. valentia is in the Rhuddanian of Scotland ( Cocks 1968).

We only recognise two species of Leptaena from the Wenlock and Ludlow of the East Baltic, Leptaena depressa and L. altera . Leptaena venzavensis was originally erected under Rugoleptaena by Rybnikova (1966: 76) from the Pagėgiai Horizon of Latvia, which she declared as Lower Ludlow, but which is now known to be Upper Ludlow. We here synonymise that species with Leptaena depressa . L. depressa is up to four times the size of L. altera and occurs in the shallow−water assemblages of the East Baltic ( Figs. 4P–R, 5A–D). L. depressa , whose type locality is in the Upper Wenlock of England, is accompanied in Gotland by the much−quoted L. rhomboidalis ( Wahlenberg, 1818) : the differences between the two species were discussed and illustrated by Bassett and Cocks (1974: 14), and chiefly consist of the enhanced genicular rim and stronger rugae in L. rhomboidalis ; but the latter is not known from Baltica east of Gotland. Leptaena is present in the Pridoli of Lithuania; but, as discussed above, most of the specimens known only show the valve exteriors, and thus may be either Leptaena or Lepidoleptaena . Leptaena depressa is known from the Švenčionys (Telychian), Paprieniai, Jačionys, Birštonas, Verknė and Nevėţis formations (Sheinwoodian and Homerian) in east Lithuania; the upper part of the Riga (Homerian), Gėluva (Homerian), Dubysa (Gorstian) and Minija (Lower Pridoli) formations in central Lithuania, and the Rusnė, Dubysa, Pagėgiai, Mituva, Ventspils (Gorstian and Ludfordian), Minija and Jūra (Pridoli) formations in west Lithuania (Appendix).

Rybnikova (1966: 78; 1967: 193) described Leptaena altera as from the Wenlock–Lower Ludlow of Latvia, and we reillustrate her types here ( Fig. 4H, I). The species may be the same as L. holcrofti which Bassett described (1974: 121) from the Upper Wenlock of Wales. L. altera differs in size from slightly larger specimens in shallower−water to smaller specimens from deeper−water facies ( Fig. 4H–O). Its distribution in Lithuania is in the Švenčionys (Telychian), Paprieniai (Sheinwoodian) formations and the Jonava Beds of the Birštonas Formation (Sheinwoodian) in east Lithuania; the Jūrmala (Telychian), Riga and Gėluva (Sheinwoodian and Homerian), and the Šešupė Beds of the Dubysa formations (Gorstian) in central Lithuania; and the Siesartis (Homerian), Rusnė, Dubysa (Gorstian) and Pagėgiai (Ludfordian) formations in west Lithuania (Appendix).