Leptaena (Leptaena) Dalman, 1828

Baliński, Andrzej, 2012, The brachiopod succession through the Silurian-Devonian boundary beds at Dnistrove, Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 897-924 : 902

Subgenus Leptaena (Leptaena) Dalman, 1828

Type species: Leptaena rugosa Dalman, 1828 ; Dalmanitina beds, Ashgil, Ordovician; Fårdalaberg (Varvsberget), Västergötland, Sweden.

Leptaena (Leptaena) aff. altera Rybnikova, 1966 Fig. 4A–J, L .

Material.—27 shells, partly embedded in sediment, 5 ventral valves, 4 dorsal valves, and about 60 fragments; all from beds 47–48 of the studied section, i.e., at 5.5 m above the S–D boundary.

Description.—Shell rather small, attaining usually 12–14 mm in width, transversally subrectangular outline, with length approaching 51–62% of width; anterior margin weakly arched, lateral margins straight to weakly concave, cardinal extremities extend forming pointed alae.

Ventral valve with a very gently convex posteror region and concave the rest of the disk, sometimes with a very weak median longitudianl elevation; peripheral rim elevated, anterolaterally forming often swellings or bulges; trail geniculated medialy at 70–90 ° from commissural plane, attaining 51–91% of disk length, medially gently concave (emarginate); umbo small, with distinct supra−apical hollowed outgrowth (pedicle tube); interarea very low, ranging 0.4–0.8 mm in height, apsacline, medially triangular, laterally sublinear, delthyrim covered apically by short convex pseudodeltidium, rest of delthyrium filled up with massive chilidium ( Fig. 4E–G ).

Dorsal disk flattened, but with very shallow median sulcus and gently convex flanks; trail separated from disk with a shallow peripheral trough which anterolaterally forms two shallow hollows corresponding to the bulges on the ventral valve; trail frequently with very weak medial convexity; interarea very low, 0.2–0.3 mm in height, linear, anacline; notothyrium covered by a convex, medially deeply grooved chilidium.

Interior of the ventral valve with subcircular muscle field, bounded laterally by ridges and divided in its anterior half by a short median ridge ( Fig. 4I, J ); dental plates obsolescent by secondary shell material, developed as thickenned ridges ( Fig. 4I, J ); teeth small, wide, crenulated ( Fig. 4J 2 ); internal surface outside the muscle area covered by sparse pustulae arranged in radial rows.

Dorsal interior with sub−triangular cardinal process lobes divided by a deep slit and expanding in width anteriorly; sockets triangular, usually shallow, broad, bounded posteriorly by the edge of the interarea and medially by sharp and straight inner socket ridges ( Fig. 4L ); adductor scars deeply impressed posteriorly in the thickened shell wall, poorly delineated anteriorly, and medially separated by wide, thickened myophragm ( Fig. 4L ); internal surface of the disk with thick tubercles around muscle field and much finer anteriorly and laterally.

Ornament multicostellate, costellae faint, delicate, 3.5–4.5 per mm, those on lateral parts of the trail sometimes thiner, up to 7 per mm; rugae usually weak, sometimes imperceptible.

Remarks.—The species differs from the majority of other leptaenids in having small shell dimensions, a transverse, subrectangular outline, usually straight to gently concave (emarginate) anterior margin, inconspicuous to absent concentric rugae, rather weak radial costellae, and an elevated peripheral rim on the ventral valve frequently forming swellings at anterolateral corners. A well preserved, distinct, supra−apical hollowed outgrowth (pedicle tube) on the studied ventral valves ( Fig. 4C 2 ) is very similar to that described and illustrated by Kozłowski (1929: 92, fig. 26) in Glossoleptaena emarginata (Barande, 1879) from the middle part of the Borshchiv Horizon. The studied specimens differ from G. emarginata in not having a strongly emarginate anterior margin, which is one of the diagnostic features of the latter taxon.

Externally the specimens from Dnistrove are almost identical to Leptaena (Leptaena) altera Rybnikova, 1966 from the Pagëgiai Formation (Ludlow) of Ezere borehole and the Wenlock of Akniste borehole, Latvia ( Rybnikova 1966). Musteikis and Cocks (2004: fig. 4H–O) re−illustrated two of the three type specimens of Rybnikova’s paper and figured several other specimens from the Telychian–Ludfordian of Lithuania. Rubel (2011) described and illustrated several specimens of the species from the Wenlock of Estonia, and Telychian– Ludfordian of Lithuania. The authors have also shown the external variability of L. (L.) altera . Although the holotype of the species ( Rybnikova 1966: pl. 1: 4; Musteikis and Cocks 2004: fig. 4H) is slightly larger and has a more semicircular shell outline than the specimens from Dnistrove, all other specimens of L. altera from the Baltic region illustrated by Musteikis and Cocks (2004) and Rubel (2011) show the same details of the external morphology and interior as the material here studied. The remarkable similarity between these forms is very suggestive despite evident difference in their stratigraphic ranges. This is the main reason that the present earliest Devonian material from Dnistrove is described as only related to the Llandovery–Ludlow L. altera .

The Podolian specimens show also some resemblance to Letagonia joachimiana Havlíček, 1967 from the Liteň and Kopanina formations (Silurian) of Bohemia ( Havlíček 1967: 105–106, pl. 19: 7–9, 12, 13, 17–20, fig. 41A; Havlíček and Štorch 1990: 77, pls. 13: 3, 15: 6, 7). The differences between the two species seem to be minor and are expressed mainly in a more rectangular shell outline of the former. It is possible that both species are also closely related although the latter is again older stratigraphically.

Figures

Fig. 4. Strophomenide brachiopods from the Silurian–Devonian boundary beds at Dnistrove (Podolia, Ukraine). A–J, L. Leptaena (Leptaena) aff. altera Rybnikova, 1966. A, B. Two nearly complete shells, ZPAL Bp. 56/42 (A) and ZPAL Bp. 56/43 (B), in dorsal (A1, B1), ventral (A2, B2), lateral (A3, B3), posterior (A4, B4), and anterior (A5, B5) views. C. Shell ZPAL Bp. 56/46 in ventral view (C1) and enlargement showing the pedicle tube (C2). D. Enlargement of the posterior region of the ventral valve ZPAL Bp. 56/32d showing interarea, delthyrium, and well preserved teeth. E–G. Enlargement of the posterior region of three shells, ZPAL Bp. 56/32c (E), ZPAL Bp. 56/32b (F), and ZPAL Bp. 56/32a (G); ventral valves uppermost. H. Almost complete shell ZPAL Bp. 56/45 in ventral view. I. Enlargement of the interior of ventral valve ZPAL Bp. 56/32e. J. Ventral valve ZPAL Bp. 56/44 with partially shown interior in general (J1) and enlarged (J2) views. L. Interior of incomplete dorsal valve ZPAL Bp. 56/32f. K, N–O. Cymostrophia (Protocymostrophia) costatula (Barrande, 1847). K. Incomplete shell ZPAL Bp. 56/18 in dorsal (A1) and ventral (A2) views. N. Interior of dorsal valve ZPAL Bp. 56/21. O. Interior of an incomplete ventral valve ZPAL Bp. 56/19. P. Exterior of an incomplete ventral valve ZPAL Bp. 56/20. M.?Mesodouvillina sp.; exterior of ventral valve ZPAL Bp. 56/53. Beds 47–48 (A–L, N–P), 102 (M). Scale bars 2 mm.

Fig. 2. Field photo of the section at Dnistrove vil− lage. The trenched interval reveals the clayey− calcareous Silurian–Devonian boundary beds. Numbers refer to the beds from which brachio− pod material was collected (see also Fig. 3).