Tiramnia davidi Poletaev, 2024, sp. nov.

Tiramnia davidi sp. nov. urn:lsid:zoobank.org:act: 0C805CE0-5009-4C43-BF07-087B0BBEB785 Fig. 3 Martinia (Pseudomartinia) sp. A. – Aisenverg 1950: 137, pl. 2 figs 9–10. Martinia (Pseudomartinia) sp. – Aisenverg 1951: 43, pl. 11 fig. 2. Martinia balkhashica – Poletaev 2008: 28, pl. 1 figs 1–5. Tiramnia sp. – Poletaev 2018: 23, pl. 13 figs 6–10. non Martinia balkhashica – Beznosova 1968: 191, pl. 29 fig. 15. Diagnosis Medium-sized shell with angular edges of trough-shaped anterior part of sulcus of the ventral valve and a shallow furrow along axis of fold distinctly limited in the anterior part of the dorsal valve. Etymology Named in honor of David E. Aisenverg, who first distinguished this species. Material examined Holotype (Fig. 3A) UKRAINE • Donetsk Region, Khartsis’k District, Vovcha River, Illinka village; late Bashkirian Smolyanynivka Formation, Н 4 limestone layer; Aisenverg 1951 Coll.; NMNH 1433/85. Paratypes (Fig. 3B–E) UKRAINE • 12 specs; Luhansk Region, Bila River, Gorodyshche village; late Bashkirian, Smolyanynivka Formation; stratigraphic interval between the H 3 limestone layer to the H 6 limestone layer; Poletaev 2018 Coll.; NMNH 2446/102 to 2446/113. Description Medium-sized shell (24 mm wide, 23 mm long and 17 mm thick in the holotype), rounded pentagonal in outline and rather strongly and uniformly biconvex in lateral profile; hinge line shorter than the largest width at mid shell. Umbonal part of ventral valve broad, well-developed, with steep lateral slopes. Cardinal extremities rounded. Beak is well developed, inclined, projecting above hinge line. Ventral interarea short, narrow, triangle, smooth. Sulcus distinctly developed, rather wide, shallow with weak axial groove and trough-like tongue; uniplicate anterior margin with distinctly angular margins of tongue at junction with lateral slopes. Dorsal valve somewhat less convex than ventral one. Dorsal umbo small, well expressed. Fold broad, indistinct in posterior part, but distinctly developted in the anterior part, where its slopes fall steeply. Fold also bears a slight median furrow. Shell surface mostly smooth, weak lines of growth are visible, and rare traces of short weak radial ribs near the anterior commissure. Microsculpture not observed on the specimens. Internal structure of the ventral valve with subrhombic deepened muscular imprints in the umbonal region, as well as traces of single axial and several ramiform lateral imprints of the vascular system (see Fig. 3E). Remarks The new species differs from other known species of Tiramnia in having a more pronounced sulcus with a trough-like profile of the tongue and a distinct fold with a median groove. Based on morphological similarity and close geological age, the specimens under study were previously assigned to the Bashkirian Martinia balkhashica Beznosova, 1968 from eastern Kazakhstan (Poletaev 2008). However, the holotype of M. balkhashica has an interior with twin vascula media (Beznosova 1968: pl. 29 fig. 16), which clearly shows that the Kazakh species belongs to Jilinmartinia. Other representatives of Tiramnia, similar to T. semiglobosa, differ from the new species by the rounded contours of the shells, sometimes drop-shaped, with an elongated ventral apex, a weak, poorly defined sulcus, often with a median groove. Martinia buckmani Yanishevsky, 1918 from the late Serpukhovian of Central Asia and early Bashkirian of the Donets Basin and the Urals is very similar to the new species. However, M. buckmani is distinguished by its smaller size, swollen shell with a strongly protruding and wrapped apical part of the ventral valve, and apparently by its internal structure. Tiramnia davidi sp. nov. differs from Postmartinia juresanensis (Stepanov, 1948) from the Pennsylvanian of Bashkortostan by the angular rather than rounded lateral profile of the clearly limited anterior end of the sulcus and its much less developed continuation (tongue), as well as by its stratigraphic and geographic occurrence.