Skenidioides tatyanae, Baliński, 2012

Baliński, Andrzej, 2012, The brachiopod succession through the Silurian-Devonian boundary beds at Dnistrove, Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 897-924 : 907-908

publication ID	https://doi.org/ 10.4202/app.2011.0138
persistent identifier	https://treatment.plazi.org/id/03E20437-BD51-FFE9-FF3B-FEB59586F8EF
treatment provided by	Felipe
scientific name	Skenidioides tatyanae
status	sp. nov.

Treatment

Skenidioides tatyanae sp. nov.

Fig. 6 View Fig .

1929 Scenidium Lewisii (Davidson) ; Kozłowski 1929: 47–50, pl. 1: 20, 21; figs. 5–7, 37I.

1968 Skenidioides lewisi Davidson ; Modzalevskaya 1968: pl. 27: 4–7.

1985 Skenidioides sp. nov.; Nikiforova et. al. 1985: 9.

Etymology: Species dedicated to Tatyana Lvovna Modzalevskaya in recognition of her work on Podolian brachiopods.

Holotype: Complete shell ZPAL Bp. 56/118; illustrated in Fig. 6A View Fig .

Type locality: Trench east of Dnistrove village , Podolia, Ukraine. Coordinates 48 ° 32'16.9”N, 26 ° 14'21.4”E GoogleMaps .

Type horizon: Beds 47–48, Khudykivtsi Beds of the Borshchiv Horizon (Early Devonian).

Diagnosis.— Skenidioides with unisulcate anterior commissure, obtuse to gently rounded cardinal extremities, apsacline and usually concave ventral interarea, delthyrial angle attaining 31–38 °, 26–31 costellae at the anterior margin; dorsal septalium deep, dorsal median septum reaching about 68% of the valve length.

Material.—Three complete shells, 57 ventral and 32 dorsal valves. Dimensions of the holotype (in mm): length 3.3, width 4.2, thickness 2.3, height of ventral interarea 1.2, width of hinge margin 3.7. The species is quite common at about 5.5 m (beds 47–48) above the S–D boundary. A few specimens have been also found 1.5 m (bed 7), 1.3 m (bed 5), and 0.7 m (bed −1) above the boundary. The species was also found in the Khudykivtsi beds at the Khudykivtsi village.

Description.—Shell small, up to about 5 mm in length, ventribiconvex, transversely elliptical to subpentagonal in outline; anterior commissure gently unisulcate, anterior margin arched to slightly emarginate, hinge margin straight and wide but narrower than maximum shell width, cardinal extremities obtuse to gently rounded, lateral margins rounded. Ventral valve sub−pyramidal with rather blunt beak, interarea high, apsacline, concave to only slightly concave, delthyrium open, sides diverging at 31–38 °. Dorsal valve transversely elliptical, weakly convex, with wide, shallow, and poorly bounded laterally median sulcus occupying about 1/3 of the valve width; dorsal interarea wide and low, flat, anacline to nearly catacline.

Shell covered by strong, rounded costellae increasing by rare bifurcations or intercalations; ventral valve frequently with wider median intercostal space in which very thin, delicate single or paired costellae are usually developed; 26–31 costellae at the anterior margin. Growth lamellae rare, poorly developed, but anteriorly more obvious and crowded. Micro−ornamentation of very fine, dense concentric lines, about 60 per 1 mm.

Ventral interior with deep, concave spondylium supported apically by wide, septum−like median thickening ( Fig. 6N View Fig ); teeth plate−like, wide ( Fig. 6G, I View Fig ). Dorsal valve with deep septalium bisected by thin median septum and triangular, elongated cardinal processes at apex; median septum high, reaching about 68% of the valve length ( Fig. 6M, O View Fig ).

Remarks.—The specimens studied here are most probably conspecific with those described in great details and illustrated by Kozłowski (1929: 47–50, figs. 5–7, 37I; pl. 1: 20, 21) from the middle part of the Borshchiv Horizon as Scenidium lewisi (Davidson, 1848) . Boucot et al. (1966: 364) suggested that cardinalia of the specimens from Podolia are of Skenidioides rather than Skenidium . Modzalevskaya (1968: pl. 27: 4–7) illustrated one shell from the Tajna beds (= Khudykivtsi Beds) at Dnistrove as Skenidioides lewisi . Nikiforova et al. (1985) suggested that Kozłowski’s (1929) material, as well as their own new collections from the upper Khudykivtsi and Mytkiv beds, represent a new species that differ from Wenlock–Ludlow S. lewisii in a number of features. Indeed, Skenidioides lewisii (Davidson, 1848) from the Wenlock Limestone and Wenlock Shale of Dudley and Walsall ( England) re−described by Bassett (1972: 35–37, pls. 5: 10, 6: 1–12) differs from the especies here described in having more restricted dorsal sulcus, higher and less concave ventral interarea, narrower delthyrium, narrower dorsal septalium, proportionaly longer dorsal median septum, and usually thicker costellae.

From Skenidioides robertsensis Johnson, Boucot, and Murphy, 1973 described from the Early Devonian of central Nevada ( Johnson et al. 1973: 15, 16; pl. 10: 1–13) the new species differs in having a less transverse shell, narrower hinge margin with less acute cardinal extremities, and more concave, apsacline ventral interarea. The latter is readily distinguishable from Skenidioides operosa Johnson, Boucot, and Murphy, 1976 from the Ludlovian of central Nevada ( Johnson et al. 1976: 23, 24, pl. 29: 1–15), the Wenlock of Arctic Canada ( Zhang 1989), and the Mackenzie Mountains of northwestern Canada ( Jin and Chatterton 1997) in having less transverse shell, less acute cardinal extremities, shorter dorsal median septum, and finer, more numerous costellae. Skenidioides polonicus (Gürich, 1896) from the Middle Devonian of the Holy Cross Mountains, Poland, which was redescribed by Biernat (1959: 16–25, pl. 1: 1–9, figs. 6–8; text−plates 1–2) and Halamski (2009: 72–74, pls. 10: 1–38, 12: 30; fig. 10) differs from the species described here mainly by having much thicker shell costellation, longer dorsal median septum, and more acute, frequently mucronate cardinal extremities. Skenidioides tatyanae sp. nov. shows great external similarity to Skenidioides henryhousensis Amsden, 1958 from the Henryhouse Formation (late Silurian) of Oklahoma, USA (see Amsden 1958), but differs from the latter by shorter and more rounded cardinal extremities, more concave ventral interarea, and narrower and stronger dorsal sulcus.

As suggested Nikiforova et al. (1985) the Silurian species described by Kozłowski (1929) from the Skala Horizon as Scenidium orthisiforme Kozłowski, 1929 probably belongs to Skenidioides . This form is poorly known and thus a nomen dubium ( Nikiforova et al. 1985). It differs from the species

http://dx.doi.org/10.4202/app.2011.0138

here described by a more convex dorsal valve, the absence of the dorsal median sulcus, more rounded lateral margins, lower ventral interarea, and finer costation.

Stratigraphic and geographic range.— Kozłowski (1929: 50) found this species in several other localities from Podolia representing “l’étage de Borszczów”. Nikiforova et al. (1985: 9) collected new specimens representing this species from the Khudykivtsi and Mytkiv beds.