Plectodonta (Plectodonta) Kozłowski, 1929

Baliński, Andrzej, 2012, The brachiopod succession through the Silurian-Devonian boundary beds at Dnistrove, Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 897-924 : 904-906

publication ID	https://doi.org/ 10.4202/app.2011.0138
persistent identifier	https://treatment.plazi.org/id/03E20437-BD5C-FFEB-FCA8-FED1907DFDD4
treatment provided by	Felipe
scientific name	Plectodonta (Plectodonta) Kozłowski, 1929
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Subgenus Plectodonta (Plectodonta) Kozłowski, 1929

Plectodonta (Plectodonta) mariae Kozłowski, 1929 Plectodonta (Plectodonta) mariae pantherae subsp. nov.

Fig. 5A–J, M–P View Fig .

1978 Plectodonta aff. maria Kozłowski ; Modzalevskaya 1968: pl. 27: 39.

2010 Plectodonta sp. ; Baliński 2010: 696, figs. 1, 2, 3A.

Etymology: After the spotted colour pattern resembling that in the panther.

Holotype: Complete articulated shell ZPAL Bp. 56/99; illustrated in Fig. 5A 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.—Shell small, attaining 5.6 mm in width, transversely subrectangular to semicircular, with slightly alate cardinal extremities; ventral sulcus and dorsal fold; inner pair of the dorsal side septa diverge at 31 °, beginning at some distance from the socket plates; ornament parvicostellate, about 70 to 100 costellae in adult specimens and 3–9 fine costellae between each major costella.

Material.—Two isolated shells and 175 specimens (mostly ventral valves) embedded in sediment. The subspecies is quite common at 5.5 m above the S–D boundary (beds 47–48). Two specimens have been revealed also at 0.4 m above the boundary and 3 other at 1.15 m below the boundary.

Description.—Shell small up to about 4.6 mm in length and 5.6 mm in width, with a length to width ratio of 0.51–0.70, having concavoconvex, transversely subrectangular alate to semicircular outline; anterior commissure rectimarginate, anterior margin arched, lateral margins rounded, usually weakly concave near the hinge line forming short, pointed ears or, less frequently, cardinal extremities angular; hinge line straight and wide, usually equals the greatest width of the shell, finely denticulate.

Ventral valve gently convex in lateral profile with slightly swollen umbonal region; beak small, blunt; interarea low, up to ca. 0.4 mm in height, apsacline, almost flat to weakly concave, margins of delthyrium diverge at about 88 °, apically closed by small convex pseudodeltidium; supraapical forament very small, bordered by slightly elevated ring; sulcus wide but poorly delimited laterally, with a median primary costa, on some specimens better marked at about midlength (about 0.5–2 mm from the beak, i.e., where lateral borders of the sulcus accomodate side septa of the dorsal valve), variable in profile from U− to V−shaped in cross−section, starting at umbo.

Dorsal valve weakly concave, with the greatest concavity in the posterior region near fairly prominent, elongate dorsal protegular node; fold well marked and wide occupying about 1/4–1/3 of the valve width, triangular in cross−section; interarea very low, linear, flat, hypercline; chilidium not preserved.

Dorsal interior visible on one juvenile specimen 1.8 mm long ( Fig. 5M View Fig ), with papillose surface; papillae coarse on the

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

central region, fine peripherally; socket plates small, widely divergent; two prominent side septa diverge at 31 °, beginning posteriorly at a distance of 0.4 mm from the socket plates; second pair of side septa very weak and shorter, almost imperceptible due to the immaturity of the specimen; cardinal process trifid in posterior view, occupying large portion of the delthyrium.

Ventral interior poorly preserved, but a short and thick median septum in the most posterior region of a valve is visible; surface finely papillose ( Fig. 5E View Fig ).

Shell ornament unequally parvicostellate; about 70 to 100 costellae in adult specimens at the anterior margin and 3–9 fine costellae between each major costella (usually 3–5); growth lines sometimes visible, rather weak ( Fig. 5C, O View Fig ).

Remarks.—The subspecies from Dnistrove is common in the lowermost Devonian, but a few specimens also occur in the uppermost Silurian, i.e., just immediately below the S–D boundary. It is most closely related to the nominative subspecies of the type species of the genus, i.e., Plectodonta (P.) mariae mariae which seems to be a direct descendant of the former. P. (P.) mariae mariae occurs abundantly in the succeeding Mytkiv Horizon of several localities in Podolia ( Kozłowski 1929; Nikiforova et al. 1985). The new subspecies differs from the nominative subspecies by a smaller, less transversely extended shell, a better marked ventral sulcus and dorsal fold, greater number of radial costellae (70 to 100 in P. [P.] mariae pantherae, 45–50 in P. [P.] mariae mariae ; Jahnke et al. 1989, however, give a total number of 95–100 costellae for the Kozłowski’s [1929] subspecies), wider interspaces between major costellae, and more numerous fine costellae between each major costella (3–9 in P. [P.] mariae pantherae, 2–4 in P. [P.] mariae mariae ). Internally both subspecies seem also very similar. The internal features of a single available dorsal valve of P. mariae pantherae are less pronounced than those in large specimen of Kozłowski’s nominate subspecies (see Kozłowski 1929: fig. 34A), but they appear virtually identical with those in juvenile specimen of P. (P.) mariae mariae representing the same individual stage of growth (see Kozłowski 1929: fig. 36C) as the former shown here on Fig. 5M View Fig .

The present subspecies differs from Plectodonta (P.) mimica ( Barrande, 1879) from the Lochkovian of Bohemia ( Havlíček 1967) in being somewhat smaller and by having a slightly more rectangular and alate shell outline and dorsal inner side septa appearing further anteriorly. According to Jahnke et al. (1989: 155) the Bohemian form should be regarded as a subspecies of P. (P.) mariae . The inadequatelly known Plectodonta moderatrix Havlíček and Storch, 1990 from the Kopanina Formation (Ludlow) of Bohemia ( Havlíček and Storch 1990) is stratigraphically much older than P. mariae pantherae and differs in having less numerous costellae. Plectodonta petila Amsden, 1958 from the Haragan Formation (Lower Devonian) of Oklahma ( USA; see Amsden 1958) is slightly larger than P. (P.) mariae pantherae and has rounded cardinal extremities. P. (P.) sanglangensis Xian, 1978 from the Tangxiang Formation of Guangxi, South China ( Vogel et al. 1989) differs from the present subspecies in being smaller and having a longitudinal median furrow on the interior of the dorsal valve. P. (P.?) heterosinus Vogel, Xu, and Langenstrassen, 1989 and P. (P.) biplexa Xu, 1979 , both from the same formation (see Vogel et al. 1989), are readily distinguished from the new subspecies by the extremely transverse and alate outline and narrow, strong sulcus and fold of the former, and by the elongate shell of the latter. P. (P.) orientalis Vogel, Xu, and Langenstrassen, 1989 also from the same formation and area, differs in having fascicostellate ornamentation.