Gracianella (Sublepida) paulula, 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 : 915-918

Gracianella (Sublepida) paulula sp. nov.

Fig. 14A–G.

Etymology: From Latin paulula , in reference to the very small shell.

Holotype: Complete shell ZPAL Bp. 56/183; illustrated in Fig. 14F.

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

Type horizon: Bed −4, lowest part of the Khudykivtsi Beds of the Borshchiv Horizon (Early Devonian).

Diagnosis.—Very small, lenticular, subcircular in outline with pointed ventral beak; distinct, apsacline to nearly orthocline ventral interarea, delthyrium open; ribs thick, about 8–10 on each flank, increasing by intercalation and bifurcation on both valves; ventral midrib pair prominent, divided by deep interspace with a thin, thread−like median costella; dorsal valve with well marked, wide sulcus and thick and simple median rib; lacking distinct concentric lamellose ornamentation.

Material.—109 complete shells and 53 fragmentary specimens. This species is quite common in the shale at 0.4 m (bed −4) above the S–D boundary. A few specimens were also found at 1.5 m (bed 7) above the boundary, as well as 0.2 m (bed −10) below it. In the studied section this short−ranging species occures about 1.1 m above the last occurrence of Dnestrina gutta and 1.2 m above Dayia bohemica (see Fig. 3 ).

Description.—Shell small, up to about 4 mm in length, biconvex, lenticular, subtriangular posteriorly and subcircular anterolaterally in outline, approximately as wide as long or slightly wider, widest slightly anteriorly to midlength; anterior margin rounded to weakly emarginate, hinge line short, almost straight, anterior commissure weakly sulcate.

Ventral valve weakly convex, slightly carinate, subtriangular in anterior profile; ventral beak pointed, interarea distinct, flat, apsacline to nearly orthocline, up to 0.6 mm in height; delthyrial cover not preserved. Dorsal valve weakly convex, subcircular to transversely elliptical, with weak to moderate median sulcus originating near umbo and widening markedly anteriorly.

Shell ornamented with thick radial ribs originating near umbos and delicate concentric, non−lamellose growth lines; ribs thickening anteriorly, separated by equal in width interspaces, increasing by intercalation and bifurcation on both valves; about 8–10 ribs on each flank; ventral midrib pair slightly carinate, thicker and more strongly elevated than lateral ribs, bifurcating laterally usually 3–4 times, separated by deeper and wider median interspace, the bottom of the interspace with a simple, very thin and low thread−like median costella; dorsal median rib simple, thicker than lateral ones, rarely with a very weak median furrow.

Remarks.—The main features of the specimens are very small and prominently ribbed shell, generally rounded outline with

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

gently rounded to slightly emarginate anterior commissure, 66, pls. 27: 42, 48; 28: 1–32). The former differs in having a weakly carinate ventral midrib pair with deep median inter− shell which is more expanded laterally, a shorter hinge margin, space occupied by thin median costella, sulcate dorsal valve and in the greatest shell width situated slightly more anteriwith distinct simple median rib, apsacline to orthocline ventral orly. Furthermore, the presence of a ventral median costella, interarea, and open delthyrium. Although the specimens are which is very characteristic for the new species, was not mennumerous they are frequently compressed by sediment com− tioned in the original description of A. canadensis nor could be paction and sparitic filling of the interior, making study of the discernible on the figures in Smith (1980). Recently the genus shell interior impossible. The small shell dimensions and char− Arctispira was considered as a possible subjective synonym of acter of the shell ribbing suggest assignment to Gracianella Ogilviella Lenz, 1968 ( Copper 2002: 1432).

(Sublepida) Mizens and Sapelnikov, 1982. The present species differs from Gracianella plicumbra While erecting Gracianella (Sublepida), Mizens and Johnson and Boucot, 1967 from the Ludlow of the Robert Sapelnikov (1982) included four species within the subge− Mountains (Nevada, USA: Johnson and Boucot 1967; Johnnus, Terebratula sublepida Verneuil, 1845 (type species), son et al. 1976) and Bohemia ( Havlíček and Štorch 1990), Gracianella plicumbra Johnson and Boucot, 1967 , G. crista and the Pridoli of Yukon Territory ( Canada: Jackson et Johnson and Boucot, 1967, and G. cryptumbra Johnson , al.1978; Lenz 1982) in having a wider shell, less rounded an− Boucot, and Murphy, 1973. The type species G. (Sublepida) terior commissure, wider dorsal sulcus with strong median sublepida was originally described by Verneuil (1845) from rib, and a distinct midrib pair on the ventral valve. From the uppermost Silurian of the east slope of the Urals, Russia Gracianella crista Johnson and Boucot, 1967 , from the Lud− (a probable topotype shell is shown here in Fig. 14K; see also low of the Robert Mountains (Nevada, USA: Johnson and Mizens and Sapelnikov 1982; Breivel and Breivel 1988). Boucot 1967; Johnson et al. 1976) and Bohemia (Havlíček Conspecific specimens have been reported by Modzalev− and Štorch 1990), the new species is distinguished by a more skaya (1968: pl. 15: 4–7) and Nikiforova et al. (1972: 89, circular instead of subrhomoidal shell outline, weaker ribs, 227) from the uppermost Silurian beds of the section at a wider dorsal sulcus with strong median rib, and clearly Dnistrove as Atrypa sublepida and Zygospiraella sublepida , divided median ventral ribs. Some finely ribbed specimens respectively. It should be noted, however, that Gracianella of a highly variable G. cryptumbra Johnson, Boucot , and (Sublepida) sublepida occurs at Dnistrove about 0.6–3.6 m Murphy, 1973 from the Pridoli of the Robert Mountains (Nebelow the S–D boundary (see Nikiforova 1977: fig. 3), vada, USA: Johnson et al. 1973) and Yukon Territory (Canwhereas G. (S.) paulula sp. nov. was found in the interval ada: Lenz 1977; Jackson et al. 1978) resembles G. (S.) spanning from 1.5 m above the boundary to 0.2 m below it. paulula sp. nov, but the latter can be easily distinguished by Due to the poorly exposed lowermost part of the section at its ribbing pattern. More often, however, the shells of the for− Dnistrove, G. (S.) sublepida was not found during the present mer species have fewer rather weak, and sometimes coalesinvestigations; however, one shell of the species from the lo− cent ribs, and a very broad, low median ventral rib.

cality from the Tatyana Modzalevskaya’s (1968) collection is here presented for comparison ( Fig. 14L). Thus, the ranges Suborder Lissatrypidina Copper, 1996

of the two species mentioned do not overlap, but are sepa−

Superfamily Lissatrypoidea Twenhofel, 1914

rated by an interval about 0.4 m thick in which Dnestrina gutta is very characteristic ( Fig. 3 ; see also Nikiforova 1977: Family Lissatrypidae Twenhofel, 1914

fig. 3). G. (S.) paulula sp. nov. differs from G. (S.) sublepida Genus Lissatrypa Twenhofel, 1914

mainly in having a smaller shell, different ribbing pattern

Type species: Lissatrypa atheroidea Twenhofel, 1914 ; Jupiter Forma− (presence of distinct ventral midrib pair divided by deep tion, Llandovery, Silurian; Anticosti Island, Canada.

interspace), open delthyrium, and the absence of conspicu−

ous, undulating growth lamellae. Lissatrypa leprosa Kozłowski, 1929

G. (S.) paulula sp. nov. seems to be most similar externally Fig. 15C .

to Arctispira canadensis Smith, 1980 described from the Early 1929 Lissatrypa leprosa sp. n.; Kozłowski 1929: 167–169, pl. 5: 15–21; Lochkovian of the Canadian Arctic Archipelago ( Smith 1980: figs. 54, 55.

H

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1968 Lissatrypa leprosa Kozl. ; Modzalevskaya 1968: pl. 27: 44–47.

1984 Lissatrypa neglecta sp. n.; Havlíček 1984: 220, pl. 1:6, 7; pl. 2: 3–8.

1985 Lissatrypa leprosa Kozłowski, 1929 ; Nikiforova et al. 1985: 42–44, pls. 10: 4, 5; 11: 1–8; fig. 5.

1989. Lissatrypa leprosa Kozłowski 1929 ; Jahnke et a. 1989: 162–163, pls. 1: 19–21; 2: 1–3; fig. 23.

1990 Lissatrypa leprosa Kozłowski, 1929 ; Havlíček and Štorch 1990: 193, pl. 65: 9.

Material.—One almost complete shell was found in a loose block of limestone near the bottom of the Khudykivtsi Beds together with Clorinda pseudolinguifera Kozłowski, 1929 and Sphaerirhynchia gibbosa ( Nikiforova, 1954) .

Remarks.—Although the single available specimens can not be studied internally, its exterior is indistinguishable from that of L. leprosa Kozłowski, 1929 . Nikiforova et al. (1985) selected a neotype of the species from the Mytkiv (= Mitkov) Beds of the Borshchiv Horizon at Ustje and re−figured several well−preserved specimens externally and internally.

Copper (2004: 110–111) listed 41 species assigned tentatively to the genus Lissatrypa Twenhofel, 1914 .

Stratigraphic and geographic range.—According to Nikiforova et al. (1985) the range of L. leprosa extends through the Khudykivtsi and lower Mytkiv beds. Havlíček and Štorch (1990) described the species in the top of the Pridoli Formation and the lowermost part of the Lochkov Formation in Bohemia. The species was also described by Jahnke et al. (1989) from the Lower Devonian of Yunnan Province, China.

Figures

Fig. 3. Distribution of brachiopod species and their frequency in the section at Dnistrove village. Secular pattern of δ13C (black curve) and δ18O (grey curve) values after Małkowski and Racki (2009) and Racki et al (2012). Lithology after Nikiforova et al. (1972: fig. 82).

Fig. 15. Atrypide brachiopods from the Silurian–Devonian boundary beds at Dnistrove (Podolia, Ukraine). A, B. Sepatrypa (Septatrypa) secreta Kozłowski, 1929. Two shells ZPAL Bp. 56/74 (A) and ZPAL Bp. 56/75 (B) in dorsal (A1, B1), ventral (A2, B2), lateral (A3, B3), posterior (A4, B4), and anterior (A5, B5) views. C. Lissatrypa leprosa Kozłowski, 1929; shell ZPAL Bp. 56/100a in dorsal (C1), ventral (C2), lateral (C3), posterior (C4), and anterior (C5) views. Beds 47–48 (A, B) and loose block between the trench and well (C). Scale bars 1 mm.