Subglobosochonetes cf. malevkensis ( Sokolskaja, 1950 )

Sun, Yuanlin & Baliński, Andrzej, 2008, Silicified Mississippian brachiopods from Muhua, southern China: Lingulids, craniids, strophomenids, productids, orthotetids, and orthids, Acta Palaeontologica Polonica 53 (3), pp. 485-524 : 498-500

Subglobosochonetes cf. malevkensis ( Sokolskaja, 1950)

Fig. 9 .

cf. 1950 Chonetes (Rugosochonetes) malevkensis sp. nov.; Sokolskaja

1950: 23–27, pl. 1: 1–16, figs. 10–11. cf. 1976 Subglobosochonetes cf. malevkensis (Sokolskaja) ; Afanasjeva

1976: 68–69, pl. 5: 4.

Material.—Nine complete to slightly fragmented shells, 29 dorsal and 49 ventral valves.

Description.—Shell small to medium in size for genus, subquadrate in outline, attaining 5–9 mm in width and 4–6.8 mm in length, with length:width ratio 0.72–0.80; greatest width near the hinge line; cardinal extremities subangular with poorly differentiated small ears; concavo−convex in lateral profile, about one−fourth as deep as long; three to four pairs of orthomorph oblique spines symmetrically arranged at an angle varying 48–58 ° from the hinge line; shell surface smooth for the first 1 to 1.5 mm from the beaks and anteriorly with round radial costellae that increase mainly by bifurcation on ventral valve and intercalation on dorsal valve; there are 5–6 costellae per mm at 3 mm distance from ventral beak and 3.5–5.5 near mid−anterior margin; interspaces narrower than width of costellae; concentric growth lines very fine, evenly and closely spaced.

Ventral valve moderately convex, the greatest convexity in the umbonal region, attaining about one−fourth of valve length; small elongate protegular pit evident near the beak; ventral interarea flat, apsacline, with small pseudodeltidium.

Dorsal valve slightly concave; dorsal interarea hypercline with small chilidium; elongate protegular node prominent on the dorsal beak ( Fig. 9A, B, G, H 1 ).

Ventral valve interior with short, plate−like teeth that are subparallel to hinge line; median septum high, posteriorly confined but commonly extended anteriorly as low ridge for about one−fifth of valve length; adductor scars indistinct; endospines confined to marginal sector of the valve, the strongest near ears, arranged in radial rows corresponding to interspaces of costellae.

Dorsal interior with small, bilobate, posteriorly directed cardinal process, which bears anteriorly a deep and large pit; inner socket ridges slender, straight, or slightly curved, anteriorly divergent at 118–130 °; anderidia long, narrow, anteriorly divergent at 50–54 °; median septum and accessory septa absent; endospines radially arranged, prominent in middle and anterior sectors of valve ( Fig. 9G 3, H 3).

Remarks.—The studied specimens are similar to Subglobosochonetes malevkensis in shell outline and ornamentation. There are small differences in the length/width ratio and the angle of divergence of the anderidia, which is slightly greater in the Chinese specimens. S. malevkensis differs also in possessing a more inflated posterior region of the ventral valve. See Subglobosochonetes gedongguanensis sp. nov. for comparison with that species.

Although the external outline and ornamentation of the Chinese specimens are very similar to those of most species of Rugosochonetes , the lack of median septum in the dorsal valve may exclude the possibility that the species is a rugosochonetid.

Subglobosochonetes cf. malevkensis is similar in general outline of the shell to S. acutiliratus (Girty, 1928) described from the Price Formation by Carter and Kammer (1990). The species from the Muhua Formation can be differentiated by its smaller size, more closely spaced and rounded costellae, and absence of dorsal accessory septa. It also differs from S. jerseyensis Carter, 1988 in having more regular shell costellation and in lacking accessory septa and brachial ridges (see Carter 1988).

Stratigraphic and geographic range.—The species has been recovered from sample MH1, M2−8, M2/2, M2/3, M2/4, and D4. Subglobosochonetes malevkensis is known from the Tournaisian of the East European Platform, Russia ( Sokolskaja 1950; Afanasjeva 1976).

Figures

Fig. 9. Chonetoid brachiopod Subglobosochonetes cf. malevkensis (Sokolskaja, 1950) from the Muhua Formation. A, B. Shells, PKUM02−0228 (A) and PKUM02−−0229 (B) in ventral (A1, B1) and dorsal (A2, B2) views. C, D. Ventral valves, PKUM02−0230 (C) and PKUM02−0231 (D), in exterior (C1, D1) and interior (C2, D2) views. E. Ventral valve, PKUM02−0232, in exterior (E1), interior (E2), lateral (E3), anterior (E4), posterior (E5), and enlarged interior (E6) views. F. Ventral valve, PKUM02−0233, in exterior (F1), interior (F2), lateral (F3), anterior (F4), and enlarged interior (E5) views. G, H, K. Dorsal valves, PKUM02−0234 (G), PKUM02−0235 (H), and PKUM02−0236 (K), in exterior (G1, H1, K1), interior (G2, H2, K2), and enlarged interior (G3, H3, K3) views. I. Incomplete shell, PKUM02−0237, in ventral (I1), dorsal (I2), and enlarged dorsal (I3) views. J. Dorsal valve, PKUM02−0238, in interior view (J1) and enlargement (J2). Samples M2/2 and M2/4 (A–H), M2−8 (I), M2/3 (J, K). Scale bars 1 mm.

Fig. 1. Outcrops of the early Carboniferous (Mississippian) formations between the villages of Muhua and Gedongguan, Guizhou Province, China. A. View of the outcrop taken from the road leading from Gedongguan (bottom) to Muhua (far left). B. Closer view of the outcrop showing the Devonian–Carboniferous boundary and the Mississippian formations (location of the MH1 and MH2 samples marked). C. Oolitic packstone lens of the Muhua Formation and closer location of the MH2 sample. D. Outcrop G near Gedongguan village (location of the GB and GT samples marked).