Subglobosochonetes gedongguanensis, Sun & Baliński, 2008

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 : 496-498

Subglobosochonetes gedongguanensis sp. nov.

Fig. 8 .

Etymology: Named after the type locality.

Holotype: PKUM02−0218 , complete shell illustrated in Fig. 8H .

Type locality: G site, about 20 m west to Gedongguan village , Muhua (Guizhou province, South China) .

Type horizon: Middle of the upper part of the Muhua Formation, correlated with the middle Tournaisian, Siphonodella crenulata Zone.

Diagnosis.— Subglobosochonetes with rounded sub−triangular to semicircular shell outline, well differentiated small ears and relatively fine radial ornamentation of six to seven costellae per mm at 3 mm in distance from the beak. Two to four pairs of spines symmetrically arranged and orthomorph oblique at about 75 °; anderidia anteriorly divergent at 56–60 °.

Material.—Eleven complete to slightly damaged shells, 2 dorsal, and 12 ventral valves.

Description.—Shell small in size, generally attaining 4–6 mm (occasionally over 7 mm) in width and 3.5–7 mm in length, rounded sub−triangular to semicircular in outline, slightly wider than long, with length/width ratio 0.83–0.90, greatest width at the hinge line; cardinal extremities subangular with small, poorly defined and slightly concave ears; concavo−convex in lateral profile; two to four pairs of spines symmetrically arranged and orthomorph oblique at about 75 °; shell surface smooth in the posterior region at 1–1.5 mm from the beak, anteriorly with round radial costellae commonly increasing by branching on ventral valve and intercalation on dorsal valve, interspaces narrower than width of costellae, six to seven costellae per mm at 3 mm distance from the beak and 5–6.5 near mid−anterior margin; concentric growth lines very fine, evenly and closely spaced.

Ventral valve strongly and evenly convex in lateral profile, the greatest convexity occurring in the umbonal region; convexity exceeds one−third of the valve length; small elongate protegular pit evident near the beak; ventral interarea flat, apsacline to orthocline, with small apical pseudodeltidium ( Fig. 8C 3, M 4 ).

Dorsal valve deeply concave posteriorly with gradually decreasing curvature to the anterior and lateral margins; dorsal interarea hypercline with small chilidium; elongate protegular node prominent on the dorsal beak ( Fig. 8A 2 , B 1 ).

Ventral valve interior with short, plate−like teeth, subparallel to hinge line ( Fig. 8C 3, M 4 ); median septum high, short, posteriorly confined, but commonly extends anteriorly as low ridge for less than one−fourth of the valve length; adductor scars indistinct; endospines radially arranged in rows corresponding to interspaces of costellae, frequently confined to valve marginal area, especially strong on lateral flanks ( Fig. 8L 3).

Interior of dorsal valve with small, bilobate, posteriorly directed cardinal process, anteriorly with deep and large cardinal process pit; inner socket ridges slender, straight, anteriorly divergent at 120–130 °; anderidia long, narrow, anteriorly divergent usually at 56–60 ° ( Fig. 8B 3, I 2 ); median septum and accessory septa not present; endospines radially arranged, prominent at middle−anterior sector of the valve.

Remarks.— The general external morphology of the new species is very similar to Globosochonetes Brunton, 1966 , but the dorsal valve interior of the new species is without median septum and accessory septa, and is thus more similar to Caenanoplia Carter, 1967 and Subglobosochonetes Afanasjeva, 1976 . However, Caenanoplia has weak rounded costellae while those in our new species are well marked.

The new species differs from the type species of the genus, i.e., Subglobosochonetes malevkensis ( Sokolskaja, 1950) from Tournaisian of the East European Platform ( Russia) by having less transverse shell outline, finer radial ornamentation, larger divergent angle of the anderidia and much shorter ventral median septum (above one−half in S. malevkensis ; see Sokolskaja 1950, Afanasjeva 1976).

Subglobosochonetes gedongguanensis sp. nov. is similar in shell shape to some adult specimens of Subglobosochonetes cf. malevkensis ( Sokolskaja, 1950) described below. The former, however, is characterized by more rounded and less wide shell outline, while the latter is evidently wider than long, has more rectangular shell outline (especially juveniles), and less concave dorsal valve.

S. gedongguanensis sp. nov. is very similar externally to S. norquayensis Carter, 1987 from Tournaisian of Western Alberta, Canada, especially in a general shell outline, dimensions, and its ornamentation (see Carter 1987). It seems, however, that the Canadian species has much shorter ventral interarea and in consequence less angular, nearly straight posterior margin of the ventral valve. The new species is also more elongate and has a shorter ventral median septum and narrowly divergent anderidia in comparison to S. jerseyensis Carter, 1988 described from the Glen Park Formation of Missouri ( Carter 1988). From S. acutiliratus (Girty, 1928) of the Price Formation (Mississippian) of the United States (see Carter and Kammer 1990) the Chinese species is slightly smaller, more elongate, and has fewer costellae. Moreover, the costellae are subangular in the former but rounded in the latter species. Internally, S. gedongguanensis sp. nov. differs in lacking accessory septa in the dorsal valve. From Caenanoplia martinezi Martinez−Chacon and Winkler Prins, 1977 [= Caenanoplia (Subglobosochonetes) martinezi according to Martinez−Chacon and Winkler Prins 1993] described from the Namurian of Spain ( Martinez−Chacon and Winkler Prins 1977) the new species differs in being more strongly concavo−convex, having a longer shell and stronger but finer costellation. Stratigraphic and geographic range.—This species was found only in samples GB and GT.

Figures

Fig. 8. Chonetoid brachiopod Subglobosochonetes gedongguanensis sp. nov. from the Muhua Formation. A. Shell, PKUM02−0214, in ventral (A1) and dorsal (A2) views. B. Dorsal valve, PKUM02−0215, in exterior (B1), interior (B2), and enlarged interior (B3) views. C. Ventral valve, PKUM02−0216, in exterior (C1), interior (C2), and enlarged interior (C3) views. D, H. Shells, PKUM02−0217 (D) and PKUM02−0218 (H, holotype), in ventral (D1, H1), dorsal (D2, H2), and posterior (D3, H3) views. E–G, J. Shells, PKUM02−0219 (E), PKUM02−0220 (F), PKUM02−0221 (G), and PKUM02−0222 (J), in ventral views.I. Incomplete dorsal valve, PKUM02−0223, in exterior (I1) and interior (I2) views. K. Ventral valve, PKUM02−0224, in exterior (K1) and interior (K2) views. L. Ventral valve, PKUM02−0225, in exterior (L1), lateral (L2), interior (L3), and posterior (L4) views. M. Ventral valve, PKUM02−0226, in exterior (M1), lateral (M2), interior (M3), and enlarged interior (M4) views. N. Ventral valve, PKUM02−0227, in exterior (N1), interior (N2), lateral (N3), and posterior (N4) views. Samples GB and GT. Scale bars 1 mm.

Fig. 4. Different results of the silicification of brachiopod valves from the Muhua Formation. A. Subradial lamellae of silica arising along fibres of the secondary shell layer as seen on the external surface of shell. B. Net−like pattern formed by silica on the internal surface of the terebratulid valve reflecting the mosaic of the terminal faces of fibres (see text for further explanation). C. Punctate shell microstructure preserved on a silicified terebratulid valve. D. Spiriferoid ventral valve silicified with “spongy” silica; general view (D1) and enlargement (D2).

Fig. 2. Lithological sections of the Tournaisian Muhua Formation at five sampling sites around Gedongguan village and their locations near Muhua, Guizhou Province, China (modified after Dzik 1997).

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).