Globosochonetes gracilis, 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 : 500

Globosochonetes gracilis sp. nov.

Fig. 10 .

Etymology: From the Latin gracile, meaning “slender” or “lean”; referring to the fine costellation of the shell.

Holotype: PKUM02−0240 ,an almost complete shell illustrated in Fig.10B .

Type locality: Muhua section, between villages of Muhua and Gedongguan (Guizhou province, South China).

Type horizon: Muhua Formation, correlated with the middle Tournaisian, Siphonodella crenulata Zone.

Diagnosis.— Globosochonetes with semicircular shell outline, ventral umbo not prominent, and fine radial ornamentation with density of 8–10 costellae per mm near mid−anterior margin; two to four pairs of orthomorph oblique spines inclined at 53–80 ° to hinge line. Anderidia divergent at 61–78 °.

Material.—Twenty−five complete or fragmented shells, 18 dorsal and 35 ventral valves.

Description.—Shell small in size, attaining 4–5.7 mm in width and 3.5–5 mm in length, semicircular in outline, slightly wider than long, with length/width ratio 0.82–0.90, greatest width at the hinge line; ears small and poorly defined; concavo−convex in lateral profile; two to four pairs of orthomorph oblique spines usually asymmetrically arranged, but on some rare specimens they seem to be more or less symmetrical (compare Brunton 1968: figs. 36–39); spines extend at 53–80 ° to hinge line; shell surface with fine radial costellae which increase by branching on ventral and intercalation on dorsal valve; eight to ten costellae per mm near mid−anterior margin.

Ventral valve strongly and evenly convex in lateral profile, the greatest convexity in the umbonal region, attaining a depth of over one−third of valve length; small elongate protegular pit evident near the beak; ventral interarea flat, orthocline to anacline with small pseudodeltidium.

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.

Ventral valve interior with short, plate−like teeth, subparallel to hinge line; median septum high, confined to umbonal region, usually continues anteriorly as low ridge for less than one−fourth of valve length; adductor scars indistinct; endospines confined to periphery of valve area, arranged in radial rows corresponding to intercostal spaces.

Dorsal interior with small, bilobate cardinal process, posteriorly directed, anteriorly with deep but small alveolus; inner socket ridges slender and straight or slightly curved, divergent anteriorly at 141–160 °; median septum absent; endospines radially arranged, strongest in central region lateral to the accessory septa; accessory septa divergent anteriorly at 21–30 °, formed by two rows of enlarged endospines increasing in size anteriorly; anderidia long, narrow, divergent anteriorly at 60–78 ° ( Fig. 10I ).

Remarks.—The new species is similar to the type species of the genus, i.e., Globosochonetes parseptus Brunton, 1968 in size and shape of the shell (see Brunton 1968). The former differs mainly in having much finer radial costellae and less inflated ventral umbo. Furthermore, the anderidia of the new species diverge from each other at 60–78 °, while in G. parseptus they are divergent at 90 °. It is also noteworthy that short dorsal median septum (breviseptum) reported in gerontic specimens of G. parseptus ( Brunton 1968: 49) is not observed in specimens from Muhua.

Previously, only one species, G. zhongnanensis Yang, 1984 , was reported from South China ( Yang 1984). Its dorsal accessory septa are plate−like, and diverge anteriorly at 6–20 ° while in the new species here described the dorsal accessory septa are ridge like and diverge at 27–30 °. The former attains larger shell size and more transverse outline.

Stratigraphic and geographic range.— Globosochonetes gracilis sp. nov. was found in samples GB and GT.

Figures

Fig. 10. Chonetoid brachiopod Globosochonetes gracilis sp. nov. from the Muhua Formation. A. Ventral valve, PKUM02−0239, in exterior (A1), lateral (A2), and posterior (A3) views. B. Holotype, PKUM02−0240, in ventral (B1), dorsal (B2), lateral (B3), posterior (B4), and anterior (B5) views. C. Shell, PKUM02−0241, in ventral (C1), dorsal (C2), and lateral (C3) views. D. Shell, PKUM02−0242, in ventral (D1), posterior (D2), and anterior (D3) views. E. Ventral valve, PKUM02−0243, in exterior (E1), interior (E2), anterodorsal (E3), and enlarged interior (E4) views, and enlargement showing external ornamentation (E5). F. Ventral valve, PKUM02−0244, in interior view (F1) and enlargement (F2). G, H, K, L. Dorsal valves, PKUM02−0245 (G), PKUM02− 0246 (H), PKUM02−0247 (K), and PKUM02−0248 (L), in exterior (G1, H1, K1, L1) and interior (G2, H2, K2, L2) views. I. Dorsal valve, PKUM02−0249, in exterior (I1), interior (I2), and enlarged interior (I3) views. J. Shell with almost removed dorsal valve, PKUM02−0250, in dorsal view. M. Dorsal valve, PKUM02−0251, in interior view. Samples GB and GT. Scale bars 1 mm.