Holynetes caurongensis, Racheboeuf & Phuong & Hung & Feist & Janvier, 2006

Racheboeuf, Patrick, Phuong, Ta Hoa, Hung, Nguyen Huu, Feist, Monique & Janvier, Philippe, 2006, Brachiopods, crustaceans, vertebrates, and charophytes from the Devonian Ly Hoa, Nam Can and Dong Tho formations of Central Vietnam, Geodiversitas 28 (1), pp. 5-36 : 28-30

? Holynetes caurongensis n. sp. ( Fig. 18 J-O)

HOLOTYPE. — External mould of a ventral valve ( BT 231 j; Fig. 18J ).

ETYMOLOGY. — From the type locality, Cau Rong.

LOCALITY AND AGE. — Cau Rong, Minh Hoa district, Quang Binh Province, Central Vietnam. Outcrop along the road between Qui Dat and Ly Hoa. Lower part of the Dong Tho Formation, Givetian ( Fig. 17B ).

MATERIAL EXAMINED. — 48 decalcified isolated internal and external moulds of both valves, and external moulds of articulated shells, most of them tectonically distorted (some specimens are illustrated in Figure 18 J-O)

DIAGNOSIS. — Three orthomorph perpendicular spines on the right side of the shell only; costellae originating by bifurcation on ventral valve, and by intercalation on dorsal valve; costellae number 11-14 per 2 mm; dorsal valve interior with stout isolated endospines, deep and large cardinal process pit, and poorly developed (or absent?) accessory septa.

DESCRIPTION

Shell small and transverse in outline (maximum width: 8.0 mm; corresponding length: 5.6 mm), with maximum width at hinge line. Shell concavoconvex with longitudinal profile of ventral valve regularly, and markedly arched. Posterolateral branches of commissures at about 90° to hinge line. Length/width ratio decreasing with growth from 0.83 (ventral valve 1.7 mm long) to about 0.65 for ventral valves more than 4.5 mm long. Ventral interarea apsacline and dorsal one hypercline; both interareas flat and lying in the same plane. Small but prominent pseudodeltidium and chilidium.Ornament of narrow, rounded and relatively high radial ribs with intervals of the same width. Costellae originating by bifurcation on ventral valve, and by intercalation on dorsal valve. A ventral valve (4.8 mm long) exhibits a total of 55 costellae along the commissure, eight of them originating by bifurcation; costellae number 11-14 per 2 mm at anterior margin. Spines orthomorph, high-angled (about 90°), on the right side of the shell only, respectively at 0.2 mm, 0.7 mm and 2.5 mm from beak.

Ventral valve interior with a short and thin myophragm, not overpassing one-fourth the valve length. Teeth and muscle field not observed. Visceral cavity with scarce, isolated, relatively strong endospines in its posterolateral parts. Periphery of the valve impressed by the external ornament.

Dorsal valve interior with deep cardinal process pit and relatively stout, elongated and rounded inner socket ridges. Other characters are not observed.

DISCUSSION

Because of the poor development, or of the possible absence of accessory septa, this new species is only tentatively assigned to the genus Holynetes . However, among Eifelian representatives of the genus, H. holynensis Havliček & Racheboeuf, 1979 from the Czech Republic, and H. cf. holynensis from Spain, exhibit variably developed, often hardly perceptible accessory septa ( Havliček & Racheboeuf 1979; Racheboeuf 1981). The lack of complete, well preserved dorsal interiors of the specimens from Vietnam precludes definite statement about the real lack of accessory septa in the new species.

? Holynetes caurongensis n. sp. closely resembles H. stephaniae Racheboeuf, 1981 (Givetian, Kersadiou Formation, western Massif Armoricain, France) in size, shape, outline, and ornament. However H. stephaniae bears a unique spine on the right side of the shell, and it exhibits a pair of well developed, long and blade-like accessory septa. The new species can be distinguished from H. holynensis by the presence of three spines instead of two in the latter, as well as by its ornament; in H. holynensis ventral radial ribs increase both by intercalation and bifurcation on the ventral valve, and mostly by bifurcation on the dorsal valve.

ENVIRONMENT

In its type locality,? Holynetes caurongensis n. sp. is only associated with locally abundant dacryoconarids (homoctenids), bivalves (aviculopectinids) and rare, small, smooth ostracods. The pink colour of the sediment is due to weathering. Originally, it probably consisted of dark to black shale and silty shale. The faunal assemblage is suggestive of a relatively deep benthic assemblage position, corresponding to the benthic assemblages BA3 or BA 4 in an outer shelf position.

DISTRIBUTION

To date,? Holynetes caurongensis n. sp. is only known from the Givetian of Central Vietnam. The genus Holynetes is currently known from the Eifelian of the Czech Republic ( H. holynensis ; Choteč limestone, Pinacites jugleri Horizon) and the Eifelian-Givetian of the Massif Armoricain, France ( H. musculosus ; Saint-Fiacre Formation, Eifelian, Costatus costatus conodont Zone, and H. stephaniae ; Kersadiou Formation, Givetian, Varcus conodont zone; Lethiers & Racheboeuf 1999). In Spain, H. cf. holynensis was reported from the Gustalapiedra Formation of Palencia and from the Moyuela and Barreras formations of the Iberian Chains, i.e. in upper Eifelian to lower Givetian beds (Garcia-Alcalde et al. 2000). Outside Europe, the genus was reported from the Lower Devonian of Guangxi, South China ( H. abnormis, Zlichovian ) by Vogel et al. (1989), but this generic assignment, although probable, remains questionable.

Family CHONETIDAE Bronn, 1862

Figures

FIG. 18. — A-I, Corbicularia noongdensis n. sp.,middle part of the Nam Can Formation,Givetian-Frasnian,Noong De (Bactelemi) Pass, Ky Son District, Nghe An Province, Vietnam; A-C, three ventral valve interiors, latex casts (BT 231a, b and c, respectively); D, dorsal side of an articulated shell, latex cast (BT 231d); E-G, three ventral valve internal moulds (BT 231e, f and g, respectively); H, I, two dorsal valve interiors, latex cast and internal mould (BT 231h and i, respectively); J-O, Holynetes caurongensis n. sp., lower part of the Dong Tho Formation, Givetian, Cau Rong, Minh Hoa District, Quang Binh Province, Vietnam; J, ventral valve exterior, latex cast (BT 231j); K, L, dorsal side of two articulated shells, latex casts (BT 231k and l, respectively); M, ventral valve interior (BT 231m); N, O, two incomplete dorsal interiors, latex casts (BT 231n and o, respectively). Scale bar: 5 mm.

FIG. 17. — Locality map: A, Muong Xen area, Nghe An Province (MX 1, brachiopod locality of Noong De Pass); B, Quy Dat and Cau Rong areas, Quang Binh Province (CR1, antiarch and charophyte-bearing locality of Cau Rong; CR2, brachiopod-bearing locality of Cau Rong; BN1, brachiopod locality of Ba Nuong).