Corbicularia noongdensis, 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 : 26-28

Corbicularia noongdensis n. sp. ( Fig. 18 A-I)

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

ETYMOLOGY. — From the name of the type locality, the

Noong De Pass. LOCALITY AND AGE. — Noong De (Bactelemi) Pass, Ky

Son district, Nghe An Province, Central Vietnam. Middle part of the Nam Can Formation, Givetian-Frasnian ( MX 1, Fig. 17A ).

MATERIAL EXAMINED. — More than 50 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 18A , C-I.

DIAGNOSIS. — Small shell with a maximum of 32 ventral costellae, increasing by intercalation only, with 6-7 costellae per mm along anterior margin; two pairs of orthomorph, high-angled spines.

DESCRIPTION

Very small shell (maximum length: 2.4 mm; corresponding width: 3.2 mm), weakly transverse in outline, with moderately and regularly convex ventral valve, and weakly concave dorsal valve. Maximum width at hinge line. Ventral interarea flat, apsacline with small, prominent pseudodeltidium; dorsal interarea low and hypercline with small chilidium. Ornament of low, rounded costellae with a maximum of 32 costellae along the commissure; six to seven costellae per mm along anterior margin. Costellae increasing by intercalation only in ventral valve, and by bifurcation only in dorsal one. Radial ornament crossed by very thin concentric fila.Two orthomorph, high-angled, and symmetrically displayed spines on each side of beak, respectively at about 0.7 mm and 1.8 mm from beak.

Ventral valve interior with myophragm not extending anteriorly beyond one third the valve length. Diductor scars poorly impressed; adductor scars oval to semi-elliptical in outline, well limited.Teeth not observed. Periphery of the valve impressed by the outer ornament.

Dorsal valve interior with a very short breviseptum originating anteriorly to a narrow, elongate cardinal process pit. Cardinal process not observed. Two radial rows of stouter endospines, spinose accessory septa, on each side of the breviseptum, and reaching anterior margin of the valve. Inner socklet ridges long and narrow, extending laterally parallel to hinge line. Anderidia very small but well defined, anteriorly divergent at about 90°. Muscle scars indistinct. Surface of the valve covered with relatively coarse, spaced and radially arranged endospines.

COMPARISON

By its external morphology, and its internal features as well, this new species from Vietnam is close to Corbicularia parisi ( Racheboeuf, 1981) and C. goasquellouensis ( Racheboeuf, 1981) , both from the Frasnian of the Massif Armoricain, France. However, C. noongdensis n. sp. exhibits a smaller shell, with thinner radial ribs (6 to 7 per mm). C. parisi has four costellae per mm along the anterior margin, and Corbicularia goasquellouensis five costellae in the same interval. Corbicularia noongdensis n. sp. has two spines on each side of the beak, like C. parisi , but the ventral costellae increase by intercalation only in C. noongdensis n. sp., instead of increasing by bifurcation, with rare intercalations towards the shell margin, in C. parisi . The ornament is quite similar to that of C. goasquellouensis , but the radial ribs are thinner, and the Armorican species has only one pair of spines instead of two.

DISTRIBUTION

To date, Corbicularia noongdensis n. sp. is only known from the Givetian-Frasnian of Central Vietnam. Other species of the genus Corbicularia are all late Givetian or Frasnian. In North Timan and the Volga-Ural region of Russia, Corbicularia menneri (Lyaschenko, 1958) occurs in the upper part of the lower Frasnian. C. cracoviensis Balinski, 1979 has been described from the lower Frasnian (Lower asymmetricus zone) of the Debnik Anticline, in southern Poland. In the western Massif Armoricain, C. parisi is restricted to the upper part of the Kergarvan Formation (uppermost Givetian, hermanni-cristatus zone), while C. goasquellouensis ranges through the Traonliors Formation (lower Frasnian, asymmetricus zone).

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).