Nipponithyris lauensis, Bitner, 2008

Bitner, Maria Aleksandra, 2008, New data on the recent brachiopods from the Fiji and Wallis and Futuna islands, South-West Pacific, Zoosystema 30 (2), pp. 419-461 : 446-448

Nipponithyris lauensis n. sp.

( Fig. 17 )

TYPE MATERIAL. — Fiji. Lau Ridge , BORDAU 1, stn DW 1485, holotype (MNHNBRA-3111; Fig.17 D-G).— Same data, stn DW 1408, DW 1432, DW 1485, 4 paratypes ( MNHN BRA-3112-3115 ; Fig. 17 A-C, H-M) .

TYPE LOCALITY. — Fiji Islands, Lau Ridge, BORDAU 1, Ridge”, type locality of the species.

stn DW 1485, 19°02.69’S, 178°29.80’W, 700- 707 m.

DIAGNOSIS. — Nipponithyris of medium size, strongly ETYMOLOGY. — Referring to the geographic name “Lau unisulcate, posterior part strongly thickened, teeth with swollen bases, no crura, loop trabelacur with long, slender descending branches and broad transverse band.

MATERIAL EXAMINED. — Fiji. MUSORSTOM 10, Viti Levu, stn CP 1354, 1 complete specimen. — Stn CP 1361, 1 complete specimen.

BORDAU 1, Lau Ridge, stn DW 1408, 12 complete specimens, 1 dorsal valve. — Stn CP 1409, 3 complete specimens. — Stn DW 1432, 33 complete specimens. — Stn CP 1458, 2 complete specimens. — Stn DW 1485, 3 complete specimens.

DEPTH RANGE. — 477-1226 m.

MEASUREMENTS. — See Table 17.

DESCRIPTION

Shell of medium size with maximum length 13.6 mm, rounded pentagonal, longer than wide, smooth and moderately ventribiconvex. Lateral commissures dorsally curved; anterior commissure from moderately to strongly unisulcate. Beak low, erect to slightly incurved with weakly defined beak ridges. Foramen small, circular, mesothyrid to permesothyrid. Deltidial plates conjunct forming a well-developed, visible symphytium. Posterior part of both valves strongly thickened, making the articulation of the shell very strong. Pedicle collar short. Ventral delthyrial cavity very narrow. Teeth short but strong with swollen bases, below which there are excavated grooves for the reception of the inner socket ridges that are narrow but massive. In the dorsal valve the dental sockets are very deep. Cardinal process semi-elliptical, prominent. Hinge plates are overgrown with secondary thickening ( Fig. 17H ). Crura absent; crural processes long, needle-like and curved inward (see Fig. 17H, I ). Median septum highest at the point of the union with lateral connecting bands and extending anteriorly as a low ridge. Loop trabecular; descending branches long and slender, attached to the septum by the connecting bands ( Fig. 17I ); ascending branches and a transverse band broad. Muscle scars deeply impressed in both valves.

REMARKS

The genus Nipponithyris is characterized by its rounded pentagonal outline and smooth, sulcate shell which is strongly thickened posteriorly ( Yabe & Hatai 1934a; Hatai 1940; Cooper 1973b; Laurin 1997). The studied specimens display all those characters, thus indicating their attribution to Nipponithyris but they differ from N. nipponensis Yabe & Hatai, 1934 and N. afra Cooper, 1973 in the absence of crura (see Hatai 1940; Cooper 1973b; Laurin 1997). None of the investigated specimens shows any sign of the development of the crura but their crural processes are long and needle-like, contrary to the short and bluntly pointed crural processes in N. nipponensis and N. afra ( Cooper 1973b) . Externally, in size and strong sulcation, N. lauensis n. sp. is close to N. afra , known from the Indian Ocean ( Cooper 1973b; Hiller 1996) and the Loyalty Islands, SW Pacific ( Laurin 1997).

Nipponithyris lauensis n. sp. displays also, both externally and internally, a great similarity to the species from off Vietnam, Holobrachia vietnamica described by Zezina (2001). Both those species are characterized by the absence of crura. Holobrachia vietnamica possesses, however, dental plates which differentiate this species from the Fiji material and any Nipponithyris species.

The specimens under study are also similar to N.fijiensis ( Elliott, 1961) from the Upper Miocene-Lower Pliocene deposits of Fiji ( Cooper 1978). This latter species was originally described from the Neogene of Fiji by Elliott (1961) under the name Abyssothyris fijiensis . However, examination of the internal structure ( Cooper 1978) revealed that it possesses a median septum that eliminates it from assignment to Abyssothyris . Although indicating similarity to Nipponithyris, Cooper (1978) created for this material a new genus: Dicrosia . In the revised edition of the Treatise, Dicrosia is treated as a synonym of Nipponithyris ( MacKinnon & Lee 2006b) .

Figures

FIG. 17. — Nipponithyris lauensis n. sp., Fiji, Lau Ridge, BORDAU 1: A-C, dorsal, anterior and lateral views of complete specimen, paratype (MNHN BRA-3112), stn DW 1485, 700-707 m; D-G, ventral, dorsal, laterial and anterior views of complete specimen, holotype (MNHN BRA-3111), stn DW 1485, 700-707 m; H, I, posterior part of dorsal valve interior, and enlargement (I) to show details of connecting band and crural processes, paratype (MNHN BRA-3113), SEM, stn DW 1432, 477-493 m; J, K, dorsal and side views of interior of dorsal valve to show brachial loop, paratype (MNHN BRA-3114), SEM, stn DW 1408, 550-561 m; L, M, inner views of ventral valve, tilted (M) to show swollen bases and grooves to accommodate inner socket ridges, paratype (MNHN BRA-3115), SEM, stn DW 1432, 477-493 m. Scale bars: A-G, 0.5 cm; H, I, 1 mm; J-M, 2 mm.