Leptothyrella fijiensis, 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 : 442-444

Leptothyrella fijiensis n. sp.

( Fig. 13 )

TYPE MATERIAL. — Fiji. Bligh Water , MUSORSTOM 10, stn CP 1332, holotype ( MNHN BRA-3101 ;

Fig. 13 B-E). — Fiji, MUSORSTOM 10, Bligh Water , stn CP 1332 and Viti Levu, stn CP 1354, 4 paratypes ( MNHN BRA-3102-3104 , 3106 ; Fig. 13 F-L) .

TYPE LOCALITY. — Fiji Islands, Bligh Water, MUSORS- TOM 10, 16°56.17’S, 178°07.86’E, 640- 687 m.

ETYMOLOGY. — From the geographical name “ Fiji ”, type locality of the species.

DIAGNOSIS. — Leptothyrella having rows of small tubercles at both sides of the beak, short, recessive dental plates, and descending branches attached to the rod-like septum.

MATERIAL EXAMINED. — Fiji. MUSORSTOM 10, Bligh Water, stn CP 1309, 1 ventral valve. — Stn DW 1314, 2 complete specimens. — Stn CP 1330, 1 complete specimen. — Stn CP 1331, 1 complete specimen. — Stn CP 1332, 82 complete specimens, 1 ventral valve, 1 dorsal valve. — Stn CP 1341, 23 complete specimens. — Stn DW 1345, 3 complete specimens. — Viti Levu, stn CP 1353, 62 complete specimens, 1 ventral valve, 1 dorsal valve. — Stn CP 1354, 34 complete specimens. — Stn CP 1363, 1 complete specimen. — Stn CP 1366, 1 complete specimen. — Stn CP 1369, 2 complete specimens.— Stn DW 1384, 1 complete specimen.

DEPTH RANGE. — 144- 963 m.

MEASUREMENTS. — See Table 15; see also Figure 14.

DESCRIPTION

Shell small (maximum observed length 4.4 mm), elongate oval in outline with greatest width at about midlength; dorsal valve often subcircular. Shell surface smooth, except for weakly defined concentric growth lines, and coarsely punctate. Both valves are weakly biconvex. Anterior commissure rectimarginate. Beak high, truncated by a large, triangular hypothyrid foramen bordered by very narrow deltidial plates; beak ridges strong. At both sides of the beak there are two rows of small tubercles ( Fig. 13J ). Pedicle collar sessile. Teeth small supported by short, thick dental plates ( Fig. 13K ). Dorsal valve interior with a high median septum with rod-like extremity. Inner socket ridges high but narrow. No cardinal process or hinge plates are present. Crura very long and slender; crural processes very weakly developed. Descending branches attached to the septum; ascending branches absent. Lophophore zygolophous, heavily spiculate ( Fig. 13H ).

REMARKS

The genus Leptothyrella was originally described from the Indian Ocean by Muir-Wood (1959); however the re-examination of the type material has shown that her

Length (mm)

description contains several errors (D. MacKinnon, pers.comm.). Dental plates, not seen by Muir-Wood (1959) are present in the type material. Muir-Wood (1959) also misidentified the lophophore of Leptothyrella as spirolophous when it is zygolophous.In 1981 Zezina created a new genus Phaneropora , with type species P.galatheae Zezina, 1981 from the SW Pacific ( Fig. 15 ), because based on Muir-Wood’s (1959) original description she recognized that Leptothyrella and Phaneropora are distinct genera. The characters distinguishing Phaneropora from Leptothyrella , according to Zezina (1981), were the presence of dental plates and zygolophous lophophore. However, she also included into Phaneropora the species from the Atlantic described by Davidson (1880), Magasella incerta which resembles P. galatheae (see Zezina 1981: 18) but differs in the loop development. The loop of P.galatheae possesses a gap between the crura and the septal pillar (see Fig. 15 C-E); at the distal extremity of each of the crura there is a prominent flattened area that is the rudiment of a descending branch ( Fig. 15F ). In turn, the species from the Atlantic has continuous, from crura to the septum, descending branches (Davidson 1887; Logan 1983, 1998; Gaspard 2003; Álvarez & Emig 2005). In the revised edition of the Treatise, MacKinnon & Lee (2006a: 2227) transferred Phaneropora incerta into Leptothyrella , making the absence or presence of descending branches the main criterion to separate the genera Leptothyrella and Phaneropora . In other characters those two taxa are very similar and there is still uncertainty about their status, and only a study of their genetic relationship would help in resolving this problem.

Until now only two species of Leptothyrella have been recognized: L. ignota (Muir-Wood, 1959) from the western Indian Ocean and L. incerta ( Davidson, 1880) from the Atlantic. The newly described species, L. fijiensis n. sp. is the third one known. It differs from the type species, L. ignota in slightly smaller size, more rounded outline and distinctly tuberculate beak ridges. Its valves are equally convex, while in L. ignota the convexity of the ventral valve is slightly greater than that of the dorsal valve (Muir-Wood 1959).

The Atlantic L. incerta differs from L. fijiensis n. sp. in lacking tubercles along the sides of the beak and in having strong dental plates (Davidson 1887; Logan 1983, 1998; Gaspard 2003; Álvarez & Emig 2005). Also its crura are more massive than those in the Fiji specimens.

The bathymetric range of L. fijiensis n. sp. is from 144 to 963 m. The species L. incerta and L. ignota have much wider depth range, 312-5300 m ( Logan 1983, 1988, 1998; Zezina 2000; Gaspard 2003; Álvarez & Emig 2005) and 850-2881 m (Muir- Wood 1959; Hiller 1986), respectively. The depth of the related species P. galatheae is also very wide, from 225 to 3493 m ( Zezina 1981, 1987; Foster 1989; Laurin 1997).

Figures

FIG. 13.— Leptothyrella fijiensis n. sp.,Fiji:A, dorsal view of complete young specimen (MNHN BRA-3105),Bligh Water,MUSORSTOM 10, stn DW 1314,656-660 m; B-E, ventral,dorsal,lateral and anterior views of complete specimen,holotype (MNHN BRA-3101),Bligh Water, MUSORSTOM 10, stn CP 1332, 640-687 m; F, G, inner and lateral views of dorsal valve, paratype (MNHN BRA-3102), Bligh Water, MUSORSTOM 10, stn CP 1332, 640-687 m; H, inner view of dorsal valve with preserved lophophore, paratype (MNHN BRA-3103), Viti Levu, CP 1354, 959-963 m; I-K, inner view of ventral valve, paratype (MNHN BRA-3104), Bligh Water, MUSORSTOM 10, stn CP 1332, 640-687 m, enlargement of posterior part (J), and tilted to show dental plates (K); L, dorsal view of complete specimen (MNHN BRA-3106), Bligh Water, MUSORSTOM 10, stn CP 1332, 640-687 m. All SEM. Scale bars: A, G, J, K, 500 μm; B-F, H, I, L, 1 mm.

FIG. 14.— Intraspecific variationin Leptothyrella fijiensis n.sp.Scatter diagram plotting length/ width. N, number of specimens.

FIG. 15. — Phaneropora galatheae Zezina, 1981, New Caledonia, Norfolk Ridge, BATHUS 3, stn CP 844, 908 m: A, B, dorsal view of complete specimen (MNHN BRA-3107), and enlargement (B) of the posterior part to show details of the beak; C-F, inner and lateral views of the dorsal valve (MNHN BRA-3108), and enlargement (F) of the rudiment of a descending branch. All SEM. Scale bars: A, 1 mm; B-E, 500 μm; F, 50 μm.