Dallithyris Muir-Wood, 1959

Bitner, Maria Aleksandra, 2007, Recent brachiopods from the Austral Islands, French Polynesia, South-Central Pacific, Zoosystema 29 (3), pp. 491-502 : 495-496

Genus Dallithyris Muir-Wood, 1959

TYPE SPECIES. — Dallithyris murrayi Muir-Wood, 1959 by original designation (Muir-Wood 1959: 305).

Dallithyris pacifica Bitner, 2006 ( Figs 1 I-K; 2C, D; Table 2)

Dallithyris pacifica Bitner, 2006b: 20-22 , fig. 2A-J.

MATERIAL EXAMINED. — Austral Islands. Marotiri, BENTHAUS, stn DW 1884, 2 ventral valves. — Stn DW 1885, 4 complete specimens, 6 ventral valves, 2 dorsal valves. — Neilson Reef, stn DW 1923, 1 complete specimen, 1 ventral valve. All complete specimens are empty shells.

DEPTH RANGE. — 360- 840 m.

MEASUREMENTS. — See Table 2.

REMARKS

Dallithyris pacifica was originally described from off the Fiji Islands. The studied specimens differ from those previously described in being slightly larger (maximum observed length is 32 mm). The shell is subtriangular in outline, thin, biconvex, smooth with fine numerous growth lines. The anterior commissure is rectimarginate. The beak is suberect, labiate with large permesothyrid foramen. The pedicle collar is short, excavated. The cardinalia have long, slender inner socket ridges and a small, transverse cardinal process. The outer hinge plates are wide, triangular. The loop is short with a very broad, angularly arched transverse band. D. pacifica differs from another short-looped brachiopod in the investigated material, Acrobrochus marotiriensis n. sp., in being larger and having a much thinner shell. Those two species also differ strongly internally in the width of outer hinge plates and the nature of the transverse band.

The shell ultrastructure of Dallithyris has been investigated for the first time. The analysis shows the shell consisted of three layers ( Fig. 2C, D ). The primary layer is thin (18-24 µm) and built of acicular crystallites. The secondary layer is 23-45 µm thick and made up of anvil-shaped fibres. The tertiary layer is much thicker (550-577 µm) than the two first and composed of prismatic calcite. The thickness of the shell is 620-643 µm at the observed section. Thus, Dallithyris belongs to the group of short-looped brachiopods with triple-layered shell.

In the revised edition of the Treatise only two species have been assigned to the genus Dallithyris (Logan in press). The type species, D. murrayi Muir-Wood, 1959 , from the central Indian Ocean is much bigger than D. pacifica and has a uniplicate anterior commissure. Dallithyris fulva (Blochmann, 1906) from southern Australia and Tasmania differs from D. pacifica in its outline and larger foramen ( Cooper 1983). This species occurs in shallower waters than D. pacifica .

Figures

FIG. 1. — A-H, Acrobrochus marotiriensis n. sp., Marotiri Island; A, B, dorsal and lateral views of complete specimen,paratype (MNHN BRA-3044), stn DW 1884, 570-620 m; C-E, dorsal, lateral and anterior views of complete specimen, holotype (MNHN BRA-3043), stn DW 1886, 620-1000 m; F, G, paratype (MNHN BRA-3045), stn DW 1884, 570-620 m, SEM enlargement of the loop of brachial skeleton of the dorsal valve (F) and enlargement of brachial skeleton tilted (G) to show gently folded transverse band; H, inner view of ventral valve,paratype (MNHN BRA-3046),stn DW 1886, 620-1000 m; I-K, Dallithyris pacifica Bitner, 2006, dorsal,lateral and anterior views of complete specimen (MNHN BRA-3048), Marotiri Island, stn DW 1885, 700-800 m, note gastropod boring on I. Scale bars: A-E, H, 0.5 cm; F, G, 2 mm; I-K, 1 cm.

FIG. 2. — A, B, Acrobrochus marotiriensis n. sp. (MNHN BRA-3047), SEM micrographs, Marotiri Island, stn DW 1884, 570-620 m, transverse sections of the entire shell showing thin primary layer (top),secondary layer built of anvil-shaped fibres and prismatic tertiary layer;C, D, Dallithyris pacifica Bitner,2006 (MNHN BRA-3049),SEM micrographs,Marotiri Island,stn DW 1885,700-800 m; C, transverse section of the entire shell showing the acicular primary layer (top) underlain by the fibrous secondary layer, passing into the prismatic tertiary layer; D, section of the shell showing anvil-shaped fibres of the secondary layer. Scale bars: A, B, 50 µm; C, D, 20 µm.