Dallithyris dubia Cooper, 1981

Bitner, Maria Aleksandra & Logan, Alan, 2016, Recent Brachiopoda from the Mozambique-Madagascar area, western Indian Ocean, Zoosystema 38 (1), pp. 5-41 : 14

Dallithyris dubia Cooper, 1981

( Fig. 7 ; Table 5)

Dallithyris ? dubia Cooper, 1981a: 21 , 22, pl. 4, figs 21-29. — Zezina 1985: 149; 2010: 1184.

MATERIAL EXAMINED. — Mozambique Channel. MAINBAZA, stn CP 3138, 7 bivalved specimens, 1 dorsal valve. — Stn CP 3141, 34 bivalved specimens, 2 ventral valves, 1 dorsal valve.

DEPTH RANGE. — 684- 707 m.

MEASUREMENTS. — See Table 5.

DESCRIPTION

Shell of medium size (maximum length of 21.9 mm), thin, elongate oval to subtriangular in outline, ventribiconvex. Shell surface smooth with numerous, distinct growth lines. Beak suberect, foramen large, circular, permesothyrid. Symphytium small, slightly concave, partially visible. Anterior commissure rectimarginate to incipiently uniplicate.

Ventral valve interior ( Fig. 7F ) with small teeth and short, excavate pedicle collar. Dorsal valve interior with long inner socket ridges and triangular and distinct cardinal process ( Fig. 7H, I ). Outer hinge plates relatively narrow, concave. Loop short with a broad transverse band angularly folded ( Fig. 7 G-I).

REMARKS

The genus Dallithyris , with type species D. murrayi , was first established by Muir-Wood in 1959 from specimens obtained from the Maldive Islands and Saya de Malha Bank in the Indian Ocean . Later Cooper (1981a) questionably assigned four specimens from Samper Bank to Dallithyris , recognizing the uncertainty of the loop characteristics of the type species. We assign our specimens to Dallithyris notwithstanding, based on the similarity of the loop in Fig. 7H, I to that illustrated by Cooper in his plate 4, fig. 28. Apart from the type of loop, the beak character supports assignment of D. dubia to the genus Dallithyris . In Stenosarina Cooper, 1977 the beak is nearly straight, whereas in Kanakythyris Laurin, 1997 it is incurved with a very small foramen ( Laurin 1997; Bitner 2009). Dallithyris dubia differs from its congeneric species in being much smaller (see Muir-Wood 1959; Cooper 1983; Bitner 2006b, 2007, 2008, 2014). D. dubia was found only in the Mozambique Channel in our study, where it is relatively common.

Figures

FIG. 7. — Dallithyris dubia Cooper, 1981, Mozambique Channel, Inhambane transect, MAINBAZA, stn CP 3141, 684-698 m: A, B, dorsal views of complete specimens (MNHN IB-2013-44-45); C-E, dorsal, lateral and anterior views of complete specimen (MNHN IB-2013-46); F, inner view of ventral valve (MNHN IB- 2013-47); G, inner view of dorsal valve (MNHN IB-2013-48); H-I, SEM micrographs of posterior part of dorsal valve interior (H) and tilted view (I) to show details of cardinalia and brachial skeleton (MNHN IB-2013-49). Scale bars: A-G, 5 mm; H, I, 2 mm.