Oceanithyris juveniformis Bitner & Zezina

Bitner, Maria Aleksandra, Melnik, Vjacheslav P. & Zezina, Olga N., 2013, New paedomorphic brachiopods from the abyssal zone of the north-eastern Pacific Ocean, Zootaxa 3613 (3), pp. 281-288 : 283-285

Oceanithyris juveniformis Bitner & Zezina gen. et sp. nov.

( Fig. 3 A–Q)

Diagnosis. As for the genus.

Etymology. Referring to the juvenile morphological features.

Type locality. Clarion-Clipperton Zone, north-eastern Pacific, station 41, 13.27°N, 134.45°W, depth 4790 m.

Holotype. Specimen in Fig. 3 B–E (XI-52-23 /1), collected in 2006, station 41.

Paratypes. Ventral valve in Fig. 3 M– O (XI-52-23 /4), collected in 2000, station 27; specimen in Fig. 3 F–H (XI-52-23 /2), collected in 2003, station 217; dorsal valve in Fig. 3 I–L (XI-52-23 /3), station 217.

Other material examined. Three specimens collected in 2003, stations 185–199; three specimens collected in 2006, station 41.

Depth range. 4580–4850 m.

Measurements (in mm).

Description. Shell very small (maximum length 3.1 mm), thin, translucent, punctate, elongate oval in outline with the greatest width at midvalve to slightly anterior, strongly biconvex with dorsal valve slightly more convex. Lateral commissures nearly straight, incipiently ventrally convex; anterior commissure rectimarginate. Shell surface smooth with distinct, wrinkled growth lines (5 of them are assumed to constitute annual rings), with very fine striae in the posterior part of the dorsal valve ( Fig. 3 E). Hinge line short, curved. Beak short, suberect, eroded in all examined specimens, suggesting a very short pedicle ( Fig. 3 E, G) and close attachment to the substrate. Beak ridges rounded. Foramen large, subcircular, deltidial plates rudimentary, triangular, disjunct.

Ventral valve interior with moderately large, hooked teeth directed medianly. No dental plates. No pedicle collar but a step-like thickening of the shell is observed ( Fig. 3 N). Muscle scars distinct.

Dorsal valve interior with strong, high inner socket ridges directed anteriorly. Dental sockets deep. Cardinal process and hinge plates not developed. No brachial skeleton and septum. Lophophore trocholophous with two arms forming a ring ( Fig. 3 I).

Remarks. The presence of numerous growth lines, including 4 or 5 yearly rings, clearly indicates that the studied specimens represent adult shells. In addition, two ripe eggs were found in gonads, supporting the interpretation of these specimens as mature, despite their diminutive condition.

Although these specimens display very few diagnostic morphological features, possessing only high inner socket ridges, they strongly differ in this respect from any brachiopods hitherto described, thus we propose a new genus and a new species.

Figures

FIGURE 3. Oceanithyris juveniformis Bitner & Zezina gen. et sp. nov., Clarion-Clipperton Zone, north-eastern Pacific; A, dorsal view of young complete specimen, XI- 52 - 23 / 5, Stn 41, 4790 m; B – E, ventral, dorsal and lateral views of complete specimen, and enlargement of the umbonal part (E) to show details of the beak and the rough surface anterior to the protegular node and fine striae, holotype, XI- 52 - 23 / 1, Stn 41, 4790 m; F – H, dorsal and posterior views of complete specimen, and enlargement of the umbonal part (H) to show details of the beak and the rough surface anterior to the protegular node, paratype, XI- 52 - 23 / 2, Stn 217, 4680 m; I – L, dorsal valve, inner views with preserved lophophore and after its removal, and enlargements of posterior part (K, L) to show details of cardinalia, paratype, XI- 52 - 23 / 3, Stn 217, 4680 m; M – O, ventral valve, inner view (M), and enlargement of posterior part (N) and tilted view (O) to show rudimentary deltidial plates, hooked teeth and step-like thickening of the shell, paratype, XI- 52 - 23 / 4, Stn 27, depth 4850 m.