Simpliciforma profunda Bitner & Zezina gen. et sp. nov. (Fig. 4 A–I) Diagnosis. As for the genus. Etymology. Referring to the great depth at which this brachiopod was found. Type locality. Clarion-Clipperton Zone, north-eastern Pacific, station 217, 13.91°N, 129.11°W, depth 4680 m. Holotype. Specimen in Fig. 4 E, F (XI-52-24 /1), collected in 2003, station 217. Paratypes. Specimen in Fig. 4 A (XI-52-24 /2), collected in 2000, station 27; specimen in Fig. 4 G–I (XI-52-24 / 4), collected in 2003, stations 185–199; dorsal valve with preserved lophophore in Fig. 4 B–D (XI-52-24 /3), stations 185–199. Description. Shell very small (maximum observed length 1.4 mm), thin, translucent, subpentagonal to subrectangular in outline with greatest width at anterior, biconvex with dorsal valve more convex. Shell surface smooth with numerous, distinct growth lines (Fig. 4 I). Lateral commissures straight, anterior commissure rectimarginate. Hinge line long, straight. Beak short, suberect, beak ridges sharp. Interarea very narrow. Dorsal notothyrium small, ventral delthyrium large, together forming a large, subtriangular foramen (Fig. 4 F, H). Deltidial plates very narrow, disjunct. Ventral valve only slightly larger than dorsal one with large open delthyrium. Dorsal valve interior with very low but thick, widely separated inner socket ridges that do not project beyond posterior margin. Dental sockets rounded and relatively deep. No cardinal process. No hinge plates. No brachial skeleton or median septum. Lophophore trocholophous (Fig. 4 B–D), consisting of a pair of curving arms projecting horizontally on either side of a mouth and forming a ring; filaments long. Remarks. The investigated specimens, although similar in simple morphological structure and trocholophous lophophore, are easily distinguishable from Gwynia capsula (Jeffreys, 1859) known from several localities in the eastern Atlantic and Mediterranean (Brunton & Curry 1979; Harper et al. 1996; Logan et al. 1997; Simon & Willems 1999). G. capsula is ovate and globose in outline, and its hinge line is slightly curved being shorter than in Simpliciforma profunda. The dorsal valve in G. capsula is a little larger than the ventral valve. The main difference, however, is visible internally; G. capsula possesses two postero-lateral submarginal ridges (Logan et al. 1997; Simon & Willems 1999) that are not observed in S. profunda. Also G. capsula has a large amphithyrid foramen, while in S. profunda the foramen is only slightly marked in the dorsal valve. In shell outline, straight hinge line and absence of dorsal submarginal ridges, these specimens are similar to the specimen from the Chatham Rise, New Zealand, described by Lüter (2008) as Gwynia macrodentata, but G. macrodentata differs from S. profunda in having thicker, closely spaced, inner socket ridges (see Lüter 2008, fig. 3B). In addition, dental sockets observed in S. profunda suggest that its teeth are less massive than those in G. macrodentata. As submarginal ridges in the dorsal valve are considered a diagnostic feature of the genus Gwynia, but are absent from G. macrodentata, the latter might better be placed in the genus Simpliciforma. Zezina (1976, p. 111) and Simon & Willems (1999, p. 18) have shown that material collected by the Talisman expedition to the eastern Atlantic was erroneously attributed to Gwynia capsula by Fischer & Oehlert (1891). Surprisingly, the specimen illustrated by them displays similarities to S. profunda, particularly in such features as its pointed ventral beak, straight and long hinge line and lack of submarginal ridges, however, their possible conspecifity needs further studies, especially considering a great geographical gap.

Published as part of Bitner, Maria Aleksandra, Melnik, Vjacheslav P. & Zezina, Olga N., 2013, New paedomorphic brachiopods from the abyssal zone of the north-eastern Pacific Ocean, pp. 281-288 in Zootaxa 3613 (3) on pages 285-287, DOI: 10.11646/zootaxa.3613.3.6, http://zenodo.org/record/215765