Xenobrochus islandicus, Hansen, 2024

Hansen, Jesper, 2024, Brachiopods of the Norwegian fauna northern North Atlantic and Arctic, with a focus on, Fauna norvegica 43, pp. 12-68 : 31-33

Xenobrochus islandicus n. sp.

Figures 17 , 18

Liothyris arctica – Wesenberg-Lund (1941) [partim], p. 9–10.

Waldheimiathyris cranium – Wesenberg-Lund (1941) [partim], p. 10–12.

Liothyrella arctica – Zezina (1997b) [partim], p. 72.

LSID: zoobank.org:act: 14CCB184-6D1B-4FA6-88CF-E13D04668E72

Type locality: Off W Iceland, The Danish ‘Ingolf’ expedition 1896 st. 84, 62°58’N, 25°24’W, 1192 m depth .

Etymology: From the geographical name ‘Island’ ( Iceland), around which the species was found.

Diagnosis: Xenobrochus species with elongate, pointy, egg-shaped outline and evenly rounded to slightly flattened anterior margin; beak erect with visible symphytium; permesothyrid medium large foramen; transverse cardinal process; angular brachial loop reaching about 1/3 valve length, with broad transverse band with a weak median fold.

Material: Holotype: Reykjanes Ridge , off SW Iceland, the Danish ‘ Ingolf’ expedition 1896 st. 84, 62°58’N, 25°24’W, 1192 m depth, one specimen (NHMD-1201208 (holotype) . Paratypes: Irminger Sea , off W Iceland, the Danish ‘ Ingolf’ expedition 1896 st. 10, 64°24’N, 28°50’W, 1484 m depth, one specimen NHMD-1186285; Reykjanes Ridge , Iceland Basin , the Danish ‘ Ingolf’ expedition 1896 st. 78, 60°37’N, 27°52’W, 1505 m depth, three specimens and two empty ventral valves (NHMD-1186283, NHMD-1186284); Reykjanes Ridge , off SW Iceland, the Danish ‘ Ingolf’ expedition 1896 st. 84, 62°58’N, 25°24’W, 1192 m depth, two specimens and one empty shell (NHMD-1201194); Irminger Sea , off W Iceland, the Danish ‘ Ingolf’ expedition 1896 st. 90, 64°45’N, 29°06’W, 1070 m depth, two empty shells (NHMD-1201179); Denmark Strait, W Iceland, the Danish ‘ Ingolf’ expedition 1896 st. 98, 65°38’N, 26°27’W, 260 m depth, two specimens (NHMD-1201193) .

Description: Holotype 8.2 mm long and 5.9 mm wide. L/W ratio of 1.16–1.50 and generally increasing with shell size. Shell moderately to rather strongly equi- to ventribiconvex. Outline pointy and egg-shaped with evenly rounded to slightly flattened anterior margin. No median folds or depressions developed on valves. Anterior commissure rectimarginate to slightly sulcate. Lateral commissure slightly S-curved. Fragile, transparent to semitransparent and endopunctate. Typically shiny and smooth except for faint growth lines and occasionally very weak radiating striation, as seen on the holotype ( Figure 17 a–b ). Colour whitish. Beak pointy and only slightly curved in dorsal direction. Comparatively high deltidial plates conjoined and fused to form a visible symphytium, often depressed along midline. Ridges bordering deltidial plates poorly developed or absent. Pedicle foramen moderately large permesothyrid (=beak ridges bisecting foramen in lower half) and circular to subtriangular. Pedicle short and branched in multiple rootlets. Pedicle collar short. Teeth form moderately high transverse plates. Dental plates absent. Cardinal process in dorsal valve a transverse semi-elliptical ridge on top of the inner socket ridges ( Figure 17k , ae). Outer hinge plate connecting inner socket ridges with crural bases narrow, triangular and poorly developed. Inner hinge plates absent. Brachial loop short and narrow, with subparallel sides and a broad, angular transverse band with anterolateral angles. Transverse band gently convex in anterodorsal direction, though a small indention can occur at the suture line at front edge. Crural processes barely anterior of mid-loop length, blunt and bending towards each other. Brachial loop reaches approximately 30% of valve length. No true dorsal median ridge, though imprints of the two main trunks of the vascular system may create a thickening on the valve floor extending to 30% of valve length. Dorsal muscle scars clearly visible and reaching slightly anterior of 1/4 valve length. Largest specimen 12.4 mm long and 8.3 mm wide.

Depth range: 260–1505 m depth.

Temperature range: 3.5–5.9˚C.

Salinity range: 34.9–35.4.

Substrate: Attached to Porifera spicules or bryozoan branches.

Geography: North Atlantic Ocean off E and SE Iceland.

Remarks: The specimens are assigned to the genus Xenobrochus Cooper, 1981b based on the presence of the diagnostic characters, such as a visible symphytium, small shell size with basically rectimarginate anterior commissure, transverse cardinal process, and a short loop with subparallel sides and an anteriorly convex transverse band. This makes this species the second of that genus to be recorded from the Atlantic Ocean and outside the W Pacific and Indian Ocean when following the opinion of Bitner & Molodtsova (2020) and including Abyssothyris? parva Cooper, 1977 in the genus (see Bitner 2011; Bitner & Molodtsova 2020).

To date, 10 species have been included in the genus. These are Xenobrochus africanus ( Cooper, 1973b) , X. agulhasensis ( Helmcke, 1938) , X. anomalus Cooper, 1981b , X. australis Cooper, 1981b , X. indianensis ( Cooper, 1973b) , X. naudei Hiller, 1994 , X. norfolkensis Bitner, 2011 , X. parvus ( Cooper, 1977) , X. rotundus Bitner, 2008 and X. translucidus ( Dall, 1920) .

Most of these species have a distinctly subtriangular or pentagonal outline with a somewhat truncated front, or a subcircular outline distinguishing them from the present species. This includes the subtriangular to subcircular Xenobrochus parvus , which according to descriptions and illustrations of Cooper (1977) and Bitner & Molodtsova (2020) is more equidimensional, is distinctly ventribiconvex, lacks anterolateral angles on the transverse band of the brachial loop, and seems to have a dull shell surface.

Among the remaining four, the shallow-water species X. indianensis (from the Indian Ocean and SW Pacific) appears morphologically very close, except for a distinctly more incurved beak and a narrower cardinal process.

The equally close X. agulhasensis from approximately 275 m depth off South Africa differs by a very small brachial loop reaching to about 1/4 valve length, and a generally lower L/W ratio of approximately 1.2.

X. africanus , from the Indian Ocean and SW Pacific with the holotype taken at 366 m depth, has a much more robust shell, slightly truncated front and a distinctly narrower and broadly V-shaped transverse band in the brachial loop.

X. australis , from 790 m depth in the South Indian Ocean, has a larger foramen with a longer pedicle collar, while the anterior end of the brachial loop does not bend in the dorsal direction and the L/W ratio of the shell is approximately 1.5.

Figures

Figure 17. Xenobrochus islandicus n. sp. a–d: dorsal, ventral and lateral views of specimen, and close-up image of deltidial plates fused to form symphytium, holotype NHMD-1201208, Reykjanes Ridge, 1192 m; e–g: dorsal, ventral and lateral views of paratype from batch NHMD-1201193, Denmark Strait, 260 m; h: ventral interior of paratype from batch NHMD-1186284, Reykjanes Ridge, 1505 m; i: dorsal view of paratype from batch NHMD-1201194, Reykjanes Ridge, 1192 m;j–l: dorsal interior view with close-up image of hinge region and brachial loop, and ventral interior view of paratype NHMD-1186285, Irminger Sea, 1484 m; m–p: exterior and interior views of dorsal and of ventral valve, paratype NHMD-1186283, Reykjanes Ridge, 1505 m; q: dorsal view of ventral valve of paratype from batch NHMD-1201194, Reykjanes Ridge, 1192 m; r, ad: dorsal and lateral views of paratype NHMD-1186284 with lophophore visible, Reykjanes Ridge, 1505 m; s: dorsal interior view showing vascular ridges of paratype from batch NHMD-1201194, Reykjanes Ridge, 1192 m; t: dorsal view of paratype from batch NHMD-1186284, Reykjanes Ridge, 1505 m; u: ventral interior view of paratype from batch NHMD-1186284, Reykjanes Ridge, 1505 m; v: ventral interior and ae: enlarged dorsal interior views of paratype from batch NHMD-1201194, Reykjanes Ridge, 1192 m; w–y: dorsal, ventral and lateral views of paratype from batch NHMD-1201193, Denmark Strait, 206 m; z–aa: dorsal and ventral views of paratype from batch NHMD-1201179, Irminger Sea, 1070 m; ab–ac: dorsal and ventral views of paratype from batch NHMD-1201179, Irminger Sea, 1070 m. Scalebar 1mm. Photos: Jesper Hansen.

Figure 18. Distribution of Xenobrochus islandicus n. sp.