Fallax dalliniformis Atkins, 1960a

Hansen, Jesper, 2024, Brachiopods of the Norwegian fauna northern North Atlantic and Arctic, with a focus on, Fauna norvegica 43, pp. 12-68 : 56-57

Fallax dalliniformis Atkins, 1960a

Figures 54 , 55

Fallax dalliniformis Atkins, 1960a, p. 71 –89, pl. 1, figs. 1–5.

Description: Shell inflated biconvex. Outline of adult shells triangular to distinctly truncated subpentagonal and widest close to front. Small specimens with more egg-shaped outline. Ventral umbo beak-shaped, curved and acute in profile. Anterior commissure normally parasulcate. Shell surface marked by growth lines. Shell matrix endopunctate. Colour whitish to slightly brownish, but older shells often with brownish secondary colour. Deltidial plates disjunct in specimens below 10 mm long, but then joining and restricting pedicle opening to a relatively small and egg-shaped foramen. Long pedicle collar (raised shell matrix in umbo). Short dental plates supporting strong hinge teeth. Hinge plates not extended forward to form a V-shape at connection with dorsal median septum but meet it in a right angle or with median indention. Long brachial loop with wide transverse band and supported by median septum. Long, low dorsal median septum reaching to 2/3–15/16 valve length and always visible as whitish knife-cut line on dorsal valve exterior. Abundant spicules in soft tissue (can disappear when stored too long in ethanol). Maximum shell length 29.7 mm.

Depth range: 702–2100 m depth ( Anadón 1994; Anadón et al. 2022). Although Brunton & Curry (1979) gave a depth range starting from 210 m, the origin of that figure is uncertain.

Substrate: Attached to rocks, shells or corals ( Atkins 1960a).

Geography: Celtic Sea and Bay of Biscay ( Atkins 1960a).

Remarks: This species is externally very similar to the two Dallina species. However, the ventral umbo is curved and beak-shaped with an egg-shaped foramen, the pedicle collar is long, the ventral hinge teeth are supported by dental plates and the brachial loop is always supported by the median septum.

Juvenile specimens may easily be confused with the species Glaciarcula spitzbergensis , which has previously led to the assumption that this subarctic and Arctic species was present in waters around Spain and the Scilly Isles (see discussion for Glaciarcula spitzbergensis ). However, juveniles of Fallax dalliniformis can be externally distinguished by a less elongated pedicle opening, broader outline with the early development of anterior truncation, and thinner shell wall.

Figures

Figure 54.Fallax dalliniformis. a–f: dorsal, ventral, lateral and anterior views, and dorsal and ventral interior views of holotype BMNH PI ZB 2988, Celtic Sea, Great Britain, 1060–1245 m; g–h: ventral and dorsal interior views of small specimen BMNH PI ZB 2987, Celtic Sea, Great Britain, 933–1006 m; i: dorsal view of specimen BMNH PI ZB 2990, Celtic Sea, France, 722 m; j: oblique view of dental plates and pedicle collar of specimen BMNH PI ZB 2991, Celtic Sea, Great Britain, 990–1190 m; k–o: inner, outer and lateral views of ventral valve, and inner and oblique lateral views of dorsal valve of small specimen BMNH PI ZB 790, Celtic Sea, Great Britain, 977 m. Solid scalebar 1mm and segmented scalebar 5mm.Photos: Jesper Hansen.

Figure 55. Distribution of Fallax dalliniformis.