Gwynia capsula ( Jeffreys, 1859 )

Hansen, Jesper, 2024, Brachiopods of the Norwegian fauna northern North Atlantic and Arctic, with a focus on, Fauna norvegica 43, pp. 12-68 : 37-39

Gwynia capsula ( Jeffreys, 1859)

Figures 27 , 28

Terebratula capsula Jeffreys, 1859, p. 43 , pl. 2, figs. 7a–b.

Gwynia capsula – King (1859), p. 258, figs. 1–5.

Terebratula ( Gwynia ?) capsula – Lovell (1861), p. 183.

Argiope capsula – Jeffreys (1863), p. 21.

Gwynia (Terebratula) capsula – Davidson (1880), p. 14.

Description: Minute, thin-walled shell, moderately to inflated biconvex with ventral valve usually slightly shorter but higher than dorsal. Outline egg-shaped oval to subtriangular. Rectimarginate anterior commissure. Hinge line narrow but straight. Ornamentation lacking except for growth lines. Shell matrix semitransparent and densely endopunctate. Colour white or yellowish. Pedicle opening relatively large and V- or U-shaped, extending into the dorsal

Depth range: Intertidal to 46 m depth ( Jeffreys 1859; Logan et al. 1997), but empty shells have been collected down to 100 m depth ( Harper et al. 1996).

Temperature: 11–13˚C ( Simon & Willems 1999).

Substrate: Found attached to rocks, gravel, in shells and serpulid tubes, and on shell sand and sand ( Jeffreys 1859; Davidson 1887; Harper et al. 1996; Logan et al. 1997; Francis et al. 2011).

Geography: Coastal waters of Scotland, Wales, England, Ireland, the southernmost North Sea off the Netherlands and Belgium, the coast of France in the English Channel, off SE France, the Spanish Bay of Biscay, Portugal, the Adriatic Sea at Croatia, Greece, and valve. Ventral umbo short and, in some cases, shorter than dorsal umbo. Deltidial plates rudimentary and not joined. Hinge teeth not supported by dental plates. No visible muscle scars. Dorsal valve with prominent inner and outer socket ridges, the inner of which continues as submarginal ridges on valve floor. Notothyrium (raised platform) bisected by a simple, weak, ridge-like cardinal process. Dorsal median ridge usually present, though rudimentary and restricted to the posterior-most part of the valve floor. Short, broad, but incomplete brachial loop supported by submarginal ridges on valve floor. Lophophores reaching approximately 4/5 valve length. Maximum shell length 1.5 mm.

on a seamount in the North Atlantic at approximately 30°N and 13°W ( Davidson 1887; Brunton & Curry 1979; Besteiro & Urgorri 1986; Logan et al. 1997; Simon & Willems 1999; Antoniadou & Chintiroglou 2005; Logan et al. 2007; Francis et al. 2011). A Holocene subfossil of the species has also been reported and illustrated from an isostatic raised shell banks located approximately 50 m above present-day sea level near Korshavn on the island Kirkøya at the Oslofjord, Norway ( Sars 1865).

Remarks: Due to its minute size, Gwynia capsula is extremely easy to miss—even when knowing that it is present. This small size also easily leads to confusion with juvenile specimens of other brachiopod species. Combining this with how many findings have been made around the entire British Isles, it seems very likely that the species is still living along some parts of southern Norway despite no records other than the one subfossil specimen from deposits containing a shelly faunal assemblage pointing towards the Holocene climatic optimum.

Figures

Figure 27. Gwynia capsula. a: dorsal view of specimen in batch BMNH PI. ZB 4245–4247, Belfast Bay, Ireland; b: interior view of ventral valve BMNH PI.B 12433, near Jersey; English Channel;c–e: ventral, lateral view with dorsal valve right, and posterior view of small empty shell in batch BMNH PI. ZB 4203–4207, Dog Strand, Connemara, Ireland; f–j: dorsal, ventral, lateral (with dorsal valve left), anterior and posterior views of iron oxide-coated specimen BMNH PI. ZB 3009, Jersey, English Channel. Scalebar c, d and e 0.5mm, otherwise 1mm. Photos: Jesper Hansen.

Figure 28. Distribution of Gwynia capsula.