Dallina septigera ( Lovén, 1846 )

Hansen, Jesper, 2024, Brachiopods of the Norwegian fauna northern North Atlantic and Arctic, with a focus on, Fauna norvegica 43, pp. 12-68 : 52-54

Dallina septigera ( Lovén, 1846)

Figures 1E, 1G , 50 , 51

Terebratula septigera Lovén, 1846, p. 183 .

Waldheimia septigera – Gray (1853), p. 59.

Terebratula (Waldheimia) septigera – Lovell (1861), p. 176.

Waldheimia (Waldheimia) septigera – Dall (1870), p. 112.

Terebratula (Waldheimia) septata (Philippi) – Jeffreys (1878) [partim], p. 407–409, pl. 23, figs. 1–1c.

Magellania septigera – Fischer & OEhlert (1891) [partim], p. 64–71.

Dallina septigera – Beecher (1893), p. 382.

Waldheimia (Macandrevia) septigera – Wesenberg-Lund (1939), p. 203.

Description: Shell longer than wide (L/W ratio: 1.06–1.56; mean: 1.20), and moderately to strongly biconvex. Adult shells with pentagonal to subtriangular outline and widest in anterior portion of shell. Juvenile specimens with egg-shaped outline, though typically with weakly truncated front. Umbo short and stubby in adults, but somewhat narrow in juveniles. Anterior commissure changing from rectimarginate in juveniles, over broadly unisulcate, to distinctly parasulcate in adult specimens. Lateral commissure is straight to gently curved. Shell surface as a rule smooth except for sporadic growth lines, but large specimens can show sporadic radiating weak striation resembling that of Hemithiris . Shell white or straw-coloured. Shell matrix endopunctate. Circular, rather large and usually slightly transverse pedicle foramen developed in specimens over 6 mm long, but subtriangular to triangular deltidial plates disjunct in smaller specimens. Deltidial plates not separated from rest of shell by distinct beak ridges, but typically forms a raised ridge along midline when conjoined. Pedicle collar (thickening in pedicle tube) very short and lacking in juveniles. Ventral teeth never supported by dental plates. Hinge plates extended forward to join dorsal median septum in a V-shape. Brachial loop long and with wide transverse band. Brachial loop not attached to median septum on specimens longer than 13 mm. Long and low dorsal median septum reaches 60–84% of valve length, except in juvenile specimens, and visible as a whitish knife-cut line on dorsal valve exterior. Juvenile specimens below 1.65 mm long lack dorsal median septum. A short septum with dot-shaped basis later evolves, which very soon becomes knife-cut shaped. Spicules in tissue mostly absent. Maximum shell length 45 mm.

Depth range: 37–1800 m depth ( Fischer & OEhlert 1892; this study), but common between 180–800 m.

Temperature range: -1.1–10.2˚C ( Carpenter et al. 1869; Jeffreys 1878).

Salinity range: 34.9–36 ( Thomsen 2001; Brand et al. 2003).

Substrate: Attached to sand, gravel, corals, shells, bryozoans, serpulid tubes or Foraminifera ( Atkins 1960a; Thomsen 2001; this study). Sea bottom varies from silt- and sand-dominated to stony ( Thomsen 2001).

Geography: Southwestern-most Barents Sea and south along the Norwegian coastline to Hordaland, northernmost North Sea, NE Atlantic S of Iceland, and along the W part of the British Isles ( Lovén 1846; Fischer & OEhlert 1891, 1892; Wesenberg-Lund 1939, 1941; Atkins 1960a; Thomsen 2001; this study). The southern border of the species is uncertain since the southernmost verified specimen is from Scotland. However, Dallina septigera seems to be absent from the southern margin of the Celtic Sea.

Remarks: In his original description of Dallina septigera, Lovén (1846) did not provide illustrations. Likely for this reason, subsequent researchers simply assumed French and Spanish specimens belonged to the same species and adjusted the species description accordingly. In contrast to the strongly triangular outline of the following species, the true D. septigera is normally subpentagonal in outline. The geographical boundary between the two species appears to be on the W side of Great Britain. In her redescription of D. septigera, Atkins (1960b) did not realise that the genus included two European species, and thus based her redescription almost solely on the material of Dallina lusitanica n. sp., assuming the differences between the single available Norwegian specimen and the remaining material were intraspecific differences. In his study on material from France, Cooper (1981a) stated that the species could usually be distinguished from the morphologically close species Fallax dalliniformis by its triangular shape with its greatest width at the front when compared to the typically more subpentagonal outline with its greatest width a bit further back on the other species, thus showing that the specimens from France were representatives of the new species and not D. septigera .

A comparison with Fallax dalliniformis is provided in the discussion of that species.

Subadult and juvenile specimens are often mistaken for Macandrevia cranium but are distinguished by the earlier development of the dorsal median septum (which rapidly becomes long) and by the absence of bordering ridges along the deltidial plates.

There are presently no indications that this species is negatively influenced by ongoing ecological changes within the Norwegian region.

Figures

Figure 1. Terminology used in the paper. d–g show different shapes of the anterior commissure. Species shown: Hemithiris psittacea (a, b, f, j); juvenile Terebratulina septentrionalis (c); Glaciarcula spitzbergensis (d, h); Dallina septigera (e, g); Macandrevia cranium (i).

Figure 50. Dallina septigera. a–d: dorsal, ventral, lateral and anterior views of large specimen TSZBr 2410, Trondheimsfjord in Trøndelag, Norway; e: dorsal view of specimen, Norwegian Sea at Nordland, Norway, Mareano 2012106 st. R821-16, 553–559 m, the Arctic University Museum of Norway; f–i: dorsal, ventral, lateral and anterior views of specimen TSZ 3004 (APN), Norwegian Sea at Trøndelag, Norway, 298 m; j: ventral exterior view of shell NHMD-656590;k–m: dorsal interior and oblique views of the same, and hinge region view of ventral valve of empty shell TSZ 3181, Norwegian Sea at Nordland, Norway, Mareano 2012106 st. R821-16, 553–559 m; n–o: lateral and dorsal interior views of small shell with preserved brachidium still attached to median septum, TSZ 3209 (APN), Haltenbanken in Norwegian Sea at Trøndelag, Norway, 347 m;p–t: dorsal, ventral, lateral, posterior and anterior views of juvenile specimen TSZ 3207 (APN), Norwegian Sea at Vestland, Norway, 307 m. Solid scalebar 1mm and segmented scalebar 5mm. Photos: Jesper Hansen.

Figure 51. Distribution of Dallina septigera. Colours mark which century the specimens were collected in. Irish specimens require verification.