Seductorithyris septemtrionalis, Sandy & Hryniewicz & Hammer & Nakrem & Little, 2014

Sandy, Michael R., Hryniewicz, Krzysztof, Hammer, Øyvind, Nakrem, Hans Arne & Little, Crispin T. S., 2014, Brachiopods from Late Jurassic-Early Cretaceous hydrocarbon seep deposits, central Spitsbergen, Svalbard, Zootaxa 3884 (6), pp. 501-532 : 523-526

Seductorithyris septemtrionalis sp. nov.

Figs. 11.1–11.16 , 12

2011 v. partim ‘ Cyrtothyris ’ sp.—Hammer et al., p. 20, table 2, fig. 7.S.

Material and occurrence. Holotype: PMO 224.896; paratypes: PMO 217.198, PMO 224.892–893, PMO 224.911, PMO 224.913, PMO 227.427?, PMO 227.430. All from seep 9.

Type locality. Knorringfjellet , central Spitsbergen, N78° 18’ 49.9” E16° 10’ 58.9” .

Diagnosis. As for the genus.

Etymology. For the northern high-latitudes that Spitsbergen occupies (septem triōnēs [also septentrional]: the seven stars of the constellation of Ursa Major [the Great Bear, or the Plough]—a constellation visible in the northern hemisphere throughout the year and a useful aid to finding North; also “of the north” or “north wind”).

Dimensions of the holotype. PMO 224.896—Length 33.7 mm, width 29.8 mm, thickness 16.1 mm.

Description. Length slightly greater than width, rounded outline, with the dorsal valve almost circular. Biconvex profile with the dorsal valve tending to be flatter, but not necessarily. Smooth shell with growth lines. Erect beak. Ventral valve umbo has a broad round keel. Lateral commissure straight, anterior commissure rectimarginate.

Cardinal process developed ( Fig. 12 , sections 0.7–1.2 mm), flat, horizontal and wide hinge plates deflected dorsally (sections 1.2–2.3), crural bases develop on inner margin of hinge plates (sections 1.6–2.3) and have a clubbed appearance initially (section 1.8). Crura are initially weakly deflected dorsally (sections 2.3–2.7). Transverse band is curved and low-arched (section 6.6, this is approximately at the mid-length of the brachidium, based on the distance the loop was traced), anterior flanges of the brachidium extend significantly beyond the transverse band (at least 5.8 mm). Loop traced to at least 11.4 mm from initial section through brachiopod (ventral umbo of sectioned specimen is damaged).

Discussion. The beak of the ventral valve is erect in Seductorithyris septemtrionalis whereas in “ Cyrtothyris ” maynci from East Greenland the beak is slightly produced, suberect, and with a massive umbo dominated by a large circular foramen ( Owen 1976). The outline of the Spitsbergen material is comparable to that of rounded terebratulide species discussed by Middlemiss (1976) from the Claxby Ironstone Formation of Lincolnshire, England, such as Rouillieria tilbyensis (Davidson) and Cyrtothyris cyrta (Walker) . Both of these species show rounded and elongated morphotypes and appear however to have a more massive pedicle foramen and may develop a uniplicate anterior commissure, which helps to distinguish them from Seductorithyris septemtrionalis . The rounded outline of Seductorithyris septemtrionalis is similar to that of Pinaxiothyris campestris ( Dagys 1968, pl. 10, figs. 1–3), although the latter appears to have a slight elongation to its outline, reaching a greater size. One specimen from Spitsbergen is tentatively referred to Pinaxiothyris campestris Dagys ( Fig. 13.1–13.4 ). It is similar in outline to an elongate specimen figured by Dagys (1968, pl. 10, fig. 4). The development of the cardinal process, flat hinge plates, crura, crural processes, and transverse band show similarities between the internal characters of Seductorithyris septemtrionalis ( Fig. 12 ) and Pinaxiothyris ( Dagys, 1968, fig. 52). The latter has hinge plates that are not as wide and the crural bases do not show the slight dorsal deflection seen in Seductorithyris .

Another genus considered during classification of this species was Moutonithyris , characterized by distinctive horizontal hinge plates and crural bases derived from the inner margins of the hinge plates. However, the crescentshaped attached crura seen in Moutonithyris (e.g. Middlemiss 1976; Nekvasilova 1980) are different from the clubshaped crural bases of Seductorithyris . In addition, Moutonithyris has a lower-arched transverse band compared to that of Seductorithyris , and does not appear to share the long flanges of the brachidium seen in the latter.

The nature of the horizontal hinge-plates with the rounded crural bases could be described as clubbed (after Cox & Middlemiss 1978) due to secondary overgrowth ( Fig. 12 , section 1.8). The crura project weakly dorsally and do not conform to those of any described Mesozoic terebratulide. The hinge-plates are short in length (sections 0.7–2.7) and the loop develops rapidly to the crural processes (section 4.0) before reaching the transverse band at 6.6 mm. The loop flanges were traced for another 5.8 mm to 11.4 mm.

The sectioned specimen might represent a juvenile specimen because of its size and hence also in its loop development. The ontogenetic development of the brachidium can hardly be said to be well understood among terebratulide brachiopods, and even less so perhaps the relationship between the hinge plates and the crural bases. However, these elements of the cardinalia appear stable in juvenile brachiopods judging from serial sections of growth-series of brachiopods (e.g. Dagys 1972; Smirnova 1973 (for rhynchonellides), 1984, 2008; see also Lee et al. 2001 and Tort & Laurin 2001). In Seductorithyris the crural bases develop as rounded to cuneate processes that project weakly dorsally, or at least do not project ventrally while attached to the hinge plates, thereby making a unique configuration that suggests a new brachiopod genus that is certainly distinct from its contemporaries. This configuration of the clubbed crura is quite different from the pendant or keeled crural bases typical of representatives of the terebratuloid Gibbithyris or the loboidothyridoid Placothyris . A specimen from seep 3 ( Fig. 13.5–13.8 ; Upper Volgian) is comparable to Placothyris kegeli from Wollaston Forland, East Greenland ( Fig. 13.9–13.12 ; Valanginian, infilling matrix is a red micrite).

Stratigraphic and geographic distribution. Uppermost Ryazanian of Spitsbergen.

Figures

FIGURE 11. Seductorithyris septemtrionalis gen. et sp. nov. 1–4. Holotype, PMO 224.896, seep 9, dorsal, ventral, lateral, and anterior views. 5–8. PMO 224.911, seep 9, dorsal, ventral, lateral, and anterior views. 9–12. PMO 224.913, seep 9, dorsal, ventral, lateral, and anterior views. 13–16. PMO 227.430, seep 9, dorsal, ventral, lateral, and anterior views. Specimen serially sectioned (Figure 12). Scale bar = 1 cm.

FIGURE 12. Transverse serial sections of Seductorithyris septemtrionalis gen. and sp. nov., seep 9, PMO 227.430. Serial sections prepared perpendicular to maximum length of the specimen, numbering of sections starts at 0.0 mm the initial section through the specimen. Dotted lines delimit groups of sections drawn at the same magnification. For section 11.4 mm enlargement of brachidial flanges shown adjacent to lower magnification drawing. Dimensions of sectioned specimen: L 25.3+; W 25.1+; T 13.9 mm. Scale bars = 1 mm.

FIGURE 13. 1–4. Pinaxiothyris campestris? Dagys, PMO 227.429, seep 9, dorsal, ventral, lateral, and anterior views. 5–8. Placothyris kegeli? Harper et al., PMO 224.922, seep 3, dorsal, ventral, lateral, and anterior views. 9–12. Placothyris kegeli Harper et al., MGUH 27501, Brorson Halvø, Wollaston Forland, North–East Greenland; Albrechts Bugt Member, Lower Valanginian. Specimen serially sectioned by Sandy in Harper et al. (2005, fig. 3). 13–20. Zittelina? sp. 13–16. PMO 224.869, seep 9, dorsal, ventral, lateral, and anterior views. 17–20. PMO 224.885, seep 9, dorsal, ventral, lateral, and anterior views. Scale bars = 1 cm.