Cyrtothyris, Middlemiss, 1959

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 : 515

Cyrtothyris ? sp.

Fig. 7.5–7.8

1976? v. “ Cyrtothyris ” maynci new species—Owen, p. 13, pl. 2, fig. 1a–c, pl. 3, fig. 4a–c, text-fig. 7.

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

Material and occurrence. Seep 3 (PMO 224.870, PMO 224.904?, PMO 224.919?), seep 8 (PMO 227.428?).

Description. Rounded, circular, to gently elongate outline, equibiconvex lateral profile, rectimarginate anterior commissure.

Discussion. The material from Spitsbergen reaches larger maximum dimensions than “ Cyrtothyris ” maynci described from Greenland ( Owen 1976). Broken ventral umbones hamper identification (e.g. Fig. 7.5–7.8 ) and referral to Cyrtothyris must be considered tentative. Owen commented that the Greenland form has an incipient uniplicate anterior commissure. The Spitsbergen specimens show a rectimarginate anterior commissure to possibly weakly uniplicate ( Fig. 7.8 , anterior commissure damaged). The Greenland material is from the Falskebugt Member ( Harper et al. 2005) of Valanginian age whereas the Spitsbergen specimens are from the Upper Volgian. Cyrtothyris has not been recorded from the Upper Jurassic and these specimens may represent a new form.

Some of the Spitsbergen brachiopods are morphologically close to Cyrtothyris and Rouillieria . Species referred to these two genera may be morphologically very similar and details of the ventral umbo and profile can be important in distinguishing between them (as discussed by Middlemiss 1976). Several specimens from seep 9 are now considered to be Seductorithyris septemtrionalis ; these have an erect beak and lack the produced ventral beak of Cyrtothyris , and are typically less inflated in profile than the latter. However, one specimen from seep 9 ( Fig. 7.1–7.4 ; PMO 224.895) with a suberect ventral umbo and incipiently uniplicate anterior commissure, is referred to Cyrtothyris aff. cyrta (Walker) . Middlemiss (1976) considers that this species has a rectimarginate to uniplicate anterior commissure.

Stratigraphic and geographic distribution. Upper Volgian of Spitsbergen.

Figures

FIGURE 7. 1–4. Cyrtothyris aff. cyrta (Walker). PMO 224.895, seep 9, dorsal, ventral, lateral, and anterior views. 5–8. Cyrtothyris? sp. PMO 224.870, seep 3, dorsal, ventral, lateral, and anterior views. 9–20. Praelongithyris? aff. borealis Owen. 9–12. PMO 224.907, seep 1, dorsal, ventral, lateral, and anterior views. 13–16. PMO 224.898, seep 9, dorsal, ventral, lateral, and anterior views. Resembles juvenile Cyrtothyris. 17–20. PMO 224.886, seep 1, dorsal, ventral, lateral, and anterior views. Form resembling Moutonithyris moutoniana (d’Orbigny). 21–24. Rouillieria cf. michalkowii (Fahrenkohl). PMO 224.901, seep 9, dorsal, ventral, lateral, and anterior views.Scale bar = 1 cm.