Zittelina Rollier 1919

Zittelina ? sp.

Fig. 13.13–13.20

1976?v. “ Zittelina ” sp.—Owen, p. 16, pl. 3, fig. 1.

2011 v. ‘ Zittelina ’ sp.—Hammer et al., p. 20, table 2, fig. 7 T.

Material and occurrence. Seep 9 (PMO 217.200, PMO 224.869, PMO 224.885).

Description. Round and weakly subpentagonal outline, somewhat ventribiconvex profile, and rectimarginate, to possibly incipiently sulcate anterior commissure. Dental lamellae are present in the ventral valve umbo and a median septum is present in the dorsal valve indicating a long-looped terebratellidine. Decorticated specimens (Fig. 13.13–13.16 and Hammer et al. 2011, fig. 7T) show the presence of dental lamellae and a median septum. A smaller specimen is a possible juvenile (Fig. 13.17–13.20).

Discussion. Owen (1976, pl. 3, fig. 1) figured a specimen from the Valanginian of Wollaston Forland, East Greenland that has a pentagonal outline, inflated biconvex profile, and ventrally deflected anterior commissure but with no obvious concomitant sulcation developed on the dorsal valve. The specimen from Spitsbergen has a round outline, rectimarginate anterior commissure, and biconvex profile but with a lesser degree of inflation compared to the Greenland specimen. The relationship between the Greenland and Spitsbergen material is difficult to assess at present as only one specimen was identified from Greenland and only two mature specimens are known from Spitsbergen. The differences in outline, profile and commissure make it unlikely that the forms from Greenland and Spitsbergen are conspecific. Given the predominantly Boreal aspect of the fauna from Spitsbergen it may be that this form is allied to Rugitela Muir-Wood (considered to be the senior synonym of Russiella Makridin (Baker 2006)). The specimens lack the sulcate dorsal valve and sulcus of the anterior commissure seen in Rugitela hippopus (Roemer) that has been recorded from East Greenland (Harper et al. 2005).? Rugitela sp. (Owen 1976) was put in synonymy with Rugitela hippopus (Harper et al. 2005) but this seems rather unlikely as the latter is elongate-quadrate in outline and weakly sulcate.

The Early Cretaceous age of this material suggests that an affinity to Zittelina rather than Kingena Davidson is more likely. However, both genera are very similar in external morphology (discussed by Owen 1970). The granular shell ornament considered characteristic of Kingena has not been observed, although well-developed endopunctation has. The incipient(?) sulcation of the anterior commissure (Fig. 13.16, 13.20) might suggest an affinity to Zittelina in contrast to faint uniplication that could suggest Kingena (based on generic diagnoses given by MacKinnon et al. 2006; although a rectimarginate anterior commissure is common to both genera). Species referred to Dzirulina Nutsubidze (considered a senior subjective synonym of Belothyris Smirnova by Sandy et al. 1995) typically have a more elongate outline than for Zittelina and Kingena (discussed by Owen 1970).

Stratigraphic and geographic distribution. Uppermost Ryazanian of Spitsbergen.