Pseudomonticlarella varia Smirnova. It 1987

Pseudomonticlarella varia Smirnova, 1987

(Figs. 3.1–3.12, 4)

1987 Pseudomonticlarella varia new species —Smirnova, p. 33, pl. 4, figs. 1, 2, 5–7, text-fig. 1.

2002 Pseudomonticlarella varia Smirnova —Manceñido et al., p. 1314, fig. 891.3a– 3p.

Material and occurrence. Seep 7 (PMO 227.419), seep 13 (PMO 227.416–418), seep 10 [damaged specimen, anterior commissure not present, PMO 224.923?].

Description. Small (less than 1 cm in length, width, and thickness), smooth rhynchonellide that shows traces of capillate ornament in places. However, the decortication of specimens makes it difficult to verify the ornament of the shell exterior. The anterior commissure is bisulcate with weakly developed sulcii at the commissure; central sulcus shallower than lateral sulcii.

The following internal characters may be visible from external examination, either through translucent shell or because of decortication: dental lamellae in the ventral umbo; a median septum extending at least half the length of the dorsal valve. Internal characters were also determined from serial sections (Fig. 4): a small septalium-like structure is present due to the concave ventral surface of the hinge plates (Fig. 4, sections 0.3–0.4 mm); crura develop on the inner margins of the hinge plates (sections 0.5–0.7); crura arch ventrally (sections 0.7–1.1). Dental lamellae were not seen in the serial sections (ventral valve umbo of the sectioned specimen was damaged, Fig. 3.1–3.4), but observed from the external examination of specimens.

Discussion. The general outline and profile of the specimens from Spitsbergen are similar to those of specimens figured by Smirnova (1987, pl. 4, figs. 1–2) from the Russian Platform, in particular that in pl. 4, fig. 1. This specimen appears to be smooth, developing costae at the anterior margin. The Russian Platform specimens reach maximum lengths close to 6 mm (Smirnova 1987, p. 42) so are smaller than the Spitsbergen material that reach 8 mm in length.

Smirnova provided serial sections of two specimens of Pseudomonticlarella varia (1987, fig. 1a–b). Both series of sections show a septalium and crura that arch ventrally anteriorly. These are features that are shared with the Spitsbergen specimen (Fig. 4). The internal structures (from the initial section through the brachiopod) were traced for 4.12 mm in specimen No. 139/584 (Smirnova 1987, fig. 1a) and for 1.45 mm in specimen No. 139/590 (fig. 1b). In the Spitsbergen specimen the internal structures were traced to section 1.2 mm (i.e., 1.2 mm from the initial section through the specimen).

It was thought originally that these diminutive specimens could be juveniles of the form referred herein to Ptilorhynchia mclachlani sp. nov. However, posteriorly the ventral valve of Pseudomonticlarella varia lacks any developing or incipient costae, which are often present posteriorly on specimens of Ptilorhynchia mclachlani. Also, posteriorly, the dorsal valve of Pseudomonticlarella varia is weakly sulcate (possibly enhanced by some decortication). This sulcate condition does not appear to characterize specimens of Ptilorhynchia mclachlani. In addition, Ptilorhynchia mclachlani and Pseudomonticlarella varia are not co-occuring. Obviously they should cooccur if they are conspecific (although this could reflect a collecting failure). The specimens of Pseudomonticlarella varia all occur with similar maximum dimensions and there are currently no other intermediate-sized forms between these and larger-sized species such as Ptilorhynchia mclachlani.

Stratigraphic and geographic distribution. Upper Volgian–Lower (?Upper) Ryazanian of Spitsbergen and originally described from the Upper Volgian of the Russian Platform (Smirnova 1987).