Rouillieria michalkowii

Rouillieria cf. michalkowii (Fahrenkohl)

Figs. 7.21–7.24, 8.1–8.8, 9

1964 cf. Rouillieria michalkowii (Fahrenkohl) —Makridin, p. 249, text-figs. 83, 84, pl. XVI, figs. 7, 8, pl. XVII, figs. 1–3. 2011 v. partim ‘ Praelongithyris’ sp.—Hammer et al., p. 20, table 2.

Material and occurrence. Seep 8 (PMO 227.420, PMO 224.925?, PMO 224.927, PMO 224.930?), seep 13 (PMO 227.421–423, PMO 224.901–902, PMO 224.905).

Description. Elongate-oval to subtriangular to subpentagonal outline, evenly biconvex profile. Lateral commissure straight to gently ventrally arched. Broad, weak uniplication of the anterior commissure.

Serial sections were prepared from one specimen (Fig. 9) which show traces of a pedicle collar (Fig. 9; sections 0.5, 0.7), cardinal process (sections 2.8–3.4), and flat and broad hinge plates with ventrally directed crural bases (sections 3.2–5.0). The crura develop anteriorly at an acute angle from the hinge plates in a “v”-shaped configuration (sections 5.0–5.4). Crural processes are high-arched (sections 7.5–8.5). The transverse band is flat topped.

Discussion. The material is referred to the loboidothyridoid genus Rouillieria Makridin. However, it has not been possible to determine the length of the flanges of the brachidium with certainty. The flanges of the brachidium were tentatively traced anteriorly to section 18.8 (Fig. 9). Sparry calcite infilling of the brachiopod made tracing the flanges difficult and it is unclear if this represents the maximum length of the loop. The serial sections do not show the exceedingly long anterior flanges on the brachidium seen in Makridin (1964, fig. 83). The sectioned Spitsbergen specimen (Figs. 8.5–8.8, 9) is approximately half the length of the largest representatives figured by Makridin and this may account for the possibly shorter flanges. Some species referred to Rouillieria (Makridin 1964) do not appear to attain such large proportions as the type species.

Middlemiss (1976, text-figs. 27, 29) provided serial sections of species referred to Rouillieria from the Claxby Ironstone Formation (Valanginian–Hauterivian), Nettleton, Lincolnshire, England — R. tilbyensis (Davidson) and R. walkeri (Davidson). These serial sections show similarities to the Spitsbergen specimen (Fig. 9) in the presence of a cardinal process, relatively flat to flat hinge plates, ventrally projecting crural bases, high arching crural processes, and a high-arched transverse band. The strongly concave juvenile hinge plates considered significant by Middlemiss (1976) have not been observed in the Spitsbergen sections.

The Spitsbergen material is similar in outline to the elongate, tear-drop shaped loboidothyridoids Atelithyris crestensis Smirnova, Boreiothyris pelecypodaeformis (Moisseiev), Moisseevia sokolovi Makridin and Spasskothyris rjasanensis Smirnova. Atelithyris crestensis from the Lower Hauterivian of the Russian Platform (Smirnova 1975) tends to have a more oval outline compared to the other species and has a deflected uniplicate anterior commissure. The development of the hinge plates and crural bases is similar to Rouillieria (in Makridin 1964) but lacks the long anterior flanges of the brachidium seen in Rouillieria. However, it would appear that Atelithyris could be derived from Rouillieria. Compared with the other taxa discussed here, Spasskothyris rjasanensis is more elongate in outline and has a broad uniplicate anterior commissure. The internal morphology is distinct, and it is distinguished “from all Jurassic and Early Cretaceous genera of long-flanged terebratulids by the development of inner hinge plates... ” (Smirnova 1975, p. 346). However, these inner hinge plates appear to be strongly keeled crural bases, at least initially. The development of these inner hinge plates in Spasskothyris is quite different from that in the Boreiothyrididae, such as in Boreiothyris, where the hinge plates meet in the medial plane of the dorsal valve (see serial sections in Dagys 1968). The internal structures of Moisseevia sokolovi Makridin are unknown but judging from external morphology it appears very close to the genus Rouillieria.

Stratigraphic and geographic distribution. Upper Volgian of Spitsbergen.