Rouillieria ovoides

Rouillieria aff. ovoides (Sowerby, 1815)

Fig. 8.9–8.16

1971b aff. Rouillieria ovoides (J. Sowerby) —Ager, p. 394, pl. 14, pl. 15, fig. A.

Material and occurrence. Seep 8 (PMO 224.924, PMO 224.926, PMO 224.931–932), seep 13 (PMO 224.906, PMO 227.426).

Description. Quadrate outline, equally biconvex profile, rectimarginate anterior commissure.

Discussion. The material is homoeomorphic with other material described herein from Spitsbergen, Uralella ? cf. janimaniensis Makridin and Seductorithyris septemtrionalis gen. et sp. nov. From Uralella ? cf. janimaniensis it may be distinguished by its smaller size, more rounded hinge line and lesser inflated profile, especially with respect to the dorsal valve. Seductorithyris septemtrionalis has a rounder hinge line, rounder outline, and narrower apical angle of the ventral beak when compared to Rouillieria aff. ovoides; the latter has a more quadrate outline.

The Spitsbergen material does not show a posterior shallow sulcation or groove in the dorsal valve. This does appear to be a variable feature (Ager 1971b; Middlemiss 1976). A rounded keel to the posterior of the ventral valve is considered characteristic (Ager 1971b; Middlemiss 1976).

Ager (1971b, p. 395) considered some variability in external morphology acceptable within Rouillieria ovoides but questioned the validity of the total number of species referred to Rouillieria by Makridin (i.e., he wondered if there might be fewer, more variable species). The R. ovoides material described by Ager (1971b) was collected from Spilsby Sandstone Formation glacial erratics in Norfolk, England. Kelly (in Kelly & Rawson 1983) noted that glacial erratics of grey glauconitic Spilsby Sandstone (Kelly’s “Lithology 2” of four erratic lithotypes) contained the ammonite Paracraspedites oppressus Casey, bivalves, and ‘nests’ [preserved biocoenoses?/clumps sensu Kidwell et al. 1986] of R. ovoides. These erratics are considered to come from the upper Middle Volgian part of the succession (Kelly in Kelly & Rawson, 1983).

Middlemiss (1976) discussed the differences between species of Rouillieria from the Claxby Ironstone Formation, Lincolnshire, England. Rouillieria walkeri (Davidson) typically has a sulcate anterior commissure and R. tilbyensis (Davidson) is typically uniplicate, whereas the Spitsbergen material is rectimarginate, more like R. ovoides. There are other subtle morphological differences between R. ovoides and R. tilbyensis (Middlemiss 1976, pp. 69–70) and the latter can also be confused with Cyrtothyris cyrta.

Smirnova (1975, 1997) referred Sowerby’s Rouillieria ovoides (in Ager 1971b) to Okathyris, described from the Russian Platform (Smirnova 1975). However, she did not give in detail the reasons for this assignation. The hinge plates of Rouillieria ovoides figured by Ager (1971b, fig. 1) are narrow compared with R. michalkowii (Makridin 1964, fig. 83). Ager did show that the flanges of the brachidium for R. ovoides from England extended at least 17.8 mm beyond the crest of the transverse band (located 20.4 mm from the posterior of the sectioned specimen), indicating long loop flanges. These flanges are proportionately more comparable in length with those recorded for Rouillieria (Makridin 1964, fig. 83) whereas the loop flanges for Okathyris are shorter (Smirnova 1975, figs. 1, 3). In serial sections of Okathyris chevkinensis Smirnova (1975, fig. 1) and O. sokolovi Smirnova (1975, fig. 3) the loop flanges anterior of the transverse band are approximately 6–7 mm in length. The brachidial flanges for Rouillieria cf. michalkowii (Fig. 9 herein) are also shorter than those for the genus in Makridin (1964) and have already been discussed under that species.

Stratigraphic and geographic distribution. Upper Volgian of Spitsbergen.