Minourella gotlandica

Hussain, Rowshi, Rogers, Steven L. & Blackburn, Joel A., 2022, Ovummuridae (calcareous microfossils) from the Much Wenlock Limestone Formation, Shropshire, UK, Palaeontologia Electronica (a 27) 25 (2), pp. 1-24 : 7

Minourella gotlandica

Munnecke, Servais and Vachard, 2000

Figure 4 A-D

Material. 4665 specimens.

Diagnosis (Modified from Munnecke et al., 2000 and MacNeil and Jones, 2006). A species of Minourella , consists of one large and one small chamber, separated by a flat septum-like structure. Rare specimens exhibit an apical aperture in the smaller ventral chamber and occasionally within both ventral and dorsal chambers.

Description (Modified from Munnecke et al., 2000 and MacNeil and Jones, 2006). A globular micro-organism characterised by two asymmetrical chambers separated by a septum-like structure. The larger chamber is globular and oval shaped with the long axis parallel to the septum. The small chamber is globular and oval/elongated-oval shaped, similar to the dorsal larger chamber. The aperture within the dorsal chamber is not always present. The aperture in the ventral chamber is rare but can occur. Three specimens with an aperture at the apex of both the dorsal and ventral chamber were observed from the Wenlock Edge material.

Dimensions. Overall test height/length of 21–233 µm, overall test width of 14–284 µm, large chamber diameter height of 2–184µm and width of 20– 245 µm, the wall around the large dorsal chamber is 0.47–35.5 µm thick. The smaller ventral chamber height of 1–113µm and width of 1–103 µm the wall around the small ventral is 0.5–16 µm thick. A septum thickness of 0.4–36 µm and an aperture of 1– 14 µm in diameter.

Remarks. Diagnosis amended to include the possibility of an aperture present both within the dorsal and ventral chambers. It is noted that very rare specimens exhibit apertures within both their dorsal and ventral chambers unlike in the Gotland material in which a single aperture was reported to be visible in the dorsal chamber and conversely only from the ventral chamber within the Hay River material ( MacNeil and Jones, 2006). The specimens observed are comparable in morphotype and size to those described by Munnecke et al. (2000), however, the MWL Fm. specimens are in general: (1) larger, (2) have thicker walls with quartz overgrowths, (3) some specimens contain pyrite within their walls and/or within their chambers as a result of diagenesis, (4) larger apical apertures and exhibit apertures within the ventral chamber. The largest Minourella gotlandica specimen is noted as having a test height of 233 µm and a test length of 284 µm. A maximum wall thickness of 36 µm and a septum thickness of 19 µm. The largest aperture width noted was 14 µm.

Stratigraphical range. Lower Silurian (Homerian) to Devonian (Famennian)

Figures

FIGURE 4. Photomicrographs of Minourella gotlandica (A-D), Minourella wenlockensis (E-G) and Minorella cameroni (H) within the MWL Fm., these specimens are from Eaton Top (A, B and C), Longville Stanway Track (D and F), Moorwood Quarry (E and G) and Harton Hollow (H). (A) = M. gotlandica (mg) specimen near a bryozoan (bry) within a spiritic matrix (sp) highlighting the similarities in wall structure, (B) = M. gotlandica with a larger globular dorsal chamber (DC) separated from the smaller globular ventral chamber (VC) by a septum like structure (ss). Sparitic (sp) matrix. Specimen has a pyritic wall. (C) = M. gotlandica with an aperture (ap) within the VC. (D) = Thickened walls of the dorsal chamber with an apical aperture (ap). (E) = Sectional cut through M. wenlockensis highlighting the conical ventral chamber (VC) in contrast to the globular dorsal chamber (DC) separated by septum like structure (ss) within a sparitic (sp). (F) = Apertures present in both DC and VC. Note pyritic growth within the specimen walls. (G) = M. wenlockensis with an apical aperture (ap) within the DC. Matrix comprised of sparite. (H) = M. cameroni Longitudinal transverse sectional cut through M. cameroni exhibiting the conical nature of the test. Lateral dorsal chamber (LDC) is separated from the ventral chamber (VC) by a septum like structure (ss). Sparitic infill of the LDC and VC from the surrounding sparitic (sp) matrix. All scale bars equal 50 µm.