Minourella cameroni (MacNeil and Jones, 2006)

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 : 7-9

Minourella cameroni

MacNeil and Jones, 2006

Figure 4H

Material. 7 (27, if triangular cross-sectional cuts are considered)

Original diagnosis ( MacNeil and Jones, 2006). Conical test, one large dorsal chamber and one small ventral chamber with apertures at anterior end of test and a dorsal secondary aperture.

Description (Modified from MacNeil and Jones, 2006). A conical test dominated by a large, anteriorly expanding dorsal chamber. The septum partly divides the dorsal chamber into a large, posterior sub-chamber and small, anterior sub-chamber. The large, posterior dorsal sub-chamber can contain a secondary aperture, possibly slit-like, on the dorsal side of the test. The small sub-chamber can have a large simple aperture at the anterior dorsal end of the test, with or without lateral flanges. The small ventral chamber narrows around and over the area of the dorsal chamber that contains the septum before opening into an aperture at the anterior end.

Dimensions. MacNeil and Jones (2006) introduced measurements of overall test length of 195– 275 μm long, 70–115 μm high, dorsal chamber height of 55–105 μm high, ventral chamber 0.5–13 μm high, and walls 3–6 μm thick. Our specimen measurements were: overall test height/length of 42–298 µm, overall test width of 7–132 µm, large dorsal chamber diameter height of 20–260 µm and width of 5–71 µm and the wall around large dorsal chamber is 2–14 µm thick. The smaller ventral chamber has a height of 2–115 µm and width of 1– 32 µm and the wall around the small dorsal ventral is 2–5 µm thick. A septum thickness of 2–6 µm.

Remarks. The specimens from the MWL Fm. show no visible apertures in either of the chambers as described by MacNeil and Jones (2006). Although in some cross-sections Minourella cameroni looks identical to Minourella gotlandica , it is difficult to determine the true specimen count of the species. Many specimens of ovummurids were triangular in morphotype, and we believe these may be transverse cross-sectional cuts through Minourella cameroni . The MWL Fm. specimens in comparison to the Hay River Material are (1) lower in population, (2) thinner in septum thickness and generally smaller, (3) longer in test length with maximum test lengths of 298 μm were observed.

Stratigraphical range. Lower Silurian to Upper Devonian

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.