Original Description The test is comprised of 12 or 13 chambers arranged in a low sinistral trochospiral with about 4 1⁄3 chambers in the last whorl. Chambers are subglobular and slightly radially compressed except for the last chamber, which is almost spherical. Chambers expand fairly rapidly in size, such that the final chamber is about twice as wide as the first full chamber in the final whorl. Sutures on both sides of the test are depressed and radial with a slight degree of curvature. The aperture is a low, broad asymmetrical arch, situated in an intra-extraumbilical position. It is covered by an irregular tooth-plate, which is a composite structure formed by the fusion of the apertural lips of the final and penultimate chambers. The wall texture is microperforate and pustulose, and appears slightly translucent under the light microscope. Pustules are about 2-5 µm in diameter at their base and tend to be conical or tetrahedral. They are more prominent on the earlier chambers of the final whorl, becoming progressively smaller and less angular on later chambers. The final chamber lacks distinct pustules except in proximity to the apertural tooth-plate. The proloculus and first two or three chambers are obscured by smooth superficial calcification. Pores are distributed irregularly, but not strictly randomly, between the pustules. A single circular perforation, with a diameter of about 10 µm is present at the base of the final suture on the spiral side, and may represent a minute supplementary aperture. Size: The maximum diameter is 260 µm; the minimum diameter is 195 µm Etymology: Latin ‘‘mirabilis’’ wonderful or strange Extra details from original publication Synonymy:
Globorotalia sp. 4 Jenkins and Orr, 1972, p. 1096, Pl. 18: Figs. 712.
Tenuitella sp. Pearson, 1995, p. 53, Pl. 1, Figs. 23-24.
Tenuitella sp. Norris, 1998, p. 455, Pl. 3, Figs. 7, 11-13, 19-20.
Morphological variability. The extraordinary feature of M. mirabilis is the high level of intraspecific variation. The holotype is a relatively ‘‘conservative’’ specimen. Some aspects of the morphological variability are examined in the morphometric study reported below. Other aspects are best explained with reference to the illustrated paratype specimens on Plates 2–4.
The specimen illustrated on Plate 2, Figs 2a-c has 5 1⁄2 chambers in the final whorl and a much lower rate of chamber expansion than the holotype. The chamber shape is also noticeably different, with chambers appearing a little more radially compressed on the spiral side, but radially extended on the umbilical side, tapering toward the umbilicus.
The specimen illustrated on Plate 2, Figs 3a-c is similar to the holotype in its overall shape. However, the tooth-plate is less well developed and has only a single point, directed toward the antepenultimate suture, and the aperture is higher and more arched.
The specimen on Plate 2, Figs 4a-c has a lower trochospiral than the holotype. The chambers of the final whorl are dorsally inflated, such that the proloculus is situated in a dorsal ‘‘umbilicus’’ rather than near the apex of the test as in the holotype. The aperture extends past the periphery and slightly onto the spiral side. The tooth-plate is correspondingly elongate and irregular.
The specimens on Plate 2, Figs. 5 and 6 have been deliberately broken to reveal the morphology of the earlier chambers. Each has four chambers in the early whorl and an extraumbilical foramen with a pronounced flaring lip. The chamber shape and arrangement are similar to the supposed ancestor, Globigerinita glutinata (see below), but the aperture is more extraumbilical than is typical for that taxon.
The specimen on Plate 2, Fig. 7 shows a very pronounced triangular tooth bordered by a thin lip and an irregular penultimate chamber. Those on Plate 2, Fig. 8 and Plate 3, figs. 1-4 illustrate a feature that is commonly encountered in this species, namely a composite toothplate. This is a complex structure, having been formed by the superimposition of the tooth of the final chamber onto the tooth of the penultimate chamber. The final tooth-plate is not in contact with the preceding tooth-plate along its entire length, but arches on to it, leaving an aperture on the posterior side, which connects directly with the primary aperture. The structure thus formed is similar to that seen in some species of the unrelated Cretaceous genera Ticinella and Rotalipora.
The specimens on Plate 3, Figs 1-4 also further illustrate the variation in test shape and apertural modification in the species. Some of them have small circular pits of unknown origin on the test surface. The specimen on Plate 3, Fig 4a-c shows a series of beaded calcite deposits on the sutures, which probably cover small supplementary apertures similar to the bullae sometimes seen in Globigerinatella insueta (see, for example, Pearson, 1995, Plate 2, Fig. 8). This specimen also has a curious long slit-like supplementary aperture on the spiral side beneath the final chamber, with an irregular tooth-plate like covering. This structure is clearly allied to the primary aperture and toothplate, but does not visibly connect on the outside of the test. The double-aperture is similar to the structure seen occasionally in various planispiral planktonic foraminifera, such as Eocene Pseudohastigerina, and has been observed on a number of specimens of Mutabella. A further spiral aperture which has a more rounded shape occurs beneath the penultimate chamber.
Many of the specimens discussed above have a tendency towards a more planispiral coiling mode, at least with respect to the final whorl. Some specimens are almost axially symmetric and biumbilicate in appearance, such as that shown on Plate 3, Fig. 5. In specimens such as this, the tooth-plate tends to be much reduced, constituting a more regular apertural lip. Note also the supplementary sutural apertures of this specimen.
The specimen on Plate 3, Fig. 6 shows an enlargement of the apertures on another, peeled specimen showing that when well-formed, they tend to be circular and bordered by a regular lip, as seen in Globigerinatella insueta.
Further specimens are shown on Plate 4, Figs. 1-4. The specimen on Plate 4, Fig. 3 shows an areal aperture with a pronounced lip on the final chamber. This is a rare but not unique feature of M. mirabilis. Other specimens on the plate show an approach in morphology to Globigerinita glutinata (specimens of which are shown in Plate 4, Figs 8-10) in the early part of the stratigraphical range of M. mirabilis.
Stratigraphic Range. Probably from Zone M3 (lower Miocene) to Zone M7 (middle Miocene) (see discussion below) Editors' Notes
Pearson, P. N., Norris, R. D. & Empson, A. (2001b). Mutabella mirabilis gen. et sp. nov., a Miocene microperforate planktonic foraminifer with an extreme level of intraspecific variability. Journal of Foraminiferal Research. 31: 120-132. gs
Mutabella mirabilis compiled by the pforams@mikrotax project teamviewed: 15-8-2022