Catalog - Mermaidogenerina loopae

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Mermaidogenerina loopae

Citation: Mermaidogenerina loopae Apthorpe 2020

Type specimens: Fig. 5: 1–13; Fig. 7: 1– 4; Fig. 11: 1–4. Holotype WAM 18.22/M52-01 (Fig. 5: 1–4; Fig. 11: 1).

Type sample (& lithostrat): Dredge sample BMR 95/DR7/8

Type age (chronostrat): Earliest Bajocian - based mainly on nannoplankton

Type locality: northeastern wall of Mermaid Canyon (4530 to 3900m water depth), offshore of northwestern Australia; the dredge haul starting at 16° 18.9′ S, 118°24.3′ E. 118 22.2′ E and ending at 16 18.2′ S

Diagnosis: Smooth-walled to very low pustulose-walled, entirely triserially coiled trochospiral form, with a high, loop-shaped aperture, extraumbilical, excentrically placed near the outer edge of the final chamber, and bounded by a thick lip.

Description: Test calcareous, a medium trochospire, triserially coiled, composed of hemispherical chambers arranged at approximately 120◦ to each other (Fig. 5: 1, 7, 11; Fig. 7: 1, 3; Fig. 11). Test width expanding rapidly between the first and second whorls. Initial chambers clearly visible, with the proloculus well defined and projecting at the base of the test. Sutures in both spiral and umbilical views curved and depressed; peripheral outline lobulate. The holotype has the last chamber missing, plus the broken trace of a large terminal bulla, which appears to have covered the top half of the last three chambers (Fig. 5: 2). Most specimens photographed on the umbilical side have the scar of a bulla, now broken off (Fig. 5: 5). The aperture is a high loop-shaped asymmetrical opening, virguline in shape, extraumbilical, situated close to the outer edge of the chamber, facing the umbilicus (Fig. 5: 2–3, 5–6, 10). The aperture is rimmed by a smooth lip throughout, which on the peripheral side thickens and becomes very wide. In several specimens including the holotype, the thickened lip has the appearance of inner lamella that has “spilled out” from the interior of the chamber (Fig. 5: 3, 5, 10).

Wall structure: Wall brown in colour, smooth, shiny when seen under the binocular microscope, microperforate, with pores less than 0.5 μm in diameter, widely and evenly scattered across chamber surfaces. Most specimens have virtually no development of tubercules or pseudomuricae except within the area formerly covered by the bulla, where they are prominent and elongated (Fig. 5: 2–3, 5–6, 9, 10). This suggests that diagenesis is not responsible for the smooth outer wall, since that process has not impacted the area of the broken bulla. Paratype WAM 18.23/M11-35 has low tubercules on the fifth and sixth chambers and scattered small tubercules on earlier chambers (Fig. 5: 8–9). The wall appears to consist of inner and smooth outer lamellae, with little additional secondary lamination (Fig. 7: 1–4). The wall is smooth in some specimens, etched in others. The holotype in its last two preserved chambers shows patchy dissolution (Fig. 5: 1–2), but earlier chambers are smooth with scattered pores (Fig. 5: 4). One paratype, WAM 18.25/M52-04, has a wall surface that appears smooth at low SEM magnification and under the binocular microscope (Fig. 5: 11), but at 7000× it is seen to consist of a reticulate maze of low interlocking V-shaped ridges (Fig. 5: 12–13) with numerous fine pores between the ridges. These would not be expected if the wall had been recrystallised.

Wall composition: In specimens from sample BMR 95/DR7/8, the wall has been indicated to be aragonite, with a high organic content, according to Raman spectroscopy by Aleksey Sadekov.

Height 0.12 - 0.14 mm; diameter 0.11 - 0.17 mm (range of 5 specimens detailed in table 2)

From the high, loop-shaped aperture

Remarks Although represented by relatively few specimens (~25), this smooth triserial form does not fit within any described Jurassic or Triassic species or genus known to the author. It does not fit within the four Jurassic genera proposed by Simmons et al. (1997), nor does it fit within reticulate-walled Conoglobigerina or pustulose-walled Globuligerina (Gradstein et al., 2017a) on the grounds of its smooth microperforate wall and ornament confined to the area of the bulla. This form does not correspond to any of the genera proposed by Fuchs (1967, 1973), which have as a minimum four chambers per whorl. It also does not correspond with Conoglobigerina pupa of Wernli and Görög (2007), which has much more tightly embracing, compressed chambers (not hemispherical chambers) and a higher coil with shallow sutures. “Conoglobigerina” trilocula Wernli and Görög has a much lower spire and a more embracing coil. Conoglobigerina solaperta of Wernli and Görög has some similarities in overall shape, having three chambers in the final whorl, but the coil expands much more slowly than in Mermaidogerina. The aperture also appears to be a low arch in C. solaperta, whereas in Mermaidogerina loopae it is a high asymmetrical virguline opening. In Mermaidogerina the variation in wall appearance seen, from smooth in some specimens, to smooth and slightly pustulose in one, to ridged in another specimen (at very high magnification), suggests that an assignment to either Conoglobigerina or Globuligerina is not possible on the basis of wall ornament, which is a key defining character of these genera. The apparent absence of secondary lamination in Mermaidogerina may explain the virtual absence of pseudomuricae development, in contrast to the other two forms described here.

Mermaidogerina is the first Jurassic recorded manifestation of entirely triserial coiling, later repeated in Guembelitria and Gallitellia; its apertural shape is, however, entirely different to these younger genera, which are also much higher-spired. Examination of Pleistocene–Holocene Gallitellia vivans (to check for the possibility of sea-floor contamination) shows it is very different in test shape (a very high trochospire of loosely appressed chambers), chamber shape (spherical rather than hemispherical), apertural shape and position (a medium, symmetrical umbilical arch without a lip, centrally placed), and wall structure and ornament. The assignment of Mermaidogerina to any benthic genus seems ruled out by the high, looped-shaped aperture, the globular, hemispherical chambers and the presence of a bulla in some specimens. In size it is only slightly smaller than Globuligerina bathoniana australiana and G. altissapertura, and as most specimens consist of two whorls, the form is not a juvenile.

References:

Apthorpe, M. (2020). Middle Jurassic (Bajocian) planktonic foraminifera from the northwest Australian margin. Journal of Micropalaeontology. 39: 93-115. gs V O

CATALOG OF ORIGINAL DESCRIPTIONS: Mermaidogenerina loopae Apthorpe 2020

Mermaidogenerina loopae

References:

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