Catalog - Globorotalia bella

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Globorotalia bella

Citation: Globorotalia bella Jenkins, 1967

Type specimens: N.Z. Geological Survey Register No. TF 1527: holotype and 3 paratypes.

Type age (chronostrat): Awamoan Stage: G. trilobus trilobus zone (Lower Miocene).

Type locality: N2/546, F 14844, grid ref.: 427420. Moderately soft,calcareous, light blue-grey siltstone from Parengarenga Harbour, Northland. Collected by G. and A. Scott.

Type repository: Lower Hutt, New Zealand; New Zealand Geological Survey

Test free, low trochospiral, dextrally coiled, equatorial periphery quinquelobate, axial periphery rounded. Wall calcareous,perforate, fairly thick, slightly roughened glassy surface. Chambers spherical-ovate, inflated, increasing fairly slowly in size, 15 forming the test in about 2 1/2 whorls, 5 chambers in the final whorl; proloculus diameter 0.02 mm. Umbilicus small, shallow. Sutures on both umbilical and spiral sides radial to slightly curved. Aperture a low slit with a thick lip, interiomarginal, umbilical-extra-umbilical

Of the 55 specimens picked out of the type sample 32 are dextrally coiled and 23 sinistrally coiled. The test size varies in maximum diameter from 0.23 mm to 0.39 mm. The majority of specimens have 5 chambers in the final whorl; a few have 4. Some specimens show a tendency for the final chambers to overlap on to the spiral side giving the test a pseudo-bilateral symmetry as in the paratype TF 1527/2.

G. bella is also present in two stratigraphically higher and two stratigraphically lower Awamoan samples of the Parengarenga Harbour section. The slit-like aperture of G. bella easily distinguishes it from Globorotalia nana semivera (Hornibrook) and Globorotalia mayeri mayeri Cushman and Ellisor, which have larger and higher-arched apertures. The 5 chambers of the final whorl of G. bella distinguish it from Globorotalia nana pseudocontinuosa Jenkins and from Globorotalia nana nana Bolli, both of which have 4 chambers in the final whorl. The species it most closely resembles is Globorotalia mayeri barisanensis Le Roy, but G. bella is distinguished from it by having nearly radial sutures and a tendency for tests to produce nearly planispirally coiled tests.
Jenkins (1960) wrongly referred specimens of G. bella to Globorotalia siakensis (Le Roy), but the Lakes Entrance oil shaft specimens from south-east Australia and New Zealand specimens are easily distinguished from it by possessing the slit-like aperture. G. siakensis is very close in morphology to G. nana semivera and may prove to be synonymous with it.
The close resemblance in test morphology of G. bella and G. mayeri barisanensis suggests a close relationship, and it is possible that the former species is the ancestor, but the two taxa have never been observed together in any sample, although in the Muddy Creek section from the East Coast of the North Island there is a small overlap in their stratigraphic ranges.
Further support for this hypothesis of an evolutionary relationship comes from the type Altonian sample S167/617 from the Clifden section where there are 4 specimens of G. bella, with slightly recurved sutures on the spiral side, which appear to be intermediate between the two taxa. The specific name is taken from the Latin bella meaning pretty.

References:

Jenkins, D. G. (1967). Planktonic foraminiferal zones and new taxa from the lower Miocene to the Pleistocene of New Zealand. New Zealand Journal of Geology and Geophysics. 10(4): 1064-1078. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia bella Jenkins, 1967

Globorotalia bella

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