Catalog - Globigerina semivera

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Globigerina semivera

Citation: Globigerina semivera Hornibrook 1961

Type specimens: Pl. 223, figs. 6455-457, holotype, X 100.: no. TF1430;Holotype and fifty paratypes holotype, no. TF1430/1.

Type sample (& lithostrat): Rifle Butts Formation

Type age (chronostrat): Lower Miocene, Awamoan

Type locality: Holotype from a brown sandstone at the top of the section at Campbells Beach, All Day Bay, 97 degrees from Trig. V. Otepopo S.D., N.Z.M.S 1, Sheet S136 (Provisional Edition 1943), Grid. Ref. 478544, South Island, New Zealand.

Type repository: Lower Hutt; New Zealand Geological Survey

Test of moderate to small size, relatively flat dorsally, consisting of up to three whorls of spherical chambers increasing in diameter very rapidly in the third whorl, which has four to four and one-half chambers. Sutures radial, straight, tending to be a little incised. Aperture a wide, arched, rimmed opening in the plane of coiling, usually extending into the umbilical area. Test wall coarsely perforate, the pores surrounded by distinct rims, which are connected forming a strong hexagonal pattern.

Occurs also in the Upper Oligocene, Waitakian, ranging to the Middle Miocene, Lillburnian and possibly Walauan.

Dimensions of holotype, diameter 0.41 mm.

Variability. - There is some variation in the size and position of the aperture. Of the fifty specimens in the type sample, the ratio of coiling is twenty-seven anticlockwise to twenty-three clockwise.

Remarks. - Globigerina semivera resembles Globorotalia moyeri Cushman and Ellizor, which, according to the type figures and description, differs in being nearly flat dorsally, with distinctly recurved chambers. It appears very likely that Globigerina semivera is the intermediate between Globigerina and Globorotalia possibly having been derived from Globigerina reticulata Stache, of the Whaingaroan and Duntroonian (Lower Oligocene).

"If the Miocene Globorotalias have evolved independently from a stock of Oligocene Globigerinas, as appears very likely, the Paleocene to Eocene Globorotalia-like species (centralis, velascoensis, pseudomenarali etc.) will have to be placed in one or more new genera. Globorotalia as it is at present used, being a mere form-genus. Globigerina semivera has been referred to previously in reports and faunal lists as Globorotalia cf. seitula (Brady).

Another similar species in the Whaingaroan and Duntroonian of Southland seems to be Globigerina reticulata described ty Stache from the Whaingaroan near Raglan on the west Auckland coast. Early representatives of Globigerina dubia Egger, in the Taranaki and Lower Wanganui Series, may have been derived from semivera by an increase in the number of chambers from four to five per whorl. Typical Globigerina dubia from Recent seas has six or more chambers per whorl and is larger than the New Zealand Upper Mioicene specimens.

References:

Hornibrook, N. d. B. (1961). Tertiary Foraminifera from Oamaru District (N.Z.). Part 1 Systematics and distribution. New Zealand Geological Survey, Paleontological Bulletin. 34(1): 1-192. gs

Leckie, R. M. et al. (2018). Taxonomy, biostratigraphy, and phylogeny of Oligocene and Lower Miocene Paragloborotalia and Parasubbotina. In, Wade, B. S., Olsson, R. K., Pearson, P. N., Huber, B. T. & Berggren, W. A. (eds) Atlas of Oligocene Planktonic Foraminifera. Cushman Foundation for Foraminiferal Research, Special Publication . 46(Chap 5): 125-178. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globigerina semivera Hornibrook 1961

Globigerina semivera

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