Catalog - Guembelitria samwelli

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Guembelitria samwelli

Citation: Guembelitria samwelli Jenkins 1978

Type age (chronostrat): Lower part of the Oligocene Globigerina (G.) euapertura Zone (Jenkins, 1966) which can be correlated with the type Chattian

Type locality: At 40 46 00 S,172 15 50 E, 34 miles southwest of Cape Farewell west coast of South Island, New Zealand

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

Test free, small elongate tetrahedron with a lobate periphery and an apical angle of about 36 degrees. Wall calcareous, perforate with irregular pores between minute calcite plates. Chambers sinistrally coiled triserially, about 14 forming the test in 4 whorls; chambers increase slowly in size (fig. 1). Sutures shallow in earliest part of test but more pronounced in later whorls. Umbilicus open. Aperture small, 0.01 mm. high, arched with a rim extending from the antepenultimate chamber to more than halfway along the arch; umbilical, at base of final chamber.

The new species is named after the Welsh surgeon and poet, David Samwell, who was with Captain J. Cook on his third voyage on the H.M.S. Resolution in 1776. The ship sailed through the Roaring Forties where G. samwelli once lived, when the southern ocean was younger.

Chamber growth. The tri-serial arrangement and growth of the chambers (maximum diameters) have been plotted for two paratypes (Fig. 1). In paratype 15, after the addition of the second and third chambers which are slightly smaller in diameter than the proloculus, there is a rather steady, although slightly irregular growth with both chambers 7 and 13 being slightly smaller than the immediately preceding chambers (Fig. 1). The other paratype also shows a slightly irregular growth pattern.
Variation. The length of 20 paratypes varies from 0.09 mm to 0.16 mm, with the average at 0.13 mm. The apical angle varies from 36" to 45" and the coiling is random in the 20 paratypes.
Preservation. Tests are rather well preserved and unfilled, but a few specimens have broken final chambers.
Wall structure. The test wall is composed of minute calcite crystals (PI. 1, Fig. 3) which in section appear to be at right angles to the wall surface. The final chamber wall appears to be mono-lamellar, while earlier walls (Pl. 1, Fig. 5) could be bi-lamellar.
Secondary apertures. Small secondary apertures are present in a number of specimens and are normally formed only on a few chambers of a test (Pl. 1, Fig. 9). Well-preserved specimens from the southeast Atlantic DSDP Leg 40, Site 360 showed a secondary aperture surrounded by a rosette of calcite crystals (Jenkins, 1977). Secondary apertures have been observed in paratypes and also in specimens from Site 360, Bass Strait and South Australia.
Comparison with Paleogene species. Guembelitria samwelli is morphologically different from the following species described from the Eocene ( fide Ellis and Messina, 1940 et seq.): ( 1 ) G. columbiana Howe, described from the Eocene of Louisiana U.S.A., has a more rapid growth pattern than G. samwelli; (2) G. oveyi Ansary, from the upper Eocene of Egypt, differs in having a much larger centrally positioned aperture and ( 3 ) G . stavensis from the middle Eocene of Alabama, U.S.A., differs in having a much larger aperture, lacking secondary apertures, having more chambers forming the test and having a different wall structure (Jenkins, 1977, pl. 1, fig. 9-13).

References:

Jenkins, D. G. (1978). Guembelitria samwelli Jenkins, a new species from the Oligocene of the Southern Hemishere. Journal of Foraminiferal Research. 8(2): 132-137. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Guembelitria samwelli Jenkins 1978

Guembelitria samwelli

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