Catalog - Globorotalia miozea conoidea

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Globorotalia miozea conoidea

Citation: Globorotalia miozea conoidea Walters 1965

Type specimens: Holotype: (Fig. 8, J-L); Register No. TF 1489.

Type sample (& lithostrat): sample GMR592,

Type age (chronostrat): Lillburnian-Waiauan

Type locality: Nine Fords Stream, a right-bank tributary of the Waimata River, Sheet N98/2, grid ref. 4125515; approximately 10 miles north of Gisborne, N.Z.

Type repository: Lower hutt; N.Z. Geological Survey

Test relatively large, a low trochospire consisting o f 2½-3 whorls of slowly enlarging chambers, with 4½-5 (usually 4½) in the last whorl; equatorial profile subcircular, equatorial periphery weakly lobate; dorsal surface of the test flat or slightly convex; ventral side highly conical with flat or only slightly inflated chamber surfaces; on early chambers of the last formed whorl, axial periphery bluntly rounded and thickened, but on later chambers more sharply angled, often with a narrow cord-like keel; wall of newly formed chambers moderately thin and finely perforate, but older chambers show a progressive secondary thickening, especially on the periphery and on the ventral surface, acompanied by a coarsening of the punctation so that the ventral surface of the earliest chambers of the last formed whorl becomes characteristically knobbly; dorsally, chambers more than twice as long as broad, separated by highly curved, tangential sutures which are sometimes slightly depressed between later chambers; chambers triangular in outline on the conical ventral side, separated by more or less straight sutures which are not easily discernible near the periphery but which, towards the umbilical area, develop into irregular grooves in the thickened wall; later sutures on the ventral side often somewhat sigmoid and depressed; umbilicus closed; aperture a low slit lying along the base of the apertural face, extending from the umbilical area almost to the periphery, sometimes obscured by a small, finely punctate, "abnormal" last chamber, with a low, undistinctive aperture; septal foramen a low slit along the base of the septal face, as in all members of the G. miozea lineage, becoming more flaring at its peripheral end.

Holotype maximum diameter = 0.58 mm; height = 0.47 mm.

Remarks: This distinctive subspecies differs from G. miozea miozea Finlay by its much larger size, by its highly conical ventral side, and by its tendency to develop a definite cord-like keel, especially on later chambers. The large flat representatives of G. miozea that are common in the lower part of the Lillburnian-Waiauan are the only other members of the lineage to attain the same size as this subspecies, but the former (Fig. 8, F-H) are distinguished by their compressed biconvex shape, and by their stronger tendency towards the development of a keel (leading towards G. cf. menardii), although all gradations between the two are sometimes seen.
The juveniles of this subspecies are subglobular in shape, with a very bluntly rounded axial periphery (Fig. 8, r).

Stratigraphic Range: First seen in the top of the Clifdenian, becoming common in the lower part of the Lillburnian-Waiauan, and dominating the G. miozea populations of the overlying Southland strata. The top of its range is not well defined, since, in the Tongaporutuan-Kapitean, this form gradually loses its distinctness and merges back into the G. miozea s.l. population by becoming smaller, less conical (and more dome-like) ventrally; and by regaining to some extent the bluntness of axial periphery that characterises the greater part of the G. miozea lineage.

References:

Walters, R. (1965). The Globorotalia zealandica and G. miozea lineages. New Zealand Journal of Geology and Geophysics. 8: 109-127. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia miozea conoidea Walters 1965

Globorotalia miozea conoidea

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