Catalog - Globotruncana gansseri gandolfii

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Globotruncana gansseri gandolfii

Citation: Globotruncana gansseri gandolfii El-Naggar 1966

Type specimens: holotype (fig. 2a-d) P.45541; unfigured paratypes, P.45542.

Type age (chronostrat): Upper Cretaceous, Maestrichtian, upper Sharawna Shale. Common to very abundant in the upper part of the Globotruncana gansseri Zone (Middle Maestrichtian), rare to common in the overlying G. esnehensis Zone (Upper Maestrichtian), and dies out completely immediately below the disconformity separating that zone from the overlying basal Tertiary.

Type locality: Holotype (fig. 2a-d) and unfigured paratypes from approximately 89.5 meters above the base of the Wadi EI-Sharawna section (25°14'N 32°44'E), about 19 km. southeast of the Nile at Esna, Esna-ldfu region, Upper Egypt.

Test large, roughly quadrate in outline, planoconvex, umbilico- convex, coiled in a very low trochospire; dorsal side almost flat and somewhat imbricate although the chambers are weakly inflated and slightly overlapping; ventral side strongly inflated and distinctly protruding; equatorial periphery roughly quadrate, moderately lobate, with a single, delicately beaded keel which is slightly shifted towards the dorsal side; axial periphery subangular, subtruncate; chambers on the dorsal side about 17 in number, arranged in 2.5 dextrally coiled whorls; the initial chambers are exceedingly small, globular, slightly inflated and almost masked by the surface rugosity; they increase very slowly in size and are followed by relatively larger, subglobular, slightly inflated chambers which increase moderately in size; the last whorl is composed of 4.5, large, roughly ovoid chambers which increase moderately in size, although the last chamber is slightly smaller than the penultimate; on the ventral side the chambers are 4.5, large, subglobular, strongly inflated and distinctly protruding; sutures on the dorsal side slightly curved, almost radial, raised and beaded, although the inflation of the chambers makes them appear to be slightly depressed in part, especially towards the inner whorl; on the ventral side the sutures are straight, radial, and strongly incised umbilicus roughly quadrate in outline, relatively wide, deep, and covered by complex tegilla of which remnants are still preserved; primary apertures interiomarginal, umbilical; tegilla with accessory apertures only poorly preserved; wall calcareous, perforate except for the imperforate keel and tegilla; surface on the dorsal side rough in the early part, covered with numerous small papillae which decrease gradually towards the last chamber; on the ventral side the surface is very rough, heavily papillose or even nodose.

Maximum diameter 0.48 mm.; minimum diameter 0.33 mm.; thickness 0.28 mm

Main variation. - Chambers 13-18, arranged in 2½-3 whorls, generally dextrally coiled. Chambers in the last whorl 4-5, flat to slightly inflated, slightly to moderately im bricate, increasing slowly in size except for the last, which is either slightly smalleror slightly larger than the penultimate. The surface can either be rough throughout, heavily nodose or even spinose, or it can be delicately papillose in the early part and smooth later.

Remarks. - Gandolfi (1955) described as Globotruncana gansseri gansseri Bolli from the Colon shale of northeastern Colombia, a form which differs from the holotype of Bolli ( 1951). Such a form is abundant in the samples studied; its morphological characters and stratigraphical range warrant its separation from the central type and therefore it is here considered as a new subspecies of G. gansseri Bolli. It is named G. gansseri galldolfi, after Dr. R. Gandolfi. It is believed to have evolved from either G. gansseri gansseri Bolli or G. gansseri dicarinata Pessagno, as suggested by the morphological features and stratigraphical ranges of these forms. On the other hand G. gansseri gandolfii is morphologically related to Globotruncana arabica El-Naggar which appears slightly higher in the section and thus may possibly represent its direct descendant.

References:

El-Naggar, Z. R. (1966). Stratigraphy and planktonic foraminifera of the Upper Cretaceous-Lower Tertiary succession in the Esna-Idfu region, Nile Valley, Egypt, U. A. R. Bulletin of the British Museum (Natural History). supplement 2: 1-291. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globotruncana gansseri gandolfii El-Naggar 1966

Globotruncana gansseri gandolfii

References:

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