Catalog - Globotruncana orientalis

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Globotruncana orientalis

Citation: Globotruncana orientalis El Naggar 1966

Type specimens: holotype (fig. 4a-d) P.45549; unfigured paratypes, P.45550.

Type age (chronostrat): Upper Cretaceous, Middle Maestrichtian, Globotruncana gansseri Zone, middle Sharawna Shale. Common to abundant in the Globotruncana fornicata Zone (Lower Maestrichtian), abundant in the overlying G. gansseri Zone, decreasing in abundance towards its top, and dying out in the basal part of the G. esnehensis Zone (Upper Maestrichtian).

Type locality: Holotype (fig. 4a-d) and unfigured paratypes from approximately 78 to 81 meters above the base of the Wadi EI-Sharawna section (25° 14 1N 32°44'E), about 19 km. southeast of the Nile at Esna, Esna-Idfu region, Upper Egypt.

Test large, almost circular in outline, coiled in a relatively high trochospire; dorsal side broadly convex, ventral side flat although the chambers are very slightly inflated; equatorial periphery almost circular, slightly lobate; axial periphery angular, acute, with 2 heavily beaded keels on the early chambers of the last whorl, reduced to a single well-developed, distinctly beaded keel in the last chambers; chambers on the dorsal side arranged in 3 dextrally coiled whorls and slowly and regularly increasing in size; the initial chambers are small, inflated and elongated in the direction of coiling, the last whorl is composed of 6 large, typically crescentic chambers; on the ventral side of the chambers are 6, heavily beaded horseshoe-shaped rims, and increase so slowly in size that they thickened and heavily beaded; umbilicus roughly hexagonal in outline, wide, deep, surrounded by slightly raised, heavily beaded ridges, and covered by interiomarginal, umbilical; tegilla with accessory apertures only poorly preserved; wall calcareous, perforate, except for the imperforate keels.

Maximum diameter 0.50 mm.; minimum diameter 0.45 mm.; thickness 0.25 mm

Main variation. - Chambers on the dorsal side 18-21, arranged in 3-3½ whorls, generally dextrally coiled. The last whorl is composed of 5-7 chambers which are large, crescentic and increase slowly in size. In some specimens the ventral keel is completely reduced and the test becomes entirely single keeled at least th roughout the last whorl.

Remarks. - Globotruncana orientalis is morphologically similar to C. leupoldi Bolli, G. conica White, G. esnehensis Nakkady and Osman and to G. sharawnaensis El-Naggar. However, it is distinguished from G. leupoldi by its flat ventral side, much narrower peripheral band and less lobate equatorial periphery. It differs from G. conica White by its less conical dorsal side, its early double keel and the horseshoe-shaped ridges on the ventral side. Globotruncana esnehensis is entirely single keeled and has strongly depressed ventral sutures, while G. sharawnaensis is single keeled in the early part becoming double keeled later, has depressed ventral sutures and a rougher surface.
The forms described by Cita (1948) and Pessagno ( 1962) as G. conica White most probably belong to the present species.
Globotruncana orientalis has possibly evolved from G. area (Cushman) [Pulvinulina area, 1926] by the flattening of the ventral side and the reduction of the ventral keels on the last chambers. This is substantiated by the fact that such tendencies were clearly observed in the specimens of G. area (Cushman). On the other hand, G. orientalis has probably evolved into G. esnehensis Nakkady and Osman, although no direct evidence was recorded."

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 orientalis El Naggar 1966

Globotruncana orientalis

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