Catalog - Globotruncana arabica

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Globotruncana arabica

Citation: Globotruncana arabica El-Naggar 1966

Type specimens: holotype (fig. 6a-dc P.45570 and paratypes, P.45571.

Type sample (& lithostrat): upper Sharawna Shale

Type age (chronostrat): Upper Cretaceous, Maestrichtian. Ranges from the upper part of the Globotruncana gansseri Zone (Middle Maestrichtian) to the middle part of the Globotruncana esnehensis Zone (Upper Maestrichtian). Abundant.

Type locality: Holotype (fig. 3a-d), figured paratype (fig. 4), and unfigured paratypes from approximately 93.5 to 97 meters above the base of the Wadi El-Sharawna section (ca. lat. 25° 14' N. and 32°44 1E), about 19 km. south east of the Nile at Esna, Esna-Idfu region, Upper Egypt.

Diagnosis. - A Globotruncana with large, concavo-convex. strongly umbilico-convcx test: ent irely single keel strongly shifted towards dorsal side: chambers increasing slowly in size and distinctly inflated on ventral side: very rough surface and large umbilicus

Description Test large, subcircular, globular in outline, concavo-convex, distinctly umbilico-convex, coiled in a very low trochospire; dorsal side shallowly concave, flat in the early part, slightly tilted inward in the last whorl; ventral side strongly inflated and distinctly protruding; equatorial periphery subcircular, globular and distinctly lobate, with a single, well developed, beaded keel which is strongly shifted towards the dorsal side; axial periphery subrounded, very gently truncate; chambers on the dorsal side about 17, arranged in 3 dextrally coiled whorls; the initial chambers are small, globular, weakly inflated, almost masked by the surface rugosity and are followed by slightly larger, subglobular, weakly inflated ones; the last whorl is composed of 5 large, subglobular, compressed chambers which increase slowly in size, are slightly elongated in the direction of coiling and strongly tilted inward towards the initial part; on the ventral side the chambers are 5, large, subglobular, strongly inflated, distinctly protruding, and enlarging so slowly that they all appear roughly equal in size; sutures on the dorsal side are slightly curved, depressed in the early part, very short, slightly curved to almost straight in the later part, raised and beaded on the periphery, becoming depressed inward; on the ventral side the sutures are straight, radial, and strongly depressed; owing to the inward tilting of the dorsal surface of the last whorl the junction between the ventral and dorsal sutures can be seen from the dorsal side; umbilicus pentagonal in outline, 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 rough, heavily papillose or even nodose, especially in the early part with the roughness decreasing gradually towards the last chamber; the single marginal keel is distinctly beaded, with the beads slightly fading out on the penultimate and last chambers; the keel of each chamber encircles its periphery and then disappears into the short, depressed, dorsal sutures; the umbilicus is not bordered by a flange of any sort, although the large, scattered beads on the surface may simulate a beaded rim.

Maximum diameter 0.54 mm.; minimum diameter 0.42 mm.: maximum thickness of last chamber 0.34 mm.: minimum thickness across middle part of test 0.23 mm

Main variation. - Chambers on the dorsal side 13-18, most commonly 15, arranged in 2½-3 whorls, generally dextrally coiled; chambers in the last whorl 4½-6, slowly to moderately increasing in size.

Remarks. - Globotruncana arabica El-Naggar is distinguished by its large, concavo-convex, strongly umbilico-convex, single keeled test, its large umbilicus and rough surface. The only known Globotruncana species with a concavo-convex, umbilico-convex test are: G. concavata (Brotzen) from the Campanian-Santonian of Palestine, G. repanda Bolli 1957, from the Campanian of Trinidad, and G. bahijae El-Naggar from the Maestrichtian of the Esna-Idfu region. The first species is distinguished from G. arabica by its closely spaced double keel, less concave dorsal side, chambers which increase more rapidly in size, and by its smooth surface. The second is differentiated by its smaller test, fewer number of chambers, double keel in the early part (which may be absent in the penultimate and last chambers), much smaller early part, and less rugose surface. The third is distinguished by its less protruding ventral side and its double keel. Globotruncana arabica is morphologically closely related to G. repanda Bolli. Small specimens of G. arabica resemble G. repanda, but differ in having an entirely single keel, and chambers which increase slowly in size. By reduction of the ventral keel and increase in the size of test, in the number of chambers and in the surface rugosity, G. repanda might possibly have evolved into G. arabica. Such tendencies are clearly recorded in G. repanda, but the latter species is known to die out completely in the Upper Campanian, while G. arabica is only recorded from the Middle and Upper Maestrichtian. Thus it is suggested that G. arabica either evolved from a yet undescribed form, transitional between it and G. repanda, or that the latter also occurs in the Lower Maestrichtian, but has not yet been found.
Globotruncana arabica is also morphologically related to G. lugeoni Tilev and G. youssefi El-Naggar which occur in association with it. lt is distinguished from the former by its larger test, its strongly shifted keel towards the dorsal side, its shorter, less curved, depressed dorsal sutures and its much wider umbilicus. It differs from the latter by the fact that G. youssefi has an almost flat dorsal side. or even slightly raised initial part, longer, more curved, raised and beaded dorsal sutures and a truly marginal keel.

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 arabica El-Naggar 1966

Globotruncana arabica

References:

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