Catalog - Globotruncana adamsi

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Globotruncana adamsi

Citation: Globotruncana adamsi El-Naggar 1966

Type specimens: holotype (fig. 2a-d) P.45511 and paratypes, P.45510.

Type sample (& lithostrat): Sharawna Shale

Type age (chronostrat): Upper Cretaceous. Maestrichtian. Common to abundant throughout the Globotruncana fornicata Zone (Lower Maestrichtian ) and the Globotruncana gansseri Zone (Middle Maestrichtian).

Type locality: Holotype (fig. 2a-d ) and unfigured paratypes from approximately 58.5 m. above the base of the Abou Saboun section (25° 11.3'N. and 32°46.3'), abou t 5.6 km. north of the Nile at El Mahamid and about 24 km. southeast of the Nile at Esna, Esna-Idfu region, Upper Egypt.

Test large, spiroconvex, roughly ovoid in outline; dorsal side highly convex and moderately inflated; ventral side almost flat; equatorial periphery ovoid, slightly lobate, with two well-developed, heavily beaded marginal keels; axial periphery truncate, bluntly subangular; chambers on the dorsal side not all clear, but apparently 23 in number, arranged in 4 dextrally coiled whorls; the initial chambers are small, globular, weakly inflated and increase very slowly in size; they are followed by slightly larger, strongly inflated, globular chambers which increase moderately in size ; the last whorl is composed of 5 large chambers which increase rapidly in size, and are subglobular and strongly inflated in the early part, becoming crescentic, strongly elongated in the direction of coiling later; the last chamber is weakly plicated; on the ventral side there are 5 chambers which increase moderately in size, being subglobular in the early part, ovoid, slightly inflated and strongly overlapping later; sutures on the dorsal side slightly curved, depressed in the early part, strongly curved, raised and distinctly beaded later; on the ventral side the sutures are straight, radial, strongly depressed at first, curved forward, delicately beaded, slightly raised or running in sutural depressions later; umbilicus roughly pentagonal in outline, wide, deep, bordered by slightly raised, delicately beaded umbilical ridges, 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 keels, peripheral band and tegilla; surface delicately papillose especially on the ventral side; the two marginal keels are well-developed and heavily beaded, the ventral one is slightly shifted towards the ventral side, and thus they enclose a relatively wide, slightly inclined peripheral band which becomes progressively narrower towards the last chamber.

Maximum diameter 0.46 mm. : minimum diameter 0.36 mm.; thickness across middle part of test 0.25 mm.

Main variation. - Chambers 13-24, arranged in 3-4 whorls, generally dextrally coiled. Chambers in the last whorl 4-6. The two keels are either equally developedor the ventral one slightly weake ns towards the last chamber.
Remarks. - Globotruncana adamsi El-Naggar is morphologically similar to both C. fornicata fomicata Plummer and C. convexa Sandidge. It is distinguished from the former by its dome-shaped, distinctly spiroconvex test, and from the latter by its less truncate axial periphery, better developed marginal keels, and strongly e longnted ch ambers in the last whorl which overlap more on the ventral side.
Globotruncana adamsi El-Naggar is believed to have evolved from C. fornicata fornicata Plummer into G. contusa patelliformis Gandolfi, as suggested by the morphological features and stratigraphical ranges of these three forms. It was probably confused in the past with G. fornicata Plummer (e.g. Cita, 1948, pp. 153-4, pl.3, fig 8a-c) and with G. caliciformis (de Lapparent) [Rosalina linnei d'Orbigny subsp. caliciforme. 1918 ] (e.g. Cita, 1948, op. cit.. pp. 148, 149, pl. 3, fig. 4a-c; Bolli, 1951,p. 194-6, pl. 34, figs. 4-6; and Galdolfi, 1955,p. 46-7, pl.3, fig.1 a-c, tf. 9 (3a-c, 4a-c)). However, these forms are not included in the synonymy of the present species as they lack the distinctly elongated chambers in the last whorl and were incompletely described by their respective authors.

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 adamsi El-Naggar 1966

Globotruncana adamsi

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