Unusually large Late Campanian – Early Maastrichtian foraminifera from the Southern North Sea Basin

A low diversity fauna of unusually large (individuals of two to three times the normal size) agglutinated and calcareous benthic foraminifera is recorded from the Late Campanian – Early Maastrichtian Chalk sequences of two boreholes (Shell 49/19.1 and Shell 49/20.2) in the Southern North Sea Basin. Associated planktonic species are extremely rare. This distinctive assemblage provides a useful local stratigraphical marker.

The 5 0 0 p m fraction of the samples from 1500ft. -2000ft. in Borehole 49/19.1 and 287Oft.-3800ft. in Borehole 49120.2 yielded extraordinarily large specimens (see P1. 1, figs. Sa, b) of a low diversity fauna of agglutinated and calcareous benthic foraminifera (see below). Only rare planktonic forms were recorded. This unusual assemblage was not recovered from Borehole 44/2.1.

SY ST E MAT IC PA LA E 0 NT 0 L 0 GY
Arenobulirnina courta (Marie) (Pl. 1, fig. 1) Remarks. This species was described originally from the Campanian (rnucronata Zone) of the Paris Basin (Marie, 1941). The test is free and globular; the chambers are arranged trochospirally in three to four whorls and are inflated to very inflated in the last whorl which makes up over half of the test. The sutures are distinct and depressed, and the aperture is a single slit or semi-circular opening midway along the internal margin of the last chamber at the junction of the four sutures of the final whorl. The wall is thick and the surface rugose. The species has been compared with Bulirnina obesa Reuss, first described from the Upper Cretaceous of Poland, but the relationship between the two species remains unclear. Eggerellina brevis (d'orbigny) Remarks. This species was described originally from the Campanian of the Paris Basin (d'orbigny, 1840). It shows a wide range of morphological variation and well documented dimorphism (microspheric and megalospheric forms). However, its taxonomic history is rather confused. The type figures show clearly the characteristic dominance of the last overlapping whorl, the trilobed appearance and the 'key-hole' shaped aperture. Two and a half chambers are usually visible from the summit. The wall is agglutinated but composed of calcareous particles.

Dimensions. Average
Bigger forms Height: 0.56mm Height: 1.17mm Width : 0.49mm Width : 0.96 mm Ataxophragmium rimosum (Marsson) (PI. 1, fig. 4) Remarks. This species was described originally from the Chalk of Riigen Island, East Germany (Marsson, 1878). The test is free and subspherical; the chambers are arranged in a tightly coiled helical spire of two to three distinctly overlapping whorls and are obscured by the strong ornamentation of randomly arranged grooves which roughen the surface and distinguish the species from other members of the genus. The grooves may be the result o f dissolution of the sponge spicules that were once cemented together to form the test. The sutures are flush, and the aperture varies from a simple semicircular interiomarginal opening on the final chamber to a narrow slit extending up the apertural face.

Dimensions. Average
Bigger forms Height: 0.53 mm Width : 0.49mm Height: 0.83 mm Width : 0.77 mm Atuxophrugmium vuriabile (d'orbigny) (PI. 1 , figs. Sa, b) Remarks. This species was described originally from the Campanian of the Paris Basin (d'orbigny, 1840) and is type of the genus Ataxophragmium. It is a highly variable form with a free test which is subglobular to elongate with a variable outline but always broader than high. T h e chambers are arranged in a helical spire of two to three loosely t o tightly coiled overlapping whorls. The sutures are flush to slightly depressed. The aperture is usually a simple semi-circular opening at the inner margin of the last chamber, but it may vary in form and be a narrow, elongate slit that extends up the apertural face which is always flattened. The wall is thick and the surface smooth or slightly rugose.  1, fig. 6) Remarks. This species has a free, asymmetrical test which is sometimes subspherical tending to planoconvex. The chambers are indistinct, slightly inflated and higher than broad, and are arranged trochospirally usually in two (but sometimes in up to three) overlapping evolute whorls. There are six chambers per whorl and the last chamber of each tends to overlap the previous whorl. The sutures are distinct and flush. The aperture is a simple, sub-rectangular, deep slit extending along the interiomarginal suture of the last chamber; the apertural face is flattened. The wall is thick and finely agglutinated with much calcareous cement; the surface is smooth. Marie (1941) referred an identical form from the Campanian (mucronata Zone) of the Paris Basin to Nonionina globulosu von Hagenow, 1842. However, several authors (e.g. Marsson, 1878;Cushman, 1931 ;Visser, 1951) have concluded that von Hagenow's unfigured species was based o n a calcareous perforate form, not an agglutinated form. Therefore, Marie's species and the present form cannot be accommodated with globulosa and a new specific name is required. Sweicicki (unpublished thesis, 1980) 1, fig. 7) Remarks. This species was described originally from the Chalk of Rugen Island, East Germany (von Hagenow, 1842). The test is free, elongate and compressed. The chambers are indistinct and uninflated; the first five or six are arranged in a broadly rounded planispiral coil and are strongly overlapping on the inner coiled margin biit later, they expand moderately and uniformly and the coil becomes evolute with the last chamber or two being partially uncoiled. The sutures are initially indistinct but later distinct, depressed and curved. The apertural face is weakly inflated with a distinct depression about the aperture which is a simple, terminal circular to elliptical pit. The wall is coarsely agglutinated and the surface is roughened by many longitudinal grooves as in Ataxophragmium rimosum (see above).

Explanation of Plate 1
All figures are x 35. Specimen numbers refer to the coilections of the British Geological Survey (Keyworth).  Barnard,side view,MPK 4096;Shell 49120.2,3620ft. Fig. 9. Cibicidoides (?) voltziana (d'orbigny), spiral view, M PK 4097; Shell 49/20.2, 3200 ft. Fig. 10. Lenticulinu sp., side view, MPK 4098; Shell 49120.2, 3620ft. Fig. 11 fig. 9) Remarks. This species was described originally from the Upper Campanian of the Paris Basin and England (d'orbigny, 1840). The characteristic large planoconvex test with prominent calcite bosses make this species distinctive although its generic position is unclear. It has been referred to a number of genera including Cibicides, Cibicidoides, Gavelinella and Gavelinopsis, all of which display similarities in basic morphology but differ in their test wall structure. Most foraminifera1 tests from the Upper Chalk have undergone fine-scale recrystallisation resulting in a secondary heterogeneous granular wall structure (Reiss, 1959 ;Schlanger & Douglas, 1974) and therefore the true generic position of this species cannot be established; study of unrecrystallised material may clarify the position.

Dimensions. Average
Bigger forms Diameter: 0.51 mm Diameter: 1.02 mm Height : 0.22 mm Height : 0.45 mm Lenticulina spp. (Pl. 1, figs. 10, 11) The unornamented Lenticulina are in most cases nearly impossible to speciate. They have long stratigraphical ranges which reduces their potential as useful biostratigraphic indicators and in the present study, no effort was made to determine them beyond generic level.