Classification of the meridionally costellate Cretaceous planktonic foraminifera

The meridionally arranged rugosity, which is restricted to the Cretaceous globigerines, is given a superfamily taxonomic status. The globigerines displaying this type of ornamentation are placed in the Rugoglobigerinacea which is subdivided into two families the Rugoglobigerinidae and the Abathomphalidae. The Rugoglobigerinidae includes the forms having an umbilical primary aperture with a tegillum (Rugoglobigerina Brönnimann, Trinitella Brönnimann, and Plummerita Brönnimann). The Abathomphalidae includes the genera which have an umbilical–extraumbilical to spiro-umbilical primary aperture with a tegillum (Abathomphalus Bolli, Loeblich & Tappan, and Kassabella El-Nakhal), or a porticus (Meridionalla El-Nakhal, and Badriella n. gen.). Badriella is introduced as a new genus with B. mouradi n. sp. (= hypotype of Praeglobotruncana loeblichae Douglas) as a type species, to accommodate the meridionally costellate species with compressed chambers and umbilical–extraumbilical primary aperture, which were previously assigned to Praeglobotruncana Bermudez. The Rugoglobigerinacea may have evolved from a hedbergelline ancestor during the latest Albian via the development of the meridinally arranged rugosity.


INTRODUCTION
The Cretaceous planktonic foraminifers include several forms which display a peculiar ornamentation pattern in the form of meridionally aligned costellae. Saint-Marc (1973), Caron (1978) and Caron & Homewood (1982/1983 believed that this type of textural ornamentation was developed in response to environmental factors. Pessagno (1967), Blow (1979), Petters et al. (1983) and Belford (1983) regarded it as a genetically controlled character. This type of ornamentation is displayed by a particular group of globigerines, which are restricted to the Cretaceous and never seen before or after that period. Taxa with, and without, meridional costellae occur together (for example in Kuwait, El-Nakhal, 1973, and in Western Australia, Belford, 1983), and the view that development of these costellae is genetically controlled seems to be reasonable and, therefore, is here accepted and the character considered as an important taxonomic feature.
The taxonomic significance of meridional ornamentation has received different interpretations. Olsson (1964) and Kassab (1976) considered it to be of specific importance, whereas Brönnimann (1952), El-Nakhal (1982, Petters et al. (1983) and Belford (1983) assign it a generic importance. Pessagno (1967) and Longoria & Gamper (1975 regarded the presence of unique types of ornamentation, like meridional rugosities or polygonal areas (honeycomb ornamentation), as a family character. Due to its taxonomic importance, and to facilitate the clasification of the related genera, the meridionally aligned rugosity is here given a superfamily status. It is suggested, therefore, that all the globigerines which have meridionally aligned costellae are placed in one taxon of superfamily rank; that is the Rugoglobigerinacea Subbotina, 1959. Furthermore, on the basis of the apertural characteristics of the included genera, this superfamily is subdivided into two families: the Rugoglobigerinidae Subbotina, 1959, andthe Abathomphalidae Pessagno, 1967, to include the genera with umbilical, and umbilical-extraumblical to spiro-umbilical primary apertures, respectively (Table 1).

SYSTEMATICS
The classification followed in the present study is that of Loeblich & Tappan (1988 Range. Early Santonian-Maastrichtian.

Diagnosis. Rugoglobigerinidae with compressed later chambers.
Description. Test in flat trochospiral coil, chambers enlarging rapidly as added, early ones globular, later chambers compressed to rhomboidal in section, with imperforate carinal band or faint keel in the final whorl, umbilicate, umbilicus wide, sutures radial, depressed, axial periphery rounded, subangular to truncate, equatorial periphery lobulate; wall calcareous, perforate, surface covered with prominent pustules and costellae in meridional arrangement, less prominent on the final chamber; primary aperture interiomarginal, umbilical, bordered by an imperforate flap, those of successive chambers fusing into tegillum having both proximal and distal accessory apertures.
Description. Test with inflated triangular chambers in a low trochospire, those of the final whorl radially elongate and ending in a tubulospine, sutures radial, depressed umbilicus small, axial periphery rounded to compressed between tubulospines, equatorial periphery strongly lobulate and stellate; wall calcareous, perforate, surface with rugosities and costellae in meridional alignment; primary aperture interiomarginal, umbilical, provided with tegilla, and having both proximal and distal accessory apertures.

Diagnosis.
Abathomphalidae with compressed later chambers, primary aperture with porticus in the early stage, later with tegillum.
Description. Test in a low to flat trochospiral coil, umbilicate, umbilicus small, chambers petaloid, sutures curved and oblique, depressed to thickened and nodose on the spiral side, depressed and radial on the umbilical side, axial periphery angular to truncate, bicarinate, the two variously spaced keels bordering an imperforate carinal band, keel on the umbilical side may be reduced to a row of short transverse costellae; wall calcareous, perforate, surface with pustules and short costellae in concentric alignment on the spiral side and radial on the umbilical side; primary aperture interiormarginal, umbilical-extraumbilical, and with a porticus, portici of sucecessive chambers coalescing in the early stage, later in the adult stage joined only at few points to form tegilla having distal accessory apertures.

Diagnosis.
Abathomphalidae with globular to slightly compressed chambers, and an aperture with tegillum.
Emended description. Test coiled in a low trochospire, chambers globular to slightly compressed, umbilicate, umbilicus wide, axial periphery rounded to angled with no indication of keel or poreless margin, sutures radial, depressed on both sides, umbilicus shallow wide, primary aperture interiomarginal, umbilical-extraumbilical to spiro-umbilical covered by tegilla, surface rugose, covered with meridionally arranged pustules and costellae on both sides.

Remarks.
In his original description of Kassabella, El-Nakhal (1984) mentioned that the genus has sutural supplementary apertures on the spiral side, and the primary aperture is bordered by a well-developed lip. Loeblich & Tappan (1988) questioned the presence of true supplementary apertures and the well-developed lip. Furthermore, they indicated that the umbilicus appears to show the remnants of a tegillum. Loeblich & Tappan's (1988) notes are confirmed and accepted in the present study. The 'sutural supplementary apertures' appearing on the spiral side (Pl. 2, fig. 1) are considered to be relict apertures or vestiges of the spiro-umbilical primary aperture left uncovered by younger chambers, with limited taxonomic importance (Douglas, 1969;Longoria;1974, Blow, 1979. Also, an illustration of a better-preserved specimen shows that the primary aperture has a tegillum rather than a porticus (Pl. 2, fig. 4) (2000) examined the original drawings of the holotypes of the type species of both genera. They observed that the type species of the genus Meridionalla (i.e. Hedbergella murphyi Marianos & Zingula, 1966) does not possess the meridionally orientated costellae, the main feature on which the genus Meridionalla was erected, except for rare apparently aligned pustules on the umbilical side of the last chamber. Petrizzo & Premoli Silva (2000) added that even those rare pustules were not detected in recently taken scanning electron micrographs of the holotype. They considered this as a confirmation of the absence of the meridionallyarranged costellae in the holotype of H. murphyi. They have, therefore, interpreted the pustules observed in the original drawings as being drawing artefacts. On the other hand, the original drawings of the type species of the genus Costellagerina (i.e. Rugoglobigerina bulbosa Belford, 1960) show a clear meridionally-arranged rugosity. Consequently, they concluded that Costellagerina is the appropriate genus, while the genus Meridionalla is inappropriate, considering it as a junior synonym of Hedbergella. However, the re-examination of the original drawings of the holotype of H. murphyi Marianos & Zingula, 1966, shows that the meridional rugosity occurs on at least the first chamber of the last whorl of the spiral side, and on the first chamber and the last three chambers of the umbilical side, though the meridional rugosity seems to be poorly developed. The occurrence of the meridional rugosity on the holotype of H. murphyi, is confirmed by Douglas (1969), who labelled as Hedbergella murphyi, specimens exhibiting a surface-texture with clear costellae aligned in preferred orientation particularly on the umbilical side where the pustules and pustule ridges radiate from the umbilicus (Douglas, 1969, pl. 5, fig. 8, reproduced in Petrizzo & Premoli Silva, 2000).
Clearness and detectability of the meridionally arranged costellae depend on the degree of its development and on the position of the costellate chambers relative to the light source. In many cases, it is easy to observe the meridional rugosity when examined by the light microscope (sometimes you need to move the stage left and right to make the rugosity more clear), but when examined by the scanning electron microscope, it becomes undetectable. An example of this case is shown in Petrizzo & Premoli Silva (pl. 2, 2000). In this plate, the meridionally arranged rugosity is obseved on only the 4th chamber of the last whorl of the spiral view ( fig. 2a), despite that it is believed that the meridional rugosity is found on other chambers of the photographed test. Therefore, the rugosity which Petrizzo & Permoli Silva (2000) referred to as rare apparently aligned pustules observed on the umbilical side of the last chamber of the holotype, is actual meridionally arranged rugosity and not a drawing artefact as those two authors interpreted.
In conclusion, the genus Meridionalla El-Nakhal, 1982, is the valid genus and it remains the senior synonym, whereas Costellagerina Petters et al., 1983 is invalid and should be considered to be a junior synonym.

Range. The range of Meridionalla and its junior synonym
Costellagerina was considered to be Cenomanian-Campanian (El-Nakhal, 1982;Petters et al., 1983;Loeblich & Tappan, 1988). However, as Meridionalla lybica was recorded from strata of latest Albian to Middle Campanian age (Premoli Silva & Sliter, 1994), the range of the present genus is modified to latest Albian-Campanian.

Diagnosis. Abathomphalidae with compressed later chambers and an aperture bordered by a porticus.
Description. Test free, trochospiral, umbilicate, axial periphery subangular with nonporous carinal band, equatorial periphery lobulate, chambers compressed, sutures curved on spiral side radial on umbilical side, flush to slightly depressed, surface rugose covered with meridionally arranged rugosities on the umbilical side; aperture an interiomarginal, umbilicalextraumbilical arch, bordered by a narrow lip, supplementary apertures absent; wall calcareous, hyaline, finely perforate, radial in structure.

Remarks.
In the discussion of his new species Praeglobotruncana loeblichae, Douglas (1969) indicated that meridionally arranged rugosities displayed by that species are features suggestive of the genus Rugoglobigerina, but its apertural characteristics H. A. El-Nakhal include relationships to the rotaliporids. He added that P. loeblichae exhibits the diagnostic features of the genus Praeglobotruncana Bermudez, but its spinose surface sets it aside from the other species of Praeglobotruncana. These remarks substantiate the necessity of erection of the present new genus to accommodate the forms exhibiting characters related to the two genera Rugoglobigerina and Praeglobotruncana.
This new genus is named in honour of the author's wife Mrs Abla M. El-Badri.
Range. Forms which can be included within the present new genus include Praeglobotruncana loeblichae Douglas, 1969, Praeglobotruncana helvetica (Bolli) of Weidich (1984), and Praeglobotruncana praehelvetica (Trujillo) of Weidich (1984). P. loeblichae was recorded from the upper Turonian-lower Coniacian of California and Alaska (Douglas, 1969), whereas P. helvetica and P. praehelvetica of Weidich are recorded from the Turonian and the middle Cenomanian-Turonian of the Bavarian Alps, respectively (Weidich, 1984). Accordingly, the range of Badriella n. gen. is considered to be middle Cenomanian-early Coniacian. Description. (Described holotype Pl. 2, figs 8-10.) Test free, low trochospiral, umbilicate, spiral side slightly convex, umbilical side slightly concave, equatorial periphery lobulate, axial periphery truncate with two faint keels on the last chambers, chambers compressed, increasing gradually in size as added, last whorl with five chambers, sutures depressed, radial straight to slightly curved on both sides; aperture a low interiomarginal umbilical-extraumbilical arch, bordered by a narrow lip, umbilicus narrow, shallow; surface spinose to rugose, surface rugosity tangentially arranged on spiral side, and meridionally arranged on umbilical side; wall calcareous, hyaline, radial in structure, perforate.
Remarks. This new species was originally figured by Douglas (1969) as a juvenile hypotype of Praeglobotruncana loeblichae Douglas. This form, however, lacks the broadly truncate axial periphery, the well developed twin keels and the wide umbilicus, which characterize the holotype of P. loeblichae. Also, it has a better developed meridional rugosity on the umbilical side than the figured holotype. These morphological differences distinguish the hypotype from the holotype, and suggests that they must be separated. Similarly, Caron (1981) Douglas, 1969).

SUMMARY AND CONCLUSIONS
The meridionally arranged rugosity is a genetically controlled character whose occurrence is restricted to the Cretaceous planktonic foraminifera. It is regarded as a fundamental criterion for taxa at the superfamily level. Forms displaying this type of ornamentation are included within the superfamily Rugoglobigerinacea. On the basis of the apertural characteristics of the included genera, this superfamily is divided into two families: the Rugoglobigerinidae and the Abathomphalidae. The former family includes the genera Rugoglobigerina Brönnimann, Trinitella Brönnimann, and Plummerita Brönnimann, which have an umbilical primary aperture. The latter family includes the genera: Kassabella El-Nakhal, Abathomphalus Bolli, Loeblich & Tappan, Meridionalla El-Nakhal and Badriella n. gen., which have an umbilical-extraumbilial to spiro-umbilical primary aperture. Badriella is introduced as a new genus with B. mouradi n. sp. (= hypotype of Praeglobotruncana loeblichae Douglas) as a type species to include the abathomphalids having compressed chambers, and umbilical-extraumbilical primary aperture with porticus. The genus Meridionalla El-Nakhal is the earliest member of this group. It appeared during the latest Albian and continued up to the Late Campanian. It is considered the basic stock from which all the meridionally costellate planktonic foraminifera evolved (Fig. 1). Meridionalla most likely evolved from a hedbergelline ancestor during the latest Albian via the development of the meridionally arranged rugosity. Like most other Cretaceous planktonic foraminifera, the Rugoglobigeribacea completely disappeared at the end of the Late Cretaceous. discussion and comments on the relation between the two genera Meridionalla and Costellagerina. I am very grateful to Prof. Dr M. Caron of the University of Fribourg, Switzerland, and Dr M. Petrizzo of the Dipartimento di Scienze della Terra, Universita degli Studi di Milano, Italy, for providing me with some of their very useful publications. Also, I would like to thank the two anonymous reviewers for their very useful comments and suggestions.