Notes on the Foraminiferal Genera Laterostomella De Klasz and Rérat and Streptochilus Brönnimann and Resig

the biserial foraminiferal genera Laterostomella de Klasz & Rérat (1962) and Streptochilus Brönnimann & Resig (1971) have been described from the Miocene of Gabon and from Miocene to Recent levels of the Pacific and Indian Oceans, respectively. Both genera have since been found in the Upper Cenozoic of other regions: Laterostomella at Rockall Bank in the North Atlantic and in Papua-New-Guinea, and modern Streptochilus specimens were collected with plankton nets in the northern part of the Indian Ocean. The apertural characteristics of Laterostomella and Streptochilus show marked similarities as does the general shape of Laterostomella guembeliniformis with some Streptochilus species. However, other Laterostomella species have a very different form and isotopic data indicate that Laterostomella has a benthic and Streptochilus a planktic life habitat. We conclude that both genera are valid. For the first time SEM pictures of Laterostomella species are presented to show morphological variability, surface texture and aperture types.


SYSTEMATIC PALAEONTOLOGY
Superfamily Buliminacea Jones 1875 Family Bolivinitidae Cushman 1927 Genus Later-ostornclla de Klasz & Rerat 1962 Type Species: Luter-o.stoniellu KuemhelinijOr-mis de Klasz & Rerat 1962. Description. A translation of the original French description is as follows: "A genus belonging to the family of Buliminidae with the following peculiarities. Test: biserial with more or less globular chambers, increasing regularly in size. Greatest width in the terminal portion. Sutures: strongly depressed on the genotype, less noticeable on the second species described below. Aperture: small, slightly elongated in a depression situated laterally on the apertural face. This depression is bordered on its exterior and lower margin by lips. That ofthe exterior margin is more developed. Its upper extremity disappears in the depression forming there a "toothplate". The lip of the lower margin is attached to the previous chamber. One of its ends disappears below the lateral lip described above. Well visible on the genotype, it is short and hardly perceptible on the other species. Ornamentation: the calcareous test is finely to rather coarsely perforate depending on the species. The ornamentation of the genotype consists of small rugosites, more developed on the initial portion of the test; that of the second species of longitudinal stripes, more or less anastornosing on the initial portion of the test. The measurements of the holotypes of the

Remarks:
The original authors pointed out that Laterostomella differs from other genera by its very peculiar aperture. At that time Stainfwthia Hofker was considered to be its closest relative. However, detailed comparison with paratypes of Virgulina c'oncava Hoglund, the genotype chosen by Hofker for Stainforthia, as well as with specimens of Stainforthia wnezuelana (Nuttall) and S . dall$ormis Hofker, convinced them that Laterostomella was a separate new genus. This view has been corroborated with other specialists consulted, such as Drs. Hoglund, Le Calvez, Sigal and Todd.
Loeblich & Tappan (1 964) gave a short description of the genus, using the original figures of the genotype species: "Test biserial, chambers inflated, sutures depressed; wall calcareous, finely to coarsely perforate, with rugose to striate surface; aperture elongate, with bordering lip, situated in a cavity at one side of apertural face, outer margin somewhat flaring and infolded to form toothplate." These authors ( 1988) placed Laterostomella in the Chiloguembelinidae, but this study shows that this genus is a benthic taxon (see below).
Gabonese topotypes of L . ~uemheliniformis show a dense perforation, and ridges giving a cancellate appearance (PI. I , Figs. 9, 10). In other forms this ornamentation is attenuated and the pores are enlarged. This is probably due to dissolution, which was also observed in some Streptochilus specimens (Resig & Kroopnick, 1983;Saito et al., 1981). Closeup pictures of the terminal part show an irregular distribution of pores on the extreme top of the last chamber. On the penultimate chambers they are sometimes aligned in very shallow furrows (PI,. 1 , Fig. 12). The lip, and usually its  We also found L. pemhe1iniformi.s in DSDP Hole 116 (North Atlantic, Rockall Bank, see Fig. I). The specimens are practically identical with the Gabonese topotypes, showing the rugose to cancellate surface texture of the holotype, and the occasional, stout basal spine (cf. PI. I , Fig. 6 and PI. 2 Fig. 2).
The second Gabonese species, L. striata is characterised by 10-12 longitudinal costae, anastomosing at the early portion of the test. The test is finely perforate except for the imperforate apertural face.
A third species, L. voluta, has been described by Belford (1 966) from Papua. He noted the similarity of this form with L. striata de Klasz & RCrat from which it "differs in being twisted, less heavily ornamented and more compressed, with an oval rather than rounded outline in end view" (Belford, 1966, p. 44). Topotypes are finely perforate, show 10-12 thin surface costae on all except the last pair of biserial chambers, and look very much like L. striata. They differ in being smaller and showing a strong twisting of the test. These differences in our opinion suggest that we are dealing with ecophenotypes.
Superfamily Heterohelicacea Cushman 1927 Family Chiloguembelinidae Reiss 1963 Genus Streptochilus Bronnimann & Resig 197 1 Type Species. Bolivinu tokelauae Boersma 1969(in: Kierstad et al., 1969= B . glohulosa Cushman 1933, according to Resig & Kroopnick, 1983. Original description. "Test biserial sometimes twisted; wall calcareous perforate; aperture high arch, excentric in position, extending from the base of the last chamber onto the apertural face. On the outside margin a collar borders the aperture. Near the base of the inside margin, the collar and apertural edge are turned inward, producing a plate-like connection with the proximal margin of the collar of the previous aperture. The length of the test varies between 75 and 300 microns" (Bronnimann & Resig, 1971, p. 1288. Remarks. Although the original authors denied the existence of internal structures in near-topotypes of Chiloguemhelinamidwayensis (Cushman), they remarked that the genus probably evolved from Chiloguemhelina, which it resembles through infolding of the inner margin of the aperture, and that it can be classified within the Heterohelicidae. Kennett & Srinivasan (1983) agree with Bronnimann & Resig (1971 ) and Resig & Kroopnick (1 983) as far as the close relationship of Streptochilus and Chiloguemhelina is concerned. They suggest that S. glohigerus, the oldest species of the genus, directly evolved from Chiloguemhelina despite the long time gap between the disappearance of the latter and the first appearance of Streptochilus.
We agree with Poore & Gosnell(1985) who discussed the links between Chiloguemhelina and Streptochilus. They indicated for Chiloguemhelina that "several Upper Paleogene species have an internal apertural plate that is very similar to the internal plate typical of the Neogene genus Strqtoc'hilus while the type species of Chiloguemhelina, C. midwayensis (Cushman) lacks any internal structure or modifications" (op. cit., p. I). Their illustrations clearly show that Chiloguemhelina midwayensis midwayensis (Cushman) has an infolded apertural margin without internal plates while other species indeed do have an internal plate (i.e. S. cubensis (Palmer), S martinii (Pijpers), and S. sp. aff. S. marfinii) and should be assigned to Streptochilus.

STRATIGRAPHIC DISTRIBUTION OF LA TEROSTOMELLA
De Klasz & Rerat (1962) indicated "Lower to Upper Miocene" (upper part of Mandorovt? Formation to lower part of N'TchenguC Formation of Gabon) as the stratigraphic range for L. guemhelin$ormis, and "Lower Miocene": (upper part of Mandorove Formation to lower part of M'BCga Formation) for L. striatu.
The genus first appears in the Gabon Basin in the Glohoraotaliafohsi s.1 Zone (De Klasz & Rerat, 1968), which is to be considered by the present biostratigraphic zonation as Middle Miocene (Blow's Zones N 9-12) ( Fig. 3b x 600, showing aperture and internal plate. Fig. 4 Lateral view of specimen with slightly different surface texture, x 175. Fig. 5 Top part of a specimen showing aperture, pore free area near the aperture and randomly disposed pores on last two chambers, x 375.   Fig. 5 Aperture of specimen shown in fig. 4, x 600. Fig. 6 Aperture of specimen shown in fig. 3, x 600. Fig. 7 Aperture of Gabonese topotype, x 600.   Berggren et al., 1985;Bolli et al., 1985;Haq et al., 1987). The upper limit of the stratigraphic range of L. gi~emhe1iniformi.s is in the lower part of the N'Tchengue Formation which has been dated as Late Miocene to Pliocene on the basisofplankticforaminifera(Brun& Monteil, 1974), and ostracods (Van den Bold in de Klasz et ul., 1978). In Gabon L. guemheliniformis is thus ranging from the Middle Miocene to the Upper Miocene if not even to the lowermost Pliocene.

Explanation of Plate 3
In DSDP Hole 116 L. ~~uernhc~lin/fi,rmis specimens are abundant in the Upper Miocene (G. cmoideu Zone, sample 116/4/6/16-19cm). The top of the range of L. striatu in Gabon is in the lower part of the M'Bega Formation at the top or just above the Glohorotuliu~jh.si group Zones. Its age is therefore late Middle Miocene or early Late Miocene. This corresponds to van den Bold's Tortonian age determination based on ostracods (in: de Klasz et ul., 1978). Belford ( 1966) found Lutet~ostomellu striutu (=phenotype L. ivlutu) in the Murua Mudstone from the Malalaua-Saw Mountains area of P a p a in levels ranging from Burdigalian to Pliocene in age.

STRATIGRAPHIC DISTRIBUTION OF STREPTOCHILUS
In Figure 2 we used the Haq et a/. ( 1987) time scale as a reference for the Strepfochilus species ranges according to different authors.

ECOLOGIC INTEKPRETATION
De Klasz & Rerat ( 1 962) considered Lutet~ostomellu a benthic genus with L. sttiutu as a "typical" benthic form while L . guemhelinijot.mis has strong morphological similarities with biserial planktic genera like Heterohelis, Chiloguetnhelinu and Streptochilus. There are however intuitive arguments in favour of the benthic way of life of L. guemhe1in~or~rni.s. For instance, a stout spine at the base of the test is found in many benthic species but never in biserial forms of known planktic habitat. The fact that luter~ostomellu has not been found in deep water sediments is further argument for such a habitat.
In Gabon both species are found in a mixed benthicplanktic foraminifera1 assemblage, with benthics dominating. The fauna has been interpreted to represent Upper Slope to Outer Shelf conditions (de KIasz & Rerat, 1968). The depth of deposition of the Miocene at DSDP Site 116 is approximately 1200m (Berggren, 1972).
The most decisive evidence for the benthic nature of L. guemhelinijormis has been provided by stable oxygen isotope analysis of DSDP Hole 116 core material. The 6Ix0 value for L. gurmhelin(fiot-mis compare with the benthic and not with the planktic assemblage of the same sample (see Table I). The difference with of planktic forms is considerable and allows L guemheliniformis to be classified clearly as a benthic form. Loeblich and Tappan (1988) Resig and Kroopnick (1983, pI.1, figs. 3,4).  Resig and Kroopnick (1983, pl. 1, fig. 7), x220. Fig. 1  Bronnimann & Resig (1 97 1) considered Streptochilus as planktic forms because of their morphologic similarities to Heterohelix and Chiloguemhelina and also because of "the absence in assemblages affected by differential solution, as opposed to the concentration in those assemblages of undisputed deep water benthic species" (op. cit., p. 1264). Because they had not been reported in oceanic plankton tows, despite their presence in sub Recent deposits as determined by I4C dates. Bronnimann and Resig (1 97 1) considered the possibility that Strepptochilus became extinct. Isotopic and distributional evidence further convinced Resig & Kroopnick ( 1983) of the planktic life habit of Streptochilus.
We found Streptochilus in plankton tows from the northern Indian Ocean (D. Kroon, pers. comm.) ascertaining that the genus is a persistent constituent of world ocean plankton.
Stable oxygen isotopemeasurements on thegenotype L. guemheliniformis show that Laterostomella is a benthic genus, in spite of morphological similarities with planktic Streptochilus species. Thus Streptochilus should not be included in Laterostomella as has been done by Loeblich and Tappan (1988). Laterosromella voluta Belford is in our opinion a phenotype of L. striara de Klasz & RCrat.
The range of Laterostomella guemheliniformis given in the original description should be changed to Middle Miocene-Late Miocene (possibly even earliest Pliocene), from Early-Late Miocene. The range of L. striata is early to late Middle Miocene, possibly reaching into the early part of the Late Miocene.
The planktic habitat of Streptochilus is evident and collections of extant Streptochilus in the Indian Ocean prove that the genus is still very much alive.
We agree with Poore & Gosnell (1985) to include Upper Paleogene species in Streptochilus. These forms may have derived from Paleogene Chiloguemhelina.