Catalog - Globigerapsis tropicalis

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Other pages this level: G. indica, G. kugleri, G. tropicalis

Globigerapsis tropicalis

Citation: Globigerapsis tropicalis Blow & Banner 1962

Type sample (& lithostrat): FCRM 1645

Type age (chronostrat): Upper Eocene

Type locality: Lindi area, Tanzania

The moderately large test consists of 3 whorls of chambers arranged trochospirally. The first 2 whorls of chambers are arranged in a very low trochospire, with about 5 chambers in each whorl; the chambers are moderately inflated and partially embracing, the amount of embrace apparently decreasing slowly and regularly when seen in dorsal aspect. The succeeding whorl consists of 3 to 4 chambers, which are abruptly and rapidly enlarging, are coiled in an increasingly high trochospire, and which increasingly embrace the earlier test ventrally. The last-formed adult chamber envelops all or most of the earlier ventral surface of the test. The penultimate chamber possesses a single, intraumbilical, arched aperture, which may be bordered by a thickened rim; the true umbilicus of the earlier chambers may be seen within the penultimate primary aperture. The adult test possesses 3 or 4 arched semicircular apertures placed in the basal suture of the last chamber at its junctions with the intercameral sutures of the earlier chambers. These adult apertures are aproximately equal size, and each possesses a thickened rim. The adult test is globose, the final chamber constituting appoximately one-third to one-half of the total test. The initial sutures are weakly depressed, the dorsal intercameral sutures being curved, and the spiral suture weakly lobulate; during ontogeny, the sutures become increasingly depressed,the intercameral sutures become moderately lobulate. The wall is thin compared to Globigerapsis index (Finlay) [Globigerinoides index, 1939]; it is finely and uniformly perforate, but it sometimes becomes weakly cancellate, especially dorsally.

Maximum diameter of holotype: 0.37 mm.

Remarks: Globigerapsis tropicalis differs from G. index (Finlay) in lacking the very thick walls, deeply incised sutures, and heavily granular surface characteristic of the latter species. Hornibrook (1958, p. 34, pl. 1, figs. 11-14) re-illustrated and briefly discussed Finlay's type specimens of 'Globigerinoides' index. F. C. Dilley has kindly shown us topotypic specimens from the 'Bartonian' stage of the Middle Eocene of South Island, New Zealand; these specimens confirm the presence of thick walls and 'deeply cleft sutures' in this species, as stated by both Finlay (1939) and Hornibrook (1958). Globigerapsis tropicalis not only lacks these characters, but is morphologically indistinguishable from specimens seen by us in the Hospital Hill Marl (Navet formation) ofsouthern Trinidad; such specimens were illustrated by Bolli (1957 c), and are believed to be synonymous. Specimens which are correctly referable to G. index (Finlay) occur abundantly in coastal Tanganyika, at horizons which may safely be correlated with the lower Middle Eocene. G. index appears to be absent in the Upper Eocene of East Africa and Trinidad, and it is probable that other records from the Upper Eocene (e.g. from Cuba, Syria, Israel and southeastern Australia-Hornibrook, 1958, p. 34) should refer to G. tropicalis.

Comparison of Finlay's holotype of G. index (reillustrated by Hornibrook, 1958) with Bolli's (1957 c, figs. 15-18) equally immature specimens, shows that the distinctions between G. index and G. tropicalis are apparent even before the adult stage of ontogeny is reached (Plate VIle, o).

Globigerapsis tropicalis differs from G. semiinvoluta (Keijzer) in possessing less embracing and better separated chambers throughout all stages of ontogeny, a less tightly coiled trochospire, slightly greater hispidity in the juvenile stage, smaller multiple apertures in the adult stage, less distinctive aperturallips in both the juvenile and adult stages, and a much less inflated and embracing final chamber.

'Globigerinoides' subconglobatus Shutskaya (given as 'Chalilov MS' by Shutskaya, 1958, pp. 83-88, pl. I, figs. 1-11), a form of Globigerapsis from the Middle and Upper Eocene of the Caucasus, is claimed by Shutskaya to be ancestral to the form referred to by us as G. tropicalis. We have seen specimens similar to G. index subconglobatus (Shutskaya) in the Middle Eocene of East Africa, these forms differing from both G. index index and G. tropicalis in possessing less rapidly enlarging and inflated chambers, smaller adult apertures and a more compact and subglobular test.

It is highly likely that Globigerapsis troptcalis is ancestral to G. semi-invo!uta.

STRATIGRAPHICAL RANGE In the area of coastal Tanganyika, G!obigerapsis tropicalis ranges from the Middle Eocene to the top of the G..semi-involuta Zone (lower part of the Upper Eocene). Subbotina (1953) a?d Gla~ssner (1937) recorded a similar range for this form 111 the Caucasus and the former author used it to define her 'Globfgerinoides conglobatus Zone' ofthe.lo:ver Upper Eocene. Bolli (1957c) records a similar range in southern Trinidad. G!obigerapsi~ index (Finlay) appears to possess a distinctly d~fferent stratigraphical distribution; all records which can be verified suggest that it does not occur above the Middle Eocene and additional evidence from Africa suggests that it may occur as low as the uppermost part of the Lower Eocene.

References:

Blow, W. H. & Banner, F. T. (1962). Blow, W. H. & Banner, F. T. (1962). The mid-Tertiary (Upper Eocene to Aquitanian) Globigerinaceae. In, Eames, F. E. , Banner, F. T. , Blow, W. H. & Clarke, W. J. (eds) Fundamentals of mid-Tertiary Stratigraphical Correlation. Cambridge University Press, Cambridge 61-151. In, Eames, F. E., Banner, F. T., Blow, W. H. & Clarke, W. J. (eds) Fundamentals of mid-Tertiary Stratigraphical Correlation. Cambridge University Press, Cambridge 61-151. gs

Premoli Silva, I., Wade, B. S. & Pearson, P. N. (2006). Premoli Silva, I., Wade, B. S. & Pearson, P. N. (2006). Taxonomy, biostratigraphy, and phylogeny of Globigerinatheka and Orbulinoides. In, Pearson, P. N. , Olsson, R. K. , Hemleben, C. , Huber, B. T. & Berggren, W. A. (eds) Atlas of Eocene Planktonic Foraminifera. Cushman Foundation for Foraminiferal Research, Special Publication. 41(Chap 7): 169-212. In, Pearson, P. N., Olsson, R. K., Hemleben, C., Huber, B. T. & Berggren, W. A. (eds) Atlas of Eocene Planktonic Foraminifera. Cushman Foundation for Foraminiferal Research, Special Publication . 41(Chap 7): 169-212. gs O

CATALOG OF ORIGINAL DESCRIPTIONS: Globigerapsis tropicalis Blow & Banner 1962

Globigerapsis tropicalis

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