Globorotalia exilis Blow 1969 from: Bolli, H.M.Saunders, J.B. (1985): Oligocene to Holocene low latitude planktic foraminifers. In: Plankton Stratigraphy Eds: Bolli, H.M.Saunders, J.B. p. 155-262 | . |
Notice: This catalogue page may contain unedited data.
|
Species Globorotalia exilis Blow 1969 |
|
|
|
| Alternative name: |
|
| Discussion / Comments: |
Bolli & Saunders (1985):
The species possesses a wall and peripheral keel as delicate or more so than G. pertenuis. It differs in having only 5-6 against 6-8 chambers in the last whorl. These two species are very closely related and intermediate specimens are difficult to separate. Based on the distribution in Caribbean sections, G. exilis developed from G. pertenuis by a reduction in number of chambers towards the end of the Globorotalia margaritae Zone, to continue to the top of the Globorotalia miocenica Zone, in the upper part of its range without G. pertenuis. In how far the reduction of chambers often coupled with irregular growth of charnbers (Olsson, 1973), is a gerontic phenomenon or possibly caused locally by adverse conditions still remains to be clarified. According to Blow (1969) G. exilis branched off from G. cultrata and, based on the thin, delicate type of wall structure, is the ancestor of the Pleistocene G. fimbriata. Such a view is not maintained here. As pointed out above, G. exilis is closely related to G. pertenuis by intermediate forms and from there to G. menardii B. On present evidence the dextral coiling G. exilis became extinct within the Pliocene at the top of the Globorotalia miocenica Zone, whereas G. fimbriata, which coils sinistrally, is known only from the Holocene. A relationship between these two species is therefore not established. G. exilis is further characterized by slightly inflated chambers on the spiral side, slightly more so than in G. pertenuis. Consequently, intercameral sutures appear distinctly incised with early ones not limbate; later ones are limbate often only in their external half. This is explained by Stainforth cl at. (1975), who showed that sutures became partially buried or overlapped by succeeding chambers. Intermediate forms in the Globorotalia margaritae Zone between G. menardii be G. multicamerata and G. pertenuis, and in the Globorotalia miocenica Zone between G. pertenuis and G. exilis point to a close relationship between these species. A detailed study of this group of species, taking into account all typical and transitional forms is still needed to improve on the present understanding of the relationship and stratigraphic ranges of these taxa. |
| Synonym list: |
Bolli & Saunders (1985):
|
| Stratigraphy - absolute ages: |
LAD: 2.15 ± 0 [Ma], Berggren et al. (1995) Atlantic Ocean
|
| References: |
Blow,W.H. (1969):
Late middle Eocene to Recent planktonic foraminiferal biostratigraphy.
In: Proceedings of the First International Conference on Planktonic Microfossils, Geneva 1967 Vol. 1 Eds: Bronnimann, P.Renz, H.H. p. 199-422
Bolli,H.M. and Saunders,J.B. (1985):
Oligocene to Holocene low latitude planktic foraminifers.
In: Plankton Stratigraphy Eds: Bolli, H.M.Saunders, J.B. p. 155-262
Berggren,W.A.; Hilgen,F.J.; Langereis,C.G.; Kent,D.V.; Obradovich,J.D.; Raffi,I.; Raymo,M.E. and Shackleton,N.J. (1995):
Late Neogene chronology: New perspectives in high-resolution stratigraphy . Geological Society of America Bulletin Vol. 107(11)
|
This work is licensed under a Creative Commons Attribution 2.5 License.
|
|
Taxon Concept provided by