Globorotalia (Turborotalia) crassaformis ronda Blow 1969 from: 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 | . |
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Subspecies Globorotalia (Turborotalia) crassaformis ronda Blow 1969 |
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| Diagnosis / Definition: |
Blow (1969):
Description of holotype: Test moderately large, coiled in a low trochospire with 14 15 chambers in the spire and with four chambers in the last whorl. Dorsal side of the test somewhat inflated with the dorsal surfaces of the chambers of the last whorl protruding slightly above the level of the earlier spire. Ventral side, strongly inflated, vaulted but not distinctly conical in outline. Umbilicus very small, open and deep. Dorsal intercameral sutures incised, very slightly curved to almost straight; ventral intercameral sutures slightly incised, radial to slightly sinuous. In dorsal aspect, the chambers are much longer, anteriorly posteriorly (tangentially) than radially broad; chambers closely appressed, inflated dorsally but not greatly embracing anteriorly posteriorly. The dorsal peripheral shoulders of the chambers smoothly rounded over the peripheral margin so giving a generally rounded appearance to the test. The spire opens slowly and regularly but the taxon is tightly coiled. Aperture, umbilical extraumbilical, a low arch bordered by an imperforate lip. Wall, calcareous, radial hyaline [62a], at least, in the inner parts. Some outer parts, at least, of the wall of the earlier chambers overlain by a felted network of lath shaped, anhedral calcareous sclerites. Wall pustulose in the later parts of the test, these pustules becoming more numerous and fusing to give a "roughened" outer surface composed of densely arranged calcareous, lath like, sclerites over some of the earlier parts of the test.
The outer layer of densely packed sclerites forms a discontinuous and partial sheath like structure which is not normally perforate in the sense that it contains the normal wallpores but it does contain an anastomosing, reticulate and ramifying meshwork of fine caniculae similar to those seen ramifying the imperforate carinae of Globorotalia (sensu stricto). The wall of the last chamber in the holotype does not have the sheath like structure but this is not necessarily the case in other specimens of the taxon. Maximum diameter of holotype 0.48 mm. |
| Discussion / Comments: |
Blow (1969):
Remarks: BE and others [64] have dismissed the "thickening" of the wall seen in specimens of "Globorotalia truncatulinoides" as being an ecophenotypic response to a change of habitat (migration into deeper water) during the ontogeny of the form. This "thickening" is present in G. (T.) crassaformis ronda, G. (T.) tosaensis tosaensis as well as in G. (G.) truncatulinoides pachytheca. It is now seen from stereoscan electron microscope studies not to be a simple case of secretion of exogenous calcareous material formed by a secondary, process unrelated to the genetic pattern of the taxon. The sheath like structure covering at least a part of the test (but not usually the last chamber) is now seen to have a complex structure with an anastomosing system of fine caniculae which could not result from a purely uncontrolled non genetic, secondary mechanism. If the deposition was merely due to a secondary secretion of calcite by reason of ecophenotypic response to a gradual migration to another habitat, then the various specimens would not be expected to show such a uniformity of structure as is observed. It is now seen that the sheath is a well organised structure which is remarkably constant in morphology. G. (T.) crassaformis ronda is essentially similar in morphology to G. (T.) crassaformis oceanica but differs in having slightly more embracing and more closely appressed chambers, a tighter coil, a smaller umbilicus and, most significantly, in possessing, at least, the partial sheath like mass of densely packed sclerites which give a "thickened appearance" to the test. This development of the sheath like structure of densely packed sclerites is discussed in reference note no. 64, P. 418, (see also reference note nos. 62 and 62a); reference should be made to these reference notes and to the discussion given for G. (T.) tosaensis tosaensis (p. 393) at this place in the discussion of G. (T.) crassaformis ronda. G. (T.) crassaformis ronda develops from G. (T.) crassaformis oceanica within the earlier part of Zone N.17 and the two taxa are frequently associated in the same samples obtained from a variety of environments. Thus, ronda and oceanica have been observed together in samples from beds whose depth of deposition must have been significantly less than 200 metres and probably not much deeper than 50 metres. On the other hand, both ronda and oceanica occur together in samples from deep sea cores whose depositional depth are greater than 2,500 metres. Again, in the stratigraphic record of ronda and oceanica, there is an interval of time (late Zone N.16 early Zone N.17) where typical G. (T.) crassaformis ronda is consistently absent in samples from very many areas and from a great variety of environments. These observations strongly discount the hypotheses put forward by BE and others [64] as to the "thickening" (i.e., vel formation of a sheath like structure) being purely and simply due to ecophenotypic response of a single uniform genetic entity.
G. (T.) crassaformis ronda differs from G. (T.) crassaformis crassaformis in a variety of characters, notably in possessing a rounded dorsal peripheral margin, tighter coiling, more closely appressed chambers as viewed dorsally, a ventral side which is not so sharply conical in outline and, in having chambers which in dorsal aspect are much longer anteriorly posteriorly (tangentially) than broad radially.
Stratigraphical Range: G. (T.) crassaformis ronda ranges from within the early part of Zone N.i7 to within Zone N.22. Typical specimens of ronda have not yet been observed in Recent or Subrecent sediments although it probably occurs in the earlier part of the interval ascribed to Zone N.23 of some deep sea cores. The holotype of G. (T.) crassaformis ronda was obtained from sample WHB.181A, Buff Bay beds Of CUSHMAN and JARVIS, i.e., Bowden formation (not Buff Bay formation Of HILL, 1899), Jamaica; Zone N.19. |
| Synonym list: |
Blow (1969):
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| 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
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