Catalog - Globorotalia crassaformis ronda

This page provides data from the catalog of type descriptions. The catalog is sorted alphabetically. Use the current identification link to go back to the main database.

Globorotalia crassaformis ronda

Citation: Globorotalia crassaformis subsp. ronda Blow 1969

Type sample (& lithostrat): Bowden Formation; sample WHB.181A = ?ER146/42

Type age (chronostrat): Pliocene, Zone N. 19

Type locality: SW side of the Coast Road east of Buff Bay, approximately 76m SE of the bridge crossing the railroad, Jamaica, West lndies;

Type repository: London; NHM (originally in BP Sunbury)

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-poteriorly (tangentially) than radially broad; chamber closely apprcseel. inflated dorsally but not greatly embracing anteriorly-posteriorly. The dorsal-peripheral shoulders of the chambers moothly 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, 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, euhedral 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, sclerite over some of the earlier parts of the test. The outer layer of densely packed selerites 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 ansatomosing, 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.

Remarks. - Bé and Ericson (1963, Ann NY Acad Sci. pp. 65-81) and Bé and Lott (1964, Science, pp. 823, 824) have dismissed the 'thickening' of the wall seen in specimens of 'Globororalia 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. lt 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 chnmbers, 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.... 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 (Iate Zone N16 to early Zone N17) 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 Ericson (1963) and Be and Lott (1964) 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.) crassafonnis 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.

References:

Blow, W. H. (1969). Late middle Eocene to Recent planktonic foraminiferal biostratigraphy. In, Bronnimann, P. & Renz, H. H. (eds) Proceedings of the First International Conference on Planktonic Microfossils, Geneva, 1967. E J Brill, Leiden 380-381. gs

Missing or ambiguous references: Bé & Ericson 1963; Bé & Lott 1964;

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia crassaformis subsp. ronda Blow 1969

Globorotalia crassaformis ronda

References:

Add Comment

Comments