Catalog - Globorotalia (Globorotalia) cultrata exilis

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Globorotalia (Globorotalia) cultrata exilis

Citation: Globorotalia (Globorotalia) cultrata exilis Blow, 1969

Type sample (& lithostrat): Bowden Formation

Type age (chronostrat): Pliocene, zone N. 1 9 Sphaeroidinella dehiscens dehiscens Globoquadrina altispira altispira Partial-range zone). Ranges from Zone N. 1 8 (Upper Miocene) to Zone N. 21 (Pliocene)

Type locality: Holotype (pl. l , figs. 1-3) from a brown silty marl exposed behind the sugar store at Bowden Warf (sample ER.156), Bowden, southeastern Jamaica, West Indies [fide Robinson, 1969, Geol. Soc. Jamaica, Jour. , vol. 10, pp. 16, 1 7, fig. 5 and figured paratype (pl. 42, figs. l , 5) from the Bowden Formation (sample E R. 538, the upper part of the probable equivalents of the "Pteropod Marl"). Folly Point, northeastern coast of Jamaica, West Indies.

Test large, coiled in a low trochospire with 16 chambers comprising the spire and with five chambers in the last whorl. Test depressed, ventral side slightly more inflated than the dorsal side with an acute carinate peripheral margin. Dorsal chambers surfaces slightly inflated, not flat and with distinctly incised intercameral sutures. The earlier dorsal intercameral sutures not limbate and the later dorsal intercameral sutures only weakly and partially limbate or totally non-limbate; sutures not raised even when weakly or partially limbate. When present, the partial limbation confined to the distal ends of the dorsal intercameral sutures. Dorsal intercameral sutures, curved and with a distinct retorse distal portion. Ventral intercameral sutures incised, subradial to sinuous. Umbilicus, open and deep but not wide. Aperture, interiomarginal, umbilical-extraumbilical, a low slit-like openmg with a distinct lip. Wall calcareous, radial hyaline, thin and delicate, densely but very finely perforate. Wall surface smooth without pustules except in the immediate area of the aperture.

Maximum diameter of holotype 0.75 mm.

This taxon is distinguished from Globorotalia (G.) cultrata cultrata (d 'Orbigny) [Rotalina cultrata 1839] by virtue of its very thin and delicate wall and by the almost complete absence of limbation over the dorsal intercameral sutures. The holotype shows rather more limbation over parts of the dorsal intercameral sutures than is normally seen but, as will be noted from the illustration (figs. 1-3), the limbation is not continuous over the whole length of all the dorsal intercameral sutures whilst it is totally absent over the dorsal intercameral sutures of the earlier chambers. In these respects G. (G.) cultrata exilis closely resembles G. (G.) praemenardii praemenardii. However, G. (G ) cultrata exilis is usually bigger at a given stage in ontogeny (judging from the number of chambers in the trochospire) than G. (G.) praemenardii praemenardii. In relationship to the large test, the very thin and delicate walls of exilis may be compared to the more robust and thicker walls of praemenardii (sensu stricto).

G. (G.) cultrata exilis possesses a general chamber shape and arrangement (including the slight but definite dorsal chamber inflation) similar to that seen in G (G.) fimbriata. Both fimbriata and exilis have the same thin, delicate type of wall structure. However, fimbriata shows limbation over the whole of all the later dorsal intercameral sutures which is different from that seen in exilis. Nevertheless, fimbriata, like exilis, usually shows no limbation over the dorsal intercameral sutures of the earlier chambers of the penultimate whorl. The peripheral carina in both fimbriata and exilis is comparatively thin and quite delicate but, of course, the latter form shows no spike-like outgrowths on the carina which are so characteristic of G. (G.) fimbriata.
The re-ocrrence of specimens showing the weak development of both the carina and the dorsal limbation is, at first sight, surprising and seems to represent a reversal of the normal trend to more massive and greater development of carinal structures and dorsal limbation seen so frequently in the different lineages of the Globorotaliidae. This, there appears to be no direct phylogenetic link between G. (G ) praenænarclii praemenardii and G. (G.) cultrata exilis and, indeed, if there were such a link it would still represent a significant reversal of the trend towards more massive carinal development. Further, the evolution from exilis to fimbriata does show the usual trend of increasing development of carina and dorsal limbation. It is hardly likely that a direct phylogenetic lineage would include both a reversed and normal trend operating over such a comparatively short time-interval as the Pliocene. Thus, at present the immediate origin of G (G.) cultrata exilis is still unknown but surely, from the resemblances inherent in the general morphology of the two forms, it is likely that it was derived from G (G.) cultrata (s.l.). The paradox presented here seems to be only explicable in terms of a sudden neotonous development of exilis from cultrata (s.s.) with the persistence of a phylogenetically primitive, and early ontogenetic condition from the embryonic, or pre-neanic, ontogenetic stage into the adult. Thus, the absence of dorsal limbation is seen to be primitive in the phylo-ontogenesis of the whole archeomenardii-cultrata lineage and the absence of dorsal intercameral limbation in exilis may be due to neotony and the resultant persistence of the embryonic, or pre-neanic condition into the later stages of ontogeny. Following this sudden neotonous break in the morphogenesis of this side branch of the G. (G.) cultrata (s.l.) lineage, there is a resumption of the normal palingenetic trend towards the development of an increasingly more massive carina and a progressively greater development of dorsal intercameral sutural limbation in specimens from the later parts of this evolutionary side branch leading from G. (G.) cultrata exilis to G. (G.) fimbriata.

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

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia (Globorotalia) cultrata exilis Blow, 1969

Globorotalia (Globorotalia) cultrata exilis

References:

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