Catalog - Globoquadrina dehiscens praedehiscens

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Globoquadrina dehiscens praedehiscens

Citation: Globoquadrina dehiscens praedehiscens Blow & Banner 1962

Type locality: from the type locality of the Globorotalia kugleri Zone, Cipero formation southern Trinidad (see Bolli, 1957 b, p. 100).

The moderately large test consists of about three whorls of rapidly enlarging, moderately inflated, partially embracing chambers, coiled in a low trochospire. Four chambers are typically present in each of the early whorls, characteristically reducing to three and a half or three in the last whorl. The equatorial profile of the test is subcircular and the equatorial periphery is very weakly and broadly lobulate. In axial profile the test is globosely subconical. The dorsal surface is only very slightly convex, whereas the ventral side is highly vaulted. The axial periphery is smoothly and broadly rounded; the dorsal surface of each chamber, especially in late ontogeny, is both narrow and slightly convex, and the broadly rounded dorsaperipheral shoulder constitutes the greatest diameter of the test. Dorsally the chambers are initially semicircular to reniform, but they rapidly become much longer than broad and distinctly depressed. In axial view the adult chambers clearly show this depression, and are narrowly ovoid. Only three chambers are typically visible ventrally in the adult. The dorsal sutures are initially obscure, but they later become distinctly and narrowly depressed. The dorsal intercameral sutures are initially curved, meeting the moderately lobulate spiral suture at broad angles, but they later become even more weakly curved or subradial. The ventral intercameral sutures are fairly deeply but narrowly depressed. In the adult the umbilicus is small but deep and open; it is approximately triangular in shape, being sharply delimited by the steeply sloping umbilical margins of the chambers of the last whorl. The apertural face of the last chamber is narrow, distinctly flattened, faces the umbilicus, and is quite sharply delimited from the rest of the chamber surface. The final (primary) aperture is a very low arch extending to the extreme limits of the umbilicus; it is bordered by a flap-like lip which becomes narrowly triangular at its mid-point, and which projects into the umbilicus as an umbilical 'tooth'. Early ontogenetic stages possess low, arched primary apertures which are umbilical extraumbilical in extent, bordered by thickened rim-like lips (cf. Globigerina oligocaenica, Plate XG, L-N). The wall of the test is thick and moderately coarsely perforate, and it possesses a rough, 'granular' surface. The test is distinctly hispid in the vicinity of the umbilicus.

Maximum diameter of holotype: 0·49 mm;

Globoquadrina rohri (Bolli), Blow, 1959, Bull. Amer. Paleont. 39, no. 178, p. 185, pl. 11, figs. 57a-<: (not Globigerina rohri Bolli, 1957).

This subspecies differs from Globoquadrina dehiscens dehiscens (Chapman, Parr and Collins) (syn. Globorotalia quadraria Cushman & Ellisor 1939) in possessing an intraumbilical aperture at an earlier ontogenetic stage. The ontogenetic change from an umbiltcal-extraumbilical to an intraumbilical apertural position, occurring later in the ontogeny of G. dehiscens dehiscens, indicates that the evolution from Globigerina tripartita tripartita, through Globoquadrina dehiscens praedehiscens to G. dehiscens dehiscens, is proterogenetic. Globoquadrina dehiscens praedehiscens also differs from G. dehiscens dehiscens in possessing more rapidly enlarging chambers, typically with only three visible ventrally in the last whorl, a more triangular umbilicus bordered, in regularly developed specimens, by a lower and narrower apertural face, which directly faces the umbilicus. The shape of the test is more rounded and less quadrate in G. dehiscens praedehiscens.

Stratigraphical range: In southern Trinidad, this subspecies first occurs in about the middle of the Globigerina ouachitaensis ciperoensis Zone, and ranges up to about the lower part of the Globigerinita stainforthi Zone. It has been observed in equivalent horizons in eastern Falcon, Venezuela, and in about the middle part of the Aquitanian of south-east Sicily, Tanganyika and New Zealand. It is likely that Bolli's stratigraphically higher records of Globigerina rohri and stratigraphically lower records of Globoquadrina dehiscens both refer to this subspecies (see p. 97).

References:

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. gs

Wade, B. S., Pearson, P. N., Olsson, R. K., Fraass, A. J., Leckie, R. M. & Hemleben, C. (2018c). Taxonomy, biostratigraphy, and phylogeny of Oligocene and Lower Miocene Dentoglobigerina and Globoquadrina. In, Wade, B. S., Olsson, R. K., Pearson, P. N., Huber, B. T. & Berggren, W. A. (eds) Atlas of Oligocene Planktonic Foraminifera. Cushman Foundation for Foraminiferal Research, Special Publication . 46(Chap 11): 331-384. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globoquadrina dehiscens praedehiscens Blow & Banner 1962

Globoquadrina dehiscens praedehiscens

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