Catalog - Pulleniatina obliquiloculata finalis

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Pulleniatina obliquiloculata finalis

Citation: Pulleniatina obliquiloculata finalis Banner & Blow 1967

Type locality: South Atlantic Ocean, Challenger Station No. 344; 7° 54' 30" S. lat., 14° 28' 20" W. long.; near Ascension Island, at 420 fathoms in depth on the Mid-Atlantic Ridge; dredged on 3rd April, 1876.

The holotype (plate 3, figure 5a- b) is a tumid, globose, thick-walled test displaying four chambers in ventral view and a fifth chamber in dorsal view. These chambers appear to be regularly enlarging, and they form a coil which is completely involute vcntrally and almost wholly involute dorsally part of only one chamber of the penultimate whorl remaining exposed. The chambers are globose, and their peripheries are very broadly round,ed. The walls of the chambers are smooth and wholly perforate, except for a periphero-ventral area facing and within the aperture, which is granular and irregularly imperforate. Dorsally, that part of the penultimate whorl which is visible constitutes the most coarsely perforate area of the test, the pores becoming more numerous and of smaller size progressively throughout the last whorl. The pore size and distribution on each chamber is very similar on both sides of the test. The dorsal sutures are broadly and weakly depressed, and the ventral sutures are almost completely flush with the chamber surface. The last-formed chamber embraces the umbilical ends of the previous chambers on the ventral side of the test, and its innermost margin joins that of the antepenultimate chamber in a linear suture; the umbilicus is completely closed by this embracement, and the umbilical depression is very shallow. The innermost end of the dorsal surface of the last-formed chamber also covers the dorsal inner end of the penultimate chamber, and the dorsal surface of the test has a shallow concavity approximately equal in depth to the one on the ventral surface. The final aperture is a high arch, entirely extraumbilical in position, extending from the ventral surface, at a point just posterior to the posterior intercameral suture of the antepenultimate chamber, across the periphery of the test and onto the dorsal surface as far as the spiral suture of the antepenultimate chamber. The aperture possesses no lip, but the narrow apertural face is turned inwards so that the aperture is situated as a re-entrant. The apertural face is thickened, hyaline and rimlike, and possesses scattered g ranules. The angle between the terminal face of the last chamber and the in-turned apertural face is bluntly acute. The aperture is broadest at its mid-point, where it is about twice as broad as the apertural face itself. Through the aperture, the granular internal surface of the chamber floor and the broad ultimate septal aperture can be seen. The maximum diameter of the tesrt is 0.72 mm., and the maximum thickness is 0.63 mm.

Discussion: P. obliquiloculata finalis is abundant in the type sample from Challenger Station 344, and these specimens have made possible a study of both the external and internal features of typical and atypical tests. Many specimens occur here which are less dorsally involute than the holotype and have more than one chamber of the penultimate whorl visible dorsally. Such a specimen was included by Brady ( !884, pi. 84, fig. 16, from Challenger Station 224, North Pacific, at 1850 fathoms in depth) in his concept of P. obliquiloculata (without differentiated subspecies). All clearly differ from P. obliquiloculata obliquiloculata, as lectotypified by A. R. Loeblich, in possessing a test which is dorsally involute throughout the whole of the last whorl. Concomitant with this, a much higher and broader final aperture, and a reduced number of chambers in the last whorl are common and characteristic features. As noted above (and see plate 2, figures 4- 9), dissection of specimens of P. obliquiloculata finalis has shown that tests similar to the holotype possess about four whorls of chambers. The first whorl is turborotaline, with a true umbilicus surrounded by five chambers and with an umbilical-extraumbilical aperture. The second whorl is "pseudo-globigerine"", with four chambers around an involute, embracing umbilical depression and with an extraumbilical (but ""pseudoumbilical"") aperture. The third whorl is also ventrally wholly involute, with an umbilical depression, but it has five chambers and a clearly extraumbilical aperture. The fourth whorl, marking the development of the adult stage usually has four chambers and the wholly extraumbilical, ventral-peripheral-dorsal aperture of P. obliquiloculata firzalis. All septal apertures, save those of the first whorl (where they are turborotaline, narrow and lipped), are highly arched and lack true apertural lips, although thickened rims may be present. In the sample from Challenger Station 344, relatively very rare specimens occur which are very high-spired (plate 2, figure 10). These forms show by their apertural size, position and direction that they belong to P. obliquiloculata finalis, but they differ from the typical form of the subspecies in their high-spired, evolute early whorls. Even in some of these forms, the final whorl dorsally embraces the penultimate whorl as far as the spiral suture between the penultimate and the antepenultimate whorls, but it does not enclose the area of the proloculus. Heavy thickening of the wall over the early chambers makes the early intercameral sutures difficult to see, and not enough specimens of this form have been obtained for adequate dissection, but it appears that in such tests five chambers are visible dorsally in all whorls except the last. We suspect that these specimens represent a microspheric form of P. obliquiloculata finalis. Microspheric and megalospheric generations of the other taxa of Pulleniatina have not yet been recognised, and their existence has not been indicated by assemblages seen by us.

Observed distribution: Specimens referable to Pullcniatina obliquiloculata finalis occur commonly in the surface sediments of the present day ocean basins and arc represented in both the Atlantic-Caribbean province and the Indo-Pacific province. However, in the latter province the specimens are frequently larger and form a very significant component of the planktonic foraminiferal faunas. In most cases, the dredged sediment collected by such expeditions as that of H. M. S. Challenger is referable to post-Zone N. 22 horizons, although sediment referable to Zone N. 22 was also dredged. In deep-sea core samples, subspecies finalis occurs prior to the development of the typical form of Globigerina calida ( s. s.) and Sphaeroidinella dehiscens excavata, both of which we use, primarily, to recognise the base of Zone N. 23. Accordingly, P. obliquiloculata finalis is recognised as first occurring within Zone N. 22, and this is confirmed by the occurrence ofjinalis within the highest part of the Navy Island Member of the Manchioneal Formation of J amaica. Further, in the higher parts of the Playa Grande Formation (Cabo Blanco Group) of northern Venezuela, referable to Zone N. 22, forms which are evidently transitional between P. obliquiloculata obliquiloculata and P. obliquiloculata finalis occur in horizons which we consider are near the middle part of Zone N. 22."

References:

Banner, F. T. & Blow, W. H. (1967). The origin, evolution and taxonomy of the foraminiferal genus Pulleniatina Cushman, 1927. Micropaleontology. 13(2): 133-162. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Pulleniatina obliquiloculata finalis Banner & Blow 1967

Pulleniatina obliquiloculata finalis

References:

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