Catalog - Wondersella athersuchi

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Wondersella athersuchi

Citation: Wondersella athersuchi Banner and Strank 1987

Type specimens: Holotype slide reg. no. P52040, Other paratypes P52041 - P52043

Type sample (& lithostrat): Shuaiba Formation, Well Zakum-1, core at 7037ft;

Type locality: offshore Abu Dhabi.

The first two to three whorls consist of regularly enlarging chambers in a uniform trochospire 0.25 - 0.3mm in diameter. The spiral side of the trochospire is wholly evolute and convex, with an apical angle averaging about 120" (Pl. 1, fig. 3) but ranging from about 110° (Pl. 1,fig. 2) in very high-spired forms to about 140° (Pl. 1, figs. 1, 6) in relatively low spired specimens. The trochospire has five chambers in each whorl (Pl. 2, figs. 2, 3, 5; Pl. 3, fig. 8) sometimes reducing to four as the chambers become longer and more depressed (Pl. 2, figs. 7, 9). The chambers are initially globular (Pl. 1,figs. 1-9;Pl. 2, figs. 1-3,5;Pl. 3, figs. 9-13) becoming laterally compressed and ovoid in axial section (higher than broad) (Pl. I , figs. 1-9; Pl. 3figs. 1-4, 6, 7, 10) and more reniform in the equatorial plane (Pl. 2, figs. 2, 3, 8, 10). The trochospire has a deep, parallel-sided, open umbilicus, with diameter about 30 - 40% that of the umbilical depression and 20 - 25% that of the trochospiral test. The aperture is umbilical-extraumbilical (Pl. 1, fig. 2; P1. 2, figs. 2, 6, 10; P1. 3, figs. 5-8) and may possess a sharply reflexed porticus (Pl. 1, fig. 2; P1. 3, fig. 3) especially developed in its posterior (more umbilical) part. The spiral and intercameral sutures are distinctly depressed but are not incised or limbate. The test surface is smooth and finely perforate (diameter about 5pm; i.e. of hed- bergellid rather than globuligerinid dimensions - see Banner, 1982, pp. 199-200).

The onset of streptospirality (during or at the end of the third whorl) is marked by increased compression of the chambers, which encroach upon and partly or wholly cover the umbilicus (p1. 1, figs. 8, 9). The chambers also rapidly become compressed dorso- ventrally and acquire peripheral angularity (Pl. 1, figs. 5,6,8,9;P1.3, figs. 1,4,7)without the development of any keel or muricocarina. The posterior parts of the chambers broaden and often become elongate into backwardly-pointing ogival forms (Pl. 2, figs. 2, 4, 7, 8).‘This radial elongation rapidly accelerates as the chambers become more umbilically placed (Pl. 1, figs. 6 , 8 , 9 ; P1. 2, figs. 1 , 4 , 5-10) and the chambers become pointed-spatulate, peripherally acutely angular and compressed, but still with convex surfaces both ventral- ly and dorsally. Each embraces about one-half of the preceding, so that axial sections may cut two or even three successive chambers, giving a false impression of rectilinearity and even biseriality (Pl. 1, fig. 8; P1. 3, figs. 11-13).The shape of the streptospiral chambers is undoubtedly variable between specimens, but their apparent breadth in axial thin section is affected by their asymmetry in equatorial plan (Pl. 2, figs. 6, 8; compare P1. 1, figs. 3, 6 and Pl. 3, figs. 4, 7). The encroachment upon, and then the closure, of the umbilicus by streptospirality, forces the aperture of each later streptospiral chamber to become extraumbi- lical. It is not known if this taxon retains a porticus, but there are indications (Pl. 2, fig. 5) that it does. The flaring, streptospiral chambers may occupy half the total test diameter (which reaches 0.4 - 0.5mm); the test remains smooth and uniformly perforate, and the test walls do not alter their thickness, structure and surface texture with the change of coiling mode and chamber shape.

The external appearance of typical W. athersuchi is reconstructed in Fig. 3; in other, wholly conspecific forms, the final chambers may be more ventrally directed (extremes shown on P1. 3, figs. 5, 5a, 6).

After Dr. J. Athersuch in recognition of his (proprietary) biostratigraphic research on the Middle East.

Remarks. The extraordinary shape of the later cham- bers in W .athemuchi cannot be due to post-deposition- a1 compression and distortion. The sediments in which the assemblages occur, although microbedded, contain abundant tests in all orientations (Pl. 4) and yet it is only the chambers of the last one or two whorls which are of exceptional shape (the early growth stages of all are regular, uniform trochospirals). Also, the other microfossils (e.g. Marssonella, PI. 4, fig. 1; fish vertebra, P1. 4, fig. 3) show no significant distortion. Finally, it would be impossible for post-mortem de- formation to create the elongate, falciform shapes of the later chambers in equatorial section (PI. 2, figs. 2, 4, 7) or the umbilical closures in axial section (PI. 1, figs. 7-9), especially without deformation of the ontogenetically earlier chambers.

Discussion. The nearest morphological analogy to the irregularly placed, flaring, compressed chambers of the last whorl of Wondersella are the similarly flaring, compressed and sometimes abnormally placed final chamber (rarely, two chambers) of Globigerinoides quadrilobatus (d’Orbigny) forma sacculifer (Brady) (Early Miocene - Recent worldwide), G. quadrilobatus fistulosus (Schubert) and G. quadrilobatus hystricosus Belford (both Pliocene, Indo-Pacific); however, in none of these is streptospirality and umbilical closure attained, and the phylogenetic separation of these forms from Wondersella is likely to preclude even homology between the terminal structures. In any case, the significance of sac-like terminal chambers in G. quadrilobatus is itself unknown: attempts to correlate the distribution of G. quadrilobatus f. sacculifer in the Gulf of Elat with seawater temperature, density and nutrition have met with no success (Reiss & Halicz, 1976) and no explanation could be offered for the frequency of individuals with sac-like chambers (Reiss, 1977).

The abundance of Wondersella within its stratigraphic interval (representing at least 30,000 years of geological time in the Zakum Section) suggests that it is not an abnormal phenotype of another (unknown) Hedbergella, but an opportunistic incursion of an ecologically (and, therefore, palaeobiogeographically) restricted species. The benthic foraminiferal fauna of the Wondersella horizon is poor both in numbers and diversity; the presence of pyrite, limonite and fish debris (Pl. 4, fig. 3) also suggests oxygen-deficient bottom waters, confirmed by lack of metazoan debris and bioturbation in the microbedded micrite (Pl. 4). It is probable that the waters were, during this phase of shelf deepening, strongly density-stratified, perhaps due to incursions of highly saline waters into this subtropical gulf and on its deepening carbonate shelf. It may be that Wondersella athersuchi was a species characterising warm, highly saline outer shelf waters of low turbidity in central, equatorial Tethys during Latest Aptian time.

Phylogenetic position of W. athersuchi subsequently revised (Banner and Desai 1988; Banner and Strank 1988)

References:

Banner, F. T. & Strank, A. R. E. (1987). On Wondersella athersuchi, a new stratigraphically significant hedbergellid foraminiferan from the Cretaceous Shuaiba Formation of the Middle East. Journal of Micropalaeontology. 6: 39-48. gs V O

CATALOG OF ORIGINAL DESCRIPTIONS: Wondersella athersuchi Banner and Strank 1987

Wondersella athersuchi

References:

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