Catalog - Globigerinoides sicanus praesicanus

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Globigerinoides sicanus praesicanus

Citation: Globigerinoides sicanus praesicanus Brönnimann & Resig 1971

Type age (chronostrat): Lower Miocene

Type locality: South West Pacific, Ontong Java Plateau or Soloman Rise New Guinea: DSDP Leg 7, Hole 64.1

Type repository: Washington; USNM

The medium-sized test is a low apparently somewhat streptospiral trochospire with three large axially compressed subglobular chambers both in the ultimate and the penultimate whorls. The arrangement of the early chambers is indistinct. The final chamber is flattened on the apertural side and makes up slightly more than one-half of the test. Its maximum diameter on the apertural side is about 475 microns. It shows a tendency to envelope the preceding chambers. In spiral and in umbilical view the outline of the test is broad oval to subcircular, in axial view broadly convex not pointed over the spiral, slightly concave over the umbilical side. The sutures are well incised. The large umbilicus is shallow and occupied by a large arched aperture of about 200 microns in width and about 75 microns in height which is devoid of lip or rim but whose margin is covered with strong pointed pustules. There are two small triangular supplementary sutural
apertures on the spiral side at the intersections of the sutures between the penultimate whorl and the chambers of the final whorl. These openings may be partially closed by protruding pustules. Their maximum diameter is about 20 to 25 microns. The wall is calcareous and its surface is strongly pustulate and perforate.

The maximum diameter of the holotype is 525 microns, its axial height about 375 microns. The holotype is from Hole 64.1, Core 4, Section 2 at 33-35 centimeters, Zone N. 6 and is illustrated on Plate 10,Figure 8.

Globigerinoides sicanus praesicanus n. subsp., resembles in its umbilical morphology Globigerina sellii Borsetti, 1959, and Globigerina tapuriensis Blow and Banner, 1962. Reference is made to the illustrations of Blow (1969, pi. 16, fig. 7 and 8 (G. tapuriensis); pl. 19, fig. 4 (primitive form of G. sellii), fig. 6 (typical G.sellii)), of Blow and Banner (1962), pi. X, fig. L, M and N (G. selli given as Globigerina oligocaenica Blowand Banner) and fig. H. J. and K (G. tapuriensis given as Globigerina tripartita tapuriensis Blow and Banner) and of Soediano (1969, PI. Ill, fig. 1A-C (given as intermediate forms between G. tripartita and G. sellii)).

However, the supplementary sutural openings clearly distinguish Globigerinoides sicanus praesicanus,n. subsp., from the listed globigerinas, with which it might be confounded if the openings were not visible.
By the arched, large primary aperture, the shallow but well-developed umbilical depression and the apparent slight streptospiral coiling Globigerinoides sicanus praesicanus differs from Globigerinoides quadrilobatus trilobus (Reuss), 1850. Globigerinoides sicanus sicanus de Stefani, 1950, has a much larger ultimate chamber than the new subspecies and a slit-like primary aperture, which is much lower than in Globigerinoides sicanus praesicanus, n.subsp. Typical Globigerinoides sicanus praesicanus occurs stratigraphically below the first appearance of Globigerinoides sicanus sicanus in N. 6. It seems that Globigerinoides sicanus sicanus evolved from Globigerinoides sicanus praesicanus by evolutionary changes affecting the size of the ultimate chamber, the umbilical depression which gradually shallows and disappears, and the primary aperture which changes from a fairly large, high-arched opening to an elongate slit. Reference is made to Blow's re-illustration of the holotype of Globigerinoides sicanus sicanus (1969, pi. 3 fig. 10 and 11) and to the
illustrations of Globigerinoides bisphericus Todd (=Globigerinoides sicanus sicanus de Stefani) in Loeblich et al. (1957), pl. 27, fig. 1 and lb) which clearly show these differences. An intermediate stage between Globigerinoides sicanus praesicanus and Globigerina sicanus sicanus which already possesses a sicanus-type primary aperture is illustrated on Plate 10, Figures 1, 4 and 7. Parallel with the described evolutionary changes, the size of the test as a whole increases. The holotype of Globigerinoides sicanus sicanus shows a maximum diameter of about 600 microns (fide Blow, 1969), the specimens illustrated by Loeblich etal. about 675 microns whereas paratypes of Globigerinoides sicanus praesicanus from Hole 64.1, Core 3, Section 1, 33-35 centimeters and Core 4, Section 3, 17-19 centimeters, as illustrated on Plate 10, Figures 5 and 6, have maximum diameters of only 400 to 550 microns. The evolutionary trend from Globigerinoides sicanus praesicanus to Globigerinoides sicanus sicanus is documented in our material by a series of transitional
forms. The scanning micrographs, Plate 10, Figures 5, 6 and 8, show that the surface is provides with blunt
pustules, probably with a central canal, and that 4 to 6 pustules fuse basally and laterally to form a polygonal
wall around a tunnel-like depression with a rounded wall pore in its center.

References:

Brönnimann, P. & Resig, J. (1971). A Neogene globigerinacean biochronologic time-scale of the southwestern Pacific. Initial Reports of the Deep Sea Drilling Project. 7(2): 1235-1469. gs O

CATALOG OF ORIGINAL DESCRIPTIONS: Globigerinoides sicanus praesicanus Brönnimann & Resig 1971

Globigerinoides sicanus praesicanus

References:

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