Catalog - Sphaeroidinella ionica ionica

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Sphaeroidinella ionica ionica

Citation: Sphaeroidinella ionica ionica Cita & Ciaranfi 1972

Type specimens: Pl. 74, figs. 1-3; Pl. 75, figs. 1-4

Type sample (& lithostrat): Sample DSDP 13-125-4-6 (80-82 cm).

Type age (chronostrat): Upper Pliocene, Globorotalia inflata Interval-zone.

Type locality: DSDP site 125 (Ionian Basin, Mediterranean Ridge).

Type repository: Milan; Collection of the Micropaleontological Laboratory, University of Milan.

Test trochospirally coiled; chambers rapidly increasing in size, subglobular to radially elongated, inflated, arranged in 2-2½ whorls. In the last whorl, four to five chambers are present, the last formed one often having an elongated shape. Wall calcareous, coarsely perforate, with a well developed cortex (sensu Blow, 1959); when the cortex is abraded, the wall appears superficially spinose, as shown in the last chamber of the holotype (Pl. 74, figs. 1a, b) and in the specimen illustrated in fig. 3 of Pl. 74. Intercameral sutures slightly depressed, radial to gently curved on both sides. The total number of chambers is about 10, however the initial ones are in most cases obscured and indistinct. Primary aperture interiomarginal, intraumbilical in position; one or more supplementary apertures are present; both the primary and the supplementary apertures are usually bordered by a rim which may be secondarily thickened with imperforate shell material.

holotype; maximum equatorial diameter 0.78 mm.; equatorial diameter perpendicular to the previous one 0.64 mm.; height of the last formed chamber 0,45 mm.; width of the last formed chamber 0.46 mm. In the specimens investigated, the maximum equatorial diameter ranges from a minimum of 0.70 mm to a maximum of 0.84 mm.

1971 Sphaeroidinella sp. Bizon & Mirkou, p. 296, pl. 42, fig. 2. 699
1972 Sphaeroidinellopsis aff. subdehiscens paenedehiscens - Bizon & Bizon, pp. 289-292, figs. 1-5.

REMARKS ON THE EVOLUTION OF THE TAXON. The derivation of Sphaeroidinella ionica sp. n. from Sphaeroidinellopsis seminulina is discussed in the preceding page, to which reference is made.

Sphaeroidinella ionica ionica and Sphaeroidinellopsis seminulina co-occur in the Sphaeroidinellopsis Acme-zone of the Lower Pliocene, as recorded in the Tyrrhenian succession (DSDP Site n. 132, see Pl. 74, figs. 2-5). The two named taxa also co-occur in the Sphaeroidinellopsis subdehiscens Interval-zone of the Upper Pliocene, as recorded in the Ionian Basin (DSDP Sample 125A-4-2, 140-142 cm) and in the Globorotalia margaritae margaritae Lineage-zone, as recorded in the Balearic Basin (DSDP Sample 124-5 cc).

In the specimens of Sphaeroidinella ionica ionica from the topmost part of the Pliocene (as the holotype), the supplementary aperture (s) are more definite and greater in size than in the earlier Pliocene specimens, as those illustrated in figs. 2 and 3, Pl. 74. The rim bordering the supplementary aperture (s) is more developed. This evolutionary trend gives rise to Sphaeroidinella ionica evoluta (see description below) which has deep and elongated depressions into which the primary and supplementary apertures open, and also well developed flanges.

The evolution from Sphaeroidinella ionica ionica to S. ionica evoluta is observed in Core DSDP 13-125-4, close to the Pliocene/Pleistocene boundary, which is locally identified by means of three different and independent criteria, namely (a) first evolutionary occurrence of Globorotalia truncatulinoides, recorded in section 4 of Core 4, (b) extinction horizon of Discoaster brouweri, recorded in the upper part of section 2 of Core 4 and (c) magnetic reversal from positive (above) to negative (below), as interpreted by Ryan (1973) and identified with the lower boundary of the Olduvai event, recorded in the lower part of section 3 of Core 4.

COMPARISON WITH OTHER TAXA. Sphaeroidinella ionica ionica differs from S. dehiscens (Parker & Jones) for having four to five chambers in the last whorl instead of three; for having a subsquare to subrounded equatorial outline instead of an elongated and/or subrectangular one and for the different (slower) growth pattern.

Sphaeroidinella ionica ionica differs from S. sphaeroides Lamb & Beard, recently described from the late Neogene (Pliocene and Pleistocene) of the Gulf of Mexico, for the higher number of chambers in the last whorl (4-5 instead of 3), for the flatter and more lobulated shape of the test, for the less embracing chambers.

Bizon and Mirkou (1971) recently figured a specimen of Sphaeroidinella from the Lower Pliocene of Zanthe (Greece). The taxon is recorded along with Globorotalia margaritae, Sphaeroidinellopsis sp. and a number of other species, also recorded along with the new taxon here described in the deep-sea Mediterranean sediments. According to the line drawing published, the greek specimen is very similar to the most primitive forms of our Sphaeroidinella ionica ionica, occurring in the Lower Pliocene (see for instance fig. 3 of Pl. 74). The specimens figured by Bizon and Bizon (1972) as Sphaeroidinellopsis aff. subdehiscens paenedehiscens Blow are surely conspecific with the new taxon. They are referred to the genus Sphaeroidinellopsis though possessing supplementary apertures which are described as follows (see op. cit., p. 291): « echancrures de la paroi reliant 2 ou 3 pores, a l'emplacement des sutures spirales et interloculaires ». The specimens illustrated are from the lowermost Pliocene of Corfu (Greece); they are quite similar in all the various morphological characters, as well as for their dimensions, to the specimens recorded from the lowermost Pliocene of the Tyrrhenian Basin, as those figured in Plate 74 (figs. 2 and 3).

References:

Bizon, G. & Bizon, J. -J. (1972). Atlas des Pricipaux Foraminiferes Planctoniques du Bassin Mediterraneen Oligocene a Quaternaire. Technip, France. -. gs

Bizon, G. & Mirkou, R. (1971). Les Foraminiferes planctoniques du Pliocene de l'Ile de Zanthe. Annales Géologiques des Pays Helléniques. 23: 285-296. gs

Cita, M. B. & Ciaranfi, N. (1972). Studi sui Pliocene e sugli strati di passaggio dal Miocene al Pliocene, 2; a new species of Sphaeroidinella from late Neogene deep-sea Mediterranean sediments (DSDP Leg 13). Rivista Italiana di Paleontologia e Stratigrafia. 79: 693-710. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Sphaeroidinella ionica ionica Cita & Ciaranfi 1972

Sphaeroidinella ionica ionica

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