Catalog - Sphaeroidinella ionica evoluta

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Sphaeroidinella ionica evoluta

Citation: Sphaeroidinella ionica evoluta Cita & Ciaranfi 1972

Type sample (& lithostrat): Sample DSDP 13-125-4-4 (6-8 cm).

Type age (chronostrat): Lower Pleistocene, Globorotalia truncatulinoides Total-range-zone

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

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

Test trochospirally coiled with subglobular chambers, laterally slightly compressed, rapidly increasing in size. Four to five chambers are present in the last whorl, the last-formed one commonly being smaller than the preceding one, or abortive. Intercameral sutures radially arranged and depressed; in most cases obscured by the depressions in form of cracks or deep excavations which extend from the umbilical area along the sutures on the umbilical side; from the spiral suture along the supplementary apertures extending to the intercameral sutures on the spiral side. Wall calcareous, coarsely perforate, with a thick, well developed cortex; the final (abortive) chamber appears thinner walled and less coarsely perforate than the previous ones. Primary aperture interiomarginal, irregularly shaped, opening into the deep and wide excavation (see figs. 1 and 2, Pl. 76) which characterizes the subspecies. Supplementary apertures interiomarginal, sutural in position, opening as well into the deep excavations of the test. Both on the spiral and on the umbilical side well developed flanges are vis ible along the rims of the deep excavations, consisting of imperforate shell material (see figs. 2 and 4, Pl. 76).

holotype: equatorial diameter passing through the last-formed chamber 1.6 mm. equatorial diameter perpendicular to the previous one 1.1 mm. In the specimens investigated, the maximum equatorial diameter ranges from 0.6 mm to over 1 mm.

Remarks. The evolution of Sphaeroidinella ionica evoluta from Sphaeroidinella ionica ionica has been discussed in the description of the latter taxon, to which reference is. made. The main difference consists in the greater development of the apertural ftanges and in the occurrence of deep excavations. Also, a larger size seems to characterize the Pleistocene subspecies, though this observation needs further support, since the taxon is too rare to make sure statements on this particular point.

Sphaeroidinella ionica evoluta is very similar to S. dehiscens excavata Banner & Blow from the Recent sediments of the Pacific Ocean for the apertural characters, also including the deep excavations in which the primary and supplementary apertures open and the well developed flanges. It differs from it because the latter shows only three chambers in the last whorl, with the last formed one always very well developed and embracing, unlike the last formed chamber of S. ionica evoluta, which is often abortive. The occurrence of an abortive final chamber is generally considered a sign of senility of the species. Specimens referred to Sphaeroidinella excavata Blow (sic) have recently been figured by Hofker (1972), coming from Recent sediments of the Caribbean. Some of the figured specimens appear very similar to Sphaeroidinella ionica evoluta (see Hofker, op. cit., pl. 7, figs. A, B and C): they have four chambers in the last whorl, with the last formed one small and abortive, and an equatorial outline subsquare, as is the case for S. ionica evoluta, instead of elongated and subrectangular, as is the case for S. dehiscens excavata. If the suggested synonymy is correct, than the taxon under discussion should be represented also in the Caribbean region.

According to Hofker (op. cit.), the « Sphaeroidinella genus» gradually increases in size from the Miocene to the Recent, while the wall structure does not evolve at all. Our observations on the Mediterranean faunas support his statement. The holotype of Sphaeroidinella ionica evoluta in fact exceeds 1 mm in size.

References:

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 evoluta Cita & Ciaranfi 1972

Sphaeroidinella ionica evoluta

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