Spiroclypeus sirottii, Less & Özcan, 2008

Less, György & Özcan, Ercan, 2008, The late Eocene evolution of nummulitid foraminifer Spiroclypeus in the Western Tethys, Acta Palaeontologica Polonica 53 (2), pp. 303-316 : 310-312

Spiroclypeus sirottii sp. nov.

Fig. 7A–N, P, Q, T.

1994 Spiroclypeus granulosus Boussac, 1906 ; Papazzoni 1994: pl. 2: 2. 1995 Spiroclypeus carpaticus ( Uhlig, 1886) ; Papazzoni and Sirotti 1995: pl. 2: 11, 12.

2004 Spiroclypeus sirottii sp. nov.; Less and Gyalog 2004: pl. (black and white) 2: 4.

2007 Spiroclypeus sirottii sp. nov.; Özcan et al. 2007: pl. 1: 20.

Derivation of the name: In honor of the late Professor Achille Sirotti (Modena), an expert on larger Foraminifera.

Holotype: MÁFI E. 9591 ( Fig. 7K), a megalospheric specimen split along the equatorial plane.

Type locality: Mossano (N Italy, Colli Berici), Marne di Priabona , sample Mossano 6.

Type horizon: Basal Priabonian, SBZ 19 A.

Material.—18 well−preserved megalospheric specimens split along the equatorial plane.

Diagnosis.—Involute, lense−shaped test with poorly developed lateral chamberlets. Most of the surface is covered by granules except at the shell’s margin where usually a rectangular sutural network can be seen. The proloculus is small; the coiling of the spiral is rather tight. The number of undivided postembryonic chambers (parameter X) is usually 2–7; its mean value exceeds 2.7 which distinguishes it from Spiroclypeus carpaticus ( Uhlig, 1886) . The secondary chamberlets are more or less regularly arranged, somewhat sparsely spaced and of very slightly hexagonal shape.

Description

External features ( Fig. 7B, D, G).—The test is small (1.5 to 4 mm in diameter), involute, biconvex, lense−shaped with a very slightly inflated, broad umbonal part occupying 70– 90% of the test’s diameter. It is covered by granules, which are very slightly larger in the centre than towards the shell’s periphery where they are absent and where a regular, rectangular network of primary and secondary septa can sometimes be seen. No significant difference in size was recognized between A− and B−forms.

Internal features.—The equatorial section of A−forms: The proloculus is small (P = 50–150 µm, P mean of different populations varies between 80 and 115 µm) and (after a kidney−shaped deuteroconch of similar size) is followed by a rather tightly coiled spiral comprising 2.5 to 4 whorls (d = 350–720 µm, d mean = 490–650 µm; D = 700–1350 µm, D mean = 920– 1160 µm; K = 38–61, K mean = 49–56). The chambers are very high; the primary septa are very strongly curved backward. After a few undivided (operculinid) postembryonic chambers (X = 2–7, very rarely 1 or 8 to 10; X mean = 2.7–5.5) all the successive chambers are subdivided into secondary chamberlets that are rather regularly arranged, somewhat sparsely spaced (S = 2–5, very rarely 6 or 7; S mean = 2.3–4.3) and of very slightly hexagonal shape.

The equatorial section of B−forms ( Fig. 7N): Based on three specimens from samples Mossano 6 and 7, the size of the proloculus is 10–15 µm. It is followed by 17–22 undivided (operculinid) chambers before the appearance of the first subdivided chamber after which some undivided chambers may reappear in a few specimens. The arrangement and shape of chamberlets in the adult stage do not differ from those of the A−forms.

Axial section ( Fig. 7H, I): The test is inflated, involute and biconvex. Piles are well visible, lateral chamberlets are visible but poorly developed.

Remarks.—The name Spiroclypeus sirottii was informally used by Less and Gyalog (2004) for the population from sample Úrhida 10 and by Özcan et al. (2007) for the population from sample Şarköy 4, although no description was provided, so the taxon is introduced formally herein. The figures of Papazzoni (1994) and Papazzoni and Sirotti (1995) cited in the synonymy list illustrate specimens from nearby samples Mossano 5 to 7. These populations were re−evaluated in this paper and placed in synonymy with the present species.

Spiroclypeus sirottii can be confused only with S. carpaticus , however, the mean value of the post−embryonic pre−heterosteginid chambers (parameter X) for the A−forms is above 2.7 for S. sirottii . The other discriminative parameters are the mean value of the number of chamberlets in chamber 14 (parameter S) which is below 4.3 and that of the diameter of the first two whorls which is below 1160 µm. In axial section the rather poorly developed lateral chamberlets also help discriminate the two taxa.

In accord with Hottinger (1977) we suppose that Spiroclypeus (and consequently S. sirottii , the first representative of the genus known so far) evolved from involute Heterostegina , very probably from H. reticulata since it has a similar structure in equatorial section, but lacks lateral chamberlets and appeared in stratigraphically lower horizons. At the same time there can be little doubt on the derivation of S. carpaticus from S. sirottii . The identical structure of their equatorial sections, the more advanced numerical characteristics of S. carpaticus compared to S. sirottii (see Table 1 and Figs. 4–6 ), and the presence of well−developed lateral chamberlets at S. carpaticus and of course the relative stratigraphic position of the two taxa suggests S. carpaticus evolved from S. sirottii .

Geographic and stratigraphic distribution.—Early Priabonian (SBZ 19) of N Italy (Mossano, samples 5, 6 and 8; Verona, Castel San Felice: Villa Le Are, Villa Devoto and the outcrop of the hairpin bend), W Hungary (Úrhida, sample 10), NW Turkey (Şarköy).

Figures

Fig. 4. Distribution of spiroclypeid populations (mean values at the 95.44% confidence level) on the S–X (heterosteginid escalation versus operculinid reduction) bivariate plot (both scales are logarithmic).

Fig. 5. Distribution of spiroclypeid populations (mean values at the 95.44% confidence level) on the P–X (proloculus diameter versus operculinid reduction) bivariate plot (X is on logarithmic scale).

Fig. 6. Distribution of spiroclypeid populations (mean values at the 95.44% confidence level) on the D–X (second whorl diameter versus operculinid reduction) bivariate plot (X is on logarithmic scale).