Heterostegina gracilis Herb, 1978

Less, György, Özcan, Ercan, Papazzoni, Cesare A. & Stockar, Rudolf, 2008, The middle to late Eocene evolution of nummulitid foraminifer Heterostegina in the Western Tethys, Acta Palaeontologica Polonica 53 (2), pp. 317-350 : 338-341

Heterostegina gracilis Herb, 1978

Fig. 15L–V .

1957 Spiroclypeus granulosus Boussac, 1906 ; Bieda 1957: 207–208, 223–224, pl. 12: 1, 2, 4, 7, 8.

1963 Spiroclypeus granulosus Boussac, 1906 ; Bieda 1963: 106–108, pl. 17: 9–11.

1964 Heterostegina cf. praecursor Tan, 1932 ; Hottinger 1964: pl. 7: 3 a–c.

1977 Heterostegina reticulata Rütimeyer, 1850 ; Hottinger 1977: textfig. 44A–D.

1978 Heterostegina gracilis sp. nov.; Herb 1978: 761–762, text−figs. 31–34, 37.

part 1986 Spiroclypeus granulosus Boussac, 1906 ; Grigoryan 1986: 122, pl. 38: 7–11 (non 6).

Diagnosis.—Involute, slightly biconvex test with oval contour, rarely with a central pile but always with granules on the surface except at the periphery of the test where a rectangular sutural network is visible. The proloculus is relatively large, the spiral is very loose. The number of pre−heterosteginid post−embryonic chambers (parameter X) is usually 1 and the number of chamberlets in chamber 14 (parameter S) generally exceeds 9–10. The chamberlets are characteristically delicate, regularly arranged and almost rectangular.

Description External features ( Fig. 15O, P, S ).—The test is involute,

biconvex, rather flat and medium−sized (3–6 mm) with slightly oval contour. The central part of the surface of the test is covered by granules that are formed at the intersections of primary and secondary septal sutures (see also Herb 1978: fig. 37). The central pile could be found only in some specimens from sample Possagno 1. The septal sutures of the alar prolongations are indistinct but a regular rectangular network of primary and secondary septal sutures may be seen in the peripheral part of the test.

Internal features.—The equatorial section of A−forms: The relatively large proloculus is followed by the second chamber of similar size and then by a very loosely coiled spiral. The chambers are very high, very densely spaced and almost evenly arched. The number of post−embryonic undivided chambers (parameter X) is usually 1, rarely 2; the number of chamberlets in chamber 14 (parameter S) generally exceeds 9–10. The secondary septa are always complete; operculinid chambers do not re−appear after the first heterosteginid chamber. The chamberlets are characteristically delicate, regularly arranged and almost rectangular. Numerical features are tabulated Table 5.

Microspheric specimens have not been found yet.

Axial section: Relying on Herb’s (1978: fig. 32) drawing, the test is biconvex and involute with elongated last whorl and with distinct piles. They are decreasing in size from the centre towards the periphery of the shell where finally they are miss−

MÁFI E. 9536 (F), MÁFI E. 9537 (G). H, I. Úrhida 7 ( Hungary), A−form, equatorial sections, MÁFI E. 9538 (H), MÁFI E. 9539 (I). J. Biralu ( Armenia), MÁFI E. 9540, A−form, equatorial section. K, L. Úrhida 8 ( Hungary), A−form, equatorial section, MÁFI E. 9541 (K), MÁFI E. 9542 (L). M–R. Heterostegina reticulata reticulata Rütimeyer, 1850 , latest Bartonian, SBZ 18 C. M–O. Úrhida 9 ( Hungary), equatorial sections, MÁFI E. 9543, A−form (M), MÁFI E. 9544, A−form (N), MÁFI E. 9545, B−form (O). P–R. Oberbergli ( Switzerland), topotypes. P. MÁFI E. 9546, naturally broken surface. Q, R. A−form, equatorial sections, MÁFI E. 9547 (Q), MÁFI E. 9548 (R) .

ing. The subdivision of the chambers into secondary chamberlets is well recognizable in all the whorls.

Remarks.—The forms described above can be undoubtedly identified with Herb’s (1978) “ gracilis ” since we could investigate the material from sample Possagno 2 closely corresponding to sample 728b in Herb and Hekel (1975), which is the type level of this species. Its validity is confirmed, since Heterostegina gracilis differs from H. reticulata by the presence of granulations, by its looser spiral, by the significantly smaller (more delicate) secondary chamberlets whose number in chamber 14 (parameter S) is significantly higher (see also Fig. 9 ). The proloculus is also somewhat larger than that of H. reticulata (see Fig. 10 ).

In samples Possagno 1 and 2 Heterostegina gracilis occurs together with H. reticulata italica . In these samples they can be separated not only by their qualitative features (see above) but also biometrically (see Fig. 7 ). Herb’s (1978) specimens from Possagno are self−evidently put into the synonymy list while Hottinger’s (1964, 1977) forms from Benidorm are coming from the same sample which we have studied.

As it can be seen from the synonymy list Heterostegina gracilis can be confused with Spiroclypeus because of their similar surface with granules. The difference between the two forms is manifested not only in the absence or presence of lateral chamberlets (the latter are diagnostic for Spiroclypeus ) but also in the tightness of their spirals. The confusion is partly because Boussac (1906) failed to provide an adequate figure of the equatorial section of his S. granulosus and until Roveda (1961), the character of the spiral of these forms from Priabona was not well−known. We could not find any real Heterostegina in Priabona, but Spiroclypeus with much tighter spiral (than that of Heterostegina gracilis ) has been abundantly found. The Priabona material is described and figured in Less and Özcan (2008) and the two forms under discussion can be compared and clearly distinguished. This is the reason why Bieda’s (1957, 1963) and Grigoryan’s (1986) forms can be identified as H. gracilis .

Heterostegina gracilis has derived most probably from H. reticulata italica since they occur together and almost overlap in their internal morphology in samples Possagno 1 and 2 (see Fig. 7 ). Moreover, on the surface of H. gracilis from sample Possagno 1, the central pile (characteristic for H. reticulata ) can also be observed (in the case of some specimens from sample Possagno 2 an indistinct umbo can also be recognized). Some kind of intraspecific evolution can be recognized between the population of sample Possagno 1 and the other three ones (see Table 1) in the increase in the size of the proloculus (P) and consequently of the diameter of the first whorl (d) and, less significantly, in the increase of parameter S. Since the size of the proloculus can be controlled not only stratigraphically but also ecologically we do not attempt to

A−form, equatorial sections, MÁFI E. 9559 (K), MÁFI E. 9560 (L), MÁFI E. 9561 (M). N, R. Verona (N Italy), Castel San Felice, Villa Le Are , A−form, equatorial sections, MÁFI E. 9562 (N), MÁFI E. 9563 (R). O. Şarköy 2 (NW Turkey), ITU O/ŞAR.2−2, A−form, equatorial section. P. Şarköy A (NW Turkey), MÁFI E. 9564, A−form, equatorial section. Q. Şarköy 4 (NW Turkey), ITU O/ŞAR.4−83, A−form, equatorial section.

subdivide H. gracilis into chronosubspecies yet. According to our recent knowledge H. gracilis became extinct with no successors at the very end of the Eocene.

Geographic and stratigraphic distribution.—Late Priabonian (SBZ 20) of Italy (Calcare di Santa Giustina, samples Possagno 1 and 2), Spain (Benidorm), and Armenia (Urtsadzor), and of the Polish (Andrychów, etc.) and Slovakian (Raslavice, etc.) Carpathians.

Figures

Fig. 15. Priabonian Heterostegina from different Western Tethyan localities. A–C. Heterostegina reticulata mossanensis ssp. nov., earliest Priabonian, SBZ 19 A. A. Verona (N Italy), Castel San Felice, hairpin bend: MÁFI E. 9565, A−form, equatorial section. B, C: Mossano 7 (N Italy), A−form, equatorial sections, MÁFI E. 9566 (B), MÁFI E. 9567 (C). D–K. Heterostegina reticulata italica Herb, 1978. D, H. Mossano 8 (N Italy), late early Priabonian, SBZ 19 B, A−form, equatorial sections, MÁFI E. 9568 (D), MÁFI E. 9569 (H). E–G, I. Noszvaj (Hungary), “middle” Priabonian, SBZ 19/20. E–G. A−form, equatorial sections, MÁFI E. 9570 (E), MÁFI E. 9571 (F), MÁFI E. 9494 (G). I. MÁFI E. 9572, external view. J, K. Possagno 1 (NE Italy), late Priabonian, Ą

Fig. 9. Distribution of the Eocene heterosteginid populations (mean values at the 95.44% confidence level) on the S–X (density of chamberlets in chamber 14 versus number of undivided post−embryonic chambers) bivariate plot (both scales are logarithmic).

Fig. 10. Distribution of the Eocene heterosteginid populations (mean values at the 95.44% confidence level) on the P–X (proloculus diameter versus number of undivided post−embryonic chambers) bivariate plot (X is on logarithmic scale)

Fig. 7. Distribution of heterosteginid specimens from sample Possagno 1 on the P–S (proloculus diameter versus density of chamberlets in chamber 14) bivariate plot.Solid circles, Heterostegina reticulata; open circles, H. gracilis.