Heterostegina reticulata Rütimeyer, 1850

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 : 334-335

Heterostegina reticulata Rütimeyer, 1850

Figs. 11J–L, N–R, 12–14, 15A–K .

Emended diagnosis.—Involute, biconvex test with central pile and slightly sigmoid septal sutures passing sooner or later into septal network towards the edges. The size of the proloculus is increasing in stratigraphic order from small to medium−sized, chamberlets (with no incomplete secondary septa) are changing simultaneously from rather irregularly arranged and rhomboid to regularly arranged and almost rectangular. Their number in chamber 14 (parameter S) also increases in stratigraphic order but usually does not exceed 7–8. The number of operculinid (undivided) chambers is strongly reduced during the phylogenesis. Based on this, the species is subdivided into seven chronosubspecies as follows:

H. reticulata tronensis ssp. nov. X mean> 17 H. reticulata hungarica ssp. nov. X mean = 11–17 H. reticulata multifida X mean = 7.2–11 H. reticulata helvetica X mean = 4.4–7.2 H. reticulata reticulata X mean = 2.8–4.4 H. reticulata mossanensis ssp. nov. X mean = 1.7–2.8 H. reticulata italica X mean <1.7

Description

External features ( Figs. 11Q, 13B, 15I ).—The test is involute, moderately thick, biconvex and medium−sized (3–6 mm) with oval contour. The distinct central pile is somewhat eccentrically placed. Septal sutures are slightly sigmoid, passing sooner or later into a hexagonal network of primary and secondary sutures towards the periphery of the whorls. No significant difference in adult size between the megalo− and microspheric forms could be observed.

Internal features.—The equatorial section of A−forms: The small to medium−sized proloculus is followed by the second chamber of similar size and then by a loosely coiled spiral. The chambers are high, densely spaced and almost evenly arched. The first appearance of chamberlets is stratigraphically controlled (see the diagnosis), undivided chambers may reappear only in the early stage of the phylogenetic development. The chamberlets (with fully developed secondary septa) are changing in stratigraphic order from rather irregularly arranged and rhomboid to regularly arranged and almost rectangular. Numerical features are tabulated in Table 5.

The equatorial section of B−forms ( Figs. 12E, 13O, 14D, J ): In the few microspheric forms listed in Table 4 the size of the proloculus is 10–15 µm. It is followed by 18–37 undivided chambers before the appearance of the first subdivided chamber after which operculinid chambers can rarely reappear. The reduction of parameter X in stratigraphical order can be reasonably supposed (see Table 4). The arrangement and shape of the chamberlets in the adult stage do not differ from those of the A−forms.

Axial section ( Figs. 11P, 13A): Relying on our own, Herb’s (1978: figs. 12, 13, 30), and Stockar’s (1999: pl. 3: 6) preparations, the test is biconvex and involute with a wide last whorl and with a distinct central pile. The subdivision of the chambers into chamberlets is evident especially in the last whorl.

Remarks.—Four names, “ reticulata ”, “ helvetica ”, “ multifida ”, and “ italica ” in historical order can be identified with the forms described above. They may correspond to different evolutionary stages of the species (see in detail at the description of the subspecies). Since “ reticulata ” is in priority among them, it is therefore the valid name for the species. To distinguish Heterostegina reticulata from H. armenica and H. gracilis , see our remarks in the description of the latter two forms. Their segregation is illustrated also in Figs. 7–10 .

The early representatives of H. reticulata occur together with the Operculina gomezi −group in almost all the localities. The latter differs from H. reticulata only in lacking complete secondary chamberlets. Since this involute Operculina appears first in lower stratigraphical levels than H. reticulata (it can be directly observed in Keçili and also in Mossano where the San Pancrazio locality lacking Heterostegina contains O. bericensis ), they can be considered as possible ancestors of the latter. The other possibility is an intermediate step between them represented by H. armenica . The intraspecific evolution is expressed significantly in the decrease of parameter X and in the increase of parameter S. The size of the proloculus (P) and that of the diameter of the first and first and a half whorls (d and D) also increase but with significant, probably ecologically−controlled deviations. The spiral becomes somewhat looser during the evolution of the species; however this change is the least evident.

Heterostegina reticulata gave rise to H. gracilis at about the middle of the Priabonian, then became extinct with no successors at the very end of the Eocene. Our preliminary studies on Oligocene involute Heterostegina (see in details below) suggest that they are not related directly to any of the Eocene forms. At the same time the derivation of genus Spiroclypeus from Heterostegina reticulata cannot be excluded since the former have a similar heterosteginid spiral but bear lateral chamberlets. Thus the Spiroclypeus morphology is more complex. Moreover, it appears in stratigraphically higher horizons than H. reticulata .

Geographic and stratigraphic distribution.—The middle late Bartonian to terminal Priabonian (SBZ 18 B−20) time−interval of the northern margin of the western Tethys, from Spain to Armenia (see in details at the particular subspecies).

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. 14. Latest Bartonian and early Priabonian Heterostegina from different Western Tethyan localities. A–F. Heterostegina reticulata reticulata Rütimeyer, 1850, latest Bartonian, SBZ 18 C. A–E. Mossano 3 (N Italy), equatorial sections, MÁFI E. 9549, A−form (A), MÁFI E. 9550, A−form (B), MÁFI E. 9551, A−form (C), MÁFI E. 9552, B−form (D), MÁFI E. 9553, A−form (E). F. Vedi (Armenia), MÁFI E. 9554, A−form, equatorial section. G–R. Heterostegina reticulata mossanensis ssp. nov., earliest Priabonian, SBZ 19 A. G–J. Mossano 6, equatorial sections, holotype, MÁFI E. 9555, A−form (G), paratype, MÁFI E. 9556, A−form (H), paratype, MÁFI E. 9557, A−form (I), paratype, MÁFI E. 9558 (J), B−form. K–M. Úrhida 10 (Hungary), Ą

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.

Fig. 8. Distribution of heterosteginid specimens from sample Keçili 11 on the P–X (proloculus diameter versus number of undivided post−embryonic chambers) bivariate plot (X is on logarithmic scale). Solid circles, Heterostegina armenica; open circles, H. reticulata.

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)