Dictyoconus Blanckenhorn, 1900

Rashidi, Felix Schlagintweit Koorosh & Babadipour, Movlud, 2016, Orbitolinid Foraminifera From The Late Maastrichtian Of The Tarbur Formation (Zagros Zone, Sw Iran), Acta Palaeontologica Romaniae 12 (2), pp. 29-46 : 34-38

Genus Dictyoconus Blanckenhorn, 1900

Type species: Patellina egyptiensis Chapman, 1900 Dictyoconus bakhtiari n. sp.

1948 Dictyoconus cf. arietinus Silvestri – Henson, p. 34, pl. 7, fig. 1, text-fig. 3, Maastrichtian of Qatar.

Figs. 3 c-d pars, 7–10

Holotype: Slightly oblique axial section of a megalospheric specimen illustrated in Fig. 10d , and detail in Fig. 10a . Thin-section 2NG 112.

Origin of the name: The species name refers to the Bakhtiaris (Bakhtiyari in Persian), a tribe living in the area of the central Zagros Mountains, southwestern part of Iran.

Type locality: Naghan section, SW Iran ( Figs. 1–2 ).

Type level: Late Maastrichtian of the Tarbur Formation.

Diagnosis: Representative of Dictyoconus with medium to high conico-convex test in megalospheric (d/h ratio often around 1), and reflexed conico-convexe tests of large diameter (d/h> 2) in microspheric forms. Megalospheric forms with eccentrically positioned simple embryonic apparatus formed by a subspherical proloculus and a hemispherical deuteroconch occasionally with tiny rudimentary exoskeleton. Marginal zone with two generations of horizontal (rafters) and three orders of vertical partitions (beams) forming a dense and narrowly spaced subepidermal network. Endoskeleton of numerous, narrowly spaced pillars, alternating between successive chambers. Marginal foramina inclined to the cone axis; cribrate foramina in the central zone perpendicular to septa.

Description: Test distinctly dimorphic with medium to high conico-convexe megalospheric (A form) and low conico-convexe microspheric specimens (B form) of large diameter and reflexed sides. The following description essentially refers to A forms, as only two reliably recognizable B forms were observed ( Fig. 8g , Fig. 10f ). The cone surface is smooth except the laterally slightly protruding initial spire ( Fig. 8a ). The eccentrically positioned spire ( Figs. 7a , 9a ) consists of numerous chambers whose exact number is unknown (> 10). It starts with a subspherical proloculus followed by a semispherical to kidney-shaped deuteroconch partially enclosing the former laterally ( Fig. 7a , Fig. 10a–b ). The deuteroconch occasionally displays the presence of very short, tiny, rudimentary plates ( Fig. 10b ). Both protoconch and deuterconch are connected by means of a single foramen. The chambers following the spiral stage display convex chambers throughout with convexity increasing during ontogeny. Marginal zone with three, occasionally four horizontal partitions (rafters). In transverse sections, the marginal chamberlets are (sub)rectangular and commonly display three thin vertical partitions (intercalary beams) each. One longer in the middle and two shorter secondaries on the sides. The endoskeleton consists of numerous, narrowly spaced pillars alternating in position between successive chambers. The pillars display round shapes in sections above the chamber floor becoming semicircular to sickle-shaped at the ceiling. In the central zone, foramina are disposed in alternating position between the chambers. Oblique marginal apertures at the transition marginal/central zones.

Remarks and comparisons: For all Lower Cretaceous species ( Carsey, 1926: D. walnutensis ; Arnaud-Vanneau, 1980: D.? vercorii ; Peybernes, 1980: D. tunesianus ; Schroeder, 1965: D. pachymarginalis ) we note that these are smaller in dimensions than D. bakhtiari and that their embryo is situated close to the slightly inclined apex. In D. bakhtiari and also the Paleogene forms, in contrast, the embryo is situated below the apex at the beginning of a well-developed spire. A deuteroconch with exoskeleton is only reported from the Aptian (Bedoulian-Gargasian) Dictyoconus? pachymarginalis Schroeder. Considering the complexity of the exoskeleton as generic criterion (e.g., Hottinger and Drobne, 1980; Vecchio and Hottinger, 2007; Vicedo et al., 2014), Dicytoconus tunesianus (Peybernès) lacking horizontal partitions (rafters) should be excluded from the genus Dictyoconus ( Peybernès, 1980) . The comparably large-sized D. walnutensis just has one rafter and one intercalary beam in the marginal zone ( Davies, 1939; Maync, 1955b). The taxonomic revision of the group of Lower Cretaceous Dictyoconus species however is beyond the scope of the present paper.

Henson (1948) reported D. bakhtiari n. sp. as Dictyoconus cf. arietinus Silvestri, 1939 (a form described originally from the Eocene of Smalia) from Maastrichtian limestones with Loftusia and Omphalocyclus from the Maastrichtian of Qatar (Dukhan no. 1 well). The dimensions (height about 1.8 mm, diameter 1.3 mm) correspond to D. bakhtiari . Also the number of chambers in the last 1 mm (10 chambers), and the size of the embryo (0.17 mm) indicated by Henson fit well (see Tab. 1). As Henson noted (op. cit., p. 35), the “diference in age and size might justify separation of the Qatar forms under a new name, but I prefer to give a qualified determination until further records are available”. It is worth mentioning here that Dictyoconus arietinus represents a junior synonym of D. egyptiensis (Cushman) ( Hottinger and Drobne, 1980; Serra-Kiel et al., 2016).

The relationship of the (Lower) Cretaceous representatives of Dictyoconus Blanckenhorn to the Paleogene forms is still some matter of debate. Schroeder (1965, p. 978) on the one hand noted the principal same structural features of both, but on the other hand assumed a polyphyletic origin because of the rather long stratigraphic gap between them. Prior to the description of the Maastrichtian D. bakhtiari , the reported gap stretched the Late Albian/Cenomanian to Thanetian interval (e.g., Hottinger and Drobne, 1980: D. turriculus ; Serra-Kiel et al., 1998; Sirel, 2009: D. baskilensis ). The Eocene Dictyoconus egyptiensis (Chapman) and D. indicus Davies possess a biconch with simple (non septulate) protoconch and deuterconch with exoskeleton ( Douglass, 1960; Hottinger and Drobne, 1980; Hottinger, 2007; Serra-Kiel et al., 2016). The specific distinction between the two may be tricky, and is mainly related to the absence of a pillared deuteroconch and a simpler exoskeleton in D. egyptiensis (Serra Kiel et al., 2016) . Both taxa are larger than D. bakhtiari , have less chambers per last mm axial length (e.g., 5–7 in D. egyptiensis ) and larger size of embryo (e.g., proloculus 0.2–0.35 mm in D. egyptiensis ; see Hottinger and Drobne, 1980). In any case, with the presence of marginal apertures, rectangular chamberlets in the marginal zone, a complex exoskeleton, and last but not least the rudimentary exoskeleton in the deuteroconch, D. bakhtiari is closer to the large-sized Paleogene than the Lower Cretaceous representatives.

Figures

Fig. 3 Microfacies of some charactersistic samples from the Mandegan (a) and Naghan sections (b–f). a Packstone with benthic foraminifera Omphalocyclus macroporus (Lamarck) (O), and oblique transverse section of Dictyoconella? minima Henson on the right. Thin-section Rt 105. b Grainstone/packstone with debris of rudist shells, and larger benthic foraminifera Loftusia sp. (L), and transverse section of Dictyoconella? minima Henson (D). Thin-section 2NG 17. c Wackestone/packstone with benthic foraminifera among two specimens of Dictyoconus bakhtiari n. sp. (see also Fig. 10c, e). d Floatstone with corals, rudists, and tangential section of Dictyoconus bakhtiari n. sp. (Di). Thin-section 2NG 87-2. e–f Packstone with Pseudorbitolina cf. marthae Douvillé, and Omphalocyclus macroporus (Lamarck) (O) in f. Thin-sections 2NG 47 and NG 47-1.

Fig. 10 Dictyoconus bakhtiari n. sp. from the late Maastrichtian of the Tarbur Formation, Naghan section, SW Iran. a, d Slightly oblique axial section of a megalospheric specimen (holotype!). b Centered axial section of a juvenile megalospheric specimen. Note the presence of tiny rudimentary septules in the deuteroconch. c Oblique axial section, partially passing through the initial spire with protoconch. e Oblique transverse section. f Tangential-oblique section of microspheric specimen. Abbreviations: de = deuteroconch, pr = protoconch, s = septum, pi = pillar, uz = undivided zone (between marginal and central zone). Thin-sections: 2NG 112 (a, d), 2NG 69 (b), 2NG 87 (c, e), 2NG 83 (f).

Fig. 1 Location map of the two studied sections.

Fig. 2 Field view of the Naghan section located approximately 50 km south west of Naghan town near the Gandomkar village.

Fig. 8 Dictyoconus bakhtiari n. sp. from the late Maastrichtian of the Tarbur Formation, Mandegan section (c) and Naghan section (all others), SW Iran. a Oblique section passing the eccentric initial spire. b–e, h–j Oblique sections; affecting five chambers in b. Note the broadening of the distal end of the beams in b–d, i. f Tangential-oblique section. g Tangential oblique section of a microspheric specimen. Abbreviations: be = beam, fo = foramen, is = initial spire, pi = pillar, s = septum. Thin-sections: Ng 53-2 (a), 2NG 52 (b), Rt 62-1 (c), Ng 53-1 (d, h), Ng 52-1 (e), 2NG 32 (f, j), Ng 87-2 (g), 2NG 53 (i).

Fig. 7 Dictyoconus bakhtiari n. sp. from the late Maastrichtian of the Tarbur Formation, Naghan section, SW Iran. a, e Axial section of megalospheric specimen. Detail of biconch in e. b Tangential section passing through the initial spire with embryo. c Tangential section passing (from above to below) the initial spire with embryo, the marginal zone with large quadrangular pattern of primary septules (beams) (1, in yellow), and the complex pattern of intercalary beams and rafters (2, orange). d Fragmentary tangential section. Note the rounded transverse sections of the chamberlets produced by the primary septules (beams) in the lower middle part of the test. f–g, j, Subaxial sections. Detail of the marginal zone (from g) showing rafters in j. h Oblique section. i, k Subaxial sections. Abbreviations: b = beam, deu = deuteroconch, mf = marginal foramen, p = protoconch, pi = pillar, r = rafter, s = septum. 1: dense subepidermal network of higher order beams (intercalary beams) and rafters. 2: coarse rectangular subepidermal network of primary rafters and beams. Thin-sections: 2NG32 (a, e), Ng 112 (b-c), Ng 15 (d), 2NG52 (f), 2NG53 (g, j, k), Ng 73 (h), Ng 83-3 (i).

Fig. 9 Dictyoconus bakhtiari n. sp. from the late Maastrichtian of the Tarbur Formation, Naghan section, SW Iran. a Oblique section. b–c Oblique sections cutting the initial spire with the embryo. d, g Subaxial sections. e–f, j Axial sections. h Obliquetransverse section. i Tangential section. Abbreviations: be = beam, de = deuteroconch, ib = intercalary beam, p = protoconch. Thinsections: Ng 21 (a), Ng 85 (b), Ng 53-1 (c), Ng 52-2 (d, h), 2Ng 87-2 (e-f), Ng 53-2 (g), Ng 55 (i), 2NG 27 (j).