Dictyoconella? minima Henson, 1948

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

Dictyoconella? minima Henson, 1948

Figs. 3a–b pars, 5–6

*1948 Dictyoconella minima n. sp. – Henson, p. 25, pl. 11, fig. 3, 8-10.

2008 Dictyoconella minima Henson – Boudagher Fadel, pl. 5.7, figs. 1-3 (paratypes).

2016 Montseciella sp. – Afghah, fig. 5d (not indicated in the legend of the figure but in the text on page 175; Tarbur Formation of south Zagros Basin).

Description: Test medium conical (apical angles 50–90 degrees); both sides and cone base slightly convex. Some transverse sections display an oval outline ( Figs. 3b , 5j ).

The initial part is characterized by a small spire (?trochospiral,?planspiral) of few chambers and with acute margin; its axis is almost parallel to the test side wall ( Fig. 6e, g, l ). The embryo is poorly visible and its detailed structure unknown ( Fig. 6h–i ). The exoskeleton consists of horizontal (rafters) and radial vertical partitions (beams) subdividing the chamber margins. The beams are continuous from one chamber to the next ( Fig. 5c, 5e, 5n–m ). They extend to a marginal through marking the boundary to the central zone. Their distal ends slightly bend upwards in direction to the apex ( Fig. 5k ). There are 2 to 3 shorter intercalary beams intercalated between the beams ( Fig. 5b, 5o–p ). There are two rafters of inequal length per chamberlet in the marginal zone ( Fig. 5a ). Shallow-tangential sections display a pattern of subrounded alveolar compartments (subepidermal network) ( Fig. 5f ). The boundary between the marginal and central zones is marked by a marginal through, in close proximity to the marginal foramina ( Fig. 5g, 5j, 5p ). The latter with oblique axis ( Fig. 5k ) form a row of circular openings corresponding to the number of beams in transverse sections ( Fig. 5o ). Indistinct parts of the central zone are filled with opaque micritic masses (fused pillars and/or secondary deposits) masking the original structure ( Fig. 5a, 5h , middle part of the cone; Fig. 6b–e ). In other parts of the central zone vertical interseptal elements (pillars) alternating between successive chambers are discernible (see Fig. 5h , youngest chambers; Fig. 6k ). The non-marginal foramenal axes in the central zone maybe vertical (as the pillars) but also oblique.

Dimensions (in mm; data from Henson, 1948, between brackets):

Test diameter (D): 0.7–1.9 mm (0.7–1.8 mm)

Test height (H): 0.8–1.4 (1.0– 1.5 mm)

D/H: 0.8–1.35

Numbers of chambers per 1 mm axial length: 12–16, mostly 15–16 (15)

Remarks: Henson (1948, p. 25) noted that the test of D. minima is conical but slightly compressed, so that the base shows an elliptical outline. Elliptical (or oval) outline of the test base is rare in orbitolinids. It is reported from the early Albian Mesorbitolina ovalis throughout its ontogeny (see Görög and Arnaud-Vanneau, 1996), and the juvenile part of the late Albian Carinoconus casterasi (Bilotte, Canérot, Moullade & Peybernès) (see Schroeder, 1985). The acute margin of the initial spire of D.? minima however shows affinities to Carinoconus (see further remarks below) thus offering an explanation that only transverse sections of the juvenile test part might be ellipsoidal in outline.

Another typical feature of D.? minima are the dark, irregular distributed microgranular masses of fused pillars (and/or secondary deposits?) in the central zone. This was also observed by Henson (1948, p. 26), stating that interseptal structures are sporadic and poorly defined in the central part of the test. Among the Orbitolinidae , secondary deposits in the central zone are an extreme exception. An example is the genus Carinoconus Cherchi & Schroeder , with the type-species C. casterasi from the Late Albian of Spain (Bilotte, Canérot, Moullade & Peybernès) (Cherchi and Schroeder, 1983; Schroeder, 1985; Loeblich and Tappan, 1987). A second species has been described as Carinoconus iraqiensis by Bernaus and Masse (2006) from the Cenomanian of Iraq. Marginal foramina and main partitions (beams) arranged in vertical lines in subsequent chambers however are not reported from the genus Carinoconus . Without further details on the structure of the embryonic apparatus and the initial part in centered sections however a reliable attribution of Dictyoconella? minima to any known ( Carinoconus ?, or new?) orbitolinid genus is not possible for the moment being.

Henson (1948) reported D.? minima “most probably” from the late Cenomanian or Turonian of Qatar (Dukhan no. 2 and 3 wells; Wasia Group, see figs. 1-2 in Sugden and Standring, 1975). In addition, Henson (op. cit, p. 26) noted “a single specimen, possibly of this species” from the Maastrichtian of Iraq. For the Dukhan oil field of Qatar, Hewaidy and Al-Hitmi (1994) established a Late Cenomanian “ Dictyoconella minima total range Zone”. Curiously, Boudagher-Fadel (2008, chart 5.2) indicated the range of D.? minima as Late Cenomanian–Santonian.

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. 5 Dictyoconella? minima Henson from the late Maastrichtian of the Tarbur Formation, SW Iran (Naghan section: a– b, j, l; Mandegan section: c–i, k, m–p). a Subaxial section. b, i, l Oblique sections. Note oval test outline in k. c–e, m Deep tangential sections displaying primary septules (beams) continuous from one chamber to the next. f Shallow tangential section displaying subepidermal network of beams and rafters. g, j, l, o–p Different transverse sections, some slightly oblique. Note five marginal foramina arranged in a radial circle (inside yellow marked rectangle), and oval test outline in j. h Subaxial section. Note alternation of pillars by stairway-like arrangement of foramina (yellow line) and opaque fused pillars/secondary deposits in the central zone. k Detail from h showing marginal foramina arranged in vertical lines between successive chambers. n Tangential-oblique section. Abbreviations: b = beam, ib = intercalary beam, fo = foramina, mf = marginal foramina, mt = marginal through, pi = pillar, s = septum. Thin-sections: Ng 196 (a), Ng 192 (b), Rt 108-3 (c–e, g, n), Rt 104 (f), Rt 96 (h, k), Rt 108-6 (i), Ng 187 (j), Ng 186 (l), Rt 107 (m), Rt 102-2 (o), Rt 104 (p).

Fig. 6 Dictyoconella? minima Henson from the late Maastrichtian of the Tarbur Formation, SW Iran (Mandegan section: b; Naghan section: a, c–k). a Oblique transverse section. b Oblique section cutting the marginal (upper part) and central zones (lower part) displaying irregularly distributed fused pillars/secondary deposits. c, k Subaxial sections. d–g Axial sections showing initial spire. h–i. Details (from e and g) of the initial spire.j Tangential section. l oblique section passing the initial spire. Abbreviations: p = protoconch, sd = secondary deposits/fused pillars. Thin-sections: 2NG 17 (a, k), Rt 100 (b), Ng 180-1 (c), 2NG 179-1 (d), 2NG 168 (e, h), 2NG 175 (f), 2NG 174 (g, i), 2NG 177 (j), 2NG 98-2 (l).