Gusicella minima ( Henson, 1948 ) SCHLAGINTWEIT & RASHIDI, 2021

SCHLAGINTWEIT, FELIX & RASHIDI, KOOROSH, 2021, Dictyoconella Henson, 1948, Upper Cretaceous Larger Benthic Foraminifera: A Taxonomic Revision With The Establishment Of Gusicella Gen. Nov. (Type-Species Dictyoconella Minima Henson), Acta Palaeontologica Romaniae 17 (2), pp. 3-13 : 10-12

Gusicella minima ( Henson, 1948) comb. nov.

Figs. 2 c-d, 7-9

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

1998 Dictyoconella minima Henson – Whittaker et al., pl. 4, fig. 3, pl. 5, figs. 1-2, pl. 48, figs. 1-4.

2008 Dictyoconella minima Henson – Schlagintweit et al., p.34, fig. 3a-b pars, 5-6.

Description: Test medium conical (apical angle from 50 to 90 degrees); both sides and cone base slightly convex. Assumed microspheric specimens broader and larger ( Fig. 8a , 9a, 9f, 9j ). Transverse sections circular ( Fig. 8p ) or ovoidal-compressed ( Fig. 7l , 8j ). The initial part is characterized by a small spire of few chambers arranged in half a whorl that is closely attaching (almost parallel) to the side test wall ( Fig. 7d, g, i , 9b ). The acute apex is tilted, marking the early development in a downwards turning to the cone base (e.g. Fig. 7g ). The megalospheric embryo appears as a single subspherical chamber (protoconch) showing short septules at its upper part facing the cone base ( Fig. 7 g-i). The chambers are subdivided into marginal and central zones. The exoskeleton consists of horizontal (rafters) and radial vertical partitions (beams) subdividing the chamber margins. There are one to three rafters in the marginal zone (e.g. Fig. 7e ). In case of two rafters, the upper one (in direction to the apex) is twice as long as the lower one. In case of three rafters, the middle one is roughly twice as long as the others. There are two to three intercalary beams between the longer main partitions (beams) ( Fig. 8p , 9d ). The latter are continuous from one chamber to the next ( Fig. 8c, 8m , 9i ). They extend to the undivided marginal through marking the boundary to the central zone ( Fig. 7a , 9d ). Here, the septa slightly bend upwards in direction to the apex forming a “buttress on marginal ridge” sensu Davies (1930) as seen in axial sections ( Fig. 8k ). Shallow-tangential sections display a pattern of subrounded alveolar compartments (subepidermal network) ( Fig. 7j , 8f , 9e ). At their distal ends the primary beams are moderately thickening. The marginal chamberlets are tapering distally terminating inwards with a marginal foramen, oriented at about 45 degrees to the main axis. In transverse sections they form a circular ring ( Fig. 7l , 8k, 8o ). Foramina can form a continuous line running parallel to the outer test surface as seen in axial sections ( Fig. 7e , 8a ). Certain portions of the central zone are filled with opaque micritic masses (fused pillars and/or secondary deposits) that obliterate the original structure ( Fig. 7 b-c, 8h). The pillars of the central zone are comparably narrowly spaced and alternate between consecutive chambers ( Fig. 7 c-d, 8a, h, 9f). In the central part, the foramina present a cribrate distribution and are vertically arranged.

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

Test diameter (D): 0.7–1.9 mm (0.7–1.8 mm). Note: specimens with a test diameter> 1.5 mm appear to be miscrospheric specimens.

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

D/H: 0.8–1.35

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

Remarks on stratigraphy: 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”. The “zonal taxon” however, has not been illustrated and therefore, cannot be commented. Within the framework of our observations in the Cenomanian Sarvak Formation of Iraq (e.g. Yazdi-Moghadam and Schlagintweit, 2020), no specimens of G. minima have been observed. There are specimens of an orbitolinid displaying a pillared central zone and a complex exoskeleton that needs further taxonomic study and evaluation, and it is different from G. minima . So far, only the occurrence in the Maastrichtian is well constrained and older occurrences require further study.

Figures

Fig. 2 Microfacies with Dictyoconella complanata Henson (a-b) and Gusicella minima (Henson) gen. et comb. nov. (c- d) from the Maastrichtian Tarbur Formation of the Naghan (a-b) and Mandegan sections (c-d), SW Iran (Zagros zone). Abbreviations: D = Dictyoconella, Di = Dicyclina, G = Gyroconulina, Gu = Gusicella, L = Loftusia, O = Omphalocyclus, P = Pseudonummoloculina, Ps = Pseudocymopolia, T = Tarburina. Thin sections: 2Ng 81 (a), 2Ng 81-4 (b), Rt 109 (c), Rt 105 (d).

Fig. 8 Gusicella minima (Henson) gen. et comb. nov. 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. c–e, m Deep tangential sections displaying primary septules (beams) continuous (aligned) 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 in o). Note oval test outline in j. 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, f = foramen, m.f. = marginal foramen, m.t. = 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. 9 Gusicella minima (Henson) gen. et comb. nov. from the late Maastrichtian of the Tarbur Formation, SW Iran (Naghan section: c–f, h–k); Mandegan section: a–b, g, l). a subaxial section of an assumed microspheric specimen. b–c axial sections showing initial spire with acute margin beneath the apex. d oblique section showing the marginal trough between the marginal and central zones as well as a circular row of marginal apertures between. e tangential section showing subepidermal network; septa in the lower part marked by white dash lines. f subaxial section of a possibly microspheric specimen. g transverse section. h subaxial section. i tangential section showing aligned main beams. j subaxial section of a possibly microspheric specimen. k tangential section. l slightly oblique transverse section showing several orders of vertical partitions (beams, intercalary beams), marginal trough, and pillared central zone. Abbreviations: m.f. = marginal foramen, m.t. = marginal through, pi = pillar. Thin sections: Rt 105 (a), 2Ng 17 (c), Rt 96 (b), 2Ng 167 (d), 2Ng 174 (e), 2Ng 118-1 (f), Rt 108-2b (g), 2Ng 169 (h), 2Ng 191 (i), 2Ng 176 (j), 2Ng 146 (k), Rt 111-1 (l).

Fig. 7 Gusicella minima (Henson) gen. et comb. nov. from the late Maastrichtian of the Tarbur Formation, SW Iran (Mandegan section: b, l; 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. Slightly oblique transverse sections showing subdivision of the marginal zone and circle of marginal apertures (left and above). Abbreviations: m.f. = marginal foramen, m.t. = marginal trough, pr = protoconch, s.d. = 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), Rt 113 (l).