Virgatosphinctes s.l. sp.

Pandey, Dhirendra Kumar, Fuersich, Franz T., Alberti, Matthias, Das, Ranajit & Saez, Federico Oloriz, 2022, First population-level study of the ammonite genus Hildoglochiceras Spath, and the Lower Tithonian record of the Hildoglochiceras Horizon in the Kachchh Basin, India, Zitteliana 96, pp. 1-49 : 1

Virgatosphinctes s.l. sp.

Fig. 11J-L

Perisphinctes (Virgatosphinctes) raja sp. nov., cf. 1910 - Uhlig: 316, pl. 50, fig. 1a-d.

Perisphinctes (Virgatosphinctes) minusculus sp. nov., cf. 1910 - Uhlig: 317, pl. 56, fig. 2a-c.

Material.

One specimen, Hildoglochiceras Bed of Jara Dome (Lower Tithonian); KSKV2019Jara/80.

Description.

Shell large (the fragment is approximately 9 cm in diameter and judging by its curvature represents around one-fourth of the phragmocone with a possible final diameter of 13 cm), evolute, slightly depressed (H: 35, T: 40.7, H/T: 0.85) or (H: 40, T: 41.6, H/T: 0.96) suboval whorl section with distinct but obtusely rounded umbilical shoulder, steep umbilical wall, lateral surface that converges smoothly in the obtusely rounded ventral region. Maximum whorl thickness slightly above umbilical shoulder. Ornamentation consisting of distant, thick, prorsiradiate, fascipartite/fasciculate ribs. Primary ribs originating from umbilical wall rursiradially, bending prorsiradially at umbilical shoulder, displaying a slight forward concavity on lateral surface, branching into thin, densely crowded five to six secondary ribs at mid-lateral height. Secondary ribs following the same course as primary ribs, one or two free secondary ribs inserted between adjacent primary ribs, maximum number of secondaries produced by a single primary rib may not exceed seven. Secondary ribs crossing ventral region with slight forward-directed sinuosity.

Remarks.

The specimen represents a small fragment of the phragmocone with moderately preserved suture lines. Due to the fragmentary nature, ornamentation of inner whorls and of the body chamber is not known. The distant, thick, prorsiradiate, fascipartite/fasciculate ribs with forward-directed concavity on the lateral surface, branching into several fine secondary ribs, the suboval whorl section and H/T ratio are similar to Perisphinctes (Virgatosphinctes) minusculus Uhlig (1910: 317, pl. 56, fig. 2a-c; D: 108, H: 32, T: 36, H/T: 0.88, U: 47). However, the number of secondary ribs produced by one primary rib ranges from 9-10 instead of up to seven in the present specimen.

The ornamentation and whorl section of the outer whorl also match Perisphinctes (Virgatosphinctes) raja Uhlig (1910: 316, pl. 50, fig. 1a-d), but P. (V.) raja has a thicker shell (D: 143, H: 49, T: 57, H/T: 0.85, U: 58.3) and distant ribs in the inner whorls. The H/T ratio in the present specimen is intermediate between the two species discussed above (see also Uhlig 1910: 318).

No true virgatotomy s. str. is recognizable in the present specimen, and divisions seem to be rather fascipartite/fasciculate. Since 5-7 secondaries occur in particular divisions on the phragmocone, a greater number could be expected on the body chamber.

This fragment of phragmocone is too incomplete for a conclusive interpretation. The significant feature is the wide-oval whorl section, which rarely occurs in typical Virgatosphinctes , if Énay’s (2009) classification is assumed (even with caution). Virgatosphinctes sp. C ( Énay 2009: 168, pl. 32, fig. 4) is the only species showing a wide-oval whorl section, but its ribbing seems to be more crowded and with shorter primary ribs ( Énay 2009: pl. 33, fig. 3). Also of interest is its early proliferation of secondary ribs, and the provenance from the "Couches à Virgatosphinctes et Aulacosphinctoides " at Nupra, in assumed Lower Tithonian below Nepalese Hildoglochiceras beds.

Biostratigraphy.

Uhlig (1910) described the two species mentioned above without precise stratigraphy from Spiti and Chidamu areas, respectively. Based on the assemblage recorded here, the comparative analysis with species of known stratigraphy, and the proposed biostratigraphic interpretation for the Hildoglochiceras described, the specimen is assigned to indeterminate horizons within the Lower but not lowermost Tithonian (three-fold division), correlated with indeterminate horizons within the lower part of the Tethyan Albertinum/Darwini Zone, less probably with the upper Darwini to lower Semiforme-Verruciferum zones (Fig. 2 ).

Figures

Figure 2. Litho- and biostratigraphy of the Kachchh Mainland.

Figure 11. A-F. Taramelliceratinae gen. and sp. ind. KSKV 2019 Jara / 17. A. Internal cast, left side view of a fragment of phragmocone showing remnants of blunt secondaries along the periphery; B. Right side view showing moderately coarse ornamentation across the flank, with a shallow and discontinuous lateral groove-like depression and selective collapse areas; C. Suture line drawn from right lateral side, showing smoothed peripheral frilling; D. Line diagram showing oval whorl-section; E. Apertural view; F. Ventral view showing unornamented, subtly raised on the mid-line, and flanked by the external ends of ribs, some of which show incipient oblique-radial swellings; G-I. Aulacosphinctoides sp. ind. KSKV 2020 Jara / 13; G. Ventral view showing broad ventral region, sedondary ribs crossing ventral region almost straight; H. Enlarged right side view of inner whorls (nucleus) showing moderately thick prosiradiate ribs and constrictions; I. Right side view of body chamber showing regularly bifurcating and occasionally, single primary rib, the nucleus part is represented by mould of the cast shown in fig. H; J-L. Virgatosphinctes s. l. sp. KSKV 2019 Jara / 80; J. Line diagram showing suboval whorl section with obtusely rounded ventral region, distinct but obtusely rounded umbilical shoulder and steep umbilical wall; K. Right side view showing distant, thick, prorsiradiate, primary ribs branching into thin, densely crowded five to six secondary ribs at mid-lateral height and displaying a slight forward concavity; L. Ventral view showing broadly rounded ventral region and secondary ribs crossing ventral region with slight forward-directed sinuosity.