Arasella minuta (Zakharov, 1983)

Korn, Dieter, Hairapetian, Vachik, Ghaderi, Abbas, Leda, Lucyna, Schobben, Martin & Akbari, Amir, 2021, The Changhsingian (Late Permian) ammonoids from Baghuk Mountain (Central Iran), European Journal of Taxonomy 776, pp. 1-106 : 24-25

Arasella minuta (Zakharov, 1983)

Fig. 20 ; Table 5

Sinoceltites ? minutus Zakharov in Kotlyar et al., 1983: 153, pl. 15 fig. 1, 2.

Arasella minuta – Korn in Ghaderi et al. 2014: text-fig. 7I. — Korn & Ghaderi in Korn et al. 2016: 887, text-fig. 44.

Diagnosis

Species of Arasella with conch reaching 35 mm dm. Subadult and adult stage with circular whorl profile (ww/wh =0.85–1.05) and rounded venter; 20–22 sharp ribs on the flanks. External lobe with V-shaped prongs; adventive and lateral lobes broadly rounded.

Type material

Holotype AZERBAIJAN • Nakhichevan Province, Akhura; probably top of the Paratirolites Limestone ; illustrated by Zakharov (in Kotlyar et al. 1983: pl. 15 fig. 1); BPI 4/813 .

Material examined

IRAN • 1 specimen; Esfahan Province, Baghuk Mountain C section ; Hambast Formation; MB.C.29720 • 3 specimens; Esfahan Province, Baghuk Mountain E section ; Hambast Formation; MB.C.29721 to MB.C.29723 • 8 specimens; Esfahan Province, Baghuk Mountain G section ; Hambast Formation; MB.C.29724 to MB.C.29729 • 1 specimen; Esfahan Province, Baghuk Mountain H section ; Hambast Formation; MB.C.29730 • 2 specimens; Esfahan Province, Baghuk Mountain ; Hambast Formation; MB.C.29731 to MB.C.29732 .

Description

Specimen MB.C.29720 has 25 mm conch diameter and is moderately well preserved ( Fig. 20A ). Less than half of the last volution belongs to the body chamber. The extremely and evolute conch (uw/ dm =0.48) shows a nearly circular whorl profile of the body chamber. The sculpture of the phragmocone and the terminal body chamber does not differ markedly; the phragmocone has slightly coarser but more rounded radial and sharp ribs on the flank, while the ribs on the body chamber are sharper and coarsest in the ventrolateral area. There are about 15 ribs per half volution.

Remarks

Species of Arasella minuta can easily be separated from all other contemporaneous ammonoids from the Iranian localities because of its very simple suture line with rounded adventive and lateral lobes and the short external lobe ( Korn et al. 2016). Arasella falcata sp. nov. possesses a much weaker sculpture with ribs coarsest in in dorsolateral area.

Stratigraphic range

Topmost part of the Hambast Formation; immediately below the extinction horizon ( Arasella minuta Zone ).

Figures

Fig. 20. Arasella minuta (Zakharov, 1983). A. Lateral and dorsal view, specimen MB.C.29720, section C, -0.05 m. B. Lateral view, specimen MB.C.29728, section G, -0.05 m. C. Arasella falcata Korn & Hairapetian sp. nov.; right = specimen MB.C.29731; left=paratype MB.C.29737, section C, -0.05 m. Scale bar units= 1 mm.