Arasella falcata Korn & Hairapetian, 2021

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 : 26-27

Arasella falcata Korn & Hairapetian sp. nov.

urn:lsid:zoobank.org:act:B2DA940A-842C-452B-8906-A29E07291FFE

Fig. 21 ; Table 6

Diagnosis

Species of Arasella with conch reaching 35 mm dm. Subadult and adult stage with compressed whorl profile (ww/wh=0.60–0.70) and rounded venter; 25 curved ribs which are coarsest on the inner flanks.

Etymology

From the Latin ‘ falx ’, meaning ‘sickle’, because of the rib course.

Material examined

Holotype IRAN • Esfahan Province, Baghuk Mountain C section ; Hambast Formation, 0.05 m below top; illustrated in Fig. 21A ; MB.C.29734 .

Paratypes IRAN • 1 specimen; Esfahan Province, Baghuk Mountain 1 section ; Hambast Formation; MB.C.29733 • 1 specimen; Esfahan Province, Baghuk Mountain G section ; Hambast Formation; MB.C.29735 •

1 specimen; Esfahan Province, Baghuk Mountain H section ; Hambast Formation; MB.C.29736 • 1 specimen; Esfahan Province, Baghuk Mountain ; Hambast Formation; MB.C.29737 .

Description

Holotype MB.C.29734 is a somewhat corroded specimen with a conch diameter of 24 mm ( Fig. 21A ). It is extremely discoidal (ww/dm =0.20) and subevolute (uw/dm =0.43) with a compressed whorl profile (ww/wh =0.63). The flanks are nearly parallel and the venter is rounded. The last volution shows about 25 ribs, which are sharpest and coarsest on the inner flank; they turn slightly forward and become weaker on the outer flank to diminish.

Remarks

Arasella minuta shows much coarser ribs, particularly on the outer flanks, while A. falcata sp. nov. possesses a much weaker sculpture with ribs strongest in in dorsolateral area.

Stratigraphic range

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

Figures

Fig. 21. Arasella falcata Korn & Hairapetian sp. nov. A. Lateral and dorsal view, holotype MB.C.29734, section C, -0.05 m. B. Lateral view, paratype MB.C.29733, section 1, -0.05 m. C. Lateral views, paratype MB.C.29736, section H, -0.05 m. Scale bar units =1 mm.