Domatoceras canonium, Korn & Hairapetian, 2025

Domatoceras canonium sp. nov.

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Figs 5–6 View Fig View Fig ; Table 2 View Table 2

Domatoceras sp. – Korn et al. 2021b: text-fig. 17e.

Diagnosis

Species of Domatoceras with extremely discoidal, subevolute conch (ww/dm ~ 0.25; uw/dm ~ 0.35), weakly compressed whorl profile (ww/wh ~0.65) and extremely high coiling rate (WER ~2.75) at a conch diameter of 70–80 mm. Whorl profile with weakly concave venter, angular ventrolateral shoulder, flattened convergent flanks, rounded umbilical margin and oblique umbilical wall. Whorl overlap extremely small. Without sculpture, but with small ventrolateral tubercles in the juvenile stage. Suture line with a small and narrow external lobe and a much larger and deeper, broadly rounded lateral lobe.

Etymology

From the Greek ‘κανονίης’=‘slender’; because of the slender conch.

Type material

Holotype

IRAN – Esfahan Province • Baghuk Mountain ; Hambast Formation, Vedioceras beds ( late Wuchiapingian); 2010; Hairapetian leg.; illustrated in Fig. 5 View Fig ; MB.C.32105 .

Paratypes

IRAN – Esfahan Province • 1 specimen; same data as for holotype; MB.C.32106 • 1 specimen; same data as for holotype; 2011; Korn et al. leg.; illustrated in Fig. 6 View Fig ; MB.C.30227 • 2 specimens; same data as for holotype; 2011; Korn et al. leg.; MB.C.32107, MB.C.32108.

Description

Holotype MB.C.32105 is a slightly corroded, fully chambered internal mould with a conch diameter of 134 mm ( Fig. 5A View Fig ); the total conch diameter inclusive the body chamber must have been at least 200 mm. The specimen has two and a half whorls, but the early ontogenetic stage of the conch is poorly preserved. The conch is, at 134 mm diameter, extremely discoidal and subevolute (ww/dm =0.22; uw/dm=0.37) with an extremely high coiling rate (WER =2.58); the whorl profile is compressed (ww/wh =0.57) with a weakly concave venter, an angular ventrolateral shoulder, flattened and convergent flanks, a broadly rounded umbilical margin and a convex and oblique umbilical wall. The last whorl hardly embraces the preceding; the whorl overlap zone is extremely small ( Fig. 5B View Fig ). The specimen shows ontogenetic changes in the conch geometry quite clearly. This can mainly be seen in the lowering of the coiling rate; at a conch diameter of 50 mm, the whorl expansion rate is 2.73, at 80 mm diameter 2.72 and at 134 mm diameter only 2.58.

The conch of the holotype is largely void of sculpture. Only the first whorl has small tubercles on the outer flank. The suture line is characterised by a very small external lobe (owing to the narrow venter) and a much larger, broadly rounded lateral lobe ( Fig. 5C View Fig ). The length of the chambers decreases on the last whorl of the phragmocone. On the first quarter of this whorl there are six septa, on the second seven, on the third there are already nine and on the last there are eleven (CLI decreases from 15 to 8 during one volution). This increasing septal crowding is probably an indication of adulthood.

Paratype MB.C.30227 is a rather well-preserved, fully chambered internal mould with a conch diameter of 71 mm ( Fig. 6B View Fig ). It largely agrees with the holotype with regard to its conch geometry, if the same conch dimensions are taken into account. However, paratype MB.C.30227 shows a slightly narrower umbilicus (uw/dm= 0.34 at 71 mm dm). The shape of the umbilical margin changes during ontogeny. Up to a diameter of 30 mm it is still subangular; with increasing growth of the conch, it becomes less and less distinct and at 71 mm diameter it is evenly rounded ( Fig. 6A View Fig ).

The paratype shows a sculpture with small ventrolateral tubercles in the juvenile stage; these tubercles are clearly visible up to a conch diameter of 30 mm. The suture line shows a small rounded external lobe, a broadly rounded lateral lobe about twice as deep and a very small umbilical lobe ( Fig. 6C View Fig ).

Remarks

Domatoceras canonium sp. nov. differs from D. ocomphalum sp. nov. and D. myloide sp. nov. in the shape of the umbilicus. Its wall is convex and oblique in D. canonium but flattened and steep in D. ocomphalum and D. myloide . The umbilical margin is rounded in D. canonium but subangular in D. ocomphalum and D. myloide . Furthermore, D. canonium possesses ventrolateral tubercles in the juvenile stage, which are absent in the other two species.

Domatoceras canonium sp. nov. differs from the species known from Julfa, such as D. elegantulum and D. multituberculatum , in the extremely small whorl overlap. Another difference is the extremely high coiling rate of about 2.75 at a conch diameter of about 80 mm in D. canonium , while this rate reaches only 2.60 in the species from Julfa.