Ocunautilus coelodesmus Korn & Hairapetian, 2025, gen. et sp. nov.

Ocunautilus coelodesmus gen. et sp. nov. urn:lsid:zoobank.org:act: FC07FF9A-4B9C-4162-8BFD-9B43A2E3052C Fig. 9; Table 5 Diagnosis Species of Ocunautilus gen. nov. with thickly discoidal, subinvolute conch (ww/dm ~ 0.55; uw/dm ~0.25), weakly depressed whorl profile (ww/wh ~1.10) and extraordinarily high coiling rate (WER ~3.40) at a conch diameter of 60 mm. Whorl profile inverted trapezoidal with moderately concave venter, subangular ventrolateral shoulder, flattened and strongly convergent flanks, angular umbilical margin and steep and flat umbilical wall. Whorl overlap small (IZR ~0.08). Without sculpture, but juvenile conch with weak, backwardly directed ribs on the inner flank, ending in low and blunt nodes on the midflank. Suture line with a rather deep, rounded V-shaped external lobe, an angular ventrolateral saddle, a shallow and broadly rounded lateral lobe and a very small umbilical lobe. Etymology From the Greek ‘κόιλος’=‘hollow’ and ‘δεσμός’=‘band, brace’; because of the shape of the venter. Type material Holotype IRAN – Esfahan Province • Baghuk Mountain; Hambast Formation, Araxoceras beds (early Wuchiapingian); 2011; Korn et al. leg.; illustrated in Fig. 9; MB.C.32113. Paratypes IRAN – Esfahan Province • 1 specimen; same data as for holotype; MB.C.32114. Description Holotype MB.C.32113 is an incomplete, almost fully chambered specimen with a conch diameter of 57 mm (Fig. 9B). It is preserved as an internal mould, but shows some shell remains. The conch is, at a conch diameter of 57 mm, thickly discoidal and subinvolute (ww/dm =0.55; uw/dm= 0.23) with an extraordinarily high coiling rate (WER=3.38). There occurs an ontogenetic decrease of the coiling rate; at a conch diameter of 37.5 mm, it has even a value of 3.70. The whorl profile is weakly depressed (ww/ wh= 1.12) with a weakly concave venter that is bordered by a sharp ventrolateral shoulder; the flanks are nearly flat and strongly convergent. The umbilical margin is subangular and the umbilical wall is steep and flat (Fig. 9A). The whorl overlap is small (IZR =0.08). The few preserved shell remains of the holotype show no ornamentation or sculpture. The internal mould, however, has very small tubercles on the angular ventrolateral shoulder in the preadult stage up to a diameter of about 35 mm. The flanks bear very shallow, from the umbilical margin backwardly directed ribs, which strengthen in the middle of the flank to form very low, broad conical nodes. The suture line is dependent on the cross-sectional shape of the whorl; it has a rather deep, rounded V-shaped external lobe, an angular saddle on the ventrolateral shoulder and a broadly rounded lateral lobe (Fig. 9C). The last quarter volution of the phragmocone has about seven chambers (CLI =13). Remarks Ocunautilus coelodesmus gen. et sp. nov. occupies an intermediate position between O. diplodocus gen. et sp. nov. and O. tachytrephus gen. et sp. nov. as far as the concavity of the venter is concerned; in O. diplodocus the venter is much more deeply concave and in O. tachytrephus it is nearly flat. Ocunautilus coelodesmus gen. et sp. nov. has very similar conch proportions to O. tachytrephus, but differs in the concave umbilical wall, which is weakly convex in O. tachytrephus.