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

Ocunautilus tachytrephus gen. et sp. nov. urn:lsid:zoobank.org:act: 3C21BF76-5C82-4F5E-B39F-43499C76950D Fig. 10; Table 6 Diagnosis Species of Ocunautilus gen. nov. with thickly discoidal, subinvolute conch (ww/dm ~ 0.50; uw/dm ~0.25), weakly depressed whorl profile (ww/wh ~1.10) and extraordinarily high coiling rate (WER ~ 3.35) at a conch diameter of 80 mm. Whorl profile inverted trapezoidal with weakly concave venter, angular ventrolateral shoulder, flattened and weakly convergent flanks, angular umbilical margin and steep and weakly convex, flattened umbilical wall. Whorl overlap very small (IZR ~ 0.04). Without sculpture, but first whorl with low, blunt nodes on the ventrolateral shoulder. Suture line with a very shallow external lobe, a subangular ventrolateral saddle, a shallow and broadly rounded lateral lobe and a very small umbilical lobe. Etymology From the Greek ‘ταχύς’= ‘rapid’ and ‘τρέφω’=‘enlarge’; because of the high coiling rate. Type material Holotype IRAN – Esfahan Province • Baghuk Mountain; Hambast Formation, Araxoceras beds (early Wuchiapingian); 2011; Korn et al. leg.; illustrated in Fig. 10; MB.C.32115. Description Holotype MB.C.32115 is a fully chambered, partially corroded specimen with a conch diameter of 85 mm (Fig. 10A). It has slightly more than two volutions; the first volution has a diameter of about 24 mm and an umbilical foramen of about 4 mm diameter. The conch is, at a conch diameter of 85 mm, thickly discoidal and subinvolute (ww/dm = 0.51; uw/dm =0.24) with an extraordinarily high coiling rate (WER= 3.35). Measurements from a position half a volution before result in similar ratios of conch geometry, but there occurs an ontogenetic decrease of the coiling rate; at a conch diameter of 48 mm, it has a value of about 3.50. The whorl profile is weakly depressed (ww/wh =1.08) with a very weakly concave venter that is bordered by a sharp ventrolateral shoulder; the flanks are nearly flat and convergent. The umbilical margin is angular and the umbilical wall is steep and weakly convex. The whorl overlap is very small (IZR =0.04). The specimen shows that the venter is still convex at a conch diameter of 24 mm, although it is already slightly flattened; the venter is completely applanate at a conch diameter of 48 mm (Fig. 10B). The conch is largely free of sculpture and ornament. On the first half of the last volution, the ventrolateral shoulder bears very low and blunt conical nodes. The suture line extends with a broadly rounded external lobe, a subangular ventrolateral saddle, a shallow lateral lobe and a very small umbilical lobe (Fig. 10C). Remarks Ocunautilus tachytrephus gen. et sp. nov. differs from the other two species O. diplodocus gen. et sp. nov. and O. coelodesmus gen. et sp. nov. from Baghuk Mountain in the shape of the umbilical wall (convex in O. tachytrephus, but concave in the other species) and in the only weakly developed concavity of the venter. Ocunautilus tachytrephus has similar conch proportions as O. coelodesmus, but shows a flattening of the venter only at a larger conch diameter. This flattening occurs at about 35 mm in O. tachytrephus, but already at about 20 mm in O. coelodesmus. Pseudotitanoceras armeniacus (Abich, 1878) has similar conch proportions, but differs from O. tachytrephus gen. et sp. nov. by a distinctly concave venter as well as the presence of tubercles on the ventrolateral shoulder and umbilical margin. Some of the specimens without tubercles mentioned by Shimansky (1965) may belong to O. coelodesmus gen. et sp. nov.