Lutonautilus elachus Korn & Hairapetian, 2025, gen. et sp. nov.

Lutonautilus elachus gen. et sp. nov. urn:lsid:zoobank.org:act: 7AC16980-96B5-465E-A223-3295CE49F01E Fig. 21; Table 15 Pleuronautilus sp. – Korn et al. 2021: text-fig. 17c. Diagnosis Species of Lutonautilus gen. nov. with thickly discoidal, subevolute conch (ww/dm ~ 0.45; uw/dm ~0.32), weakly depressed whorl profile (ww/wh ~ 1.15) and very high coiling rate (WER ~ 2.50) at a conch diameter of 50 mm. Whorl profile with nearly parallel flanks; venter flattened, flanks slightly flattened, umbilical margin narrowly rounded, umbilical wall flat. Sculpture with about 20 blunt lateral ribs per volution. Suture line with a shallow external lobe and a shallow, broadly rounded lateral lobe. Etymology From the Greek ‘ἐλαχύς’=‘small’; because of the rather small conch. Type material Holotype IRAN – Esfahan Province • Baghuk Mountain; Hambast Formation, Vedioceras beds (late Wuchiapingian); 2011; Korn et al. leg.; illustrated in Fig. 21A–C; MB.C.30225. Paratype IRAN – Esfahan Province • 1 specimen; same data as for holotype; 2014; Korn et al. leg.; illustrated in Fig. 21D–E; MB.C.32130. Description Holotype MB.C.30225 is a somewhat corroded, largely chambered specimen with a conch diameter of 48 mm (Fig. 21A). The conch is thickly discoidal and subevolute (ww/dm= 0.47; uw/dm =0.31) with a very high coiling rate (WER=2.50). The whorl profile is depressed rectangular (ww/wh =1.18) with flattened venter and almost parallel flanks. Both the ventrolateral shoulder and the umbilical margin are rounded; the umbilical wall is flat and perpendicular to the plane of symmetry. The sculpture of the holotype is not well-preserved because of corrosion. However, about 20 blunt ribs are present; the ribs appear to be wider than their interspaces. These ribs start on the umbilical margin and run with very shallow sinus across the flank. They become coarser in the middle of the flank and form radially elongated nodes in the ventrolateral area. The suture line shows a very broad, shallow external lobe and a slightly deeper lateral lobe (Fig. 21C). Remarks Lutonautilus elachus gen. et sp. nov. differs from L. cratus gen. et sp. nov. in the stouter whorl profile (ww/wh ~ 1.15 in L. elachus but only 0.95–1.05 in L. cratus), the nearly parallel flanks (convergent in L. cratus), the narrower umbilicus (uw/dm ~ 0.30 in L. elachus but 0.35–0.40 in L. cratus). Furthermore, the ribs begin at the umbilical margin in P. elachus, but in the outer area of the umbilical wall in L. cratus. Lutonautilus elachus gen. et sp. nov. differs from L. cymus gen. et sp. nov. in the shape of the ribs, which in L. elachus are almost straight on the flanks, while they extend with a concave curve in L. cymus. The ribs of L. elachus are coarse in the midflank area, while they are weakly developed in the middle of the flank in L. cymus and reinforced like nodes on the outer and inner flanks.