Fididomatoceras intracostatum Korn & Ghaderi, 2025, gen. et sp. nov.

Fididomatoceras intracostatum gen. et sp. nov. urn:lsid:zoobank.org:act: 3DDD3943-57AC-4E12-9036-70812D1C1220 Fig. 13; Table 5 Diagnosis Species of Fididomatoceras gen. nov. with thinly discoidal, subinvolute conch (ww/dm ~ 0.40; uw/ dm ~ 0.27), weakly compressed whorl profile (ww/wh ~ 0.90) and extremely high coiling rate (WER ~ 2.55) at a conch diameter of 30 mm. Whorl profile inverted trapezoidal or lyriform with convergent flanks; venter flattened, flanks flattened or slightly concave in the outer area, umbilical margin narrowly rounded. Sculpture in the juvenile stage with blunt ribs on the flanks. Suture line with very narrow external lobe and slightly larger and deeper, broadly rounded lateral lobe. Etymology Combination of the Latin ‘ intra ’ (prepos.) = ‘inner’ and ‘ costatum ’ (adj., n.) =‘ribbed’; because of the juvenile sculpture. Type material Holotype IRAN – East Azerbaijan • Ali Bashi N; Araxoceras Beds of the Julfa Formation (early Wuchiapingian); 2018; Korn et al. leg.; illustrated in Fig. 13A–B; MB.C.32017. Paratype IRAN – West Azerbaijan • 1 specimen; Aras Valley; Araxoceras Beds of the Julfa Formation (early Wuchiapingian); 2018; Ghaderi leg.; illustrated in Fig. 13C–E; MB.C.32018. Description Holotype MB.C.32017 is the better preserved of the two comparatively small specimens (Fig. 13A). It has a conch diameter of 32 mm and is thinly discoidal and subinvolute (ww/dm=0.42; uw/dm=0.26). The coiling rate is extremely high (WER= 2.61) and the imprint zone is small. The whorl profile is weakly compressed (ww/wh =0.89) and shows a narrowly rounded umbilical margin and a steep, slightly flattened umbilical wall. The conch is widest near the umbilical margin; from here the flanks slowly converge towards the angular ventrolateral shoulder, which is preserved as a weakly elevated ridge. The venter becomes flat only in the last half of the volution, beginning at a conch diameter of about 22 mm. Before this, the venter is broadly rounded (Fig. 13B). Shell remains are visible on large areas of the specimen, they show periodically strengthened growth lines extending with a broad arch in backward direction across the flanks and form a deep, angular ventral sinus. Two thirds of the last volution belong to this stage, while the previous ontogenetic stage before possesses shallow, backwardly directed ribs on the flanks. Paratype MB.C.32018 has a conch diameter of 31 mm (Fig. 13C). It is thinly discoidal and subinvolute (ww/dm= 0.39; uw/dm =0.30) with a compressed whorl profile (ww/wh =0.87). The venter is rounded up to a conch diameter of 27 mm and becomes flattened thereafter (Fig. 13D). This means that it reaches the stage with pronounced ventrolateral shoulders and the applanate venter at a larger conch diameter than the holotype. The suture line shows a very shallow, broadly rounded external lobe and a slightly deeper, rounded lateral lobe (Fig. 13E). Remarks Fididomatoceras intracostatum gen. et sp. nov. can hardly be confused with another nautilid species of Transcaucasia because of its very characteristic morphology. Fididomatoceras intracostatum differs from F. gracile gen. et comb. nov. in the presence of ribs in the juvenile stage and in the more distinct ventrolateral shoulder, which is angular in F. intracostatum but narrowly rounded in F. gracile. Furthermore, F. intracostatum has a lower coiling rate (WER <2.75) than F. gracile (WER> 3.00).