Tainoceras hystatum Korn & Hairapetian, 2025, sp. nov.

Tainoceras hystatum sp. nov. urn:lsid:zoobank.org:act: BE14C8AB-8E7D-4D26-B1BA-64AFA2B52E70 Fig. 23; Table 17 Diagnosis Species of Tainoceras with thinly discoidal, evolute conch (ww/dm ~ 0.44; uw/dm ~ 0.32), weakly depressed whorl profile (ww/wh ~1.05) and extremely high coiling rate (WER ~ 2.55) at a conch diameter of 40 mm. Whorl profile octagonal with shallow longitudinal ventral groove. Sculpture with two rows of tubercles on the ventrolateral shoulder and one row on the umbilical margin. Suture line undulating with very shallow lobes and low saddles. Etymology From the Greek ‘ὕστατος’=‘latest’; because the species is the stratigraphically latest of the nautiloids from Baghuk Mountain. Type material Holotype IRAN – Esfahan Province • Baghuk Mountain section H; Hambast Formation, Paratirolites beds (late Changhsingian), 0.40 m below top; 2013; Korn et al. leg.; illustrated in Fig. 23; MB.C.32134. Paratype IRAN – Esfahan Province • 1 specimen; same data as for holotype; 2011; Korn et al. leg.; MB.C.32135. Description Holotype MB.C.32134 is a somewhat corroded specimen with a conch diameter of 40 mm (Fig. 23A). It is fairly well preserved only on its left side as an internal mould. The specimen is completely chambered. The conch is thinly discoidal and subevolute (ww/dm=0.43; uw/dm= 0.33) with an extremely high coiling rate (WER =2.55). The whorl profile is generally octagonal and slightly depressed (ww/wh=1.09). It has a flattened venter with a shallow longitudinal depression; the ventrolateral shoulder is subangular and the flanks are almost flat and parallel. The umbilical margin is prominent and the umbilical wall is oblique and flat. The areas between the angular margins of the whorl profile are flattened (Fig. 23B). The sculpture consists of three rows of tubercles, one on the umbilical margin, one on the ventrolateral shoulder and one delimiting the midventral longitudinal groove. The first and the last ones are only very weakly developed; the middle one is much stronger and consists of blunt conical nodes. They are connected to the tubercles on the umbilical wall by very low ribs. The suture line is strongly dependent on the whorl profile; it has shallow lobes at the flattened areas and low saddles at the pronounced margins (Fig. 23C). Remarks Tainoceras hystatum sp. nov. shows a morphology of conch and sculpture that can be described as conservative. Even some of the Late Carboniferous species, such as the type species T. quadrangulum, show an octagonal shape of the whorl profile with two rows of tubercles each adjacent to the median longitudinal groove. However, T. quadrangulum has coarser ventrolateral nodes and a narrower umbilicus (uw/dm <0.30) than T. hystatum (uw/dm>0.30). Tainoceras hystatum sp. nov. differs by its wider umbilicus and particularly by the parallel flanks, which are either convergent or divergent in many other species of the genus Tainoceras. Another distinguishing feature is the shape of the tubercles, which are much smaller in T. hystatum than in most other species.