Fididomatoceras gracile (Shimansky, 1965), 2025

Korn, Dieter & Ghaderi, Abbas, 2025, Late Permian nautiloids from Julfa (NW Iran), European Journal of Taxonomy 1018, pp. 1-113 : 22-23

Fididomatoceras gracile (Shimansky, 1965) gen. et comb. nov.

Fig. 12 ; Table 4

Domatoceras gracile Shimansky, 1965b: 160 , pl. 16 fig. 1.

Diagnosis

Species of Fididomatoceras gen. nov. with thinly discoidal, subinvolute conch (ww/dm ~0.35; uw/dm ~0.25), weakly compressed whorl profile (ww/wh ~0.70) and extremely high coiling rate (WER ~ 3.05) at a conch diameter of 40–55 mm. Whorl profile inverted trapezoidal with convergent flanks; venter and flanks flattened, umbilical margin broadly rounded. Suture line with broadly rounded, moderately deep external lobe and slightly larger and deeper, broadly rounded lateral lobe.

Type material

Holotype

AZERBAIJAN • Dorasham 1; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); illustrated by Shimansky (1965b: pl. 16 fig. 1); PIN 1572/211 .

Material examined

IRAN – East Azerbaijan • 1 specimen; Ali Bashi N; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2018; Korn et al. leg.; illustrated in Fig. 12 ; MB.C.32016.

Description

Specimen MB.C.32016 is a fairly complete, but rather poorly preserved specimen with a conch diameter of 58 mm ( Fig. 12A, C ). On the right side it is heavily encrusted by a tabular coral colony. The conch is thinly discoidal and subinvolute (ww/dm=0.35; uw/dm=0.26) with a compressed whorl profile (ww/ wh = 0.70). The venter is weakly flattened and bordered by a subangular ventrolateral shoulder. The umbilical margin is rounded, from here the sinuous flanks converge to the venter ( Fig. 12B ). The suture line has a relatively large and deep, broadly rounded external lobe, a narrowly rounded ventrolateral saddle and a broadly rounded, shallow lateral lobe that occupies the entire flank ( Fig. 12D ).

Remarks

Shimansky (1965b) discussed the external similarity of Fididomatoceras gracile gen. et comb. nov. and the Late Carboniferous species Stenodomatoceras moorei ( Miller, Dunbar & Condra, 1933) . He stated that differences between these species mainly regard very different conch sizes and the much more involute conch of S. moorei . He thus regarded this as a case of homeomorphy. However, the size of the specimens can hardly be used to distinguish species or even genera. The holotype of F. gracile is only 40 mm in diameter and is probably a preadult specimen. The difference in umbilical width is a better separating criterion; the holotype of S. moorei has, at a conch diameter of 40 mm, a uw/dm ratio of 0.21, whereas this is 0.28 in the holotype of F. gracile .

A hypothetical criterion for differentiation between Fididomatoceras gen. nov. and Stenodomatoceras could be the position of the siphuncle; this is close to the venter in S. moorei and also in S. kleihegei (Miller, Lane & Unklesbay, 1947) . However, the position of the siphuncle in F. gracile gen. et comb. nov. is not known. Stenodomatoceras kleihegei and S. gardi (Murphy, 1970) possess a concave venter bordered by a slightly raised ventrolateral shoulder, which is narrowly rounded or subangular in F. gracile . Because of these differences, but mainly because of the large stratigraphic distance in occurrence of the three known Late Carboniferous North American species of Stenodomatoceras and F. gracile , we follow the interpretation of Shimansky (1965b) here and do not classify F. gracile in Stenodomatoceras . Instead, we place it in the new genus Fididomatoceras described here.

Fididomatoceras gracile gen. et comb. nov. can be easily distinguished from most of the species of the family Domatoceratidae from Transcaucasia by the narrower umbilicus, the more widely embracing whorls and the higher coiling rate (WER> 3.00 in F. gracile , but usually 2.25–2.50 in the other species). Fididomatoceras gracile differs from F. intracostatum gen. et sp. nov. in the absence of ribs in the juvenile stage and in the less distinct ventrolateral shoulder, which is narrowly rounded in F. gracile but angular in F. intracostatum . Furthermore, F. gracile has a higher coiling rate (WER> 3.00) than F. intracostatum (WER <2.75).

Figures

Fig. 12. Fididomatoceras gracile (Shimansky, 1965) gen. et comb. nov., specimen MB.C.32016 (Korn et al. 2018 Coll.) from the Araxoceras Beds of the Julfa Formation at Ali Bashi N. A. Lateral view of left side. B. Reconstruction of apertural view. C. Lateral view of right side. D. Suture line at dm =58.0 mm, ww =17.0 mm, wh =21.0 mm. Scale bar units=1 mm.

Tables

Table 4. Conch dimensions (in mm) and ratios of Fididomatoceras gracile Shimansky, 1965 gen. et comb. nov.

Nr.	dm	ww	wh	uw	ah	ww/dm	ww/wh	uw/dm	WER	IZR
MB.C.32016	58.4	20.5	29.2	15.4	25.0	0.35	0.70	0.26	3.06	0.14
PIN 1572/211	40.0	15.0	20.5	10.0	17.0	0.38	0.73	0.25	3.02	0.17