Domatoceras elegantulum, Korn & Ghaderi, 2025

Domatoceras elegantulum sp. nov.

urn:lsid:zoobank.org:act:

Figs 7–8 View Fig View Fig ; Table 1 View Table 1

Nautilus parallelus – von Arthaber 1900: 213, pl. 18 fig. 2.

Domatoceras parallelus – Shimansky 1965a: 41, pl. 15 fig. 10.

Domatoceras parallelum – Teichert & Kummel 1973: 421, pl. 2 figs 1–2, 9–10. — Gliwa et al. 2020: text-fig. 17a.

Diagnosis

Species of Domatoceras with extremely discoidal, subevolute conch (ww/dm ~0.25; uw/dm ~ 0.35), weakly compressed, inverted trapezoidal whorl profile (ww/wh ~0.70) and very high to extremely high coiling rate (WER =2.30–2.60) at a conch diameter of 75–100 mm. Whorl profile with flattened venter, subangular ventrolateral shoulder, gently convergent, flattened flanks and rounded umbilical margin. Ornament with fine growth lines, without ribs or nodes. Suture line with a narrow and shallow external lobe and a much larger and deeper, broadly rounded lateral lobe.

Etymology

The epithet comes from the Latin ‘ elegantulum ’ (adj., n.) = ‘very fine’; because of the elegant conch shape.

Type material

Holotype

IRAN – West Azerbaijan • Aras Valley; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2011; Korn et al. leg.; illustrated in Fig. 7 View Fig ; MB.C.29346 .

Paratypes

IRAN – West Azerbaijan • 1 specimen; same data as for holotype; illustrated in Fig. 8A–D View Fig ; MB.C.31989 • 4 specimens; same data as for holotype; 2018; Ghaderi leg.; MB.C.31993 to MB.C.31996 . – East Azerbaijan • 1 specimen; Zal; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian ); 2018; Ghaderi leg.; illustrated in Fig. 8E–F View Fig ; MB.C.31990 • 1 specimen; Ali Bashi 4; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2010; Korn et al. leg.; MB.C.31991 • 1 specimen; Ali Bashi N; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2011; Korn et al. leg.; MB.C.31992 .

Description

Holotype MB.C.29346 is the largest of the fairly complete specimens ( Fig. 7B View Fig ). It has a conch diameter of nearly 100 mm and is chambered except for a short segment that belongs to the body chamber. The conch is extremely discoidal (ww/dm =0.26) and subevolute (uw/dm= 0.36) with a very high coiling rate (WER=2.32). The whorl profile is weakly compressed (ww/wh =0.71) and inverted trapezoidal; it is widest at the rounded umbilical margin, from where the flattened flanks slowly converge towards the subangular ventrolateral shoulder ( Fig. 7A View Fig ). The venter is flat at the largest diameter but slightly concave earlier on the last preserved volution. The suture line shows a rather narrow and broadly rounded external lobe with flanks diverging slightly more than 90 degrees ( Fig. 7C View Fig ). The curvature of the ventrolateral saddle is similar to that of the external lobe and is positioned at the ventrolateral shoulder. The flanks are occupied by the broadly rounded lateral lobe, which is about half as deep as wide. The last volution of the phragmocone has 22 septa, which are rather regularly spaced ( SD ~16 degrees).

Paratype MB.C.31989 ( Fig. 8A–C View Fig ) is a nearly complete internal mould specimen with a conch diameter of 75 mm. Only the last whorl is fairly well preserved, two thirds of this belong to the phragmocone and one third to the body chamber. The specimen closely resembles, in the conch proportions, the holotype and is extremely discoidal and subevolute with a compressed whorl profile (ww/dm=0.26; uw/dm= 0.35; ww/wh= 0.66). The venter changes from flatly rounded to nearly flat during the last volution; in parallel, the ventrolateral shoulder changes from narrowly rounded to subangular. The flanks are flattened and slowly converge toward the venter. The suture line has a very shallow external lobe and a shallow lateral lobe ( Fig. 8D View Fig ). The last half volution of the phragmocone possesses 13 septa standing in rather regular distances ( SD ~14 degrees).

The smaller fragment MB.C.31990 ( 14 mm whorl height) provides an insight in the juvenile outline of the whorl profile and the suture line ( Fig. 8F View Fig ). It demonstrates that the shape of the whorl profile and also the course of the suture line probably does not change much during ontogeny.

Remarks

The new species has some resemblance with “ Nautilus parallelus Abich, 1878 ”, but it can be seen from the description and illustration by Abich (1878: 17, pl. 2 fig. 2) that the umbilicus is rather wide (uw/ dm ~0.45) in that species and that the flanks are almost parallel. This species has thus a much wider umbilicus than D. elegantulum sp. nov. (uw/dm ~ 0.35) and differs also in the parallel flanks (convergent in D. elegantulum ).

Domatoceras elegantulum sp. nov. differs from D. multituberculatum sp. nov. in the lack of ventrolateral nodes of the inner whorls, the slightly less evolute (uw/dm ~ 0.35 in contrast to ~ 0.38) and slenderer conch, the more compressed whorl profile and the broadly rounded umbilical margin.

Domatoceras elegantulum sp. nov. differs from D. convergens by the less strongly convergent flanks and the slightly wider whorl profile (ww/wh ~ 0.65 in contrast to D. convergens with ~ 0.60) at a conch diameter of 75 mm. The venter is much wider in D. elegantulum when compared to D. convergens .