Serometacoceras verae ( von Arthaber, 1900 ), 2025

Serometacoceras verae ( von Arthaber, 1900) gen. et comb. nov.

Figs 19–20 View Fig View Fig ; Table 11 View Table 11

Pleuronautilus Verae von Arthaber, 1900: 216 , pl. 18 fig. 4.

Pleuronautilus verae – Shimansky 1965a: 41.

Pleuronautilus ( Pleuronautilus) verae – Kummel 1953: 36.

Pleuronautilus dzhulfensis Shimansky, 1965b: 158 , pl. 15 figs 5–6.

Nautilus Pichleri – Abich 1878: 21 View in CoL , pl. 4 fig. 2.

Pleuronautilus sp. indet. 2 – Teichert & Kummel 1973: 418, pl. 4 figs 3–4.

Diagnosis

Species of Serometacoceras gen. nov. with thickly discoidal, subevolute conch (ww/dm ~ 0.50; uw/dm ~ 0.35), weakly depressed whorl profile (ww/wh ~ 1.25) and very high coiling rate (WER =2.40–2.50) at a conch diameter of 50 mm. Whorl profile nearly rectangular, usually with gently convergent flanks; venter broadly convex, flanks flattened. Sculpture with about 15 low ventrolateral nodes per volution; the nodes are connected by slightly curved ribs with weak nodes on the umbilical margin, sometimes with an additional row of nodes on the midflank. Suture line nearly straight on the venter and with a broadly rounded lateral lobe.

Material examined

IRAN – West Azerbaijan • 1 specimen; Aras Valley; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2012; Korn et al. leg.; illustrated in Fig. 19A–C View Fig ; MB.C.32044 • 3 specimens; same data as for preceding; 2018; Ghaderi leg.; MB.C.32046 to MB.C.32048 . – East Azerbaijan • 1 specimen; Ali Bashi main valley ; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2018; Ghaderi leg.; illustrated in Fig. 19D–E View Fig ; MB.C.32045 • 1 specimen; Ali Bashi 4; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2018; Ghaderi leg.; MB.C.32049 • 2 specimens; Ali Bashi; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); illustrated in Fig. 20A–B View Fig ; GLM#GH1002, GLM#GH1004 .

Description

Specimen MB.C.32044 is an incomplete conch, consisting of slightly less than half of a volution, with about 50 mm ( Fig. 19A View Fig ). The specimen allows the examination of the cross section of the last two whorls up to 40 mm diameter ( Fig. 19B View Fig ). Of particular interest is that shell remains are preserved on the last whorl; these show that shell thickenings at the umbilical margin, the inner flank and in the ventrolateral area cause a modification in the whorl profile. These thickenings cause tubercles at the umbilical margin and the middle of the flank. The whorl profile of this specimen shows that the conch is widest at the umbilical margin, from where the flanks slowly converge towards the broadly rounded venter, separated by a distinct but rounded ventrolateral shoulder. The siphuncle has a subcentral position with a slight shift towards the venter. It is also slightly displaced towards the right side of the conch.

Specimen MB.C.32045 is the best in the newly collected material, although only half of a whorl is preserved ( Fig. 19D View Fig ). It has a conch diameter of 52 mm and consists of the last part of the phragmocone and part of the body chamber. It is thinly discoidal and subevolute (ww/dm =0.48; uw/dm = 0.36) with a slightly depressed, subquadrate whorl profile (ww/wh = 1.27). On half of a volution, there are eight short, coarse protracting ribs. They emerge on the inner flank at a short distance from the umbilical margin and become coarser on the flank; they terminate in forming prominent and pointed, radially elongate tubercles at the ventrolateral shoulder. The suture line extends with linear course across the venter and forms a wide and shallow lobe on the flank ( Fig. 19E View Fig ). The septa are densely arranged; they are spaced at intervals averaging about 11 degrees.

Remarks

Serometacoceras verae gen. et comb. nov. was almost always placed in the genus Pleuronautilus by earlier authors; this was because of the rather coarse ribs and rows of nodes on the flanks, which shows some similarity with the type species P. trinodosus Mojsisovics, 1882 . However, the great morphological similarity of S. verae and S. dorashamense gen. et comb. nov. makes such a classification problematic. A separation of the two species into different genera would only make sense if an evolutionary transformation from S. dorashamense to S. verae was the origin of the genus Pleuronautilus . However, the empirical data is not sufficient for such an assumption.

The fragment illustrated by Abich (1878: pl. 4 fig. 2) as “ Nautilus Pichleri von Hauer ” has a whorl height of 18 mm and belongs to an apparently subevolute specimen. It shows straight ribs on the flank; these thicken into nodes in the ventrolateral area. The specimen is most likely to be attributed to Serometacoceras verae gen. et comb. nov.

“ Pleuronautilus dzhulfensis ” was described by Shimansky (1965b) with a rather small holotype of a conch diameter of only 42 mm. This specimen agrees well with the figure of the type specimen of S. verae gen. et comb. nov. in both conch geometry and sculpture. S. dzhulfensis should therefore be considered a synonym.

Serometacoceras dorashamense gen. et comb. nov. is a similar species but differs from S. verae gen. et comb. nov. in possessing weaker umbilical nodes; in addition, S. verae sometimes has an additional row of nodes on the middle flank, in contrast to S. dorashamense . Another distinguishing feature is the shape of the venter; it is broadly rounded in S. verae and flattened in S. dorashamense .