Nicimitoceras subacre ( Vöhringer, 1960 )

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Oberrödinghausen (Early Carboniferous; Rhenish Mountains, Germany), European Journal of Taxonomy 882, pp. 1-230 : 97-100

Nicimitoceras subacre ( Vöhringer, 1960)

Figs 59–60 ; Tables 55–56

Imitoceras subacre Vöhringer, 1960: 120 , pl. 1 fig. 5, text-fig. 2.

Imitoceras subacre – Weyer 1977: 169, pl. 2 fig. 5.

Nicimitoceras subacre – Korn 1993: 585; 1994: 53, text-figs 58c–d, 59c, 60c, 64d.

non Imitoceras (Imitoceras) subacre – Ruan 1981: 73, pl. 16 figs 13–15, 18–20.

Diagnosis

Species of Nicimitoceras with a conch reaching 70 mm diameter. Conch thinly pachyconic, involute at 5 mm dm (ww/dm ~0.65; uw/dm ~0.10); thickly discoidal, involute at 15 mm dm (ww/dm ~0.55; uw/ dm ~0.00); thinly discoidal, involute at 30 mm dm (ww/dm ~0.40; uw/dm = 0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~0.70); coiling rate high (WER ~2.10). Venter broadly rounded in the early stage, narrowly rounded in the preadult and adult stage. Growth lines extremely fine, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with V-shaped external lobe and twice as deep, symmetrically V-shaped adventive lobe with blunt base.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3b/c; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 1 fig. 5) and Korn (1994: text-fig. 58c); re-illustrated here in Fig. 59B ; GPIT-PV-63854.

Paratypes

GERMANY • 3 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3e; Vöhringer Coll.; GPIT-PV-63852, GPIT-PV-63853, GPIT-PV-63855 .

Additional material

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3d1b; Weyer 1993–1994 Coll.; MB.C.31130.1–2 .

Description

Holotype GPIT-PV-63854 is a rather well-preserved specimen with almost 33 mm conch diameter, showing both the shell ornament and the suture line ( Fig. 59B ). The conch has the shape of a thick lens (ww/wh = 0.37); the umbilicus is closed and the coiling rate is high (WER = 2.04). Remains of the shell indicate that its surface is almost smooth; there are no constrictions or internal shell thickenings. Only parts of the suture line can be seen; there is an adventive lobe that is rounded at its base.

Paratype GPIT-PV-63853 has a corroded body chamber and therefore only very small shell remains are preserved; they show lamellar growth lines in the ventrolateral area. The internal mould does not show constrictions. The suture line has a narrow external lobe with slightly divergent flanks. It is about two thirds the depth of the much wider, blunt and V-shaped adventive lobe ( Fig. 60B ).

Paratype GPIT-PV-63852 also has a corroded body chamber and no shell remains are preserved; moreover, the internal mould does not show constrictions. The specimen shows a weak indication of a narrower venter only at its maximum conch diameter of 39 mm.

Specimen MB.C.31130.1 is an incomplete but otherwise rather well-preserved specimen with nearly 43 mm conch diameter ( Fig. 59C ). It is slightly stouter than the type material (ww/dm = 0.42; ww/wh = 0.72) but otherwise closely resembling. The specimen shows the transition from the continuously rounded venter to the significantly narrowed venter only on the last volution. The shell of the specimen appears to be smooth.

The sectioned paratype GPIT-PV-63855 gives insight into the ontogenetic conch development from the earliest juvenile stage up to a conch diameter of 37 mm; however, some parts of the last whorls had to be reconstructed ( Fig. 60A ). The whorl profile changes continuously during ontogeny. An initial crescent-shaped whorl profile is replaced by a C-shaped (2–4 mm dm) and followed by a horseshoe-shaped (5–8 mm dm) profile; finally, the conch shape is suboxyconic with convex flanks and a subacute venter.

The ontogenetic trajectories of the conch parameters ww/dm and ww/wh are rather simple and show a continuous decrease of the values during ontogeny ( Fig. 60C–D ). Between 2 and 4 mm conch diameter, the ww/dm ratio decreases from ~0.90 to ~0.40 and the ww/wh ratio from ~2.20 to ~0.65. In contrast to this, the WER has a similar value of around 1.70 up to about 5 mm conch diameter, followed by an increase to more than 2.00 at 9 mm dm ( Fig. 60E ). Thereafter it remains rather stable.

Remarks

Nicimitoceras subacre is easily distinguished from the other species of the genus Nicimitoceras , but also from the species of the closely related genus Stockumites , by the very broad, blunt adventive lobe. From N. heterolobatum with similar conch morphology, N. subacre is also distinguished by the subinvolute juvenile stage, which is evolute in N. heterolobatum .

Figures

Fig. 59. Nicimitoceras subacre (Vöhringer, 1960) from the Oberrödinghausen railway cutting, both Vöhringer Coll. A. Paratype GPIT-PV-63853 (Vöhringer Coll.) from bed 3e. B. Holotype GPIT- PV-63854 (Vöhringer Coll.) from bed 3b/c. C. Specimen MB.C.31130.1 (Weyer 1993–1994 Coll.) from bed 3d1b. Scale bar units = 1 mm.

Fig. 60. Nicimitoceras subacre (Vöhringer, 1960) from the Oberrödinghausen railway cutting. A. Cross section of paratype GPIT-PV-63855 from bed 3e. B. Suture line of paratype GPIT-PV-63853 from bed 3e, at ww =12.5 mm, wh =14.0 mm. C–E. Ontogenetic development of the conch width index (ww/ dm), umbilical width index (uw/dm), whorl width index (ww/wh) and whorl expansion rate (WER) of selected specimens. Scale bar units=1 mm.