Nicimitoceras trochiforme ( 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 : 94-97

Nicimitoceras trochiforme ( Vöhringer, 1960)

Figs 57–58 ; Tables 53–54

Imitoceras trochiforme Vöhringer, 1960: 119 , pl. 1 fig. 4, text-fig. 1.

Imitoceras trochiforme – Bartzsch & Weyer 1982: 21.

Nicimitoceras trochiforme – Korn 1994: 59, text-figs 58a, 59a–b, 60a–b. — Sprey 2002: 52, pl. 4 fig. 8, text-fig. 17c.

non Nicimitoceras trochiforme – Bockwinkel & Ebbighausen 2006: 107, text-figs 23, 24a–b, e–f.

Diagnosis

Species of Nicimitoceras with a conch reaching 70 mm diameter. Conch thickly pachyconic, involute at 5 mm dm (ww/dm ~0.75; uw/dm ~0.10); thickly discoidal, involute at 15 mm dm (ww/dm ~0.55; uw/dm ~0.02); thinly discoidal, involute at 30 mm dm (ww/dm ~0.45; uw/dm = 0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~0.85); coiling rate high (WER ~2.05). Venter broadly rounded in the early and subadult stage, narrowly rounded in the adult stage. Growth lines extremely fine, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with lanceolate external lobe and twice as deep, asymmetric adventive lobe with steep ventral flank.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3c ; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 1 fig. 4), Korn (1994: text-fig. 58a) and Sprey (2002: pl. 4 fig. 8); re-illustrated here in Fig. 57 ; GPIT-PV-63848.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3d ; Vöhringer Coll.; GPIT-PV-63849, GPIT-PV-63951 • 1 specimen; Rhenish Mountains , Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 2 ; Vöhringer Coll.; GPIT-PV-63992.

Additional material

GERMANY • 1 specimen; Rhenish Mountains , Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 2 ; Vöhringer Coll.; MB.C.31129 .

Description

Holotype GPIT-PV-63848 is a rather complete but somewhat corroded specimen with 56 mm conch diameter ( Fig. 57 ). It is a thinly discoidal conch (ww/dm = 0.37) with a closed umbilicus and a moderately high coiling rate (WER ~1.90). The whorls are widest in the area of the inner flank near the umbilicus; from there the broadly rounded flanks converge to the rather narrowly rounded venter. The body chamber is corroded and there are no shell remains preserved; the internal mould does not show constrictions. The suture line has a small, narrow, lanceolate external lobe. The adventive lobe is almost twice as deep and very asymmetrical with an almost vertical ventral and a distinctly convex dorsal flank ( Fig. 58D ).

The cross sections of the paratypes GPIT-PV-63851 ( Fig. 58B ) and GPIT-PV-63992 ( Fig. 58C ) allow the study of conch morphology up to a diameter of 23 mm. Particularly paratype GPIT-PV-63851 shows the transition from the early juvenile stage (up to about 4 mm dm), which has a crescent-shaped whorl profile, to the middle stage with increasing whorl height. At a conch diameter of 4 mm dm, the umbilicus begins to close rapidly.

The growth trajectories of the sectioned specimens are rather simple in their course. The ww/dm and ww/wh ratios are monophasic with a continuous decrease; for example, the ww/dm ratio decreases from ~1.00 at 1 mm dm to a value of ~0.37 at 56 mm ( Fig. 58E–G ).

Remarks

Nicimitoceras trochiforme has a similar conch like N. heterolobatum , but differs in possessing less widely umbilicate juvenile whorls. The whorl profile of both species also differs; while in N. trochiforme the profile is widest near the umbilicus, in N. heterolobatum it is widest in the midflank area. In addition, the adventive lobe is very asymmetrical in N. trochiforme , but almost V-shaped in N. heterolobatum .

Figures

Fig. 57. Nicimitoceras trochiforme (Vöhringer, 1960), holotype GPIT-PV-63848 (Vöhringer Coll.) from the Oberrödinghausen railway cutting, bed 3c. Scale bar units =1 mm.

Fig. 58. Nicimitoceras trochiforme (Vöhringer, 1960) from the Oberrödinghausen railway cutting, all Vöhringer Coll. A. Cross section of specimen MB.C.31129 from bed 2. B. Cross section of paratype GPIT-PV-63851 from bed 3d. C. Cross section of paratype GPIT-PV-63992 from bed 2. D. Suture line of holotype GPIT-PV-63848 from bed 3c, at ww = 17.0 mm, wh= 21.5 mm. E–G. 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.