Nicimitoceras heterolobatum ( 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 : 102-105

Nicimitoceras heterolobatum ( Vöhringer, 1960)

Figs 62–64 ; Tables 59–62

Imitoceras heterolobatum Vöhringer, 1960: 136 , pl. 3 fig. 4, text-fig. 15.

Prionoceras (Imitoceras) heterolobatum – Weyer 1965: pl. 7 fig. 3.

Nicimitoceras heterolobatum – Korn 1994: 57, text-figs 58b, g, 59d–f, 60e–f.

Acutimitoceras (Streeliceras) heterolobatum – Becker 1996: 37.

Diagnosis (widely umbilicate morphotype)

Species of Nicimitoceras with a conch reaching 70 mm diameter. Conch thinly discoidal, subevolute at 5 mm dm (ww/dm ~0.45; uw/dm ~0.40); thinly discoidal, involute at 15 mm dm (ww/dm ~0.45; uw/ dm ~0.03); thinly discoidal, involute at 30 mm dm (ww/dm ~0.40; uw/dm = 0.02). Whorl profile at 30 mm dm weakly compressed (ww/wh ~0.75); coiling rate moderate (WER ~1.95). 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; internal shell thickenings restricted to the venter and outer flanks. Suture line with lanceolate external lobe and twice as deep, symmetrically V-shaped adventive lobe with angular base.

Diagnosis (narrowly umbilicate morphotype)

Species of Nicimitoceras with a conch reaching 70 mm diameter. Conch thinly discoidal, subinvolute at 5 mm dm (ww/dm ~0.50; uw/dm ~0.20); thinly discoidal, involute at 15 mm dm (ww/dm ~0.50; 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 ~2.05). 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; internal shell thickenings restricted to the venter and outer flanks. Suture line with lanceolate external lobe and twice as deep, symmetrically V-shaped adventive lobe with angular base.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3b/c ; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 3 fig. 4) and Korn (1994: text-fig. 58b); re-illustrated here in Fig. 62 ; GPIT-PV-63893.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2; Vöhringer Coll.; GPIT-PV-63869, GPIT-PV-63895 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3; Vöhringer Coll.; GPIT-PV-63897 • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3e; Vöhringer Coll.; GPIT-PV-63898, GPIT-PV-63977 .

Additional material

GERMANY • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 1; Vöhringer Coll.; MB.C.31134 • 2 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 2 ; Vöhringer Coll.; MB.C.31135.1–2 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3a ; Vöhringer Coll.; MB.C.31136 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3b ; Vöhringer Coll.; MB.C.31137 • 3 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3c ; Vöhringer Coll.; MB.C.31138.1– 3 • 2 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3d; Vöhringer Coll.; MB.C.31139.1–2 • 2 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , loose material; Vöhringer Coll.; MB.C.31140.1–2 • 2 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3a ; Weyer 1993–1994 Coll.; MB.C.31141.1–2 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3b ; Weyer 1993–1994 Coll.; MB.C.31142 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3d2 ; Weyer 1993–1994 Coll.; MB.C.31143 • 1 specimen; Rhenish Mountains, Oese , old quarry ; Hangenberg Limestone , bed 32; Weyer & Korn 2000 Coll.; MB.C.5257.

Description

Holotype GPIT-PV-63893 is a complete conch with a diameter of 42 mm; it is fully septate and partly covered with shell material ( Fig. 62 ). It has the shape of a thick lentil (ww/dm = 0.38), a completely closed umbilicus and a moderately high aperture (WER = 1.98). The shape of the whorl profile shows that this is widest in the inner flank area, from where the flanks weakly converge towards the umbilicus and more strongly converge toward the narrowly rounded venter. The shell is almost smooth and there are no constrictions on shell and internal mould.

The suture line of the holotype has a very small external lobe with parallel flanks and a rounded base. On the flank follow a small inverted U-shaped ventrolateral saddle, 1.5 times as wide as the external lobe and the adventive lobe. This is the dominant element of the suture line; it is twice as deep and wide as the external lobe. It is almost symmetric and V-shaped with weakly curved flanks ( Fig. 63F ).

The material can be separated into two groups with respect to the umbilical width of the inner whorls. In the first group, the uw/dm ratio exceeds a value of 0.50 at 2 mm conch diameter and at 5 mm diameter, the uw/dm ratio is still about 0.40 ( Fig. 63G–I ). In the second group, the uw/dm ratio reaches 0.45 at 2 mm diameter, but already at 5 mm diameter it amounts only 0.20 caused by stronger overlap upon the preceding whorls ( Fig. 64J–L ).

Vöhringer already produced four cross sections, of which he used only one for his publication( Vöhringer 1960: text-fig. 15a). This belongs to the morphogroup with widely umbilicate juvenile whorls. These sectioned specimens are MB.C.31135.1 (9 mm dm; Fig. 63D ), GPIT-PV-63977 (14 mm dm; Fig. 63B ) MB.C.31139.1 (18 mm dm; Fig. 63C ) and GPIT-PV-63897 (36 mm dm, but only up to 18 mm dm sufficiently preserved; Fig. 63A ). Specimen GPIT-PV-63977 shows the very short interval of evolute inner whorls, which reach at a conch diameter of about 2 mm a uw/dm ratio of 0.50 for only two volutions, followed by a rapid decrease of the uw/dm ratio to only 0.03 at 14 mm dm.

The ontogenetic trajectories can be described using the four sectioned specimens. The ww/dm ratio has a weakly triphasic ontogeny, in which the second phase is characterised by a stagnant ww/dm ratio of 0.45–0.50 between 3 and 20 mm conch diameter. The coiling rate shows, after an early juvenile decrease to WER = 1.60 at 2 mm conch diameter, a continuous increase to 2.10 at 20 mm diameter. It remains with minor changes, up to 42 mm diameter, at this level ( Fig. 63G–I ).

The second morphogroup is represented by eight cross sections ( Fig. 64A–H ), most of which were already produced by Vöhringer. They show that the variation is rather small in the ww/dm, uw/dm and ww/wh ratios, while the coiling rate is rather variable. The specimens have the highest uw/dm ratio at 2 mm conch diameter and thereafter, the umbilicus is almost completely closed until a conch diameter of 10 mm.

Remarks

Nicimitoceras heterolobatum does not seem to be a rare species in the section of Oberrödinghausen, but most of the specimens are rather small (less than 20 mm conch diameter). These smaller specimens, of which numerous cross sections have been made, can be classified into two morphogroups based on the width of the umbilicus. The problem with this subdivision is that the holotype cannot be assigned because the geometry of the inner whorls is not known. This problem must remain unsolved at present. The close morphological agreement of the rather large sectioned specimen GPIT-PV-63897 with the holotype suggests that the holotype belongs to the morphogroup with very evolute inner whorls.

There are several species of Nicimitoceras with similar preadult adult morphology but with different ontogenetic trajectories in the Hangenberg Limestone. One of them is N. trochiforme , which possesses a much shorter widely umbilical early juvenile stage than N. heterolobatum , of which specimens of both morphogroups have a longer widely umbilicate juvenile stage.

Figures

Fig. 62. Nicimitoceras heterolobatum (Vöhringer, 1960), holotype GPIT-PV-63893 (Vöhringer Coll.) from the Oberrödinghausen railway cutting, bed 3b/c. Scale bar units=1 mm.

Fig. 63. Nicimitoceras heterolobatum (Vöhringer, 1960) from the Oberrödinghausen railway cutting (widely umbilicate morphotype). A. Cross section of paratype GPIT-PV-63897 from bed 3e. B. Cross section of paratype GPIT-PV-63977 from bed 3c. C. Cross section of specimen MB.C.31139.1 from bed 3d. D. Cross section of specimen MB.C.31135.1 from bed 2. E. Suture line of specimen MB.C.31139.1 from bed 3d, at ww =7.3 mm, wh =8.1 mm. F. Suture line of holotype GPIT-PV-63893 from bed 3b/c, at dm =36.7 mm, ww =16.0 mm, wh= 22.5 mm. G–I. 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.

Fig. 64. Nicimitoceras heterolobatum (Vöhringer, 1960) from the Oberrödinghausen railway cutting (narrowly umbilicate morphotype). A. Cross section of specimen MB.C.31140.1 from unknown bed. B. Cross section of specimen MB.C.31140.2 from unknown bed. C. Cross section of specimen MB.C.31142 from bed 3b. D. Cross section of specimen MB.C.31138.1 from bed 3c. E. Cross section of specimen MB.C.31134 from bed 1. F. Cross section of specimen MB.C.31138.2 from bed 3c. G. Cross section of specimen MB.C.31135.2 from bed 2. H. Cross section of paratype GPIT-PV-63898 from bed 3e. I. Suture line of paratype GPIT-PV-63898 from bed 3e, at dm =9.5 mm, ww =5.8 mm, wh= 5.6 mm. J–L. 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.