Stockumites similis ( Vöhringer, 1960 ) Korn & Weyer, 2023

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 : 78-81

Stockumites similis ( Vöhringer, 1960) comb. nov.

Figs 48–49 ; Tables 42–43

Imitoceras liratum simile Vöhringer, 1960: 127 , pl. 2 fig. 4, text-fig. 7.

Prionoceras (Imitoceras) liratum simile – Weyer 1965: 446, pl. 8 figs 1–2.

Acutimitoceras simile – Korn 1994: 49, text-figs 46a–b, 47d, 50g, 54a.

non Imitoceras (Imitoceras) simile – Ruan 1981: 75, pl. 16 figs 10–12.

Diagnosis

Species of Stockumites with a conch reaching 40 mm diameter. Conch at 5 mm dm thickly pachyconic, involute (ww/dm ~0.75; uw/dm ~0.10); at 15 mm dm thinly pachyconic, involute (ww/dm ~0.65; uw/dm ~0.00); at 30 mm dm thickly discoidal, involute (ww/dm ~0.55; uw/dm ~0.02). Whorl profile at 30 mm dm weakly depressed (ww/wh ~1.00); coiling rate moderately high (WER ~1.95). Venter broadly rounded, umbilical margin broadly rounded. Growth lines very fine, narrow-standing, with convex course. Weak constrictions on the shell surface; weak internal shell thickenings. Suture line with narrowly lanceolate external lobe and narrowly V-shaped adventive lobe.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 1; Vöhringer Coll .; illustrated by Vöhringer (1960: pl. 2 fig. 4) and Korn (1994: text-fig. 46b); re-illustrated here in Fig. 48B ; GPIT-PV-63894.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 1; Vöhringer Coll.; GPIT-PV-63874, GPIT-PV-63878 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3c; Vöhringer Coll.; GPIT-PV-63876 .

Additional material

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone,bed2;Vöhringer Coll.;MB.C.31114.1–2 • 1specimen; Rhenish Mountains , Oberrödinghausen , railway cutting; Hangenberg Limestone, bed 2a; Weyer 1993–1994 Coll.; MB.C.31115 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 2b; Weyer 1993–1994 Coll.; MB.C.31116 • 2 specimens; Rhenish Mountains, Oberrödinghausen , road cutting; Hangenberg Limestone , bed 2; Korn 1977 Coll.; MB.C.31117.1–2 • 1 specimen; Rhenish Mountains, Oberrödinghausen , road cutting; Hangenberg Limestone , bed 1; Korn & Weyer 2000 Coll.; MB.C.31118.

Description

Holotype GPIT-PV-63894 is a specimen with partially preserved shell, 29 mm in diameter ( Fig. 48B ). It is thickly discoidal (ww/dm = 0.56) with an almost completely closed umbilicus and a moderate coiling rate (WER = 1.85). The whorl profile is characterised by a broadly rounded venter that merges continuously into the broadly rounded flanks. The umbilical margin is also evenly rounded. The shell surface bears extremely fine growth lines extending with a convex arch across the flanks. Broad but shallow shell constrictions run parallel to the growth lines ( Fig. 49D ).

Paratype GPIT-PV-63874 with 39 mm conch diameter is more slender (ww/dm = 0.51) and has a higher coiling rate (WER = 2.04). This specimen has very fine growth lines and shell constrictions, which are additionally reinforced on the inner side of the shell and therefore cause deep constrictions of the internal mould ( Fig. 48A ). The suture line of paratype GPIT-PV-63876 shows a lanceolate external lobe with weakly divergent flanks. It is almost as wide as the narrowly rounded ventrolateral saddle and the V-shaped adventive lobe ( Fig. 49C ).

The cross sections of specimen MB.C.31115 and paratype GPIT-PV-63878 allow the study of conch ontogeny up to a diameter of 30 mm ( Fig. 49 A-B). The whorl profile is crescent-shaped up to about 2 mm in diameter; after that, a transformation into a C-shaped and finally horseshoe-shaped form takes place very quickly. Both cross sections show very similar trajectories; the difference in the early juvenile stage could be due to a non-central location of the section of paratype GPIT-PV-63878.

Remarks

Stockumites similis was introduced as a subspecies of the Late Devonian species “ Imitoceras liratum ” by Vöhringer (1960). However, apart from being prionoceratids, these two species share only the thickly discoidal to pachyconic conch with shell constrictions, while the inner whorls have a very dissimilar morphology. Furthermore, the adult conch has a very low aperture in Mimimitoceras liratum ; the WER is only around 1.50 in contrast to 1.95 in S. similis .

Stockumites similis differs from S. exilis by the stouter conch; at 15 mm conch diameter, the ww/dm ratio is 0.65 for S. similis , but only 0.55 for S. exilis . In addition, the coiling rate is apparently higher in S. similis than in S. exilis (WER ~ 1.95 in S. similis but only ~ 1.75 in S. exilis ).

Other species with shell constrictions are S. convexus and S. parallelus sp. nov. Both species are more slender at 15 mm conch diameter (ww/dm = 0.55). In addition, both bear distinctly coarser growth lines, which are weakly biconvex in S. parallelus in contrast to S. similis .

Figures

Fig. 48. Stockumites similis (Vöhringer, 1960) from the Oberrödinghausen railway cutting, both Vöhringer Coll. A. Paratype GPIT-PV-63874 from bed 1. B. Holotype GPIT-PV-63894 from bed 1. Scale bar units= 1 mm.

Fig. 49. Stockumites similis (Vöhringer, 1960) from the Oberrödinghausen railway cutting. A. Cross section of specimen MB.C.31115 (Weyer 1993–1994 Coll.) from bed 2a. B. Cross section of paratype GPIT-PV-63878 (Vöhringer Coll.) from bed 1. C. Suture line of paratype GPIT-PV-63876 (Vöhringer Coll.) from bed 3c, at ww=12.2 mm, wh =9.5 mm. D. Constriction course of holotype GPIT-PV-63894 (Vöhringer Coll.) from bed 1, at ww =15.0 mm, wh =13.0 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.