Stockumites antecedens (Vöhringer, 1960) comb. nov.

Fig. 41; Tables 36–37

Imitoceras prorsum antecedens Vöhringer, 1960: 140, pl. 2 fig. 6, text-fig. 18.

Acutimitoceras prorsum antecedens – Kullmann 1983: 234, text-fig. 2b.

Acutimitoceras antecedens – Becker 1988: 205, pl. 2 figs 14–15. — Korn 1994: 42, text-figs 49d, 50c–d, 52b, 54e. — Kullmann 2000: text-fig. 4j.

Acutimitoceras (Stockumites) antecedens – Sprey 2002: 52, pl. 3 fig 6, text-fig. 17g. — Kullmann 2009: text-fig. 3.5.

Diagnosis

Species of Stockumites with a conch reaching 40 mm diameter. Conch at 4 mm dm thinly discoidal, evolute (ww/dm ~0.35; uw/dm ~0.60); at 12 mm dm thickly discoidal, involute (ww/dm ~0.50; uw/ dm ~0.10); at 20 mm dm thickly discoidal, subinvolute (ww/dm ~0.50; uw/dm ~0.20). Whorl profile at 20 mm dm weakly compressed (ww/wh ~0.90); coiling rate moderately high (WER ~1.95). Venter rounded, umbilical margin rounded. Growth lines fine, narrow-standing, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with narrowly V-shaped external lobe and narrowly V-shaped adventive lobe.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5; Vöhringer Coll .; illustrated by Vöhringer (1960, pl. 2 fig. 6) and Korn (1994, text-fig. 49d); re-illustrated here in Fig. 41A; GPIT-PV-63907.

Paratypes

GERMANY • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3d; Vöhringer Coll.; GPIT-PV-63908 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5; Vöhringer Coll.; GPIT-PV-63910 .

Description

Holotype GPIT-PV-63907 is a rather small specimen with only 15 mm conch diameter (Fig. 41A). It is thickly discoidal with an open umbilicus (ww/dm = 0.50; uw/dm = 0.20). The whorl profile is slightly depressed (ww/wh = 1.09) with broadly rounded venter and flanks as well as a broadly rounded umbilical margin.

The ornament consists of fine growth lines running with a shallow convex curve across the flanks and forming a shallow ventral sinus (Fig. 41E). In the suture line, the external lobe and the adventive lobe are about the same width. However, their shape differs; the external lobe is lanceolate, while the adventive lobe is V-shaped (Fig. 41C).

The two cross sections of paratypes GPIT-PV-63910 and GPIT-PV-63908 are very similar in the very evolute inner whorls (Fig. 41B–C), but differ markedly in the growth phase above 4 mm diameter. Paratype GPIT-PV-63910, which comes from the type horizon (bed 5) shows a rapidly increasing coiling rate up to nearly 2.00 at 10 mm diameter, while paratype GPIT-PV-63908 has a much lower coiling rate that not even reaches a value of 1.75.

The growth trajectories of the parameters ww/dm and uw/dm demonstrate their considerable ontogenetic changes (Fig. 41F–H). Due to the very evolute juvenile stage, the uw/dm ratio can reach a value of almost 0.60; the whorl profile is crescent-shaped. The stronger overlap of the volutions starting at about 7 mm conch diameter leads to a whorl profile, which gradually changes into an almost circular and finally into a horseshoe-shaped outline.

Remarks

Stockumites antecedens has, with respect to its conch morphology, a marginal position within the genus. It is the species that retains the open umbilicus of the juvenile stage the longest in ontogeny; even at about 15 mm diameter the conch is not closed (ww/dm = 0.20), while the umbilicus is already more or less completely closed in most of the other species of the genus.

Stockumites prorsus also has an open umbilicus at comparable sizes; however, this species differs from A. antecedens by the coarse, biconvex growth lines. From the basal Carboniferous strata of the Anti-Atlas, the two species S. saharae and S. endoserpens are known(Ebbighausen & Bockwinkel 2007); both also have a very evolute juvenile conch, but close the umbilicus earlier in ontogeny than S. antecedens .