Stockumites parallelus Korn & Weyer, 2023

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Gattendorf (Devonian-Carboniferous boundary; Upper Franconia, Germany), European Journal of Taxonomy 883, pp. 1-61 : 27-30

Stockumites parallelus Korn & Weyer, 2023

Figs 13–14 ; Tables 7–8

Imitoceras subbilobatum – Vöhringer, 1960: 135, text-fig. 14.

Acutimitoceras subbilobatum – Korn 1984: 76, pl. 2 figs 13–15, text-fig. 5e; 1992c: 178, pl. 1 figs 2–3, pl. 2 figs 7–8; 1994: 51, text-figs 37d, 39, 42a–c, 50a. — Kullmann 2000: text-fig. 4f. — Korn & Klug 2002: 197, text-fig. 173c, f. — Korn et al. 2003: 1125, text-fig. 3c. — Korn & Weyer 2003: pl. 2 figs 6–7, text-fig. 14d.

Stockumites parallelus – Korn & Weyer 2023: 57 , figs 7f, 35.

? Aganides ornatissimus – Schmidt 1924: 149, pl. 8 figs 3–4.

Diagnosis

Species of Stockumites with a conch reaching 70 mm diameter. Conch at 5 mm dm thinly pachyconic, subinvolute (ww/dm ~ 0.65; uw/dm ~ 0.20); at 15 mm dm thickly discoidal, involute (ww/dm ~ 0.55; uw/dm ~ 0.05); at 30 mm dm thickly discoidal, involute (ww/dm ~ 0.50; uw/dm ~ 0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~ 0.90); coiling rate moderately high (WER ~ 1.90). Venter rounded, umbilical margin rounded, flanks subparallel. Growth lines fine, narrow-standing, with slightly biconvex course. Weak constrictions on the shell surface; coarse internal shell thickenings. Suture line with lanceolate external lobe and narrowly V-shaped adventive lobe (from Korn & Weyer 2023).

Material examined

Holotype GERMANY • Rhenish Mountains , forestry road cutting 900 m east of Stockum; Stockum Limestone ( Stockumites prorsus Zone ); Korn 1982 Coll.; illustrated by Korn (1994: text-fig. 42a); SMF 43083 .

Additional material GERMANY • 26 specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; BGRB X13392 – BGRB X13395 , BGRB unnumbered • 1specimen; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21a (“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31267 • 1 specimen; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21c–d (“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31268 .

Description

The three specimens shown here, MB.C.31267 (30 mm dm; Fig. 13A ), BGRB X13392 (26 mm dm; Fig. 13B ) and BGRB X13393 (25 mm dm; Fig. 13C ), allow the study of the middle growth stage. The variation of the conch parameters is low; an ontogenetic trend towards a more slender conch is visible. At about 30 mm conch diameter, the conch is thinly discoidal (ww/dm =0.40–0.45) with a nearly completely closed umbilicus. All three show constrictions of the shell surface, which, however, become fainter with increasing size of the specimens. On the internal mould, the constrictions are considerably deeper because the shell constrictions are reinforced internally by thickening of the shell.

The cross sections of the three specimens MB.C.31268, BGRB X13394 and BGRB X13395 show very similar pictures ( Fig. 14A–C ). The innermost whorls are not preserved, but it can be assumed that they are evolute. The whorl profile is horseshoe-shaped at 6 mm, conch diameter and the widest part is on the inner half of the flank at some distance from the rather narrowly rounded umbilical margin. In the ontogenetic interval between 4 and 52 mm conch diameter, the growth trajectories are rather simple. The ww/dm ratio decreases from about 0.55 to 0.40; at the same time the ww/wh ratio decreases from 1.50 to 0.65 ( Fig. 14D ).

Remarks

As can be seen from the synonymy list, specimens of the new species were often assigned to Stockumites subbilobatus . However, the holotype of S. subbilobatus has no constrictions and is stouter than S. parallelus . With a conch diameter of 30 mm, the width-to-diameter ratio is about 0.45 for S. parallelus , but about 0.50 for S. subbilobatus .

Stockumites parallelus has the most slender conch of the genus at Gattendorf; already at a diameter of 20 mm the ww/dm ratio is less than 0.50. Another criterion good for distinguishing S. parallelus from the other species of the assemblage are the conspicuous shell constrictions, which are furthermore reinforced on the inner side of the shell and therefore lead to very deep constrictions of the internal mould.

Figures

Fig. 13. Stockumites parallelus Korn & Weyer, 2023 from Gattendorf. A. Specimen MB.C.31267 (Korn 1989 Coll.) from bed 21a. B. Specimen BGRB X13392 (Schindewolf 1934 Coll.) from bed 21. C. Specimen BGRB X13393 (Schindewolf 1934 Coll.) from bed 21. Scale bar units =1 mm.

Fig. 14. Stockumites parallelus Korn & Weyer, 2023 from Gattendorf. A. Cross section of specimen MB.C.31268 (Korn 1989 Coll.) from bed 21c–d. B. Cross section of specimen BGRB X13394 (Schindewolf 1934 Coll.) from bed 21. C. Cross section of specimen BGRB X13395 (Schindewolf 1934 Coll.) from bed 21. D–F. 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.