Stockumites convexus

Stockumites convexus (Vöhringer, 1960)

Fig. 15; Table 9

Imitoceras prorsum convexum Vöhringer, 1960: 139, pl. 2 fig. 5, text-fig. 17.

Imitoceras prorsum convexum – Weyer 1977: 172, pl. 2 figs 8–9.

Acutimitoceras convexum – Korn 1992b: 16, pl. 2 figs 17–18; 1994: 42, text-figs 49a–c, 50e, 52a, 54d, 56c. — Schönlaub et al. 1992: 16, pl. 5 figs 17–18. — Korn & Weyer 2003: 100, pl. 2 figs 1–2.

Stockumites convexus – Becker & Weyer 2004: 18, text-fig. 3g. — Korn & Weyer 2023: 64, figs7e, 39–40.

Species of Stockumites with a conch reaching 40 mm diameter. Conch at 5 mm dm thickly discoidal to thinly pachyconic, subevolute to evolute (ww/dm=0.55–0.65; uw/dm=0.40–0.50); at 15 mm dm thickly discoidal, involute (ww/dm=0.45–0.55; uw/dm=0.05–0.10); at 25 mm dm thickly discoidal, involute (ww/dm=0.45–0.55; uw/dm ~ 0.00). Whorl profile at 25 mm dm weakly compressed (ww/wh ~ 0.90); coiling rate moderate to high (WER =1.90–2.10). Venter broadly rounded, umbilical margin broadly rounded. Growth lines coarse, wide-standing, with convex course. Weak constrictions on the shell surface; coarse internal shell thickenings. Suture line with lanceolate external lobe and V-shaped adventive lobe.

Holotype GERMANY • Rhenish Mountains, Oberrödinghausen railway cutting; bed 6 (Acutimitoceras acutum Zone); Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 2 fig. 6) and Korn (1994: text-fig. 49a), re-illustrated here in Fig. 15A; GPIT-PV-63903.

Additional material GERMANY • 1 specimen; Upper Franconia, 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1916 Coll.; SMF Mbg.7572 • 1 specimen; Upper Franconia, 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; BGRB X13396.

Specimen BGRB X13396 is a fairly well preserved, only slightly distorted specimen with a diameter of 20 mm (Fig. 15B). It has been preserved almost entirely with the shell and therefore allows the ornament to be studied. The conch is thickly discoidal (ww/dm =0.53) with the umbilicus not completely closed. The whorl profile shows almost parallel, only slightly convergent flanks, a rounded umbilical margin and a continuously rounded venter. The coiling rate is comparatively low (WER =1.81). The shell bears convex, fine growth lines, which are somewhat stronger on the venter than on the flanks. In addition, the shell surface shows four constrictions spaced less than 90 degrees apart. They extend from the umbilical margin across the flanks and venter. The last of these constrictions, however, is restricted to the outer flank and the venter.

Stockumites convexus is one of the rare species in the assemblage from Gattendorf. It differs from most of the other species in its shell constrictions. Only S. parallelus also has shell constrictions, but has a more strongly compressed conch (ww/dm=0.50 at 20 mm dm) than S. convexus (ww/dm=0.55 at 20 mm dm). Furthermore, S. convexus has a slightly opened umbilicus at 20 mm conch diameter, whereas the umbilicus is completely closed in S. parallelus. But the most important is the course of the growth lines and constrictions: they are convex in S. convexus and slightly biconvex in S. parallelus.