Stockumites depressus Korn & Weyer 2023, comb. nov.

Stockumites depressus (Vöhringer, 1960) comb. nov. Figs 50–51; Tables 44–45 Imitoceras depressum Vöhringer, 1960: 130, pl. 3 fig. 5, text-fig. 10. Acutimitoceras depressum – Korn 1994: 43, text-figs 44i–j, 45d, f–g, 47c, 48e. — Ebbighausen & Bockwinkel 2007: 131, text-figs 12c–d, 13. non Imitoceras (Imitoceras) depressum – Ruan 1981: 66, pl. 13 figs 10–11. Diagnosis Species of Stockumites with a conch reaching 50 mm diameter. Conch at 5 mm dm globular, subinvolute (ww/dm ~0.85–0.95; uw/dm = 0.15–0.25); at 15 mm dm pachyconic, involute (ww/dm = 0.70–0.80; uw/ dm = 0.05–0.10); at 30 mm dm thinly pachyconic, involute (ww/dm ~0.70; uw/dm ~0.05). Whorl profile at 30 mm dm weakly depressed (ww/wh ~1.35); coiling rate moderately high (WER ~1.90). Venter broadly rounded, umbilical margin broadly rounded. Growth lines fine, wide-standing, with weakly biconvex course. Without constrictions on the shell surface; with weak internal shell thickenings. Suture line with narrowly lanceolate external lobe and V-shaped adventive lobe. Material examined Holotype GERMANY • Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 3 fig. 5) and Korn (1994: text-fig. 44i); re-illustrated here in Fig. 50B; GPIT-PV-63872. Paratypes GERMANY • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2; Vöhringer Coll.; GPIT-PV-63887, GPIT-PV-63889 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3b; Vöhringer Coll.; GPIT-PV-63886. Additional material GERMANY • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2; Vöhringer Coll.; MB.C.31119 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3a; Vöhringer Coll.; MB.C.31120. Description Holotype GPIT-PV-63872 with a diameter of 24 mm is the smaller of two rather well preserved specimens from the type series. It is thinly pachyconic (ww/dm = 0.67) with closed umbilicus and broadly arched flanks and venter (Fig. 50B). The coiling rate is moderately high (WER = 1.87). The shell surface bears fine growth lines that are weakly biconvex on the flank with a dorsolateral projection that is slightly higher than the ventrolateral projection (Fig. 51E). The ventral sinus is broad and shallow. The suture line has a narrow V-shaped external lobe, a somewhat asymmetrical, narrowly rounded ventrolateral saddle and a V-shaped adventive lobe (Fig. 51D). The larger paratype GPIT-PV-63886 with a conch diameter of 31 mm has a somewhat stouter conch shape (ww/dm = 0.71) compared to the holotype. This specimen also has very fine, slightly biconvex growth lines and like in the holotype, shell constrictions and internal shell thickenings are absent (Fig. 50A). The three cross sections shown (Fig. 51A–C) demonstrate the rather large variation within the species. The growth trajectories show only minor ontogenetic changes; up to a conch diameter of 10 mm, growth is even almost isometric. Only thereafter, the ww/dm ratio (and thus the ww/wh ratio) shows a decrease. However, the umbilicus begins to close at about 4 mm conch diameter. Remarks Stockumites depressus has a conch form most similar to that of S. intermedius. However, S. depressus has a rather narrowly rounded umbilical margin, which is clearly different from the very broadly rounded umbilical margin of S. intermedius. The inner whorls are less widely umbilicate in S. depressus when compared to S. intermedius. The growth lines are clearly different; they are fine and with a biconvex course in S. depressus but lamellar and convex in S. intermedius. Other species of Stockumites have either a stouter (S. kleinerae) or more slender conch (most of the other species of the genus).Published as part of Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Oberrödinghausen (Early Carboniferous; Rhenish Mountains, Germany), pp. 1-230 in European Journal of Taxonomy 882 on pages 81-83, DOI: 10.5852/ejt.2023.882.2177, http://zenodo.org/record/817758