Stockumites depressus ( 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 : 81-83

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).

Figures

Fig. 50. Stockumites depressus (Vöhringer, 1960) from the Oberrödinghausen railway cutting, both Vöhringer Coll. A. Paratype GPIT-PV-63886 from bed 3b. B. Holotype GPIT-PV-63872 from bed 2. Scale bar units= 1 mm.

Fig. 51. Stockumites depressus (Vöhringer, 1960) from the Oberrödinghausen railway cutting, all Vöhringer Coll. A. Cross section of paratype GPIT-PV-63889 from bed 2. B. Cross section of paratype GPIT-PV-63887 from bed 2. C. Cross section of specimen MB.C.31119 from bed 2. D. Suture line of holotype GPIT-PV-63872 from bed 2, at dm =23.0 mm, ww =11.2 mm, wh=7.5 mm. E. Growth line course of holotype GPIT-PV-63872 from bed 2, at dm = 23.0 mm, ww =11.2 mm, wh=7.5 mm. F–H. 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.