Stockumites convexus ( Vöhringer, 1960 )

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 : 64-68

Stockumites convexus ( Vöhringer, 1960)

Figs 7E , 39–40 ; Tables 34–35

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: 30 , fig. 15.

Diagnosis

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.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen railway cutting; bed 6; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 2 fig. 6) and Korn (1994: text-fig. 49a), re-illustrated here in Fig. 39A ; GPIT- PV-63903.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5; Vöhringer Coll.; GPIT-PV-63904, GPIT-PV-63905 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6; Vöhringer Coll.; GPIT-PV-63906 .

Additional material

GERMANY • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 4; Vöhringer Coll.; MB.C.31092 • 2 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 5; Vöhringer Coll.; MB.C.31093.1–2 • 7 specimens; Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 6; Vöhringer Coll.; MB.C.31094.1–7 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , loose material; Korn 1977 Coll.; MB.C.31095 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 6b2; Weyer 1993–1994 Coll.; MB.C.31096 • 1 specimen; Rhenish Mountains, Hasselbachtal ; Hangenberg Limestone , bed 59; Weyer 1993–1994 Coll.; MB.C.5241.1.

Description

Holotype GPIT-PV-63903 is a rather well-preserved specimen with 22 mm diameter; it is largely covered with shell ( Fig. 39A ). The conch is thickly discoidal (ww/dm = 0.51) with an almost closed umbilicus (uw/dm = 0.06) and a high coiling rate (WER = 2.08). The whorl profile is weakly compressed (ww/wh = 0.93) with convex, convergent flanks and a continuously rounded venter. The shell bears rather coarse, lamellar growth lines, which run with a broad convex arc across the flank and form a broad, distinct sinus on the venter ( Fig. 40H ). On the surface of the shell, there are faint constrictions that follow the course of the growth lines.

Paratype GPIT-PV-63904 ( Fig. 39C ) with 20 mm diameter is very similar to the holotype in its conch proportions and also the ornament. It clearly shows that the constrictions are strengthened on the inner side of the shell and lead to very deep constrictions of the internal mould. The suture line has a lanceolate external lobe with very weakly divergent flanks, a slightly wider parabolic ventrolateral saddle and a V-shaped adventive lobe with weakly outwardly curved flanks ( Fig. 40G ).

Smaller specimens, such as paratype GPIT-PV-63905 (12 mm dm; Fig. 39B ) and especially MB.C.31096 (18.5 mm dm; Fig. 39D ) show the transition from the juvenile stage to the middle stage, which is characterised by a rather rapid closure of the umbilicus by increased overlap of the inner flank area upon the umbilicus.

Specimen MB.C.31096 is well-preserved and deserves a detailed description. It has a conch diameter of 18.5 mm ( Fig. 39D ). It is thickly discoidal and involute (ww/dm = 0.46; uw/dm = 0.08), but shows that the umbilicus is strikingly narrowed during the last volution by strong overlap of the dorsal flank area upon the umbilicus. Half a volution before the largest diameter, the uw/dm ratio is 0.17. The umbilical margin is rounded and the flanks converge only slowly towards the broadly rounded venter; the coiling rate is high (WER = 2.08). Almost the entire specimen is covered by shell. This shows delicate growth lines, which extend with a very low and wide projection across the flank and in the ventrolateral area turn back to form a shallow ventral sinus. The shell surface bears some weak constrictions following the course of the growth lines. These constrictions begin in the inner flank area and are deepest in the ventrolateral area.

A series of cross sections demonstrates the modification of the conch from the widely umbilicate, serpenticonic juvenile stage to the discoidal subadult stage ( Fig. 40 A-G). In the middle growth stage, the flanks are almost parallel; the whorl profile is widest in the middle of the convex curved flanks. The ww/dm ratio is almost constant up to a conch diameter of 30 mm ( Fig. 40I ). The morphological variation is particularly evident in the coiling rate. This is caused by the transformation of the serpenticonic into the disc-shaped conch at slightly different conch diameters ( Fig. 40K ).

Remarks

Stockumites convexus is one of the species of the genus in which the transformation of the serpenticonic juvenile stage to the involute, discoidal middle stage occurs only at a comparatively large conch diameter of about 10 mm dm. Of the species from the Rhenish Mountains, only S. antecedens is comparable in this respect, although with an even later onset of change (at 15 mm dm).

Stockumites convexus differs from the co-occurring species S. subbilobatus by the growth lines extending convexly across the flanks, which are either almost straight or slightly biconvex in S. subbilobatus .

Figures

Fig. 7. Early Tournaisian representative ammonoids of the family Prionoceratidae from the Hasselbachtal section, all Weyer 1993–1994 Coll. A. Mimimitoceras hoennense Korn, 1993, specimen MB.C.5241.2 from bed 59. B. Globimitoceras globiforme (Vöhringer, 1960), specimen MB.C.5240.7 from bed 57. C. Stockumites voehringeri sp. nov., paratype MB.C.5235.1 from bed 46. [Illustrated by Korn & Weyer (2003, pl. 1 figs 1–2) as Acutimitoceras cf. intermedium.] D. Stockumites intermedius (Schindewolf, 1923), specimen MB.C.5248.1 from bed 77. E. Stockumites convexus (Vöhringer, 1960), specimen MB.C.5241.1 from bed 59. F. Stockumites parallelus sp. nov., paratype MB.C.5247.2 from bed 57. (Illustrated by Korn & Weyer 2003: pl. 2 figs 6–7, as Acutimitoceras subbilobatum.) Scale bar units =1 mm.

Fig. 39. Stockumites convexus (Vöhringer, 1960) from the Oberrödinghausen railway cutting. A. Holotype GPIT-PV-63903 (Vöhringer Coll.) from bed 6. B. Paratype GPIT-PV-63905 (Vöhringer Coll.) from bed 5. C. Paratype GPIT-PV-63904 (Vöhringer Coll.) from bed 5. D. Specimen MB.C.31096 (Weyer 1993–1994 Coll.) from bed 6b2. Scale bar units =1 mm.

Fig. 40. Stockumites convexus (Vöhringer, 1960) from the Oberrödinghausen railway cutting; all Vöhringer Coll. A. Cross section of paratype GPIT-PV-63906 from bed 6. B. Cross section of specimen MB.C.31094.1 from bed 6. C. Cross section of specimen MB.C.31094.2 from bed 6. D. Cross section of specimen MB.C.31093.1 from bed 5. E. Cross section of specimen MB.C.31092 from bed 4. F. Cross section of specimen MB.C.31094.3 from bed 6. G. Suture line of paratype GPIT-PV-63904 from bed 5, at ww =7.5 mm, wh=7.0 mm. H. Growth line course of holotype GPIT-PV-63903 from bed 6, at dm =16.0 mm, ww ca. 9.0 mm, wh =8.5 mm. I–K. 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.