Stockumites similis Korn & Weyer 2023, comb. nov.
Creators: name: Korn, Dieter
name: Weyer, Dieter
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Goniatitida
Prionoceratidae
Stockumites
Stockumites similis

description: Stockumites similis (Vöhringer, 1960) comb. nov.
Figs 48–49; Tables 42–43
Imitoceras liratum simile Vöhringer, 1960: 127, pl. 2 fig. 4, text-fig. 7.
Prionoceras (Imitoceras) liratum simile – Weyer 1965: 446, pl. 8 figs 1–2.
Acutimitoceras simile – Korn 1994: 49, text-figs 46a–b, 47d, 50g, 54a.
non Imitoceras (Imitoceras) simile – Ruan 1981: 75, pl. 16 figs 10–12.
Diagnosis
Species of Stockumites with a conch reaching 40 mm diameter. Conch at 5 mm dm thickly pachyconic, involute (ww/dm ~0.75; uw/dm ~0.10); at 15 mm dm thinly pachyconic, involute (ww/dm ~0.65; uw/dm ~0.00); at 30 mm dm thickly discoidal, involute (ww/dm ~0.55; uw/dm ~0.02). Whorl profile at 30 mm dm weakly depressed (ww/wh ~1.00); coiling rate moderately high (WER ~1.95). Venter broadly rounded, umbilical margin broadly rounded. Growth lines very fine, narrow-standing, with convex course. Weak constrictions on the shell surface; weak internal shell thickenings. Suture line with narrowly lanceolate external lobe and narrowly V-shaped adventive lobe.
Material examined
Holotype
GERMANY • Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 1; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 2 fig. 4) and Korn (1994: text-fig. 46b); re-illustrated here in Fig. 48B; GPIT-PV-63894.
Paratypes
GERMANY • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 1; Vöhringer Coll.; GPIT-PV-63874, GPIT-PV-63878 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3c; Vöhringer Coll.; GPIT-PV-63876.
Additional material
GERMANY • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone,bed2;Vöhringer Coll.;MB.C.31114.1–2 • 1specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2a; Weyer 1993–1994 Coll.; MB.C.31115 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2b; Weyer 1993–1994 Coll.; MB.C.31116 • 2 specimens; Rhenish Mountains, Oberrödinghausen, road cutting; Hangenberg Limestone, bed 2; Korn 1977 Coll.; MB.C.31117.1–2 • 1 specimen; Rhenish Mountains, Oberrödinghausen, road cutting; Hangenberg Limestone, bed 1; Korn & Weyer 2000 Coll.; MB.C.31118.
Description
Holotype GPIT-PV-63894 is a specimen with partially preserved shell, 29 mm in diameter (Fig. 48B). It is thickly discoidal (ww/dm = 0.56) with an almost completely closed umbilicus and a moderate coiling rate (WER = 1.85). The whorl profile is characterised by a broadly rounded venter that merges continuously into the broadly rounded flanks. The umbilical margin is also evenly rounded. The shell surface bears extremely fine growth lines extending with a convex arch across the flanks. Broad but shallow shell constrictions run parallel to the growth lines (Fig. 49D).
Paratype GPIT-PV-63874 with 39 mm conch diameter is more slender (ww/dm = 0.51) and has a higher coiling rate (WER = 2.04). This specimen has very fine growth lines and shell constrictions, which are additionally reinforced on the inner side of the shell and therefore cause deep constrictions of the internal mould (Fig. 48A). The suture line of paratype GPIT-PV-63876 shows a lanceolate external lobe with weakly divergent flanks. It is almost as wide as the narrowly rounded ventrolateral saddle and the V-shaped adventive lobe (Fig. 49C).
The cross sections of specimen MB.C.31115 and paratype GPIT-PV-63878 allow the study of conch ontogeny up to a diameter of 30 mm (Fig. 49 A-B). The whorl profile is crescent-shaped up to about 2 mm in diameter; after that, a transformation into a C-shaped and finally horseshoe-shaped form takes place very quickly. Both cross sections show very similar trajectories; the difference in the early juvenile stage could be due to a non-central location of the section of paratype GPIT-PV-63878.
Remarks
Stockumites similis was introduced as a subspecies of the Late Devonian species “ Imitoceras liratum ” by Vöhringer (1960). However, apart from being prionoceratids, these two species share only the thickly discoidal to pachyconic conch with shell constrictions, while the inner whorls have a very dissimilar morphology. Furthermore, the adult conch has a very low aperture in Mimimitoceras liratum; the WER is only around 1.50 in contrast to 1.95 in S. similis.
Stockumites similis differs from S. exilis by the stouter conch; at 15 mm conch diameter, the ww/dm ratio is 0.65 for S. similis, but only 0.55 for S. exilis. In addition, the coiling rate is apparently higher in S. similis than in S. exilis (WER ~ 1.95 in S. similis but only ~ 1.75 in S. exilis).
Other species with shell constrictions are S. convexus and S. parallelus sp. nov. Both species are more slender at 15 mm conch diameter (ww/dm = 0.55). In addition, both bear distinctly coarser growth lines, which are weakly biconvex in S. parallelus in contrast to S. similis.
description: 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 78-81, DOI: 10.5852/ejt.2023.882.2177, http://zenodo.org/record/8177581