Stockumites parallelus sp. nov.

urn:lsid:zoobank.org:act: 0D620A01-E214-47B5-A002-3DAD048B04BC

Figs 7F, 35; Tables 27–29

Imitoceras subbilobatum – Vöhringer, 1960: 135, text-fig. 14.

Acutimitoceras subbilobatum – Korn 1984: 76, pl. 2 figs 13–15, text-fig. 5e; 1992c: 178, pl. 1 figs 2–3, pl. 2 figs 7–8; 1994: 51, text-figs 37d, 39, 42a–c, 50a. — Kullmann 2000: text-fig. 4f. — Korn & Klug 2002: 197, text-fig. 173c, f. — Korn et al. 2003b: 1125, text-fig. 3c. — Korn & Weyer 2003: pl. 2 figs 6–7, text-fig. 14d.

? Aganides ornatissimus – Schmidt 1924: 149, pl. 8 figs 3–4.

Diagnosis

Species of Stockumites with a conch reaching 70 mm diameter. Conch at 5 mm dm thinly pachyconic, subinvolute (ww/dm ~0.65; uw/dm ~0.20); at 15 mm dm thickly discoidal, involute (ww/dm ~0.55; uw/dm ~0.05); at 30 mm dm thickly discoidal, involute (ww/dm ~0.50; uw/dm ~0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~0.90); coiling rate moderately high (WER ~1.90). Venter rounded, umbilical margin rounded, flanks subparallel. Growth lines fine, narrow-standing, with slightly biconvex course. Weak constrictions on the shell surface; coarse internal shell thickenings. Suture line with lanceolate external lobe and narrowly V-shaped adventive lobe.

Etymology

Named after the nearly parallel arrangement of the flanks.

Material examined

Holotype

GERMANY • Rhenish Mountains, forestry road cutting 900 m east of Stockum; Stockum Limestone ( Stockumites prorsus Zone); Korn 1982 Coll.; illustrated by Korn (1994: text-fig. 42a); SMF 43083.

Paratypes

GERMANY • 6 specimens; Rhenish Mountains, forestry road cutting 900 m east of Stockum; Stockum Limestone; Korn 1982 Coll.; SMF 43080–43082, SMF 43084–43086 • 2 specimens; Rhenish Mountains, trench II 950 m east of Stockum; Stockum Limestone; Korn 1982 Coll.; SMF 43087– 43088 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5; Vöhringer Coll.; GPIT-PV-63890 • 5 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6; Vöhringer Coll.; MB.C.31085.1–5 • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5c; Weyer 1993–1994 Coll.; MB.C.31086.1–2 • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6b2; Weyer 1993–1994 Coll.; MB.C.31087 • 1 specimen; Rhenish Mountains, Hasselbachtal; Hangenberg Limestone, bed 76; Weyer 1993–1994 Coll.; MB.C.5247.2 • 1 specimen; Rhenish Mountains, Oese, old quarry; Hangenberg Limestone, lower part; Paproth Coll.; MB.C.5292 .

Description

Holotype SMF 43083 is an exfoliated specimen with 42 mm conch diameter; it suffered from a nearsagittal calcite vein. It is thickly discoidal (ww/dm = 0.47) with an almost closed umbilicus. The internal mould shows two constrictions standing 90 degrees apart, but the last half whorl does not possess a constriction.

Paratype MB.C.31087 (Fig. 35A) is a specimen with 28 mm conch diameter preserved on only half a side; it is obviously thinly discoidal and involute. The shell surface bears widespread growth lines, which are almost straight on the flank and bend back to a ventral sinus in the ventrolateral area. Parallel to the growth lines are shallow constrictions with distances of 90 degrees.

The two sectioned paratypes GPIT-PV-63890 (28 mm dm; Fig. 35B) from Oberrödinghausen, SMF 43081 (22 mm dm; Fig. 35C) from Stockum and MB.C.5292 (19 mm dm; Fig. 35D) from Oese show sections that are largely matching. After two inner whorls with a semi-modular profile, there is a rapid transformation to a horseshoe-shaped profile with subparallel flanks.

Remarks

As can be seen from the synonymy list, specimens of the new species were often identified as Stockumites subbilobatus . However, the holotype of S. subbilobatus has no shell constrictions and is stouter than the specimens assigned here to S. parallelus sp. nov. At a conch diameter of 30 mm, the ww/dm ratio is about 0.45 in S. parallelus, but about 0.50 in S. subbilobatus . The problems with the poor preservation of the holotype of “ Goniatites subbilobatus ” are discussed by Korn & Weyer (2023).

Vöhringer (1960) united material of at least two species under “ Imitoceras subbilobatum ”. On the one hand, there were specimens from the lowermost part of the Hangenberg Limestone (beds 6 and 5), which are indeed conspecific with the original; on the other hand, there were specimens from the middle part of the Hangenberg Limestone (beds 4 to 2), which, however, have no shell constrictions (but only internal shell thickenings) and show more strongly convergent flanks. These are separated here as the new species S. voehringeri sp. nov.

Only a few useful specimens are available from the Hangenberg Limestone of the Rhenish Mountains. Stockumites parallelus sp. nov. can easily be distinguished from the other species of the genus by the combination of the slender conch with subparallel flanks, the shell constrictions and the weakly biconvex growth lines.