Stockumites subbilobatus

Stockumites subbilobatus (Münster, 1839)

Figs 10–12; Tables 5–6

Goniatites subbilobatus Münster, 1839: 21, pl. 17 fig. 1.

Imitoceras subbilobatum – Schindewolf 1952: 291. — Librovitch 1940: 13, pl. 1 fig. 4. — Weyer 1977: 177, text-fig. 2.2.

Acutimitoceras subbilobatum – Korn 1994: 51, text-fig. 56g.

non Sporadoceras subbilobatum – Frech 1897: 177 g, text-fig. 3.

non Imitoceras subbilobatum – Librovitch 1940: 13, pl. 1 figs 1–3. — Balashova 1953: 191, pl. 11 figs 1–16. — Vöhringer 1960: 135, pl. 3 fig. 3, text-fig. 14.

non Prionoceras (Imitoceras) subbilobatum – Kullmann 1960: 528, pl. 7 fig. 5, text-fig. 16a. — Weyer 1965: 446, pl. 7 fig. 2.

non Imitoceras (Imitoceras) subbilobatum – Ruan 1981: 65, pl. 12 figs 14–16.

non Acutimitoceras subbilobatum – Barskov et al. 1984: pl. 1 figs 5–6, text-fig. 3. — Korn 1984: 76, pl. 2 figs 13–15, text-fig. 5e; 1992b: 15, pl. 1 figs 28–29, pl. 2 figs 13–14, 23–24; 1992c: 178, pl. 1 figs 2–3, pl. 2 figs 7–8; 1994: 51, text-figs 37d, 39, 42a–c, 44d–f, 47a, 50a, 53a–b, 58f. — Sheng 1989: 111, pl. 33 figs 1–2. — Schönlaub et al. 1992: 15, pl. 5 figs 28–29, pl. 6 figs 13–14, 23–24. — Kullmann 2000: text-fig. 4f. — Korn & Klug 2002: 197, text-fig. 173c, f. — Korn et al. 2003: 1125, text-fig. 3c. — Korn & Weyer 2003: pl. 2 figs 6–7, text-fig. 14d. — Bockwinkel & Ebbighausen 2006: 99, text-figs 15, 16a–d. — Korn & Feist 2007: 106 fig. 6a.

non Imitoceras (Acutimitoceras) subbilobatum – Kusina 1985: 43, pl. 3 fig. 5. — Bogoslovsky 1987: pl. 5 fig. 5, pl. 6 fig. 4.

non Stockumites subbilobatus – Becker et al. 2002: pl. 4 figs 6–7. — Becker & Weyer 2004: 18, text-fig. 3c.

Diagnosis

Species of Stockumites with a conch reaching 70 mm diameter. Conch at 5 mm dm thickly discoidal and evolute (ww/dm ~ 0.50, uw/dm ~ 0.50); at 15 mm dm thinly pachyconic, subinvolute (ww/dm ~ 0.65, uw/dm ~ 0.15); at 30 mm dm thickly discoidal, involute (ww/dm ~ 0.55, 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. Growth lines fine and lamellar, wide-standing, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with lanceolate external lobe and narrowly V-shaped adventive lobe.

Material examined

Holotype GERMANY • Upper Franconia, 400 m north-west of Kirchgattendorf; Münster Coll.; illustrated by Münster (1839b: pl. 17 fig. 1) and Korn (1994: text-fig. 56g); re-illustrated here in Fig. 10; SNSB BSPG AS VII 26.

Additional material GERMANY • 31 specimens; Upper Franconia, 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; BGRB X13385 – BGRB X13391, BGRB unnumbered • 1 specimen; Upper Franconia, 400 m north-west of Kirchgattendorf; bed 21c–d (“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31266.

Description

Holotype SNSB BSPG AS VII 26 is a poorly preserved specimen with about 70 mm conch diameter (Fig. 10). About two thirds of the last whorl belong to the body chamber. Especially the last whorl is strongly distorted, which has led to a markedly deviating morphology of the body chamber. The penultimate half whorl is less affected by deformation and allows statements about the conch morphology. At 49 mm diameter the conch is thinly discoidal (ww/dm=0.44). The apparently little deformed phragmocone shows subparallel flanks, which converge rather slowly to the continuously rounded venter.

Specimen BGRB X13385 is a moderately preserved conch 41 mm in diameter (Fig. 11A); it is somewhat deformed and shows the surface of the shell on the outer and inner sides in addition to the conch shape. The conch is thickly discoidal (ww/dm= 0.48) and almost completely involute. The umbilical margin is narrowly rounded; the flanks converge to the rather narrowly rounded venter. The shell bears fine, lamellar growth lines with a convex course. There are no constrictions on the shell surface or on the internal mould.

The two specimens BGRB X13386 (40 mm dm; Fig. 11B) and BGRB X13387 (36 mm dm; Fig. 11C) hardly differ from specimen BGRB X 13385 in conch morphology and ornamentation. Specimen BGRB X13389 shows the suture line, which possesses a narrow, lanceolate external lobe, a broadly rounded, slightly asymmetric ventrolateral saddle and a V-shaped adventive lobe (Fig. 12B).

The cross section of specimen BGRB X13388 allows the study of the conch geometry and its ontogeny up to a diameter of 41 mm (Fig. 12A). The ontogenetic changes are very conspicuous and characterised by the transition of a serpenticonic juvenile stage to a goniatitoid middle and adult stage. The conch is widely umbilicate (uw/dm> 0.50) with a kidney-shaped whorl profile up to about 6 mm diameter; thereafter the uw/dm ratio becomes rapidly smaller and already at 20 mm conch diameter the umbilicus is almost completely closed. At 20 mm diameter the conch is thinly pachyconic (ww/dm ~ 0.62) with broadly rounded, nearly parallel flanks and a broadly rounded venter. The ontogenetic trajectories of the ww/dm and uw/dm ratios are markedly triphasic (Fig. 12C–E), but the phases of these two parameters are not synchronous.

Remarks

Stockumites subbilobatus is a somewhat problematic species because the holotype is poorly preserved and therefore a comparison with other specimens, which are usually considerably smaller, is rather difficult. However, the apparently low number of species occurring in the Gattendorf section allows for a discussion about the identity of this species. It had therefore to be determined which of the smaller specimens, some of which have been cross-sectioned, correspond most closely to the conch morphology of the holotype. We choose such specimens, which, at a conch diameter of about 30–50 mm, most resemble the undeformed part of the holotype.

The restriction of the species S. subbilobatus on the basis of the holotype and some other specimens from Gattendorf leads to an extensive modification of the synonymy list; most of the records previously considered to belong to this species have to be excluded.

These include first and foremost the specimens from the Rhenish Mountains, which can no longer be identified as S. subbilobatus, but these belong to at least two other species:

(1) Specimens from the Stockum Limestone, its time-equivalent strata (Stockumites prorsus Zone) and the lower part of the Hangenberg Limestone (Acutimitoceras ucatum Zone): These specimens (e.g., Korn 1984, 1994) possess shell constrictions and a more strongly compressed conch; the ww/dm ratio is 0.45 at 30 mm conch diameter, in contrast to 0.50 in S. subbilobatus. This species is newly described by Korn & Weyer (2023) as Stockumites parallelus.

(2) Specimens from the middle part of the Hangenberg Limestone (Paprothites dorsoplanus and Pseudarietites westfalicus zones). These specimens (e.g., Vöhringer 1960; Korn 1994) have internal shell thickenings and a stouter conch than S. subbilobatus (ww/dm= 0.50–0.60). This species is newly redescribed by Korn & Weyer (2023) as Stockumites voehringeri.

Furthermore, many specimens have been placed in S. subbilobatus simply based on their slender conch shape (Librovitch 1940; Kullmann 1960; Ruan 1981; Barskov et al. 1984; Kusina 1985; Bogoslovsky 1987; Korn 1992b; Becker et al. 2002; Becker & Weyer 2004). These are listed here in the synonymy list; however, a discussion of all these forms is beyond the scope of this research.

Stockumites subbilobatus belongs to the species of the genus with a rather slender conch. Among the species known from Gattendorf, however, the conchs of S. parallelus and S. hofensis sp. nov. are even more slender; at 20 mm conch diameter, the ww/dm ratio is ~ 0.60 in S. subbilobatus, but only about 0.50 in S. parallelus and S. hofensis. In addition, S. parallelus has shell constrictions and S. hofensis shows clearly convergent flanks.

Stockumites intermedius has a very similar ornament with widely spaced lamellar growth lines, but has a stouter conch (ww/dm= 0.70–0.75 at 20 mm dm) and a very broadly rounded umbilical margin (narrowly rounded in S. subbilobatus).

A conch form similar to S. subbilobatus can be seen in S. hilarus. This species also has a rather slender conch (ww/dm ~ 0.60) and a very widely umbilicate juvenile conch, but in contrast to S. subbilobatus it bears conspicuous constrictions.